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Preparation / Synthesis of alcohols:

Synthesis of Alcohols


Alcohols can be prepared by the hydration of alkenes or by the reduction of aldehydes, ketones, acids, and esters.

Hydration of alkenes

The elements of water can be added to the double - bonded carbons of an alkene in either a Markovnikov's or an anti Markovnikov's manner. As shown in the following figure, a hydrogen ion catalyzes the Markovnikov's addition.



The anti Markovnikov's addition results from a hydroboration - oxidation reaction.


Reduction of aldehydes and ketones


An aldehyde has a structural formula of R - CHO

while the structural formula of a ketone is R - COR'

            In these formulas, the R or R' group may be either an aliphatic or aromatic group. In a ketone, the R and R' groups may represent the same group or different groups. These types of compounds are best reduced by complex metal hydrides, such as lithium aluminum hydride (LiAlH4) or sodium borohydride (NaBH4).

Following are two examples of complex metal reductions:
 
 
Reduction of carboxylic acids

The reduction of a carboxylic acid R - COOH leads to the formation of a primary alcohol.



This reduction requires a very strong reducing agent, and lithium aluminum hydride is the standard choice.

 

Diborane, B2H6, also reduces carboxylic acids to alcohols.

  
 

Catalytic hydrogenation gives very poor yields and is not usually used for this type of reaction.

Reduction of esters

Esters, like carboxylic acids, are normally reduced with lithium aluminum hydride. In these reactions, two alcohols are formed. An example is the reduction of methyl benzoate to benzyl alcohol and methanol.
 
 

 Grignard reaction with aldehydes and ketones

The Grignard reaction is the only simple method available that is capable of producing primary, secondary, and tertiary alcohols. To produce a primary alcohol, the Grignard reagent is reacted with formaldehyde.



Reacting a Grignard reagent with any other aldehyde will lead to a secondary alcohol.



Finally, reacting a Grignard reagent with a ketone will generate a tertiary alcohol.


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