Various Chemical Property of Alkanes.

1. Chemical properties of
alkanes
For B- Pharm 1st year (2nd Semester)
Six Sigma Institute of Technology and sciences, Rudrapur
Presented By-Sandhya Punetha

2. Chemical reaction given by alkanes
• Alkanes gives only 2 types of chemical reaction-
1) Substitution reactions
2) Thermal and catalytic reactions

3. A) SUBSTITUTION REACTIONS-
• In these reactions, one or more of the H- atoms of alkane are substituted by
either atoms like- Chlorine, Nitro (NO2); sulphuric (SO3H) etc.
• Substitution reactions are of following types-
1) Halogenation
2) Nitration
3) Sulphonation
4) Chlorosulphonation

4. 1) Halogenation Reaction-
• In these type of reaction the hydrogen atoms of alkanes are substituted by
halogen atoms like- chlorine, bromine,iodine,fluorine.This type of reaction
is known as Halogenation reaction.
• The order of reactivity of halogen –.
𝐹2 > 𝐶𝑙2 > 𝐵𝑟2 > 𝐼2

5. Types of Halogenation Reactions-
1) Chlorination
When alkanes are treated with chlorine than the process is known as chlorination. This
type of reaction occur in alkanes at 300-400˚C in the presence of sunlight or UV
radiations.
Example- Light
𝑪𝑯 𝟒 + 𝑪𝒍 𝟐 𝑪𝑯 𝟑Cl + HCl
Methane Methyl chloride

6. • The reaction does not stop till all the hydrogen atoms present in the methane react
with chlorine in presence of light.
𝑪𝑯 𝟑Cl + 𝑪𝒍 𝟐 𝑪𝑯 𝟐 𝑪𝒍 𝟐 + HCl
Dichloromethane
𝑪𝒍 𝟐 + 𝑪𝑯 𝟐 𝑪𝒍 𝟐 CH𝑪𝒍 𝟑 + HCl
Chloroform
CH𝑪𝒍 𝟑+ 𝑪𝒍 𝟐 C𝑪𝒍 𝟒 + HCl
Tetrachloromethane

7. • Mechanism of Chlorination
The chlorination of alkanes takes place through the formation of free radical as
intermediate.
Step1- Initiation Step
Cl:Cl Cl . + Cl .
Chlorine free radicals
Step 2- Propagation Step
H
Cl . + H : C H HCl + .C𝑯 𝟑
H Methyl free radical

8. • Step 3- Termination Step
. Cl + . C𝑯 𝟑 C𝑯 𝟑-Cl
Methyl chloride
. C𝑯 𝟑 + . C𝑯 𝟑 C𝑯 𝟑- C𝑯 𝟑
Ethane

9. Important facts-
• The small amount of oxygen if present during chlorination of methane it slow
down the process as it form 𝑪𝑯 𝟑-O-O. free radical.
• Substance whose presence slow down the process when even present in small
amount are called inhibitor and this process is known as inhibition.
• The time period till which inhibition lasts is known as inhibition period.

10. 2) Bromination
When alkanes react with bromine than the reaction is known as bromination reaction.
Alkane react with bromine in the same way as chlorine react with alkanes.
Example- 𝑪𝑯 𝟒 + 𝑩𝒓 𝟐 𝑪𝑯 𝟑 𝑩𝒓 + HBr
Methane Methyl bromide

11. 3) Iodination
When iodine reacts with alkane than the reaction occur reversibly. The
hydrogen iodide formed as the byproduct is a powerful reducing agent and
is capable of reducing the iodoalkanes to alkanes.
Example-
𝑪𝑯 𝟒 + 𝑰 𝟐 𝑪𝑯 𝟑I + HI
Methyl iodide

12. • Alkanes can be iodinated in the presence of an oxidising agent such as
iodic acid ( HI𝑶 𝟑) or nitric acid which destroys the hydriodic acid (HI)
as it is formed.
Example- 5HI+ HI𝑶 𝟑 3𝑰 𝟐 + 3𝑯 𝟐O

