Saturated hydrocarbons contain carbon-
carbon or carbon-hydrogen single bonds. If
different carbon atoms are joined together to
form open chain of carbon atoms with single
bonds ,they are termed as alkanes. In organic
compounds each carbon atoms are bonded
with 4 hydrogen atoms so as to satisfy
carbon’s tetravalent quality.
Alkanes can be prepared by the catalytic hydrogenation
of unsaturated hydrocarbons in the presence of catalyst
‘Ni’ ,nickel, or ‘pt’ ,platinum at 200⁰C to 300⁰C.
FROM UNSATURATED HYDROCARBONS
•From alkyl halides
By Wurtz Reaction:
When alkyl halides are heated with sodium metal in the presence of dry
ethereal, alkanes are obtained (generally having double number of C-
atoms than in alkyl halides). This reaction is known as Wurtz reaction and
used for the preparation of symmetrical alkanes.
Alkyl halides (except fluorine) on reduction with zinc and dilute hydrochloric
acid gives alkanes
CH3-cl+ H2 CH4 +HCl
Chloromethane in presence of Zn,H+
gives methane and HCl
From carboxylic acids
• By Kolbe’s electrolytic method
Electrolysis of aqueous conc. solution of sodium or potassium salt of
carboxylic acid gives alkanes.
RCOONa——–> RCOO- + Na+
By Heating Na-salt of Carboxylic acid
When Na-salt of carboxylic acid is heated with soda lime (NaOH
& CaO), alkane is obtained having one carbon less than salt by removal
of a molecule of CO2. This reaction is also known as decarboxylation.
RCOONa + NaOH R-H + Na2CO
Sodium ethanoate (methane)
This is the principle reaction for laboratory
preparation of methane gas
1) Combustion reaction
They combust in oxygen to form carbon dioxide, water and heat. It is
this heat which makes them good fuel sources.
C2H6 (g) + 7/2 O2(g) → 2CO2(g) + 3H20(g) + heat
C8H20 (g) + 13 O2(g) → 8CO2 (g) + 10H2O(g) + heat
2) Controlled oxidation
alkanes on heating with a regulated supply of
dioxygen or air at high pressure and in the
presence of suitable catalysts give a variety of
2CH4 + O2 2CH3OH
3) Substitution reaction
They undergo substitution reactions with halogens in the presents of
CH4(g) + Cl2(g) → CH4Cl(g) + HCl(g)
CH4(g) + 4Cl2(g) → CCl4(g) + 4HCl(g)
Substitution reaction in cyclic
n-alkanes on heating in the presence of anhydrous aluminum chloride and
hydrogen chloride gas isomerise to branched chain alkanes
Unsaturated hydrocarbons are hydrocarbons that have double
or triple covalent bonds between adjacent carbon atoms.
Those with at least one carbon to carbon double bond are
called alkenes and those with at least one carbon to carbon
triple bond are called alkynes. These carbon compounds are
bonded with the required number of hydrogen so as to fulfill
the valency of carbon.
H H H
H H H
alkyles on partial reduction with calculated amount
of dihydrgen in the presence of palladised charcoal
partially deactivated with poisons like sulphur
compounds or quinoline give alkenes
From alkyl halides
alkyl halides on heating with alcoholic potash eliminate one
molecule of halogen acid to form alkenes. This reaction is known as
DEHYDROHALOGENATION ,i.e., removal of halogen acid.
From alcohols by acidic dehydration
In dehydration reactions, a molecule of water is eliminated
from an alcohol molecule by heating the alcohol in the
presence of a strong mineral acid. A double bond forms
between the adjacent carbon atoms that lost the
hydrogen ion and hydroxide group.
Dehalogenation. Vicinal dihalides, which are alkane molecules that contain two
halogen atoms on adjacent carbon atoms, can form alkenes upon reaction
From vicinal dihalides
Addition of dihydrogen
Alkenes add up to one molecule of dihydrogen gas in
the presence of finely divided nickel, palladium or
platinum to form alkanes.
Addition of halogens
The reaction with bromine (Br2)are used as a test for alkenes. Bromine solutions
are brownish red. When we add a Br2 solution to an alkene, the color of the
solution disappears because the alkenes reacts with the bromine:
Addition of water
The reaction between an alkene and water to form
an alcohol is called hydration, requires a catalyst—
usually a strong acid, such as sulfuric acid (H2SO4):
Addition of sulphuric acid
Cold conc. Sulphuric acid adds to alkenes in accordance
with markovnikov rule to form alkyl hydrogen sulphate by
the electrophilic addition reaction.
From calcium carbide
Ethyne (acetylene) preparation. Ethyne, which is commonly
called acetylene, is the simplest alkyne. Historically, it was prepared
by reacting calcium carbide with water.
From vicinal dihalides
The loss of additional hydrogen and halogen atoms from the
double‐bonded carbon atoms leads to alkyne formation. The
halogen atoms may be located on the same carbon (ageminal
dihalide) or on adjacent carbons (a vicinal dihalide).
An alkyne reacts with hydrogen in the
presence of catalyst (Pt or Ni) at 250°C,
first forming alkenes and finally alkane.
Addition of hydrogen
Addition of Halogens
Alkynes react with halogens (Cl2 or Br2) in the dark,
forming dihaloalkenes first and finally tetrahaloalkanes.
The reaction gets accelerated in the presence of light
or halogen carriers.
Addition of Halogen Acids
Alkynes reacts with halogen acids according to the
Markownikoff's rule i.e. the carbon atom carrying the
least number of hydrogen atoms will have the negative
part of the addendum attached to it.
On heating, alkynes undergo polymerization in the presence
of catalyst. The nature of products depends upon the
conditions. For example,
When ethyne (acetylene) is passed through a hot copper
tube, it polymerizes to benzene.
Oxidation of alkynes gives mono or dicarboxylic
Alkaline KMnO4 oxidizes ethyne to oxalic
These hydrocarbons are also known s ‘arenes’ or most of such
aromatic compounds are found to contain benzene ring, which are
highly saturated but in most of aromatic compounds are found to
unsaturated too .Aromatic compounds containing benzene rings are
known as benzenoids and those not containing benzene rings are
known as non-benzenoids.
Cyclic polymerization of ethyne
Ehthyne on passing through red hot iron tube at 873k undergoes
cyclic polymerization. Three molecule polymerize to form benzene
,which is the starting molecule for the preparation of derivatives of
benzene, dyes, drugs and large number of other organic
compounds. This is the best route for entering from aliphatic to
aromatic compounds .
Decarboxylation of aromatic acids
Sodium salt of benzenoic acid on heating with soda lime
Reduction of phenol
Phenol is reduced to benzene by passing its vapor over
heated zinc dust
Electrophilic substitution reactions
A nitro group is introduced into benzene ring
when benzene is heated with a mixture of con.
Nitric acid & con sulphuric acid.
Arenes react with halogens in the presence of a
Lewis acid to yield haloarenes.
Under vigorous conditions ,i.e., at high
temperature and /or pressure in the presence of
nickel catalyst , hydrogenation of benzene gives
When heated in air ,benzene burns with sooty
flame producing CO2 and H2O
C6H6 + (15/2 )O2 6 CO2 +3 H2O