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CHEMISTRY - 2202.
UNIT 3 : Organic Chemistry Part 1
Classifying Organic Compounds.
Organic chemistry is the study of the molecular compounds of carbon. (Excluding:
oxides of carbon, carbonates, carbides, and cyanides.) ; also known as carbon
chemistry. Carbon compounds are found in living organisms; such as glucose
(C6H12O6) or blood sugar. The number of carbon compounds far exceeds all other
elements. Organic compounds are held together by molecular (covalent) bonds
within their molecules.
organic compounds
hydrocarbons
hydrocarbon derivatives
Organic vs. Inorganic : Organic molecules can be huge! Their size can range from
a few atoms to millions of atoms. These large molecules can have correspondingly
large masses and can have complex arrangements or structures. However, most of
the organic compounds that you will learn about are relatively small molecules. In
comparison to organic compounds, inorganic compounds generally contain a small
number of atoms; these compounds can range in size from two to a dozen or more
atoms/ions.
Comparison of organic and inorganic compounds.
Characteristic
Organic
Inorganic
Compounds
Compounds
Melting and Boiling Points
Generally low melting and
boiling points.
Generally high melting and
boiling points.
Stability/Reactivity When
Heated.
Generally react with oxygen
in the air when heated.
(Hydrocarbon combustion)
Generally stable up to very
high temperatures.
Most are non conductors both
in pure form and in aqueous
solution.
Many are conductors in
aqueous solution and when
melted.
Electrical Conductivity
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The inorganic forms of carbon (around 10%) are in the form of oxides of carbon,
carbonates, carbides, and cyanides. The organic forms of carbon (around 90%) can be
found in natural sources or can be man-made or synthetic sources.
Sources of Organic Compounds : There are three generally accepted sources of
organic compounds :
1) Nature (living organisms ;
2) invention / human ingenuity and
3) carbonized organic matter: coal, oil, and natural gas.
1. Carbonized Organic Matter: Coal, Oil, and Natural Gas.
Hundreds of millions of years ago, the organisms that inhabited earth were quite
different than those we find here today. Plants were fast growing and lacked the
woody tissues associated with the trees that currently dominate the world's
productive ecosystems. Giant plants with broccoli-like stems grew rapidly, died, and
decayed to form rich organic soils upon which more and more plants grew.
Eventually, thick layers of decomposing organic matter accumulated in much the
same way that peat bogs do today. Over time these massive organic layers were
buried under sediment, rock, or ice where they were subjected to tremendous
pressures. In this way, they were transformed into various types of coal.
Meanwhile in Earth's prehistoric shallow seas, simple organisms like algae, bacteria
and zooplankton thrived. As these tiny organisms died, they formed thick layers of
organic matter on the sandy bottoms of these seas. Compression of layer upon
layer of this material produced rocks known as shale. Under the tremendous
pressures from the layers above, and with the shifting of earths tectonic plates,
the organic matter trapped in these rocks was converted to oil and natural gas over
millions of years. The oil and gas migrated into porous rocks like sandstones or into
large pockets of space located kilometres below the earth's surface. Thus organic
matter from the past became today's fossil fuels.
Humans have known about fossil fuels for over 6000 years; however, only during
the past 300 years have they been utilized on a large scale. Coal was the first of
the fossil fuels to be extracted from the earth on a commercial basis. It was the
fuel that drove the steam engines of the industrial revolution in the 18th, 19th,
and 20th centuries.
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Through a process called destructive distillation, coal was converted into coke,
coal tar, and coal gas. Coke was used in the smelting of ores, coal tar was refined
into over 200 different carbon compounds, and coal gas was used for things like
street lighting!
Oil emerged as the dominant energy source for transportation in the 20th century.
Natural gas is becoming the clean alternative to coal for generating electricity. It
is also widely used as home heating and appliance fuel in North America. The
economies of the western world are now completely dependent on oil and natural
gas.
To some people, the burning of fossil fuels represents a tremendous waste. Not
only does this practice contribute to the build up of carbon dioxide in the
atmosphere, it also consumes that raw materials needed to make useful substances
like plastics. By some estimates, the world will virtually exhaust its supply of oil
and natural gas by 2050 - that's within your lifetime!
2. Nature : Living Organisms.
Every living organism is a source of organic compounds. Each species is capable of
producing a wide range of compounds, some of which are unique to that single
species. The scent of a rose, the taste of a strawberry, and the fuzziness of a
peach are the results of biochemical manufacturing processes within living things.
Given that there are hundreds of thousands of species on earth, nature represents
our most important source of organic compounds.
Humans have extracted and purified thousands of useful compounds from plants
and animals. For example, the penicillin used to fight bacterial infections is
extracted from a naturally occurring mold. Acetylsalicylic acid, commonly known as
aspirin, comes from the bark of a species of willow tree. Vanilla flavouring is
extracted from dried beans that come from a species of orchid called Vanilla
planifolia. The heart drug digitalis comes from a plant called Digitalis purpurea. The
list of examples goes on for volumes of pages.
3. Invention.
Antibiotics, aspirin, vanilla flavouring, and heart drugs are examples of substances
that no longer have to be obtained directly from nature. They are manufactured in
laboratories from organic starting materials. Furthermore, experiments in which
the chemical structures of naturally occurring substances are modified has
produced organic compounds substances that do not exist anywhere in nature.
