introduction to alkanes, preparation methods and properties

1. Alkanes:
Preparation Methods &
Properties

2. Preparation Methods
1. Hydrogenation of alkenes or alkynes
2. Reduction of alkyl halides
3. Decarboxylation of carboxylic acid
4. Hydrolysis of Grignard reagents
5. Wurtz synthesis
6. Corey-House alkane synthesis
7. Kolbe’s synthesis

3. Hydrogenation of alkenes or alkynes
Alkenes or alkynes react with hydrogen in the presence of nickel
catalyst at 200-300°C to form alkanes. Other catalyst includes platinum
and palladium.

5. 2. Reduction of Alkyl Halides
Alkyl halides undergo reduction with nascent hydrogen to form alkanes

6. 3. Decarboxylation of carboxylic acid
When sodium salt of carboxylic acid is strongly heated with sodalime
(Na0H+Ca0) a molecule of carbon dioxide is split off as carbonate and an
alkane is formed.

7. 4. Hydrolysis of Grignard Reagent
Grignard reagent on treatment with water gives alkane

8. 5. Wurtz Synthesis
Higher alkanes are produced by heating an alkyl halide with sodium
metal in dry ether solution. Two molecules of alkyl halide lose their
halogen atoms as NaX. The net result is the joining of two alkyl groups
to yield a symmetrical alkane (R-R) having even number of carbon
atoms.

9. 6. Corey-House alkane synthesis
Alkyl halide is first converted to lithium dialkylcopper (LiR2Cu), which is
then treated with an alkyl halide to give an alkane.
This method is useful for the preparation of unsymmetrical alkanes.

11. 7. Kolbe’s Synthesis
When a concentrated solution of sodium salt of a carboxylic acid is
electrolyzed, an alkane is formed.
This reaction is only suitable for the preparation of symmetrical
alkanes

12. Physical properties
• First four alkanes i.e., methane, ethane, propane and butane are
gases.
• Next fifteen members are colorless liquids
• Higher alkanes are wax-like solids
• Non-polar in nature, therefore, dissolves in non-polar solvents i.e.,
carbon tetrachloride and benzene, but insoluble in polar solvents
such as water
• Alkanes are less denser (0.7 g/ml) than water (1.0 g/ml)
• The boiling point of n-alkanes increases with increasing molecular
weight in a smooth manner while melting point do not increase in a
regular fashion

13. Chemical Properties
Alkanes are relatively stable to common reagents such as acids, alkalis,
oxidizing agents etc., at room temperature. The relative stability or
inactivity of alkanes is due to the fact that the electronegativities of
carbon (2.60) and hydrogen (2.1) do not differ appreciably. Thus the bond
electrons in C—H are practically equally shared between them and the
bond is almost nonpolar. The C—C bond is completely nonpolar. Therefore,
polar reagents find no reaction sites on alkane molecules. Furthermore,
the C—C and C—H are strong bonds. This explains why alkanes are stable to
acids, alkalis, oxidizing reagents etc., at room temperature.

14. The random collisions between the molecules of potential reactants
(alkanes and reagents) occur but the energies of these collisions
are not sufficient to bring about a chemical reaction. At high
temperatures, however, the energies of collisions are much
frequently powerful enough to 'break and make' bonds.
Thus alkanes undergo most of their reactions through the formation
of the highly reactive 'free radicals' as a result of 'energetic
collisions' between their molecules at high temperature.
Alkanes give only two types of reactions.
(A) Substitution Reactions; and
(B) Thermal and Catalytic Reactions.

15. A. Substitution Reactions
In these reactions, one or more of the H-atoms of alkane are substituted by
either atoms like chlorine and bromine or by certain groups like nitro (—
NO2), sulphuric (—SO3H), etc.
Some of the most common reactions shown by alkanes are given below.
1. Halogenation:
It involves the substitution of H-atoms of alkanes by as many halogen atoms
i.e., by chlorine (chlorination) ; by bromine (bromination) by iodine
(iodination) or by fluorine (fluorination). The order of reactivity of halogens
in this regard is
F2> Cl2 > Br2 > I2

16. (a) Chlorination
Alkanes react with chlorine in the presence of ultraviolet light
or diffused sunlight or at a temperature of 300-400C, yielding
a mixture of products

17. The reaction do not stop at this stage. The remaining three
hydrogen atoms of ethyl chloride can be successfully replaced by
chlorine atoms.

18. 2. Nitration of alkanes
At ordinary temperatures alkanes do not react with nitric acid.
However, when a mixture of an alkane and nitric acid vapors is
heated at 400-500C, one hydrogen atom on the alkane is
substituted by a nitro group (—NO2).
The process is called vapor phase nitration, and yields a class of
compounds called nitroalkanes.

19. With higher alkanes, a mixture of products is obtained, some of
which result from rupturing carbon-carbon bonds.
For example, the nitration of ethane results in a mixture of
nitromethane and nitroethane

20. 4. Sulfonation
This involves the substitution of a hydrogen atom with a –SO3H group.
At ordinary temperatures neither concentrated nor fuming sulfuric acid
reacts with alkanes. However, when alkanes are subjected to prolonged
reaction with fuming H2S04, one hydrogen atom on the alkane is
replaced by the sulfonic acid group (–SO3H).
Where R=C6H13 or larger alkyl group. Lower alkanes such as methane
and ethane do not give this reaction

21. B. Thermal and Catalytic reactions
1. Combustion/Oxidation: When ignited in the presence of sufficient
excess of oxygen, alkanes burn to form carbon dioxide and water. The
combustion of these hydrocarbons is accompanied by the evolution of
large quantities of heat. This is the most important reaction because it
is the major route by which we provide energy for a wide range of
purposes. For example, we make use of it in our homes and factories
when we burn natural gas, kerosene, and other hydrocarbons.

22. 2. Pyrolysis (Cracking)
The decomposition of a compound by heat is called pyrolysis
(Greek, pyro-fire, lysis-loosening).
When alkanes are heated to a high temperature in the absence of
air, pyrolysis or a 'thermal decomposition' occurs. Large alkane
molecules are broken down or cracked to give a mixture of smaller,
lower molecular weight alkanes, alkenes and hydrogen.
Pyrolysis generally requires temperatures in the range 500-800C.
However, in the presence & of a catalyst (finely divided silica-
alumina), reactions can be carried at less high temperatures and
this is called catalytic cracking.

23. Ethane when heated to 500° in the absence of air gives a mixture of
methane, ethylene and hydrogen.
Propane when pyrolysed at 600 gives a mixture of propylene, ethylene,
methane and hydrogen.

24. 3. Isomerization
The molecular rearrangement of one isomer into one or more other
isomers - called isomerisation. Normal alkanes are converted to their
branched -chain isomers in the presence of aluminium chloride and HCl
at 25°C.

25. Similarly other less branched alkanes isomerize to the more
branched ones. Thus,
Isomerization is used to increase the branched chain content of
lower alkanes produced by cracking, because branched chain
alkanes are more valuable in than n-alkanes in motor spirit

26. 4. Aromatization
Alkanes containing six to ten carbon atoms are converted into
benzene and its homologues at high temperature and in the
presence of a catalyst. The process called aromatization takes
place by simultaneous dehydrogenation and cyclization of an
alkane to give the aromatic hydrocarbon containing the same
number of carbon atoms.

27. Thus when n-hexane is passed over Cr205 supported over alumina
at 600°, benzene is produced.