13. 4) Fluorination
Fluorine is the most reactive of the halogen toward alkane. Fluorine react
with alkanes under certain conditions. The reaction in which fluorine react
with alkane is known as fluorination.
The reactivity order of different hydrogens in alkanes are as follows-
Tertiary > Secondary > Primary

14. 2) Nitration-
At ordinary temperature alkames do not react with nitric acid. However, when
a mixture of an alkane and nitric acid vapours is heated at 400- 500˚C, one
hydrogen atom on the alkane is substituted by a nitro group ( 𝑵𝑶 𝟑). This
process is called as vapour phase nitration and yields a class of compounds
called nitroalkanes.
400-500˚C
Example- R-H + OH- N𝑶 𝟐 R-N𝑶 𝟐 + 𝑯 𝟐O
Alkane Nitric acid Nitro-alkane

15. Mechanism-
400-500˚C
Example- 𝑪𝑯 𝟑-H + OH- N𝑶 𝟐 𝑪𝑯 𝟑- N𝑶 𝟐 + 𝑯 𝟐O
Methane Nitro-methane
• Step1- 2HN𝑶 𝟑 + 𝑵 𝟐 𝑶 𝟓 ∙ N𝑶 𝟐 + ∙ N𝑶 𝟑
Free radicals
• Step2- 𝑪𝑯 𝟑-𝑪𝑯 𝟑 + ∙ N𝑶 𝟐 𝑪𝑯 𝟑-𝑪𝑯 𝟐 ∙ + H N𝑶 𝟐
Ethyl free radical

16. • Step3- 𝑪𝑯 𝟑-𝑪𝑯 𝟐 ∙ + ∙ N𝑶 𝟐 𝑪𝑯 𝟑-𝑪𝑯 𝟐 N𝑶 𝟐 + 𝑪𝑯 𝟑-𝑪𝑯 𝟐-O-N=O
Nitroethane Ethyl nitrile
• Step 4- 𝑪𝑯 𝟑-𝑪𝑯 𝟐-O-N=O 𝑪𝑯 𝟑-𝑪𝑯 𝟐-O∙ + ∙ N=O
• Step5- 𝑪𝑯 𝟑-𝑪𝑯 𝟐-O∙ ∙ 𝑪𝑯 𝟑-𝑪𝑯 𝟐 = 𝑶
Formaldehyde
Step 6- C𝑯 𝟑∙ + ∙ N𝑶 𝟐 𝑪𝑯 𝟑- N𝑶 𝟐
Nitro-methane

17. Important Fact-
• Nitroalkanes are used as a solvents, fuels and starting
materials for the synthesis of other types of aliphatic
compounds.

18. 3) Sulphonation-
• Under normal condition neither concentrated or fuming sulphuric acid react
with alkanes.
• When alkanes are subjected to a prolonged reaction with fuming sulphuric acid,
one hydrogen atom of alkane is replaced by a sulphonic acid group (S𝑶 𝟑H).
This process is known as sulphonation and the products are alkylsulphonic
acids.
R-H+ HO- S𝑶 𝟑H R- S𝑶 𝟑H + 𝑯 𝟐O
Alkylsulphonic acid

19. Example- 𝑪 𝟔 𝑯 𝟏𝟒 + HO- S𝑶 𝟑H 𝑪 𝟔 𝑯 𝟏𝟑- S𝑶 𝟑H + 𝑯 𝟐O
Hexylsulphonic acid
Mechanism-
Step 1- HO- S𝑶 𝟑H OH∙ + ∙ S𝑶 𝟑H
Step2- 𝑪 𝟔 𝑯 𝟏𝟒 + OH∙ 𝑪 𝟔 𝑯 𝟏𝟑∙ + 𝑯 𝟐 𝑶
Step3- 𝑪 𝟔 𝑯 𝟏𝟑∙ + ∙ S𝑶 𝟑H 𝑪 𝟔 𝑯 𝟏𝟑- S𝑶 𝟑H + 𝑯 𝟐O
Hexylsulphonic acid