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Each year over 250,000 new chemical compounds are discovered and many of
these are products of scientists' imaginations, exploration, and in some cases experiments gone wrong! Plastics are excellent examples of substances that are
the product of invention - they are not found anywhere in nature.
Why are there so many organic compounds? Carbon atoms form stable covalent
bonds with other carbon atoms. Carbon-to-carbon bonds are very strong - a lot of
energy is required to break them compared to other covalent bonds.
C
Electron dot diagram of a carbon atom
The number of possible organic compounds is indefinite because of:
1.
The stability of carbon to carbon bonds.
2.
The possibility for single, double, or triple carbon to carbon bonds,
and the many different combinations and arrangements of these
bonds.
3.
The variety of stable multiple carbon structures such as chains, rings,
spheres and sheets.
4.
The ability of carbon to form bonds with other elements to produce
hydrocarbon derivatives
HISTORY: Jön Jacob Berzelius, a Swedish chemist in the early 19th century,
classified compounds of the time into inorganic (from mineral sources) and organic
(from living things).
Before 1828, scientists believed that only living things had the ability to produce
organic compounds. They attributed the origins of organic compounds to a "vital
force", a key concept in the theory of vitalism. In 1828 however, Friedrich Wöhler
synthesized urea in the absence of any living agent (see below). Urea is an organic
compound that contains carbon, hydrogen, nitrogen and oxygen atoms; it is found in
urine. The synthesis of urea proved that organic compounds could be produced in a
lab. Wöhler's production of urea helped redefine organic chemistry to include
carbon containing compounds not made by living things.
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More on Wöhler's Experiment :
In 1828, Friedrich Wöhler was trying to prepare ammonium cyanate by reacting
silver cyanate with ammonium chloride.
AgOCN(aq)
silver cyanate
+
NH4Cl(aq)
➞
ammonium chloride
AgCl(s)
+ NH4OCN(aq)
silver chloride ammonium cyanate
Wöhler mixed the silver cyanate and ammonium chloride solutions together to form
a silver chloride precipitate. He filtered the silver chloride, and evaporated the
water in the filtrate to obtain some white crystals. Unexpectedly, these crystals
did not have properties of the ammonium cyanate. Instead, the crystals were
found to be identical to urea. The ammonium cyanate was not produced as expected
because during the heating process to evaporate water from the filtrate, the
atoms in the ammonium cyanate had rearranged to form urea.
60°C
NH4OCN(s)

(NH2)2CO(s)
ammonium cyanate
urea
Notice that urea and ammonium cyanate have the same empirical formula CH4ON2.
Ammonium cyanate is not an organic compound because it is ionic.
Since Wöhler, many discoveries have been made, such as: (1) acetic acid in 1845 ;
(2) sucrose in 1953 by the Canadian chemist, Raymond Lemieux.
Pure hydrocarbons:
 organic compounds that contain carbon and hydrogen atoms only.
We can divide pure hydrocarbons into two main groups:
 aliphatic hydrocarbons and aromatic hydrocarbons. (Each of these can be
further subdivided.)
 The aliphatic hydrocarbons have carbon atoms arranged in straight or
branched- chains. Rings of carbon atoms are also classified as aliphatic as
long as they lack the special carbon to carbon bonds associated with
benzene.
 Compounds that have bonding similar to benzene are classified as aromatic
hydrocarbons.
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hydrocarbons
aliphatic hydrocarbons
aromatic hydrocarbons
(including continuous-chained
and cyclic forms)
alkanes
alkenes
Benzene Compounds
(these have a special type of
bond not found in the
aliphatic hydrocarbons)
alkynes
Aliphatic hydrocarbons:
Continuous-chain hydrocarbons and their branched and cyclic forms. (These can
have combinations of single, double, or triple bonds between carbon atoms.)
Continuous-chain hydrocarbon: A hydrocarbon that contains any number of carbon
atoms in a continuous chain. (They are also known
as straight-chain hydrocarbons.)
Cyclic hydrocarbon: A hydrocarbon that consists of three or more carbon atoms
bonded together to form a ring.
Each type of aliphatic hydrocarbon has a general formula which shows the ratio of
carbon atoms to hydrogen atoms. The table below shows the general formulas for
some aliphatic hydrocarbons.
Type of
Alkane
Hydrocarbon
General formula
CnH2n+2
Alkene
Alkyne
(With one double
(With one triple
bond only)
bond only)
CnH2n
CnH2n – 2
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You can use these general formulas to determine the number of hydrogen atoms
bonded to a given number of carbon atoms. A general formula can also help you to
classify a compound when given a chemical formula.
NOTE :
Remember the general formula of a continuous-chain alkane is: CnH2n+2.
It is important to first identify the type of hydrocarbon from its
chemical formula by checking the subscripts for the ratio of carbon
to hydrogen atoms.
How to represent hydrocarbons?
 A common way to represent a hydrocarbon is to use a condensed structural
formula.
o For example, the chemical formula of C4H10 can be represented as:
CH3CH2CH2CH3 or CH3–CH2–CH2–CH3 (The dashes represent single
carbon-to-carbon bonds).
o A condensed structural formula still provides more information about
the physical arrangement of the compound than the chemical formula
 A complete structural formula — shows all atoms and bonds .
