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Nucleophilic Addition, Electromeric Effect, Aldol Condensation, Crossed Aldol Condensation

An electron-deficient species forms a sigma bond with a nucleophile during nucleophilic addition.

Nucleophilic addition

An electron-deficient species forms a sigma bond with a nucleophile during nucleophilic addition. As a result of these reactions, carbonyl groups can be converted into a wide range of functional groups in organic chemistry. Three steps can be identified in the reaction of nucleophilic addition of carbonyl compounds.
  • Each time an electrophilic carbonyl carbon combines with a nucleophile, it forms a sigma bond.
  • This has now caused a rupture in the carbon-oxygen pi bond (the oxygen is now the recipient of electrons from the carbon).
  • The alcohol derivative is produced by protonation of the alkoxide.
In order to form an alkoxide, strong nucleophiles attack the carbon-oxygen double bond directly. For nucleophilic addition to occur, it is necessary to activate the carbonyl group with an acid catalyst when weak nucleophiles are used.

Coplanar structure characterizes sp2 hybridization of carbonyl groups. The nucleophile, breaks the pi bond by attacking the C=O group. In this state, the carbonyl carbon sp3 hybridizes with the nucleophile, forming a sigma bond. A tetrahedral geometry is visible in the resulting alkoxide intermediate.

What are the reasons for the nucleophilic addition of carbonyl compounds?

Carbonyl compounds contain polar carbon-oxygen bonds. Oxygen has a higher electronegativity and density because of its higher electronegativity. This causes an oxygen atom to generate a partial negative charge and a carbon atom to generate a partial positive charge. Carbonyl carbon acts as an electrophile because of its partial positive charge. A group of acids is introduced to stabilize the partial negative charge on the oxygen atom. By attacking the carbonyl oxygen atom with a proton, the acid counteracts its negative charge and neutralizes it. Nucleophilic addition reactions between aldehydes and ketones are relatively more reactive. The R group next to a secondary carbohydrate stabilizes the carbohydrate. Aldehydes are less likely than ketones to form carbocations, so they're more likely to undergo nucleophilic attacks.

Electrometric affect

Molecular dipoles form instantly when molecules of organic compounds are attacked by an attacking reagent, a phenomenon known as the electrometric effect. The presence of a multiple bond is necessary for organic compounds to exhibit this effect. When an attacking reagent comes into contact with the atoms involved in multiple bonds, it causes the complete transfer of one electron. During the time that the attacking reagent is in contact with the organic compound, electrometric effect lasts only for a short time. A polarized molecule returns to its unpolarized state once the attacking agent is removed from the system.

Electrometric effects and their types

A single electrometric effect can be classified into two types: the +E effect and the -E effect. Based on the direction that electron pairs are transferred, this classification is made.

+E effect

Pi bonds are attacked because the electron pair of the pi bond moves towards the attacking reagent. Acid can produce the +E effect on alkenes when it is added to them. As soon as the electron pair has been transferred, the attacking reagent attaches to the atom. An electrophile attacks a positively charged atom with pi electrons, which are then transferred to the positively charged atom through the +E effect. The following illustration illustrates the +E effect in action, which occurs when ethene is protonated.

-E effect

A pi bond's electrons can be affected by moving them away from an attacking reagent. When an electron pair is lost during the electron transfer, an atom in the molecule that is negatively charged is attacked by the attacking reagent. A -E effect is normally observed when attacking reagents are nucleophiles and the electrons of the attacking reagents are transferred to atoms where they will not bond. Here is an illustration of the -E effect using nucleophiles and carbonyl compounds.

Aldol condensation

The aldehydes containing α-hydrogen obtained through aldol condensation are formed by adding a diluted base to the β hydroxy aldehydes. This reaction is called aldol condensation. Two types of carbonyl compounds react with each other to produce crossed aldol condensation.


The conjugated enone is formed by reaction of a carbonyl compound with an enolate ion, which results in a β-hydroxy ketone, aldehyde, or conjugated enone, which is then dehydrated to produce a conjugated enone. Carbon-carbon bonds are formed by the Aldol Condensation in organic synthesis.

Aldol Condensation is a general reaction that takes place in organic synthesis.

Here is a common example of an aldol condensation using hydroxide ions as the catalyst, which is common for base-catalyzed reactions.


Step 1

Aldehyde protons are deprotonated by the hydroxide ion in reverse order.

Step 2

The unreacted aldehyde is added to the enolate ion 1.

Step 3

By protonating alkoxide ion 2, water is created.

Step 4

The hydroxide ion converts a small percentage of aldol into enolate (4).

Step 5

A hydroxide ion is lost on the enolate ion (4).

We can see the aldol reaction from steps 1 through 3.

Crossed aldol condensation

A crossed aldol reaction occurs when the aldehyde or ketone molecules react with each other in a protic solvent such as water or alcohol. Carbonyl compounds react when they react with each other, a reaction called crossed aldol condensation. As long as both aldehydes contain alpha hydrogens, both are able to form carbanions and accept carbanions. It is therefore an ineffective synthetic material that combines four items. The presence of alpha hydrogen is required for aldehydes to accept carbanion ions. Two products result from this reaction. Aromatic aldehydes that do not contain an alpha position are typical substrates for the cross-aldol reaction. As the condensation product dehydrates quickly, the α, β - unsaturated ketone is formed, preventing the formation of retro-aldol.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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