20. 4) Chlorosulphonation-
• When alkanes are treated with a mixture of sulphur dioxide and chlorine in the
presence of ultraviolet light a hydrogen atom of the alkane is replaced by a
chlorosulphonyl group ( 𝑺𝑶 𝟐Cl).
• Example-
𝑪𝑯 𝟑 𝑪𝑯 𝟐 𝑪𝑯 𝟑 + 2S𝑶 𝟐+ 2𝑪𝒍 𝟐 𝑪𝑯 𝟑 𝑪𝑯 𝟐 𝑪𝑯 𝟐 𝑺𝑶 𝟐Cl +𝑪𝑯 𝟑 𝑪𝑯 𝑪𝑯 𝟑 + 2HCl
Propane 1-Propane sulphonyl 𝑺𝑶 𝟐Cl
Chloride 2-propanesulphonyl
Chloride

21. Mechanism-
Step1- 𝑪𝒍 𝟐 𝟐 𝑪𝒍 ∙
Step 2- RH + 𝑪𝒍 ∙ R∙ + HCl
Step3- R∙ + 𝑺𝑶 𝟐 R 𝑺𝑶 𝟐 ∙
Step4- R 𝑺𝑶 𝟐 ∙ + 𝑪𝒍 𝟐 R 𝑺𝑶 𝟐 Cl + Cl∙

22. Important Facts-
• Chlorosulphonation is used commercially in the preparation of
sodium alkyl sulphonate detergents from alkanes.

23. B) THERMAL AND CATALYTIC REACTIONS-
• These are those reactions which involve heat and catalyst during the
reaction. Such type of reaction are known are known as thermal and
catalytic reactons.
• These reactions includes-
• Oxidation
• Pyrolysis
• Isomerism
• Aromatisation

24. 1) Oxidation-
• When alkanes react with oxygen such reactions are known as oxidation.
• This is a type of combustion reaction.
• When alkanes are heated in presence of air, they are completely oxidised
to carbon dioxide to carbon dioxide with large amount of heat.
• Example-
𝑪𝑯 𝟒 + 2𝑶 𝟐 𝑪𝑶 𝟐 + 𝑯 𝟐O

25. Important Facts-
• This reaction is important method for large scale production of acetylene.

26. 2) Pyrolysis ( Cracking)-
• The decomposition of a compound by heat is called pyrolysis.
• This process when applied to alkanes is known as cracking.
• This type of reaction requires temperature in the range 500-800˚C in the presence of a
catalyst reactions can be carried at less high temperatures and this is called catalytic
cracking.
• Example- 3𝑪𝑯 𝟑 𝑪𝑯 𝟑 2 𝑪𝑯 𝟐= C𝑯 𝟐 + 𝑯 𝟐 + 𝟐𝑪𝑯 𝟒
Ethane Ethylene Methane

27. 3) Isomerism-
• The molecular rearrangement of one isomer into one or more other isomers
is called isomerisation.
• Example-
AlCl3-HCl
𝑪𝑯 𝟑 −C𝑯 𝟐- C𝑯 𝟐- 𝑪𝑯 𝟑 𝑪𝑯 𝟑- CH- 𝑪𝑯 𝟑
𝑪𝑯 𝟑
n- Butane isobutane-2-methylpropane

28. Important Facts-
• Isomerisation is used to increase the branched chain content
of lower alkanes.

29. 4) Aromatisation-
• Alkanes containing 6 to 10 carbons are converted into benzene and its
homologues series at the high temperature and in the presence of a
catalyst the process called aromatisation.
• Example-
+ 𝐻2
Hexane Benzene

30. “God helps those who help themselves”
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