 A practical way for us to draw a hydrocarbon is to omit the hydrogen atoms
leaving only the carbon atoms and the lines to indicate where the bonds
occur — a carbon skeleton . (This is faster and neater than drawing in all of
the hydrogen atoms.)
 Also, a line structural diagram can be used to represent long chain of C atoms — the end of each line segment represents a C - atom and H - atoms
are not shown. See the examples below.
CH2 CH2CH2CH2CH2CH2CH2CH3
or
C C C C C C C C
Alkane is a pure hydrocarbon containing only single covalent bonds between carbon
atoms. Alkanes have the general formula CnH2n+2.
A branched-chain alkane is an alkane with one or more alkyl groups.
Homologous series is a series formed by a group of compounds where each
successive member differs by a “constant unit".
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Alkyl Groups : Alkyl groups have the general formula CnH2n +1. An alkyl group
may consist of one or more single bonded carbon atoms. It looks like an alkane
except that it has one less hydrogen atom. You can identify alkyl groups on a
continuous-chain alkane by finding the longest continuous chain and then locating
any carbons that do not appear to be part of that continuous chain. The prefix
system you memorized earlier is used in the naming of alkyl groups. A numbering
system is used to describe the location of an alkyl group on a continuous chain.
To name an alkyl group:
1.
Count the number of carbons in the alkyl group.
2.
Use the appropriate prefix to indicate the number of carbon atoms.
(These prefixes are the same as those used to indicate the number of
carbon atoms in any hydrocarbon.)
3.
Add the ending -yl to the prefix.
Notice that there is a hydrogen atom missing from a terminal carbon atom of this
alkyl (propyl) group.
CH 2 CH2 CH 3
or
H
H
H
C
C
C
H
H
H
H
This means that the terminal carbon has a bonding electron available to share with
a carbon in the continuous chain. The alkyl group shown above has three carbon
atoms. When you combine the prefix prop with the ending -yl, you get the name
propyl.
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Saturated hydrocarbon is a hydrocarbon which contains only single carbon to
carbon bonds or has the maximum number of hydrogen atoms possible.
Unsaturated hydrocarbon is a hydrocarbon which contains at least one double or
triple carbon to carbon bond. Unsaturated hydrocarbons contain less hydrogen
atoms than saturated ones.
Alkenes : A double bond is formed when two atoms share two electron pairs.
Hydrocarbons which contain at least one double bond between carbon atoms are
called alkenes. The general formula of a continuous-chain alkene is CnH2n. (That's
two hydrogen atoms less than in a continuous-chain alkane.)
Alkynes are pure hydrocarbons that have at least one triple carbon to carbon bond.
Alkynes are unsaturated hydrocarbons. Although it is possible for a hydrocarbon
to have more than one triple bond, we will study alkynes that have just one triple
bond. Alkynes with one triple bond have the general formula of CnH2n - 2. A good
example of an alkyne is ethyne (C2H2) which is more commonly known as acetylene.
Ethyne is used in oxyacetylene torches for welding.
Remember that a molecular formula is used to represent alkanes, alkenes and
alkynes; predict using the general formulas for alkanes, alkenes and alkynes as
mentioned earlier.
Physical Properties (b.p. and solubility) and Molecular Geometry of Alkanes,
Alkenes, and Alkynes :
Remember that the type of chemical bonding in a substance influences the
chemical and physical properties of that substance. There are two basic types of
bonding forces: intramolecular forces like covalent bonds, and intermolecular
forces like van der Waals forces (London dispersion force and dipole-dipole force)
and hydrogen bonding. In this lesson, you will focus on the intermolecular forces
present in hydrocarbons and try to establish a relationship between the presence
of these forces in a substance and the physical properties of that substance. The
physical properties you will consider are boiling points and solubility. Boiling points
are a generally accepted measure of the strength of intermolecular forces. The
solubility of a substance is determined by its compatibility with a solvent.
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Boiling Points of Hydrocarbons :
Hydrocarbons have relatively low melting and boiling points compared to inorganic
compounds of similar size. Since boiling point reflects the strength of
intermolecular forces, you can deduce that intermolecular forces in hydrocarbons
are weaker than those of other types of substances. The question is “why”?
Intermolecular forces are attractions between the individual molecules in a
substance. There are three basic types. London dispersion force is a function of
two things: the number of electrons per molecule in a substance and molecular
geometry (shape of the molecules). Generally, the strength of London dispersion
force increases with increasing numbers of electrons per molecule; however, the
type of molecular shape can also contribute to the magnitude of this force in a
substance. Dipole-dipole force is the attraction between the opposite poles of
neighbouring polar molecules. It is a factor contributing to higher boiling points in
polar substances only. Hydrogen bonding results from the presence of a highly
polar covalent bond between a hydrogen atom and a high electronegativity atom
such as nitrogen, oxygen, or fluorine. Hydrogen bonding has a significant influence
on boiling points. London dispersion force is present in all molecular substances
whereas dipole-dipole force and hydrogen bonding are found in polar substances.
Pure hydrocarbon molecules are non-polar. Even though the bonds between carbon
and hydrogen atoms are polar (carbon has an electronegativity of 2.5 and hydrogen
has an electronegativity of 2.1), the bond dipoles in hydrocarbons are equal and
cancel each other out, making the molecules non-polar. Therefore the factor which
determines the melting and boiling points of any hydrocarbon is London dispersion
force. See the bond dipoles in the following.
