CHEM 132: INTRODUCTION TO ORGANIC CHEMISTRY
2017/2018 (PHYSIOLOGY, ANATOMY AND RADIOLOGY)
CHEMISTRY OF THE ALKANES
Alkanes are the simplest hydrocarbon made of carbon and hydrogen only. They have
the general formula CnH2n+2 where n = 1,2,3,4 etc. The first three members are represented as;
Alkanes are called saturated hydrocarbons because they are saturated with hydrogen.
In other words, they do not contain any double or triple bonds. Alkanes contain strong C-C and
C-H covalent bonds. Each carbon is bonded to enough hydrogen atoms to give maximum
covalence of 4. They are relatively chemically inert and are sometimes referred to as Paraffins
(Latin: parum affinis, which means “little affinity”)
STRUCTURE: This has been discussed under sp3 hybridization.
Nomenclature: Names and IUPAC System of naming has been discussed already.
The Table below shows the IUPAC names of the first ten Alkanes
Primary, Secondary and Tertiary Carbons. The structural formulas of alkanes contain four
types of carbons;
• primary carbon is bonded to one other carbon: (1⁰ Carbon)
o both of these carbons
• secondary carbon is bonded to two other carbons: (2⁰ Carbon)
o the center numbered 2 carbon only
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• tertiary carbon is bonded to three other carbons: (3⁰ Carbon)
o the center numbered 3 carbon only
• quaternary carbon is bonded to four other carbons: (4⁰ Carbon)
o the center numbered 4 carbon only
Hydrogen atoms attached to 1⁰, 2⁰, 3⁰ carbons are often referred to as primary, secondary and
tertiary hydrogen atoms.
ALKYL GROUPS: An alkyl group is formed by removing one hydrogen from an alkane.
They are named simply by dropping -ane from the name of the corresponding alkane and
replacing it by -yl. They are known collectively as alkyl groups. The general formula for an
alkyl group is CnH2n +1, since it contains one less hydrogen than the parent alkane, CnH2n + 2.
Examples, CH3― (Methyl group) CH3CH2— (Ethyl group).
There are two possible alkyl groups that can be made from 3 C’s
There are four possible alkyl groups that can be made from 4 C’s
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Examples of some Nomenclatures;
SIDE CHAIN SPECIFIC RULES:
a. 4-propyloctane b. 4-isoproplyoctane
CH2CH2CH3 CH3CHCH3
CH3CH2CH2CH2CHCH2CH2CH3 CH3CH2CH2CH2CHCH2CH2CH3
c. 5-butylnonane d. 5-secbutylnonane
CH2CH2CH2CH3 CH3 CHCH2CH3
CH3CH2CH2CH2CHCH2CH2CH2CH3 CH3CH2CH2CH2CHCH2CH2CH2CH3
e. 5-isobutylnonane f. 5-tertbutylnonane
CH3
CH CH3 CH3
CH2 CH3 C CH3
CH3CH2CH2CH2CHCH2CH2CH2CH3 CH3CH2CH2CH2CHCH2CH2CH2CH3
ISOMERISM OF ALKANES: The first three hydrocarbons of the series (methane, ethane
and propane) do not exhibit isomerism. The next hydrocarbon butane exists in two isomeric
forms. The number of possible isomers increases rapidly as the length of the chain increases.
These molecules are isomers of the same chemical formula. Heptane have 9 isomers, octane
18 and decane 75 isomers. Examples;
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n-butane 2-methylpropane (isobutane)
n-hexane
2-methylpentane
3-methylpentane
(isohexane)
2,2-dimethylbutane
(neohexane)
2,3-dimethyl butane
not
3,3-dimethyl butane
NATURAL SOURCE OF ALKANES: The two main sources of alkanes are natural gas and
petroleum. Alkanes on Earth are found in natural gas and petroleum, which are formed by the
decomposition of plant and animal material that has been buried for long periods in the Earth’s
crust, an environment with little oxygen. Natural gas and petroleum, therefore, are known as
fossil fuels. Natural gas consists of about 75-80% methane, 10% ethane. The remaining10-
15% is composed of small alkanes such as propane and butane. Petroleum is a chief source of
alkanes containing up to 40 carbons. Petroleum is a complex mixture of alkanes and
cycloalkanes that can be separated into fractions by distillation. The fraction that boils at the
lowest temperature (hydro-carbons containing three and four carbons) is a gas that can be
liquefied under pressure.
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METHODS OF PREPARATION
Alkanes are prepared by the following methods:
1. Hydrogenation of unsaturated hydrocarbons: Alkenes or alkynes react with hydrogen
in presence of catalyst (Ni, Pt or Pd) at 200-300o C to give alkanes. (Sabatier and Senderen’s
reaction).
