Preparation of alkanes.

Methods of Preparation of Alkanes

1. Alkanes Topic2- Preparation For B-pharm students- Presented by- sandhya punetha

2. Preparations of alkanes- Sabateier and senderens reactions- Decarboxylation reaction Reduction of alkyl halides Reduction of alcohols, aldehydes,ketones and carboxylic acids Wurtz reaction Hydrolysis of Grignard reagents Kolbe’s method

3. 1) Sabatier-and Senderens Reactions- In this type of reaction unsaturated hydrocarbons( like- alkenes; alkynes) are treated with hydrogen in the presence of nickel at 200-300˚C than same no. of alkane are produced. Example- Ni 200-300˚C Alkene + Alkane = + Ni (200-300˚C) - Ethene Ethane  

4. Important point of this reaction- Palladium and platinum can also be used in this reaction in place of nickel but doesn’t use because they are expensive. This method is used in the industry because large amount of alkynes and alkanes are obtained from cracking of petroleum.

5. Question- Give example of 3 reducing agent .

6. 2) Decarboxylation of carboxylic acid When the sodium salts of carboxylic acid is heated strongly with sodalime ( NaOH+ CaO ) than carbon dioxide and alkane are produced. This process is also known as Decarboxylation reaction. Example- R- COONa + NaOH RH + Sodium acetate Methane  

7. Important point of this reaction- In this reaction one carbon less than the reactant is formed in the product.

8. Questions- What is sodalime . What do you mean by decarboxylation.

9. 3) Reduction of alkyl halides- When alkyl halides are treated with nascent hydrogen they get converted into corresponding alkanes. Example- R-X + 2H R-H + HX Alkyl halide Alkane I + 2H - H + H-I Methyl iodide Methane  

10. Important point of this reaction- This reaction occurs best with iodides and bromides. Chlorides and fluorides are less reactive. This method is used to give pure hydrocarbons of alkanes.

11. Questions- What is Lucas reagent. Give example of various reducing agent which can be used in this reaction.

12. 4) Reduction of Alcohol; Aldehyde; Ketone and Carboxylic acid When aldehyde; ketone; carboxylic acid and alcohol are treated with hot concentrated hydriodic acid in combination with red phosphorus reduces oxygenated compounds to alkanes. Example- P (150˚C) R-OH + 2HI R-H + + O Alcohol Hydriodic Alkane acid  

13. P(150 OH + HI C + Methanol Hydriodic Methane acid P(150 Aldehyde + 4HI Alkane + 2 + O P(150 HO + 4HI + Ethanal Ethane  

14. P(150 Ketone + 4HI Alkane + 2 + O O P(150 C - C- C + 4HI C - C - C + 2 + O Propanone Propane  

15. P (200-250˚C) Carboxylic acid + 6HI Alkane + 3 + O O P(200-250˚C) C - C- OH + 6HI + 3 + O Ethanoic acid Ethane  

16. Important point of this reaction- Hydriodic acid is an effective reducing agent. This method is used to produce higher alkanes from carboxylic acid obtained from hydrolysis of fats.

17. 5) Wurtz Reaction- When two molecules of alkyl halide are treated with sodium metal in presence of dry ether than higher alkanes are produced this type of reaction is named as wurtz reaction. Example- dry ether 2R-X + 2Na R-R + 2NaX Alkyl halide Alkane dry ether 2 Br + 2 Na + 2 Na Br Methyl bromide Ethane  

18. Mechanism of Wurtz reaction- Mechanism Free Radical Ionic Mechanism Mechanism

19. Free Radical Mechanism- In this it involves the attack of a sodium atom on alkyl group like methyl bromide and form a methyl free radical. Than 2 methyl free radical combine to give ethane. dry ether 2 Br + 2 Na + 2 Na Br Methyl bromide Ethane Mechanism- Step1- Br + Na + Na Br Methyl free radical Step2- 2 C.  

20. Ionic Mechanism- This involves the intermediate formation of metal alkyls. In this methyl bromide react with 2 sodium atoms to produce methyl sodium as an intermediate and sodium bromide . When this methyl sodium react with another molecule of methyl bromide than ethane is produced. Example- 2 Br + 2 Na + 2 Na Br Methyl bromide Ethane Mechanism Step1- Br + Na + N Methyl sodium Step2- + N Ethane  

21. Important point of this reaction- When different alkyl halides are taken in this reaction than it produces a mixture of alkyl halides. The order of reactivity of alkyl halide- RCl < RBr < RI. Other metal that can be used in place of sodium are zinc and finely divided silver. Limitations - Only used for symmetrical alkanes production. Mixtures of alkanes cannot be separated because of less difference in their boiling point.

22. 6) Hydrolysis of Grignard Reagents- Alkylmagnesium halides ( Grignard Reagent) are obtained by heating alkyl halides with magnesium metal in anhydrous ether. These on treatment with water decomposes to give alkanes containing the same number of carbon atoms as the original alkyl halide. Example- dry ether R-X + Mg R-Mg-X Alkylmagnesium halide ( Grignard Reagent) R-Mg-X + H-OH R-H + ( MgOH Water Alkane  

23. 7) Kolbe’s method- When a concentrated solution of sodium or potassium salt of a carboxylic acid is electrolysed , a higher alkane is formed. Example- electrolysis 2RCOONa + 2 O R-R + 2 + 2NaOH+ Sodium carboxylate At anode At cathode 2 COONa + + 2 O + 2 + 2NaOH+ Sodium acetate Ethane Anode Cathode  

24. Important point of this reaction- This reaction is also used for symmetrical alkanes.

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