C
C
ethane
C
ethene
C
C
C
ethyne
Generally, as the number of carbon atoms per hydrocarbon molecule increases in a
homologous series, the boiling points tend to increase. The table below shows a
similar relationship using numbers of electrons per molecule in pure substances. It
shows number of electrons and boiling points of some alkanes.
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NOTE :
Compound
Number of Electrons
ethane
propane
butane
pentane
18
26
34
42
Boiling Point
( C)
-88.5
-42.0
-0.5
36.0
A similar trend occurs in the alkene and alkyne series.
Generally, continuous chain hydrocarbons containing one to four carbon atoms per
molecule are gases at room temperature, while those with five to seventeen carbon
atoms per molecule are liquids, and those with eighteen or more carbon atoms per
molecule are solids. This trend varies with the presence of one or more multiple
bonds per molecule.
Compare the boiling points of the three members of each homologous group. The
homologous group with two carbon atoms includes ethane, ethene, and ethyne; their
chemical formulas are C2H6, C2H4, and C2H2 respectively. You would expect that
since ethyne has the lowest number of electrons, it will also have the lowest boiling
point; however, it has the highest boiling point in its homologous group. This
suggests that the shape of the molecule must affect the strength of London
dispersion forces which in turn affects the boiling point. The linear shape of
ethyne molecules enables them to get closer together. The closer molecules can
get, the greater the electrostatic forces that hold them together in the
substance. In other words, London dispersion forces increase when molecules can
fit close together. Recall the shapes around the carbon to carbon bonds in ethane,
ethene and ethyne.
Shapes around Carbon Atoms Involved in Single, Double or Triple Bonds :
Do you remember how to draw electron dot (Lewis) diagrams for atoms and
molecules?
Electron dot diagrams show only the valence electrons of an atom. (The number of
valence shell electrons of an element corresponds to that element's group number
on the periodic table.) The rest of the atom (other inner electrons and the
nucleus) is represented by the element's chemical symbol.
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To draw an electron dot diagram for a molecule, determine which atom is the
central atom (the one that has the most bonding electrons) and draw it first.
Arrange the rest of the atoms around the central atom, pairing up the bonding
electrons. For example, to draw the electron dot diagram for methane, CH4, start
by drawing the diagrams for each type of atom:
C
H
Since carbon has four bonding electrons, it is the central atom, draw it first and
then arrange the hydrogen atoms around it. Notice that all electrons are paired.
H
H
C
H
H
Groups of valence electrons between atoms are called bonding groups. Pairs of
electrons that are not located between atoms are called lone pairs. See the table
below.
Number of Bonding
Number of Lone
Groups
Pairs
Example
Name of shape
tetrahedral
4
3
0
0
C
C
C
2
0
trigonal planar
linear
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Your electron dot diagram for ethane should show that each carbon atom in the
ethane molecule has four bonding groups and zero lone pairs of electrons. This
results in a geometric arrangement called a tetrahedron In a tetrahedral
arrangement, the central atom is bonded to four other atoms such that the bond
angles are about 109.5o. See the figure (tetrahedron) below.
The easiest way to illustrate a tetrahedral arrangement is to use your molecular
model kit to build a model of methane (CH4). Each of the four hydrogen atoms are
in the four "corners" of a tetrahedron (a four-sided geometrical shape).
Now look at your electron dot diagram of ethene. Each carbon atom has three
bonding groups and zero lone pairs of electrons. The electron groups around each
carbon are furthest apart at bond angles of 120°. This results in a trigonal planar
arrangement of atoms around each carbon in the molecule. All atoms in this
molecule are in the same plane, so the molecule is flat.
Finally, look at the electron dot diagram of ethyne. Each carbon atom in the
molecule has two bonding groups and zero lone pairs of electrons. This results in a
linear arrangement around each carbon. The two bonding groups are furthest
apart at a bond angle of 180o. In a linear arrangement, the central atom and two
other atoms form a straight line. The linear arrangement of the carbon atoms in
the triple bond allows the ethyne molecules to get closer together, and causes an
increase in the electrostatic attractions. This results in a higher boiling point than
in ethene and ethane.
Solubility of Pure Hydrocarbons :
Solubility of a substance is its ability to dissolve in a particular solvent. We know
that alcohol is soluble in water and that oil is not soluble in water. How can we
explain this?
The rule of thumb is that like dissolves like. In other words, non-polar compounds
tend to be soluble in non-polar solvents and polar compounds are soluble in polar
solvents.
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Water is a polar solvent; it will dissolve polar compounds like methanol. See the
structural formulas of water and methyl alcohol below showing molecular polarity.
H
H
O
H
H
water
C
H
O
H
methyl alcohol
Octane is a non-polar substance; therefore, it will not dissolve in a polar solvent
such as water, but it will dissolve in a non-polar solvent like hexane or carbon
tetrachloride, CCl4(l).
Naming and Writing Organic Compounds.
To draw a structural formula for any continuous-chain hydrocarbon:
1.
Determine the number of carbon atoms in the molecule by looking at the
subscript in the chemical formula.
2.
Draw the carbon atoms in a straight line. Draw a line between each atom to
represent a single covalent bond.
3.
