Mechanism of Nucleophilic Addition

Nucleophilic Addition Reaction is a term used to describe the addition of nucleophiles to a nucleophile. As oxygen is highly electronegative, it forms a polar bond with carbon, in which the electrons are distributed unequally among the two molecules. As a result, the oxygen atom receives a partial negative charge, whereas the carbon atom receives a partial positive charge. Since the carbon atom has a low electron density, it is vulnerable to attack by a nucleophile, which is an atom that can give electrons to the carbon atom. Also, aldehydes and ketones lack effective leaving groups when a nucleophile attacks the carbonyl carbon. Due to this, the nucleophile merely pulls the electrons toward the oxygen atom, forming a new bond between the carbon atom and the oxygen atom. The oxygen atom now has a negative charge. It can attract a hydrogen ion from the solution, resulting in the formation of alcohol on the carbonyl atom. This type of reaction is referred to as a nucleophilic addition reaction, and it is particularly prevalent in the reactions of aldehydes and ketones. When a nucleophile reacts with an electron-deficient species, a nucleophilic addition reaction occurs. A nucleophile creates a sigma bond with the electron-deficient species. They allow the conversion of carbonyl groups into various functional groups, which is useful in many applications.

Mechanism of Nucleophilic Addition Reaction

In the case of nucleophilic addition, the mechanism is as follows: A two-step process is involved in the general mechanism of the nucleophilic addition reaction.

• Step 1: Carbonyl is attacked by a nucleophile

In this reaction, the nucleophile establishes a connection with the electrophilic C=O carbon atom, which results in a rehybridisation of the carbonyl carbon from the sp² position to the sp³ position. The electronegative oxygen atom is drawn to the electrons in the pi-bond, resulting in a tetrahedral alkoxide intermediate.

• Step 2: There is no leaving group, thus addition occurs

The addition of acid causes the alkoxide to be protonated, resulting in the formation of alcohol. In the presence of strong nucleophiles, such as RMx (Grignard reagent), RLi, and RCCM, the C=O bond is directly attacked, forming the intermediate. On the other hand, weak nucleophiles, such as Water, ROH, and RNH₂, necessitate the use of an acid catalyst. Aldehydes and ketones are examples of polar compounds. These compounds also have a greater boiling point than hydrocarbons, making them more suitable as fuels. Although aldehydes and ketones have lower boiling points than alcohols, they are more volatile. There are numerous reactions involving aldehydes and ketones that are adequate for various synthetic reactions to take place.

Nucleophilic Addition Reaction in presence of different agents

• Nucleophilic Addition Reaction in presence of Water

The chemical reaction occurs when Water is added to a carbonyl molecule (aldehyde or ketone) and results in the formation of a geminal diol (hydrate). Because of the neutral environment, this is a slow reaction. However, the rate of reaction can be improved by using a catalyst, which can be either an acid or a base. Both acidic and basic conditions accelerate the reaction but both follow different mechanisms.

• Nucleophilic Addition Reaction in the Presence of the Grignard Reaction

The Grignard Reagents (R-Mg-X) react with carbonyl compounds and produce alcohols. The nucleophilic atom present in the alkyl radical R attacks the aldehyde or ketone (C=O), resulting in a single electron transfer. It forms an organometallic intermediate (Orm). After that, the protonation of the alkoxide oxygen occurs, and primary alcohol is formed in the presence of Water when formaldehyde is added (R, R’ = H). Secondary alcohol is formed when other aldehydes are added (R’=H), while tertiary alcohol is formed when ketones are added.

• Nucleophilic Addition Reaction in presence of Primary Amines

Imines are generated during the nucleophilic reaction of primary amines (ArNH₂ or R-NH₂) with ketone or aldehydes. Imines are formed in the following way: First and foremost, the protonation of the carbonyl group occurs in either an acidic or a basic reaction. Neutral nucleophiles are more likely to attack the carbonyl molecule in the acidic or base reaction. The nucleophile N then attacks the electrophilic C of the C=O from the pi-bond, triggering a reaction. In the process of removing a proton, the positive charge on the N is neutralised, resulting in the creation of a Carbinolamine as an intermediate product. Rate of Reaction: The more electrophilic the substrate, faster the addition reaction. Also lesser the steric hindrance, faster the reaction. In accordance with both these factors, aldehydes are more reactive towards nucleophilic addition than ketones. Also resonance tampers with the electrophilicity of the substrate and reduces the rate of reaction.

Points to Remember

• A nucleophilic addition reaction occurs when a chemical molecule with a double bond or triple bond combines with a nucleophile, resulting in the breakage of the double or triple bond.
• A carbocation will form in acetone, and the two CH₃ groups present in acetone will stabilise it, making the nucleophilic addition reaction a simple process.
• A carbonyl compound undergoes a nucleophilic addition reaction, and the steps involved in the mechanism are as follows: generation of the nucleophile, nucleophilic attack, protonation, and regeneration of a catalyst.
• Nucleophilic addition reaction occurs when the Carbonyl group is hybridised with Sp2 in the presence of Water. A breakdown of the pi link results in sp³ hybridisation of the carbonyl molecule.
• Comparatively, aldehydes are more reactive toward the Nucleophilic Addition Reaction than ketones.

Conclusion

With the help of additional reactions, we can change numerous bonds into various functional groups. The reaction needs to occur to convert unsaturated molecules into saturated and more functional species. Nucleophilic addition reactions are responsible for a number of other significant reactions. Nucleophilic addition reaction occurs in the case of aldehydes and ketones.