Tuesday, January 29, 2008

IIT JEE Revision - Preparation of alkanes

General methods

1. From unsaturated hydrocarbons (alkenes and alkynes)
2. From alkyl halides
3. From carboxylic acids and their salts

From unsaturated hydrocarbons (alkenes and alkynes)

By catalytic hydrogenation alkenes and alkynes are converted into alkanes.
Ni, Pt or Pd in the form of fine powder are used as catalysts. A temperature of 523-573 K needs to be employed.

Methane cannot be prepared by this method because alkenes or alkynes will have two carbons at their lowest level.

2. From alkyl halides

Wurtz reaction: When an alkyl halide (usually bromide or iodide) is treated with sodium in dry ether, a symmetrical alkane containing twice the number of carbon atoms of alkyl halide is obtained.



Reducing agents can be used to add hydrogen to the halide and remove the halogen atom.
i) Zinc + HCl is one reducing agent.
ii) Catalytic hydrogenation using Pd or Pt as catalyst
iii) Hydrogen iodide (halogen acid) in the presence of red phosphorous also acts as reducing agent. In this reaction phosphorous combines with iodine to form phosphorous triiodide.
iv) zinc copper couple and alcohol


By the use of Grignard reagent

Alkyl halides react with magnesium metal in diethyl ether to form alkyl magnesium halides which are called as Grignard reagents.

Grignard reagents are highly reactive and are easily decomposed by water or alcohol to form alkanes

RMgX + HOH (H2O) ---> RH + Mg(OH)X

3. From carboxylic acids

i) Reduction of carboxylic acid: Carboxylic acids are reduced to alkanes by hydroiodic acid (HI). In this reaction COOH group in the carboxylic acid is reduced to CH3 group.


ii) When sodium salt of a monocarboxylic acid is heated with soda lime (a mixture of NaOH and Cao in the ratio of 3:1) at about 630 K, alkane is formed.

RCOONa + NaOH -->RH + Na2CO3

In this reaction a CO2 group is removed from carboxylic acid and therefore the reaction is called decarboxylation.



iii) Kolbe's reaction

When an acqueous solution of sodium or potassium salt of carboxylic acid is eletrolysed alkane is evolved at the anode.

Kolbe's reaction can also be used like wurtz reaction for preparing alkanes with even number of carbon atoms.



The methods in this section can be summarised as

R-COONa ---> RH
R-COOK ---> R-R - Kolbe's reaction
R-COOH---> R-CH3

Industrial method: Petroleum provides the natural source of alkanes.

4 comments:

J.R said...

". From carboxylic acids

i) Reduction of carboxylic acid: Carboxylic acids are reduced to alkanes by hydroiodic acid (HI). In this reaction COOH group in the carboxylic acid is reduced to CH3 group."

..........what is your reference source aside from Gatterman comments and Vogel.. ...please list as I'm interested in your comment.....solo

KVSSNrao said...

I thank you for the comment. This blog is only a guidance material and revision material for students at 10+2 level and it is based on one standard text supported by other texts. The main text that I use for my learning this subject is "Modern's ABC of Chemistry" Dr.S.P. Jauhar. I recommend this book to aspiring candidates. This particular reaction was given in that book of class XI. Now I look for your reasons to look for more references. Is it a controversial point?

J.R said...

" I look for your reasons to look for more references. Is it a controversial point?"

It's a controversial point because there hasn't been anything published to the duplication of the reaction save what was documented in the 1907 Gatterman text or in the Vogel text...

I just thought you had a reference to a current published successful synthesis, whereas they reduce the COOH to the CH3 using the HI .....java

KVSSNrao said...

Mr. j.r.

I thank you for making me aware of this issue. I shall try to discuss the issue with some chemistry faculty and learn something about it from them. I am not basically a chemistry man, and this blog is the result of my involvement in helping my daughter in her studies. Hence my own knowledge is limited to the texts that she has to study.