Arenes - Position of substitution

In the earlier write-up about electrophilic substitution reaction, I stated the factors that affect the location of the second substitution of the electrophile.

The nature of Y affects the location of the second subsitution. If Y is electron-donating, it results in positions 2 and 4 to be favoured. While, position 3 is favoured if Y is electron-withdrawing.

When attached to benzene, these groups are electron donating groups: -OH, -OCH₃, -NH₂ (Question: Despite O and N are electronegative elements, they are electron donating here. Why?) and -alkyl substituents (e.g. -CH₃.)

However, when these groups are attached to benzene, they are electron withdrawing groups: -NO₂, -COOH, -COR (where R = alkyl) and -CHO. While halogens (e.g. -Cl, F, Br) are mildly electron withdrawing but they following the same pattern as electron donating groups.

In fact, when Y is an electron-donating group, the 4th position is favoured over the 2nd position.

From the above picture which describes the mechanism, the two steps look similar but the intermediates formed in each step are different. The mechanism suggests that to form the 2-substituted product, the 2nd electrophile must approach the H which is near to Y. While the case of the 4-substituted product, the 2nd electrophile is furthest away from Y.

As both electrophile and substituent Y have electron cloud, thus when both are placed in close proximity, there will be repulsion between their electron cloud. Therefore, it is more difficult for the 2nd electrophile to approach position 2 as compared to the 2nd electrophile approaching position 4. This difficulty is steric hinderance.

Hence, it results in the slow step of the formation of the 2-substituted product to be slower (or higher E_a). This results in the intermediate formed in the formation of the 4-substituted product, is produced in larger proportion. Indirectly, resulting in more 4-substituted product formed.

The final part of this write-up is beyond the scope of the "A" level syllabus. But it aims to explain why the nature of the substituent Y can affect the location of the 2nd substitution. Do click the pictures for the explanation. Hence, this bit of the article is trying to account for why electron donating substituents are 2, 4 directing. However, this last portion does not account for why the 4-substitution is the most favoured.


Hence, by drawing the lewis structures of the intermediates (using the pattern observed from the above diagram) will aid in the explanation to why different Y substituents direct the second electrophile to different carbon positions.

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^{Article written by Kwok YL 2009.}

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