College of Dentistry
Organic Chemistry
Assistant Lecture
Aayad Amaar
An Introduction to Organic Chemistry: The Saturated Hydrocarbons
Organic chemistry is the study of carbon-containing compounds. The bonds between
carbon and another atom are almost always covalent bonds, whereas the bonds in many
inorganic compounds are ionic bonds. Ionic bonds result from the transfer of one or more
electrons from one atom to another. Thus, ionic bonds are electrostatic, resulting from the
attraction between the positive and negative ions formed by the electron transfer. Covalent
bonds are formed by sharing one or more pairs of electrons.
The most general classification of organic compounds divides them into hydrocarbons and
substituted hydrocarbons. A hydrocarbon molecule contains only carbon and hydrogen. A
substituted hydrocarbon is one in which one or more hydrogen atoms is replaced by another
atom or group of atoms.
The Chemistry of Carbon
The number of possible carbon-containing compounds is almost limitless. Why are there so
many organic compounds? There are several reasons. First, carbon can form stable, covalent
bonds with other carbon atoms. Consider three of the allotropic forms of elemental carbon:
graphite, diamond, and buckminsterfullerene. Models of these allotropes are shown in Figure.
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Organic Chemistry
Alkanes:
Alkanes are saturated hydrocarbons; that is, alkanes contain only carbon and hydrogen
bonded together through carbon-hydrogen and carbon-carbon single bonds. CnH2n+2 is the
general formula for alkanes. In this formula, n is the number of carbon atoms in the molecule.
Structure and Physical Properties
Three types of formulas, each providing different information, are used in organic
chemistry: the molecular formula, the structural formula, and the line formula. The molecular
formula tells the kind and number of each type of atom in a molecule but does not show the
bonding
pattern.
Consider the molecular formulas for simple alkanes:
The advantage of a structural formula is that it shows the complete structure, but for large
molecules it is time-consuming to draw and requires too much space. The compromise is the
condensed formula. It shows all the atoms in a molecule and places them in a sequential
order that indicates which atoms are bonded to which. The following are the condensed
formulas for the preceding five compounds.
The simplest representation of a molecule is the line formula. In the line formula we
assume that there is a carbon atom at any location where two or more lines intersect. We also
assume that there is a carbon at the end of any line and that each carbon in the structure is
bonded to the correct number of hydrogen atoms. Compare the structural and line formulas
for butane and 2-methylpropane, shown here:
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Organic Chemistry
Example:
Nomenclature
The International Union of Pure and Applied Chemistry (I.U.P.A.C.) is the organization
responsible for establishing and maintaining a standard, universal system for naming organic
compounds. The system of nomenclature developed by this group is called the I.U.P.A.C.
Nomenclature System. The following rules are used for naming alkanes by the I.U.P.A.C.
system.
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Organic Chemistry
1. Determine the name of the parent compound, the longest continuous carbon chain in the
compound. Notice that these names are made up of a prefix related to the number of carbons
in the chain and the suffix -ane, indicating that the molecule is an alkane. Write down the
name of the parent compound, leaving space before the name to identify the substituents.
Parent chains are high lighted in yellow in the following examples:
2. Number the parent chain to give the lowest number to the carbon bonded to the first group
encountered on the parent chain, regardless of the numbers that result for the other
substituents.
3. Name and number each atom or group attached to the parent compound. The number tells
you the position of the group on the main chain, and the name tells you what type of
substituent is present at that position. For example, it may be one of the halogens [F-(fluoro),
Cl-(chloro), Br-(bromo), and I-(iodo)] or an alkyl group. In the following examples the parent
chain is highlighted in yellow:
4. If the same substituent occurs more than once in the compound, a separate position number
is given for each, and the prefixes di-, tri-, tetra-, penta-, and so forth are used, as shown in
the following examples:
5. Place the names of the substituents in alphabetical order before the name of the parent
compound, which you wrote down in Step 1. Numbers are separated by commas, and
numbers are separated from names by hyphens. By convention, halogen substituents are
placed before alkyl substituents.
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Example:
Organic Chemistry
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Organic Chemistry
ISOMERIC ALKANES: THE BUTANES
Methane is the only alkane of molecular formula CH4, ethane the only one that is C2H6, and
propane the only one that is C3H8. Beginning with C4H10, however, constitutional isomers are
possible; two alkanes have this particular molecular formula. In one, called n-butane, four
carbons are joined in a continuous chain. The n in n-butane stands for normal and means
that the carbon chain is unbranched. The second isomer has a branched carbon chain and is
called isobutane.
HIGHER n-ALKANES
n-Alkanes are alkanes that have an unbranched carbon chain. n-Pentane and n- hexaneare
n-alkanes possessing five and six carbon atoms, respectively.
Their condensed structural formulas can be abbreviated even more by indicating within
parentheses the number of methylene groups in the chain. Thus, n-pentane may be written as
CH3(CH2)3CH3 and n-hexane as CH3(CH2)4CH3. This shortcut is especially convenient with
longer-chain alkanes. The laboratory synthesis of the ultralong alkane CH3(CH2)CH3 was
achieved in 1985; imagine trying to write a structural formula for this compound in anything
other than an abbreviated way!
PROBLEM 2.2
An n-alkane of molecular formula C28H58 has been isolated from a certain fossil plant.
Write a condensed structural formula for this alkane. n-Alkanes have the general formula
CH3(CH2)xCH3 and are said to belong to a homologous series of compounds. A homologous
series is one in which successive members differ by a
group. Unbranched alkanes are
sometimes referred to as straight-chain alkanes, but, their chains are not straight but instead
tend to adopt the zigzag
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Organic Chemistry
PROBLEM 2.3 Much of the communication between insects involves chemical messengers
called pheromones. A species of cockroach secretes a substance from its mandibular glands
that alerts other cockroaches to its presence and causes them to congregate. One of the
principal components of this aggregation pheromone is the alkane shown in the bond-line
formula that follows. Give the molecular formula of this substance, and represent it by a
condensed formula.
THE C5H12 ISOMERS
Three isomeric alkanes have the molecular formula C5H12. The unbranched isomer is, as we
have seen, n-pentane. The isomer with a single methyl branch is called isopentane. The third
isomer has a three-carbon chain with two methyl branches. It is called neopentane.
Table 2.3 presents the number of possible alkane isomers as a function of the number of
carbon atoms they contain. As the table shows, the number of isomers increases enormously
with the number of carbon atoms and raises two important questions:
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Organic Chemistry
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