Hydrocarbons in Health Sciences ( by Dr. Alaa J.Mahrath)
This picture representing a petroleum refinery. Petroleum, along with natural gas, provides
nearly 90% of the organic materials for the synthesis and manufacture of synthetic fibers,
plastics, drugs, dyes, adhesives, paints, and a multitude of other products.
2.1. How Do We Write Structural Formulas of Alkanes?
Alkanes are members of a larger class of organic compounds called hydrocarbons
.
Figure (2.1): The four classes of hydrocarbons
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Hydrocarbons in Health Sciences ( by Dr. Alaa J.Mahrath)
In this picture, Butane, CH3CH2CH2CH3, is the fuel
in this lighter. Butane molecules are present in both
the liquid and gaseous states in the lighter.
Table (2.1) : The First Ten Alkanes with Unbranched Chains
2.2. What Are Constitutional Isomers?
Constitutional isomers are compounds that have the same molecular formula but different
structural formulas. By “different structural formulas” we mean that they differ in the kinds of
bonds (single, double, or triple) and/ or in the connectivity of their atoms. For the molecular
formulas CH4, C2H6, and C3H8, only one connectivity of their atoms is possible, so there are no
constitutional isomers for these molecular formulas. For the molecular formula C4H10, however,
two structural formulas are possible: In butane, the four carbons are bonded in a chain; in 2methylpropane, three carbons are bonded in a chain with the fourth carbon is a branch on the
chain. The two constitutional isomers with the molecular formula C 4H10 are drawn here both as
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Hydrocarbons in Health Sciences ( by Dr. Alaa J.Mahrath)
condensed structural formulas and as line-angle formulas. Also shown are ball-and-stick models
of each.
Example: Do the structural formulas in each of the following sets represent the same
compound or constitutional isomers?
Solution
(a) Each structural formula has an unbranched chain of six carbons; they are identical and
represent the same compound.
(b) Each structural formula has the same molecular formula, C7H16. In addition, each has a
chain of five carbons with two CH3 branches. Although the branches are identical, they
are at different locations on the chains. Therefore, these structural formulas represent
constitutional isomers.
Another example : Draw line-angle formulas for the five constitutional isomers with the
molecular formula C6H14.
Solution
Here are line-angle formulas for all constitutional isomers with six, five, and four carbons in the
longest chain. No constitutional isomers for C6H14 having only three carbons in the longest
chain are possible
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Hydrocarbons in Health Sciences ( by Dr. Alaa J.Mahrath)
.
Homework: Draw structural formulas for the three constitutional isomers with the molecular
formula C5H12.
2.3.How Do We Name Alkanes?
It's simple issues, you can go back to the background in your previous educational to review
these roles. See this table
Table 2.1: Prefixes Used in the IUPAC System to Show the Presence of 1 to 20 Carbons in an
Unbranched Chain
Let's see this examples: write the structural formula of 4-methyloctane?
Table 2.2 : Names of the Eight Most Common Alkyl Groups
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Hydrocarbons in Health Sciences ( by Dr. Alaa J.Mahrath)
Examples :
Some common Names :
2.4.Where Do We Obtain Alkanes?
The two major sources of alkanes are natural gas and petroleum. Natural gas consists of
approximately 90 to 95% methane, 5 to 10% ethane, and a mixture of other relatively lowboiling alkanes—chiefly propane, butane,and 2-methylpropane.
Petroleum is a thick, viscous liquid mixture of thousands of compounds, most of them
hydrocarbons, formed from the decomposition of marine plants and animals. Petroleum and
petroleum-derived products fuel automobiles, aircraft, and trains. They provide most of the
greases and lubricants required for the machinery utilized by our highly industrialized society.
Furthermore, petroleum, along with natural gas, provides nearly 90% of the organic raw
materials for the synthesis and manufacture of synthetic fibers, plastics, detergents, drugs,
dyes, and a multitude of other products. The fundamental separation process in refining
petroleum is fractional distillation . Practically all crude petroleum that enters a refinery goes to
distillation units, where it is heated to temperatures as high as 370 to 425°C and separated into
fractions. Each fraction contains a mixture of hydrocarbons that boils within a particular range.
2.5.What Are Cycloalkanes?
A hydrocarbon that contains carbon atoms joined to form a ring is called a cyclic hydrocarbon.
When all carbons of the ring are saturated (only carbon–carbon single bonds are present), the
hydrocarbon is called a cycloalkane. Cycloalkanes of ring sizes ranging from 3 to more than 30
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Hydrocarbons in Health Sciences ( by Dr. Alaa J.Mahrath)
carbon atoms are found in nature, and in principle there is no limit to ring size.
