Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Addition Reactions Involving Carbonyls and Nitriles
9.2 Mechanism for Addition Reactions Involving Carbonyl Compounds

Owing to the polarity of the carbonyl bond, which is present in aldehydes and ketones, the dipole moment of these compounds is greater than zero. For example, the dipole moment of acetone (2-propanone) is 2.85 debyes. Shown in (9-1) are the two resonance structures of the carbonyl functionality, note the charge separation in one of the resonance structures (the right structure), hence a minor contributor to the overall structure of the molecule; it does however illustrate that the carbon of the carbonyl group is electropositive and the oxygen is nucleophilic.

(9-1)

One observation that is readily made about compounds that contain the carbon—oxygen double bond functionality is that the pi (π) electrons are not equally distributed about the double bond, compared to an alkene. As a result, it is much easier to predict the addition of an electrophile or nucleophile to these compounds, compared to that of an alkene or alkyne. The addition of an electrophile to a carbonyl group is shown in Reaction (9-2).

(9-2)

As you can imagine, in another step, a nucleophile will bond to the electrophilic carbon, as shown in Reaction (9-3) to form an addition product.

(9-3)

Based on the predictability of the addition of an electrophile and nucleophile to the carbonyl functionality, these reactions are considered regiospecific. Of course, it is possible for the sequence of the addition steps to take place differently, and the first step involves the nucleophile adding to the electrophilic carbon of the carbonyl group, as shown in Reaction (9-4).

(9-4)

In the next step, the electrophile will add to the nucleophilic intermediate that was created by the addition of the nucleophile, as shown in Reaction (9-5) to give a final neutral product.

(9-5)

The overall hypothetical addition reaction involving the addition of a nucleophile and an electrophile to a carbonyl functionality is shown in Reaction (9-6).

(9-6)

As we have seen from previous sections, it is possible to dictate a reaction path (or mechanism) by varying the reaction conditions. If this addition reaction were carried out in the presence of excess electrophile, such as an acidic medium, the electrophilic would add first. On the other hand, if carried out in the presence of excess nucleophile, such as a basic medium, the nucleophile would add first. Thus, the sequence of the addition pathway can be dictated based on the reaction conditions.