2. Reduction of alkyl halide: Alkyl halide undergo reduction with nascent hydrogen to form
alkanes. The hydrogen for reduction may be obtained by using any of the following
reducing agents: zinc and hydrochloric acid (HCl); zinc and acetic (CH3COOH) acid;
Zn-Cu couple in ethanol; or LiAlH4, H2 gas and Ni or Pt catalyst may also be used.
R—X + 2[H] R―H + HX
Alkyl halide Alkane
Alkyl iodides are conveniently reduced by heating with HI and red Phosphorus in sealed
tube. The function of red phosphorous is to remove the iodine formed by forming PI3
otherwise it would react with alkane to give back alkyl iodide.
3. Decarboxylation of Carboxylic acids: Salt of Carboxylic Acid (e.g. sodium salt) when
heated strongly with sodalime (NaOH + CaO), decarboxylate to give alkane. The alkane
produced has one carbon less in its structure as compared to parent carbon. A molecule of
carbon (IV) oxide is split off as carbonate.
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4. Hydrolysis of Grignard Reagent: Alkyl magnesium halide (Grignard reagent) which is
obtained by treating alkyl halides with magnesium in anhydrous (dry) ether, on hydrolysis
(treatment with water) give alkane.
5. Wurtz Synthesis (Reaction): Higher alkanes are produced by heating an alkyl halide (RX)
with sodium metal in dry ether solution. Two molecules of alkyl condense with sodium
metal to give symmetrical alkane (R-R). They lose their halogen atoms as NaX.
The mechanism involves the formation of an extremely reactive organosodium
intermediate: RX + 2Na [RNa] + NaX; RX + [RNa] R―R + NaX
If two reacting alkyl halides are different, mixture of three alkanes are produced which is
difficult to separate. For example methyl chloride and ethyl chloride gives the mixture of
ethane, propane and butane. Propane from the combination of methyl chloride and ethyl
chloride; ethane from the combination of two molecules of methyl chloride; and n-butane
from two molecules of ethyl chloride.
This method is useful only for the preparation of symmetrical alkanes.
6. Corey-House Alkane synthesis: In this method an alkyl halide is first converted to lithium
dialkyl cuperate, LiR2Cu and then treated with an alkyl halide to give an alkane.
R-X + 2Li RLi +LiX
2RLi +CuI Li(R)2Cu +LiI
R2CuLi +R'X R-R' +RCu +LiX
Alkane
This method is suitable for the preparation of unsymmetrical alkanes, i.e., those of the R―R’
type. Examples:
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7. Kolbe’s Synthesis: Alkane is formed and collected at anode when a concentrated solution
of sodium or potassium salt of carboxylic acid is electrolyzed using Platinum electrodes.
This method is also suitable for the preparation of symmetrical alkanes.
8. Reduction of Alcohols, Aldehydes, Ketones and fatty acids:
a. Alcohols, aldehydes, ketones and fatty acids can be reduced with hot hydroiodic acid
and red phosphorous to give alkanes.
R-OH + 2HI R-H + H2O + I2; R-CHO + 4HI R-CH3 + H2O + 2I2
R-CO-R + 4HI R-CH2R + H2O + 2I2;
R-COOH + 6HI R-CH3 + 2H2O + 3I2
b. Aldehydes and ketones can also be reduced to alkanes by means of amalgamated zinc
and Conc. Hydrochloric acid (Clemmensen reduction)
Methane can also be prepared from aluminium carbide: Al4C3 +12HCl 3CH4 + 4AlCl3
Properties of Alkanes:
Physical Properties:
1) First four members (methane to butane) are colourless, odourless gases, next thirteen
(C5-C17) are colourless, odourless liquids while higher alkanes are colourless wax like solids.
2) These are insoluble in water as they are nonpolar compounds but soluble in organic solvents
like ether, benzene and acetone etc. Liquid alkanes are lighter than water.
3) In case of normal alkanes boiling points and specific gravities increase with rise in molecular
weights. As a rule the boiling points of alkanes having branched carbon chain are lower than
those of isomeric normal alkanes.
4) Variation in the melting point of alkanes is not regular. Alkanes with odd number of carbon
atoms have lower melting point than the next lower and next higher alkanes having even
number of carbon atoms. This is due to the greater inter molecular attraction in even numbered
alkanes having end carbon on the opposite sides of the molecules.
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Chemical properties:
1) Alkanes are extremely stable and inert substances. This is due to the fact that there is a small
difference in the electronegativity of carbon (2.60) and hydrogen (2.10). Thus the bond electron
in C-H bond are practically equally shared between them and the bond is almost
nonpolar. The C-C bond is completely nonpolar. Therefore electrophilic or nucleophilic reagent
find no site for attack on alkane molecules. Furthermore, the C-H and C-C bonds are strong
bonds. Alkanes due to these types of strong bonds remain stable to (unaffected by) acids, alkalis
and oxidizing agents at room temperature.