Figure (2.2): Fractional distillation of petroleum. The lighter, more volatile fractions are
removed from higher up the column; the heavier, less volatile fractions are removed from
lower down.
Some examples of cycloalkane
Five-membered (cyclopentane) and six-membered (cyclohexane) rings are especially abundant
in nature; for this reason, we concentrate on them in this lectures .
To name a cycloalkane, prefix the name of the corresponding open-chain alkane with cyclo-,
and name each substituent on the ring. If there is only one substituent on the ring, there is no
need to give it a location number. If there are two substituents, number the ring beginning with
the substituent of lower alphabetical order.
Solution
(a) The molecular formula of this compound is C8H16. Because there is only one substituent,
there is no need to number the atoms of the ring. The IUPAC name of this cycloalkane is
isopropylcyclopentane.
(b) The molecular formula is C11H22. To name this compound, first number the atoms of the
cyclohexane ring beginning with tert-butyl, the substituent of lower alphabetical order
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(remember, alphabetical order here is determined by the b of butyl, and not by the t of tert-).
The name of this cycloalkane is 1-tert-butyl-4-methylcyclohexane.
2.6.What Are the Shapes of Alkanes and Cycloalkanes?
A. Alkanes: for any alkane (except, of course, for methane), there are an infinite number of
conformations. The majority of molecules in any sample will be in the least crowded
conformation; the fewest will be in the most crowded conformation.
B. Cycloalkanes:
Cyclopentane :the most stable conformation of cyclopentane is the envelope conformation
shown in Figure below .
Cyclohexane :The most stable conformation of cyclohexane is the chair conformation shown in
figure below .
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Hydrocarbons in Health Sciences ( by Dr. Alaa J.Mahrath)
Figure 2.3: Chair conformation of cyclohexane showing equatorial and axial C-H bonds.
Figure 2.4 : shown the most stable conformation is equatorial methylcyclohexane .
You can see some applications of the cycloalkanes conformation in Poisonous puffer fish.
Tetrodotoxin is produced in the liver and ovaries of this fish .
Tetrodotoxin blocks the sodium ion channels, which are essential for neurotransmission . This
blockage prevents communication between neurons and muscle cells and results in weakness,
paralysis, and eventual death.
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Hydrocarbons in Health Sciences ( by Dr. Alaa J.Mahrath)
2.7.What Is Cis-Trans Isomerism in Cycloalkanes?
Cycloalkanes with substituents on two or more carbons of the ring show a type of isomerism
called cis-trans isomerism. Cycloalkane cis-trans isomers have (1) the same molecular formula
and (2) the same connectivity of their atoms. We can illustrate cis-trans isomerism in
cycloalkanes by using 1,2-dimethylcyclopentane as an example.
The prefix cis (Latin: on the same side) indicates that the substituents are on the same side of
the ring; the prefix trans (Latin: across) indicates that they are on opposite sides of the ring.
Alternatively, we can view the cyclopentane ring as a regular pentagon seen from above, with
the ring in the plane of the page. Substituents on the ring then either project toward you (that
is, they project up above the page) and are shown by solid wedges, or they project away from
you (project down below the page) and are shown by broken wedges.
We say that 1,2-dimethylcyclopentane has two stereocenters. A stereocenter is a tetrahedral
atom, most commonly carbon, at which exchange of two groups produces a stereoisomer. in
this molecule, exchange of H and CH3 groups at either stereocenter converts a trans isomer to
a cis isomer, or vice versa.
Some definitions:
Stereoisomers : Isomers that have the same connectivity of their atoms but a different
orientation of their atoms in space.
Stereocenter: A tetrahedral atom, most commonly carbon, at whichexchange of two groups
produces a stereoisomer.
Configuration: Refers to the arrangement of atoms about a stereocenter; that is, to the relative
arrangement of parts of a molecule in space.
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Hydrocarbons in Health Sciences ( by Dr. Alaa J.Mahrath)
Example : draw the stereoisomers of 1,4-dimethylcyclohexane ?
Another example: Which of the following cycloalkanes show cis-trans isomerism? For each that
does, draw both isomers. (a) Methylcyclopentane (b) 1,1-Dimethylcyclopentane ,
(c) 1,3-Dimethylcyclobutane.
Solution
(a) Methylcyclopentane does not show cis-trans isomerism; it has only one substituent on the
ring.
(b) 1,1-Dimethylcyclobutane does not show cis-trans isomerism because only one arrangement
is possible for the two methyl groups. Because both methyl groups are bonded to the same
carbon, they must be trans to each other—one above the ring, the other below it.