Draw single lines from carbon atoms to hydrogen atoms. Each carbon atom
should have four single bonds and each hydrogen must have just one single
bond.
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Nomenclature (Naming) of Aliphatic Hydrocarbons :
Prefixes used to indicate the number of carbon atoms in an organic
compound.
Number
Prefix
1
meth
2
eth
3
prop
4
but
5
pent
6
hex
7
hept
8
oct
9
non
10
dec
IUPAC Rules for Naming Aliphatic Hydrocarbons.
RULES for NAMING BRANCHED CHAIN ALKANES:
1.
Identify the parent chain — longest continuous chain of C - atoms.
2.
Number the C - atoms of the parent chain from the end closest to the
branch.
3.
Identify the branch and note their position on the chain. If a group (branch)
shows up more than once, use prefixes such as di- (2), tri- (3), tetra- (4) etc.
4.
List the names of alkyl groups in alphabetical order.
5.
Use proper punctuation —
commas are used to separate numbers
(i)
hyphens are used to separate numbers and
words
(ii)
name of alkane is written as one word
NOTE :
I:
To convert a name to a formula:
i)
Find root word in hydrocarbon name.
ii)
Number C - atoms in parent chain.
iii)
Identify alkyl groups.
iv)
Add H’s as needed.
II : Complete IUPAC name — format:
(Number of location) - (branch name) (parent chain)
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ANOTHER EXPLANATION — To name branched-chained alkanes:
1.
2.
3.
4.
5.
6.
7.
Find the longest continuous chain of carbons in the molecule and name it.
This is the parent chain of the molecule. Be careful, the longest continuous
chain is not always obvious because it may be bent. (HINT! highlight the
parent chain in some way.)
Number the carbons in the parent chain. To do so, locate the end to which
branching is closest. Designate this carbon as number 1 .
List the alkyl groups present.
If there is more than one type of alkyl group in the molecule, list their
names in alphabetical order. In alphabetical order, but precedes eth, and so
on. For the purpose of alphabetizing ignore the latin prefixes di, tri, etc.
If there are two or more of the same alkyl group, indicate this by using latin
prefixes. These prefixes are the same ones that you learned when naming
molecular compounds (di = twice, tri = three times, tetra = four times, and
penta = five times.)
Use numbers to indicate the location of the alkyl groups on the parent chain.
Every alkyl group is assigned a number.
Use proper punctuation. Commas are used to separate numbers, hyphens are
used to separate numbers and letters.
RULES for NAMING ALKENES and ALKYNES:
To name a continuous-chain alkene with one double bond:
1.
Count the number of carbon atoms in the longest continuous chain. Use the
appropriate prefix to indicate the number of carbon atoms.
2.
Add the ending -ene to the prefix. This ending indicates that there is a
double bond present in the molecule.
3.
Find the location of the double bond and assign it the lowest possible number
in the parent chain. (You may count from either left to right or right to
left; whichever gives you the lowest possible number for the double bond.)
4.
Combine the number from step 3 with the name from steps 1 and 2 and
separate them with a hyphen.
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To name a branched-chain alkene with one double bond:
1.
Count to find the longest continuous chain of carbon atoms that contains the
double bond. Name it as the corresponding alkene, giving the double bond
the lowest possible number.
2.
List the alkyl groups present. List them alphabetically. For the purpose of
alphabetizing, ignore the prefixes di, tri, etc.
3.
Use prefixes if there are two or more alkyl groups of the same type.
4.
Use a number to indicate the location of each alkyl group on the parent
chain. Remember, when assigning numbers, the double bond is given priority
over the alkyl groups.
5.
Use proper punctuation. Commas are used to separate numbers hyphens are
used to separate numbers and letters.
To name a continuous-chain alkyne with one triple bond:
1.
Count to find the longest continuous chain of carbon atoms. Use the
appropriate prefix to indicate the number of carbon atoms.
2.
Add the ending -yne to the prefix to indicate the presence of a triple bond.
3.
Assign the lowest possible number for the location of the triple bond.
4.
Add the number to the name but use a dash to separate the number from
the alkyne name.
To name a branched-chain alkyne with one triple bond:
1.
Count to find the longest continuous chain of carbon atoms that contains the
triple bond. Name it as you would the corresponding alkyne, giving the triple
bond the lowest possible number.
2.
List the alkyl groups present either alphabetically. For the purpose of
alphabetizing, ignore the prefixes di, tri, etc.
3.
Use prefixes to indicate the presence of two or more alkyl groups of the
same type.
4.
Use numbers to indicate the location of the alkyl groups on the parent chain.
When assigning numbers, the triple bond is given priority over the alkyl
groups.
5.
Use proper punctuation. Commas are used to separate numbers, hyphens are
used to separate numbers and letters.
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Alicyclic hydrocarbons :
Non-benzene rings of three or more carbon atoms are known as alicyclic
hydrocarbons and these are further classified based on the types of bonding
within the rings.
Cyclic Alkanes : Cyclic alkanes have two less hydrogen atoms than their
corresponding continuous-chain alkanes. The general formula for a cyclic alkane is
CnH2n. which is the same general formula for an alkene with one double bond. Cyclic
alkanes may have one or more side branches of carbon atoms; however, we will
focus on the unbranched cyclic alkanes in this course.
