Chemical properties of alkanes

Various Chemical Property of Alkanes.

1. Chemical properties of alkanes For B- Pharm 1 st year (2 nd 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- Substitution reactions 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- Halogenation Nitration Sulphonation 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 –. > >  

5. Types of Halogenation Reactions- 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 + Cl + 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. Cl + + HCl Dichloromethane + CH + HCl Chloroform CH + C + 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 + .C H Methyl free radical  

8. Step 3- Termination Step . Cl + . C C -Cl Methyl chloride . C + . C C - C Ethane  

9. Important facts- The small amount of oxygen if present during chlorination of methane it slow down the process as it form -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- + + 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- + I + HI Methyl iodide  

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

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 ( ). This process is called as vapour phase nitration and yields a class of compounds called nitroalkanes . 400-500˚C Example- R-H + OH- N R-N + O Alkane Nitric acid Nitro-alkane  

15. Mechanism- 400-500˚C Example- -H + OH- N - N + O Methane Nitro-methane Step1- 2HN + ∙ N + ∙ N Free radicals Step2- - + ∙ N - ∙ + H N Ethyl free radical  

16. Step3- - ∙ + ∙ N - N + - -O-N=O Nitroethane Ethyl nitrile Step 4- - -O-N=O - -O∙ + ∙ N=O Step5- - -O∙ ∙ - Formaldehyde Step 6- C ∙ + ∙ N - N 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 (S H). This process is known as sulphonation and the products are alkylsulphonic acids. R-H+ HO- S H R- S H + O Alkylsulphonic acid  

19. Example- + HO- S H - S H + O Hexylsulphonic acid Mechanism- Step 1- HO- S H OH∙ + ∙ S H Step2- + OH∙ ∙ + Step3- ∙ + ∙ S H - S H + O 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 ( Cl ). Example- + 2S + 2 Cl + + 2HCl Propane 1-Propane sulphonyl Cl Chloride 2-propanesulphonyl Chloride  

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

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- 3 2 = C + + Ethane Ethylene Methane  

27. 3) Isomerism- The molecular rearrangement of one isomer into one or more other isomers is called isomerisation . Example- AlCl3-HCl C - C - - CH- 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- + Hexane Benzene  

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