Reactions
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Reactions

Ketones have as their functional group a double bond between a carbon and an oxygen. The reactions that are characteristic of ketones are the reactions that involve this carbon-to-oxygen double bond.

Addition

The most common reaction or class of reactions for ketones is addition, although these reactions are generally classified differently. I believe that the addition reactions that ketones undergo are not as extensive as addition reactions that alkenes undergo. But these are the characteristic type of reactions for ketones.

Consider the reaction in which H2 is added across the C=O double bond. Two versions are shown here. Note that, in a way, the addition of 2 H's across the carbon-oxygen double bond to give an alcohol as a product is just the opposite of the reaction in which a ketone is formed from an alcohol by taking two hydrogens away.
|  
-C=O 

H-H

 ® |  
-C-O 
| |
H H
O
||
R-C-R

 

 + H2 ®    O-H
|
R-C-R
|
H

 

Reduction

It is not at all necessary for both of the hydrogen atoms to come from an H2 molecule.

One of the most common ones is the addition of 2 H's across the carbon-oxygen double bond to give an alcohol as a product. This is just the opposite of the reaction in which a ketone is formed from an alcohol by taking two hydrogens away, as shown below.

H  H  H  H
|  |  |  |
H-C--C--C--C-H
|  |  |  |
H  OH H  H

2-butanol

 ¾¾¾® + 2 H· H     H   H
|     |  |
H-C--C--C--C-H
|  ||  |  |
H  O  H  H

2-butanone

  Remember that the reaction in which the alcohol is changed into a ketone involves removing two hydrogen atoms. This is an oxidation reaction.

Ketones can be reduced to form alcohols by putting the two hydrogen atoms back on. The hydrogen atoms bring electrons with them. Thus, the compound is gaining electrons, therefore it is a reduction reaction. When both hydrogen atoms come from H2, you can also call it an addition reaction.

An equation for such a reaction is shown here (and in Example 17 in your workbook).

H     H   H
|     |  |
H-C--C--C--C-H
|  ||  |  |
H  O  H  H

2-butanone

+ 2 H· ¾¾¾® H  H  H   H
|  |  |  |
H-C--C--C--C-H
|  |  |  |
H  OH H  H

2-butanol

Note that this reaction is the reverse of the reaction shown earlier on this page. We can emphasize that relationship by using reversible arrows, as shown below. Note that reversing the direction of the reaction requires changing the conditions of the reaction. To reduce a ketone, you need a reducing agent. To oxidize an alcohol, you would need an oxidizing agent.

H     H  H
|     |  |
H-C--C--C--C-H
|  ||  |  |
H  O  H  H

2-butanone

+ 2 H· reduction
¾¾¾¾¾¾®
¬¾¾¾¾¾¾
oxidation		 H  H  H  H
|  |  |  |
H-C--C--C--C-H
|  |  |  |
H  OH H  H

2-butanol

This concept becomes very important in biological systems where metabolism requires oxidizing conditions (aerobic conditions). If the oxidizing agents to drive the reactions are not available (anaerobic conditions), then the process must stop, change directions, or be altered in some other manner. This issue will return in the lessons on biochemistry.

 

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