Lower alkanes undergo two types of reactions:
a) Substitution Reaction
b) Thermal and catalytic Reactions.
These reactions take place at high temperature or on absorption of light energy through the
formation of highly reactive free radicals. Some of the important reactions of alkanes are
following.
Halogenations: This involves the substitution of hydrogen atoms with halogen atoms.
a) Chlorination: Alkanes react with chlorine in presence U.V light or diffused sunlight or at
temperature 300-400oC to form the corresponding substituted products.
For example: Methane reacts with chlorine to give methyl chloride and HCl.
It is difficult to stop this reaction at first step. However, the yield of CH3Cl can be improved by
taking excess of methane. Ethane and higher alkanes react with chlorine in a similar way and
all possible substitution products are obtained.
b) Bromination: Bromine reacts with alkanes in a similar way but slowly and at higher
temperature.
c) Iodination: It is reversible. The hydrogen iodide formed during iodination reduces the
product back to reactant hence it must be carried out in the presence of strong oxidizing agent
like iodic acid or nitric acid which destroys the hydroiodic acid (HI) as it is formed.
CH4 + I2 CH3I + HI; 5HI + HIO3 3I2 + 3H2O;
d) Fluorination: Since fluorine is most reactive, fluorination under ordinary condition is
accompanied by explosion. Fluoroalkane can however be obtained from alkane by action of
fluorine diluted with Nitrogen.
2) Nitration: This involves replacement of hydrogen atom by –NO2 group. Nitration is carried
out by heating mixture of alkanes and nitric acid vapours at 400-500oC. The process is known
as Vapour Phase Nitration.
R-H + HO-NO2 R-NO2 + H2O
Since the reaction is carried out at high temperature, chain fusion also takes place during the
reaction and mixture of all possible mono nitro derivatives are obtained.
For example,
3) Sulphonation: This involves replacement of hydrogen atom by –SO3H group. It is carried
out by heating alkanes with fuming sulphuric acid or oleum at higher temperature.
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R-H + HO-SO3H R-SO3H + H2O
Alkane fuming sulphuric acid alkane sulphonic acid
Lower alkanes do not give this reaction.
4) Thermal decomposition [Pyrolysis or Cracking]: The decomposition of compound by heat
is known as pyrolysis. Thermal decomposition when applied to alkanes is known as Cracking.
This leads to the formation of lower alkanes, alkenes and hydrogen etc. For example
In presence of catalyst pyrolysis can be carried out at less higher temperature. This is called
Catalytic Cracking.
5) Isomerisation: Conversion of alkanes to its chain isomer is carried out by heating normal
alkane with anhydrous AlCl3 and HCl at 25℃. For example,
6) Aromatisation: Conversion of aliphatic compound to aromatic compounds is known as
aromatisation. Alkanes containing 6-10 carbon atoms are converted into benzene and it’s
homologous at high temperature and in the presence of catalyst. It involves
cyclization/dehydrogenation
For example:
7) Oxidation [Combustion]: When burnt in excess of air or oxygen alkanes form carbon
dioxide and water with the evolution of heat. For Example:
CH4 + 2O2 CO2 + 2H2O + heat; 2C2H6 + 7O2 4CO2 6H2O + heat
In general,
3n+1
CnH2n+2 + [ ] O2 nCO2 + (n+1) H2O + Heat
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The evolution of heat in this reaction forms the basis of the use of these hydrocarbons as source
of heat and power.
CYCLOALKANES: The general name of saturated cyclic hydrocarbons is cycloalkane.
Cycloalkanes ranging from 3-30 are found in nature, cyclopentane and cyclohexane are found
abundant in nature. They have the general formula CnH2n (n=3, 4, 5,..), hence they are isomers
alkenes
Physical Properties
-Cycloalkanes are insoluble in water but soluble in nonpolar solvents, –they have a higher
boiling point than their noncyclic counterparts with the same number of carbon atoms,- they
are more rigid and symmetrical than the noncyclic alkanes,- cyclopropane and cyclobutane are
gases at room temperature while cyclopentane - cyclooctane are liquids at room temperature
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Chemical Properties:- They undergo addition reaction especially cyclopropane and
cyclobutane
Some Nomenclatures of Cycloalkanes are listed below:
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2. b. When two or more different substituents are present, number according
to alphabetical order.
2 1
1 2
-n ot
1 - E th y l - 2 - m e th y l c y c l o h e x a n e 2 - E th y l - 1 - m eth y l c y c l o h ex a n e
3. Halogen Substituents
Halogen substituents are treated exactly like alkyl groups:
-F fluoro
- Cl chloro
- Br bromo
-I iodo
CH3
1 - C h l o r o - 2 - m e t h y l c y c l o b u ta n e
Cl
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