(c) 1,3-Dimethylcyclobutane shows cis-trans isomerism. The two methyl groups may be cis or
they may be trans.
Homework: Which of the following cycloalkanes show cis-trans isomerism? For each that does, draw both
isomers. (a) 1,3-Dimethylcyclopentane (b) Ethylcyclopentane (c) 1,3-Dimethylcyclohexane
2.8.What Are the Physical Properties of Alkanes?
A.melting and boiling points:
In general, both boiling and melting points of alkanes increase with increasing molecular
weight (Table 2.3). Alkanes containing 1 to 4 carbons are gases at room temperature. Alkanes
containing 5 to 17 carbons are colorless liquids. High-molecular-weight alkanes (those
containing 18 or more carbons) are white, waxy solids. The wax found in apple skins. Paraffin
wax, a mixture of high-molecular-weight alkanes, is used for wax candles, in lubricants, and to
seal home-canned jams, jellies. Mineral oil and Vaseline, are used as an ointment base in
pharmaceuticals and cosmetics, and as a lubricant and rust preventive.
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The boiling point of each branched-chain isomer is lower than that of hexane itself; the more
branching, the lower the boiling point. (Table 2.4).
Figure (2.5):Paraffin wax and mineral oil are mixtures of alkanes
Table 2.3: Physical Properties of Some Unbranched Alkanes
Table 2.4 : Boiling Points of the Five Isomeric Alkanes with the Molecular Formula C 6H14
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B. Solubility: A Case of “Like Dissolves Like”
Because alkanes are nonpolar compounds, they are not soluble in water, which dissolves only
ionic and polar compounds. Alkanes do not dissolve in water because they cannot form
hydrogen bonds with water.
C. Density :
The average density of the liquid alkanes is about 0.7 g/mL; that of higher-molecular-weight
alkanes is about 0.8 g/mL. All liquid and solid alkanes are less dense than water (1.000 g/mL)
and, because they are insoluble in water, they float on water.
Examples : Arrange the alkanes in each set in order of increasing boiling point.
(a) Butane, decane, and hexane .
(b) 2-Methylheptane, octane, and 2,2,4-trimethylpentane
Solution:
(a) All three compounds are unbranched alkanes. Decane has the longest carbon chain, the
strongest London forces between its molecules, and the highest boiling point. Butane
has the shortest carbon chain and the lowest boiling point.
(b) These three alkanes are constitutional isomers with the molecular formula C8H18. 2,2,4-Trimethyl
pentane is the most highly branched isomer and, therefore, has the smallest surface area and the
lowest boiling point. Octane, the unbranched isomer, has the largest surface area and the highest
boiling point.
2.9.What Are the Characteristic Reactions of Alkanes?
A. Reaction with Oxygen: Combustion
Oxidation of hydrocarbons, including alkanes and cycloalkanes, is the basis for their use as
energy sources for heat [natural gas, liquefied petroleum gas (LPG), and fuel oil] and power
(gasoline, diesel, and aviation fuel). Following are balanced equations for the complete
combustion of methane, the major component of natural gas, and propane, the major
component of LPG or bottled gas. The heat liberated when an alkane is oxidized to carbon
dioxide and water is called its heat of combustion .
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B. Reaction with Halogens: Halogenation:
If we mix methane with chlorine or bromine in the dark at room temperature nothing happens.
If, however, we heat the mixture to 100°C or higher or expose it to light, a reaction begins at
once.
2.10. What Are Some Important Haloalkanes?
One of the major uses of haloalkanes is as intermediates in the synthesis of other organic
compounds.
A. Chlorofluorocarbons:
All the haloalkanes, the chlorofluorocarbons (CFCs) manufactured under the trade name
Freons are the most widely known. CFCs are nontoxic, nonflammable, odorless, and
noncorrosive. Originally, they seemed to be ideal replacements for the hazardous compounds
such as ammonia and sulfur dioxide formerly used as heat-transfer agents in refrigeration
systems. The CFCs also found wide use as industrial cleaning solvents to prepare surfaces for
coatings, to remove cutting oils and waxes from millings, and to remove protective coatings.
B. Solvents:
Several low-molecular-weight haloalkanes are excellent solvents in which to carry out organic
reactions and to use as cleaners and degreasers. Today, the most widely used haloalkane
solvent is dichloromethane, CH2Cl2.
For more details please see the following References
1. Introduction to General Organic and Biochemistry ; 9th Edition , Frederick A. Bettelheim
and William H. Brown ..et al ,Brooks / Cole. Canada.2010.
2. General Organic and Biochemistry . 4th Edition, Denniston K. , The McGraw−Hill
Companies, 2013
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