2) Reduction

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.


 

You can find the mechanisms for both the Markovnikov's and anti-Markovnikov's addition of water in CliffsQuickReview Organic Chemistry I.

Reduction of Aldehydes and Ketones

An aldehyde has a structural formula of

while the structural formula of a ketone is

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:

Lithium aluminum hydride is a very strong reducing agent that will reduce many functional groups in addition to aldehydes and ketones. Sodium borohydride is a much weaker reducing agent that basically will reduce only aldehydes and ketones to alcohols.

You can also catalytically reduce aldehydes and ketones to produce 1 and 2 alcohols. Reduction conditions are very similar to those used to reduce alkene double bonds. If a molecule possesses both a double bond and an aldehyde or ketone functional group, reduction of the aldehyde or ketone group is best carried out using sodium borohydride. The reduction of cyclohexanone by hydrogen gas with a platinum catalyst produces cyclohexanol in good yield.

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Reduction of carboxylic acids

The reduction of a carboxylic acid:


 

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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