Cyclobutane, a cyclic alkane, can be represented in the following ways:
C
C
C
C
CH 2
CH 2
or
or
CH 2
CH 2
All three ways are acceptable. In the geometric figure, each point or corner
represents the location of a carbon atom.
To name cyclic alkanes:
1.
Count the number of carbon atoms in the ring and name the structure as you
would name the corresponding continuous-chain alkane.
2.
Add the prefix cyclo to the alkane name.
Isomers in Organic Chemistry.
Structural Isomers : Sometimes there are two or more possible structural
formulas for a single chemical formula. You might have noticed that chemicals with
different IUPAC names have the same chemical formula. For example, octane and
trimethylpentane both have the formula C8H18 ; however, these molecules have
different structural formulas. Such substances with different arrangements of
atoms for the chemical formula are called structural isomers. Structural isomers
may also have very different chemical and physical properties. For example, 1butanol and diethyl ether are isomers of C4H10O, however, 1-butanol is miscible
with water while diethyl ether is immiscible.
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There is another kind of isomer that has to be studied. Geometric isomers have
the same atoms bonded in the same way but arranged differently in space. Cis and
trans isomers would be examples of geometric isomers. They differ in their
geometrical orientation around a double bond that can't rotate. The cis isomer has
its substituents on the same side of the double bond while the trans isomer has its
substituents on opposite sides of the double bond.
cis- 1,2 - dibromoethene
trans- 1,2 - dibromoethene
Try this! Draw geometric isomers for bromochloroethene and fluoroiodoethene.
Structural isomerism is the existence of two or more compounds with the same
molecular formula but different structural formulas. The first three alkanes have
just one possible structural formula. However, butane (C4H10) has two possible
structural isomers as shown below:
C
C
C
C
butane
C
C
C
C
methylpropane or
2-methylpropane
There is no need to use a number to indicate the location of the methyl group on
methylpropane because there is only one possible location - on carbon 2.
(Remember an alkyl group cannot be attached to a terminal carbon.) You will have
to use numbers to indicate the location of the alkyl group(s) in larger isomers. You
will also notice that as the number of carbon atoms in a hydrocarbon increases, the
number of structural isomers possible also increases. There are 75 possible
isomers of C10H22, 366 319 for C20H42, and 4 111 846 763 for C30H62.
20
Cyclic Alkenes : Recall that carbon's ability to form double and triple bonds
contributes to the diversity of organic compounds. Like alkanes, many alkenes have
structural isomers. An alkene with the formula C2H4 does not have a structural
isomer because there is only one possible location for the double bond. However,
the chemical formula of C3H6 has two possible structures. One isomer is a
continuous-chain alkene and the other is a cyclic alkane. (Remember the general
formula for a cyclic alkane? It is the same as the general formula for a
continuous-chain alkene with one double bond: CnH2n.)
Structural isomers of C3H6 :
C
C
C
C
C
C
1-propene or propene
C3H6
cyclopropane
C3H6
Like alkanes, alkenes can exist in cyclic forms. A cyclic alkene with one double
bond will have two less hydrogen atoms than the corresponding continuous-chain
alkene. The general formula for a cyclic alkene is CnH2n - 2.
Structural, condensed, and geometric representations of cyclobutene.
H
H
C
C
H
CH
CH 2
or
C
C
H
H
H
C4H6 : cyclobutene
or
CH
CH 2
21
To name cyclic alkenes with one double bond:
1.
Count to find the number of carbon atoms in the ring and name as the
corresponding continuous-chain alkene. (There is no need to indicate the
location of the double bond.)
2.
Add the prefix cyclo to the alkene name.
Isomerism in Alkynes: Molecules having the general formula of CnH2n - 2 can have
structural isomers if they contain three or more carbon atoms. (There is just one
possible structure for the chemical formula C2H2.) The isomers of a hydrocarbon
with the general formula of CnH2n - 2 can be continuous-chain alkynes with one triple
bond and cyclic alkenes with one double bond. The figure below shows the
structural isomers for the chemical formula C3H4. Note that each isomer has
exactly the same number of carbon and hydrogen atoms.
Isomers of C3H4 :
C
H
C
C
C
C3H4
1-propyne or
propyne
C
C
C3H4
cyclopropene
Try this! Draw all the isomers of C6H14. (Hint: There are five possible isomers).
Core Lab : “Properties of Isomers”
22
Writing and Balancing Chemical Equations.
Organic Reaction Types for Hydrocarbons :
1.
Cracking :
large molecules
catalyst
>
heat
C17H36(l) 
2.
smaller molecules
C9H20(l) + C8H16(l)
Reforming :
small molecules   larger molecules (with more branches)
catalyst
C5H12(l) + C5H12(l)
3.
 
heat
C10H22(l) +
H2(g)
Complete Combustion / Hydrocarbon Combustion (hc) : The following
groups,(alkanes / alkenes / alkynes)can undergo complete combustion to
produce mainly CO2 and H2O. When sufficient amounts of oxygen are
available, the combustion of hydrocarbons is complete resulting in the
production of carbon dioxide and water vapour. The general form of the
equation is:
hydrocarbon + oxygen gas  carbon dioxide + water vapour
compound
+ O2(g)  most common oxides
2 C8H18(l) + 25 O2 (g)  16 CO2(g) + 18 H2O(g) + Energy
When the amount of oxygen available is insufficient, the combustion is
incomplete and poisonous carbon monoxide gas is produced.
