ELECTRONIC AND STERIC EFFECTS
Aldehydes are more reactive to nucleophiles than ketones.
Alkyl groups have an inductive effect whereby they ‘push’ electrons towards a neighboring electrophilic center and make it less electrophilic and less reactive. Ketones have two alkyl groups and are less electrophilic than aldehydes which have only one alkyl group.
The transition state for nucleophilic addition resembles the tetrahedral product. Therefore, any factor affecting the stability of the product will affect the stability of the transition state. Since the tetrahedral product is more crowded than the planar carbonyl compound, the presence of bulky alkyl groups will increase crowding and decrease stability. Since ketones have two alkyl groups to aldehyde’s one, the transition state for ketones will be less stable than the transition state for aldehydes and the reaction will proceed more slowly. Bulky alkyl groups may also hinder the approach of the nucleophile to the reaction center – the carbonyl group.
Generally it is found that aldehydes are more reactive to nucleophiles than ketones. There are two factors (electronic and steric) which explain this difference in reactivity.
The carbonyl carbon in aldehydes is more electrophilic than it is in ketones due to the substituents attached to the carbonyl carbon. A ketone has two alkyl groups attached whereas the aldehyde has only one. The carbonyl carbon is electron deficient and electrophilic since the neighboring oxygen has a greater share of the electrons in the double bond. However, neighboring alkyl groups have an inductive effect whereby they push electron density towards the carbonyl carbon and make it less electrophilic and less reactive to nucleophiles (Fig. 1).
Propanal has one alkyl group feeding electrons into the carbonyl carbon, whereas propanone has two. Therefore, the carbonyl carbon in propanal is more electrophilic than the carbonyl carbon in propanone. The more electrophilic the carbon, the more reactive it is to nucleophiles. Therefore, propanal is more reactive than propanone.
Electron inductive effects can be used to explain differing reactivities between different aldehydes. For example the fluorinated aldehyde (Fig. 2) is more reactive than ethanal. The fluorine atoms are electronegative and have an electron-withdrawing effect on the neighboring carbon, making it electron deficient. This in turn has an inductive effect on the neighboring carbonyl carbon. Since electrons are being withdrawn, the electrophilicity of the carbonyl carbon is increased, making it more reactive to nucleophiles.
Steric factors also have a role to play in the reactivity of aldehydes and ketones. There are two ways of looking at this. One way is to look at the relative ease with which the attacking nucleophile can approach the carbonyl carbon. The other is to consider how steric factors influence the stability of the transition state leading to the final product.
Let us first consider the relative ease with which a nucleophile can approach the carbonyl carbon of an aldehyde and a ketone. In order to do that, we must con-sider the bonding and the shape of these functional groups (Fig. 3). Both mole-cules have a planar carbonyl group. The atoms which are in the plane are circled in white. A nucleophile will approach the carbonyl group from above or below the plane. The diagram below shows a nucleophile attacking from above. Note that the hydrogen atoms on the neighboring methyl groups are not in the plane of the carbonyl group and so these atoms can hinder the approach of a nucleophile and thus hinder the reaction. This effect will be more significant for a ketone where there are alkyl groups on either side of the carbonyl group. An aldehyde has only one alkyl group attached and so the carbonyl group is more accessible to nucleophilic attack.
We shall now look at how steric factors affect the stability of the transition state leading to the final product. For this we shall look at the reactions of propanone and propanal with HCN to give cyanohydrin products (Fig. 4).
Both propanone and propanal are planar molecules. The cyanohydrin products are tetrahedral. Thus, the reaction leads to a marked difference in shape between the starting carbonyl compound and the cyanohydrin product. There is also a marked difference in the space available to the substituents attached to the reac-tion site – the carbonyl carbon. The tetrahedral molecule is more crowded since there are four substituents crowded round a central carbon, whereas in the planar starting material, there are only three substituents attached to the carbonyl carbon. The crowding in the tetrahedral product arising from the ketone will be greater than that arising from the aldehyde since one of the substituents from the aldehyde is a small hydrogen atom.
The ease with which nucleophilic addition takes place depends on the ease with which the transition state is formed. In nucleophilic addition, the transition state is thought to resemble the tetrahedral product more than it does the planar start-ing material. Therefore, any factor which affects the stability of the product will also affect the stability of the transition state. Since crowding is a destabilizing effect, the reaction of propanone should be more difficult than the reaction of propanal. Therefore, ketones in general will be less reactive than aldehydes.
The bigger the alkyl groups, the bigger the steric effect. For example, 3-pen-tanone is less reactive than propanone and fails to react with the weak bisulfite nucleophile whereas propanone does (Fig. 5).
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