C3 H8( g ) 
7
2
O2( g )  3 CO( g )  4 H2 O( g )
In this course, you can assume that most hydrocarbon combustion reactions
are complete.
23
4.
Addition or Hydrogenation : Alkenes and alkynes are unsaturated
hydrocarbons containing at least one double or triple bond respectively.
They do not undergo substitution reactions; instead, they undergo addition a reaction in which substituents are added to both carbons involved in the
multiple bond. Alkenes and alkynes are chemically more reactive than alkanes
because of the presence of the multiple carbon to carbon bonds; (alkenes
more reactive than alkanes ; alkynes more reactive than alkenes).
In addition reactions, no hydrogen atoms are removed from the
hydrocarbon. Substituents are bonded to the hydrocarbon using the bonding
electrons that make up the multiple bond. In alkenes, a double bond is
reduced to a single bond and in alkynes, a triple bond is reduced to either a
double or a single bond depending on the amount of the substituent available
for addition.
Alkene or Alkyne + H2(g)  Alkane
CH2 = CH - CH2 - CH3 + H - H
 CH3 - CH2 - CH2 - CH3
CH


C - CH2 - CH3 + 2 H - H
CH


C - CH2 - CH3 + H - H  CH2 = CH - CH2 - CH3

CH3 - CH2 - CH2 - CH3
NOTE : In the above addition reactions, double bond or triple bond is eliminated
and a saturated compound is formed. Addition reaction of an alkyne occurs in two
steps — 1) If # of moles of added reagent equals # of moles of alkyne, then a
substituted alkene is formed (ie.) for one mole of added reagent ; 2) If two or
moles of reagent are added, then a substituted alkane is produced.
5.
Substitution : A substitution reaction occurs when a hydrogen atom is
removed from the hydrocarbon and replaced by a halide substituent. The
products are a hydrocarbon derivative and a hydrogen halide. A key point to
remember about substitution reactions is that a hydrogen atom has to be
removed from the hydrocarbon before a substituent can be added.
Remember that one element takes the place of another element — in this
case, H). It can occur in alkanes, cycloalkanes and aromatics.
24
Alkane +
halogen   
light
Aromatic + halogen  
light
Organic halide
+
Aromatic halide +
hydrogen halide
Hydrogen halide
Industrial Processes : Hydrocarbons.
Crude oil is homogeneous mixture of many different organic compounds. The
individual compounds or groups of compounds are called fractions. When crude oil
is refined, these miscible fractions are separated from each other. Separation is
achieved by heating crude oil so that the various fractions boil off and condense at
different heights in a distillation tower (like the ones you see in Come By Chance,
NF). This process is repeated over and over in various towers until the crude oil is
separated into as many different fractions as possible.
The separation of crude oil on the basis of the different boiling points of its
fractions is called fractional distillation (or fractionation). Can you relate this
process to differences in the strengths of intermolecular forces among the
fractions in a crude oil sample?
Cracking Reactions break hydrocarbons into smaller components (in the absence of
air). There are two types :1) thermal cracking (occurs at high temperatures)
2) catalytic cracking (thermal cracking sped up by the addition of a catalyst)
Reforming (actually catalytic reforming) Reactions form larger molecules from
smaller molecules — (ie.) the opposite of cracking.
NOTE :Above reactions can be found in alkanes, alkenes, alkynes and
aromatics.
25
FRACTIONAL DISTILLATION : We know that crude oil is a complex mixture
of hydrocarbons and each hydrocarbon species is called a fraction. When crude oil
is refined, the fractions of the mixture are separated from each other. Separation
is achieved by heating the crude oil so that each fraction evaporates at its boiling
point and rises through a fractionation tower. As it rises, the fraction loses heat
and condenses. The towers are designed so that fractions condense at certain
points in the tower so that they can be reclaimed as liquids for further processing.
Generally, the low molecular weight (low molar mass) hydrocarbons evaporate first
and rise to the highest levels of the tower. The opposite is true for high molecular
weight compounds. Some compounds do not evaporate. These are reclaimed as tar
and are used to make things like asphalt.
Fractional Distillation Tower : A fractional distillation tower contains trays
positioned at various levels. Heated crude oil enters near the bottom of the tower.
The bottom of the tower is kept hot, and the temperature gradually decreases
towards the top of the tower. The lower the boiling point of a fraction, the higher
the tray on which it condenses.
Petroleum Fractions :
- Fractions with the lowest b.p.’s contain the smallest molecules due to fewer
electrons and weaker London dispersion forces.
- Fractions with higher b.p.’s contain larger molecules and stronger London
dispersion forces.
Aromatic Hydrocarbons:
The aromatic hydrocarbons are a class of compounds that include benzene,
benzene derivatives, and other cyclic structures with similar bonding to benzene.
Benzene is the best known member of this class. An aromatic hydrocarbon is a
hydrocarbon which has bonding similar to benzene.
Many aromatic hydrocarbons have distinctive odours, hence their name. They are
very important in the manufacturing of styrene plastics, octane enhancers for
gasoline, detergents, and some medicines. Most of the benzene produced
industrially is used to produce ethylbenzene which is the starting material for
styrene plastics.
26
The chemical formula for benzene is C6H6. Note that this formula does not
match any of the general formulas for aliphatic hydrocarbons we have encountered
so far. Benzene molecules consist of a ring of six carbon atoms each of which is
bonded to a hydrogen atom. The ring concept has been refined considerably since
it was first proposed by August Kekulé in 1865. He proposed a flat, hexagonal ring
of six carbons joined by alternating single and double bonds. His idea had to
explain these observations:
All the carbon-carbon bonds in the ring are the same length, somewhere in
between the longer single C-C bonds and the shorter double C=C bonds.
The benzene molecule is planar; all of its atoms are in the same plane.
Benzene behaves like alkanes in chemical reactions, not like the alkenes.
(More on the chemical reactions of hydrocarbons later.)
Since a theory based on alternating single and double bonds alone does not support
these observations, Kekulé used resonance theory to refine the model. Resonance
occurs when two equally valid structures can be drawn for a molecule as shown
below:
or
Kekulé proposed that the single and double bonds oscillated from one position to
another to give an “average” of the two structures. These “averages” are
represented in a number of ways including:
or
The current explanation for the structure of the benzene ring involves
hybridization of valence orbitals in which valence electrons are shared equally by
all six carbons in the ring. Try to picture it this way. Each carbon has four valence
electrons. One of these is used to form a single covalent bond with a hydrogen
atom. The remaining unpaired bonding electrons are used to join the carbons
27
together to form a ring. Since you know that there are no single and double
bonds, you can picture the electrons as being delocalized - that is - equally shared
among the carbon atoms of the ring. The sharing of delocalized electrons results
in very strong hybrid bonds between the carbon atoms of the ring. The benzene
molecule is commonly represented as a hexagon with an inscribed circle. The circle
can be either a solid or dotted line and in either case, it represents the delocalized
electrons which are equally shared around the carbon ring. See the common
representation of a benzene molecule below.
Physical Properties of Benzene : Benzene is a liquid at room temperature. Its
boiling point is 80.1°C , which is comparable to the boiling point of cyclohexane,
81.4°C. Benzene molecules are entirely planar. Since each carbon atom has three
bonding groups and zero lone pairs, the bond angles are 120°. Benzene is non-polar
and is therefore soluble in non-polar solvents as is the case for all other pure
hydrocarbons.
Benzene Compounds With Side Branches :
One or more hydrogen atoms of the benzene molecule may be substituted with an
alkyl group. The resulting compound is called an alkyl benzene. Up to six of
benzene's hydrogen atoms can be substituted or replaced by a substituent group.
You will focus on mono and disubstituted benzene compounds.
Monosubstituted Alkyl Benzenes : A benzene compound in which one hydrogen is
replaced by an alkyl group is called a monosubstituted alkyl benzene. See the
examples below.
28
C
C
C
CH3
Notice in the figure above that each alkyl group is attached by a terminal carbon
atom. The name of an alkyl benzene is written as one word.
To name an alkyl benzene in which the alkyl group is attached to the benzene
ring by a terminal carbon atom:
1.
2.
3.
Use benzene as the parent name.
Name the alkyl group.
Add the alkyl group's name to benzene as a prefix. (There is no need to
indicate the position of the alkyl group because all of the carbon atoms in
the benzene ring are equivalent.)
Disubstituted Alkyl Benzenes : Two hydrogen atoms from the benzene ring may be
replaced by alkyl groups to form a disubstituted alkyl benzene. The alkyl groups
may be the same or different. There are three possible arrangements for two
alkyl groups on a benzene ring, so there are three possible isomers for a given
compound. For this reason, the relative positions of the alkyl groups must be
indicated in the compound's name. See the examples that follow showing
arrangements of alkyl groups on a benzene ring.
C
C
C
1
6
C
1
6
2
5
2
2
5
3
5
3
4
1
6
4
3 C
4
C
29
Notice that the numbering of carbon atoms on the benzene ring starts at an alkyl
group to give lowest possible numbers. There are only three possible combinations:
1 and 2, 1 and 3, and 1 and 4. Convince yourself of this by counting counterclockwise.
There are two methods to indicate the alkyl groups' positions:
1.
Use numbers to indicate the position of the alkyl groups (this is the
preferred IUPAC method.)
2.
Use prefixes to indicate the position of the alkyl groups:
 ortho (abbreviated o) for attachment to carbons 1 and 2
 meta (abbreviated m) for attachment to carbons 1 and 3
 para (abbreviated p) for attachment to carbons 1 and 4
Example : The three isomers of dimethylbenzene.
C
C
C
C
C
1,2-dimethylbenzene
or
o-dimethylbenzene
1,3-dimethylbenzene
or
m-dimethylbenzene
C
1,4dimethylbenzene
or
p-dimethylbenzene
In the figure shown, the alkyl groups are the same. If the alkyl groups differ, the
same naming rules apply. The alkyl groups may be listed either alphabetically or by
order of increasing complexity. In either case, the first alkyl group in the name is
designated as carbon number one.
30
Phenyl Group: There are instances where a benzene ring is connected to a nonterminal carbon of an alkyl group and others where more than one benzene ring is
connected to an alkyl group. In these cases, the alkyl group becomes the parent
and the benzene ring(s) become the branch(es). As a branch, benzene is called a
phenyl group. For example, this structural formula represents phenylpropane.
C
C
C