Are Aldehydes Electrophilic:13 Facts You Should Know


Aldehyde, a carbonyl compound as an electrophile, is widely popular among the electrophilic group and is used as one of the most important compounds to explain the nature of electrophilicity of compounds in general.

Aldehydes are electrophilic compounds due to the presence of the polar C=O bond in its structure, in which the sp2 hybridized carbon atom act as an electrophilic center due to the electron deficiency caused by the bond electronegative oxygen atom

What makes something electrophilic?

If a species is a neutral or positively charged species, i.e. electron-deficient species, then they tend to attract electron pairs from the substrate molecule towards itself. This tendency of a species or something to attract electrons makes them electrophilic.

Electrophiles are electron-deficient, and therefore, they usually attack nucleophiles or electron-rich centers on the substrate

Attack of an electrophile on electron-rich Benzene is represented as,

are aldehydes electrophilic

are aldehydes electrophilic: attack of electrophile on benzene

Ex BF3, H+, Cl+, CH3+(carbocation), CH3. (Free radical) etc.

Is carbonyl electrophilic?

The carbonyl carbon in the carbonyl compound is electrophilic in nature due to the partial positive charge developed on it. So we can say that the carbonyl compound is electrophilic

Why carbonyl is electrophilic?

As we look into the C=O bond in carbonyl, the oxygen atom is highly electronegative than that of a carbon atom. It tends to pull the electrons towards itself, making the compound polar, and because of that, carbon attains partial positive charge and oxygen attains partial negative charge

The pi bond in between the carbon and oxygen makes the oxygen to attract more electron pair towards itself intensifying the charge separation between them making the carbonyl more electrophilic.

electrophilic carbon

Are aldehydes electrophilic?

To know are aldehydes electrophilic for that, We know that carbonyl compound is electrophilic in nature and as Aldehyde is also a part of carbonyl compound having C=O bond in its structure in which carbon is electrophilic and hence are aldehydes electrophilic in nature.

Why are aldehydes electrophilic?

The reason for why are aldehydes electrophilic is same for that of a carbonyl compound. In aldehyde the carbon attains +ve charge due to electron deficiency caused by the electronegative oxygen atom due to its tendency to attract electrons towards itself making aldehyde electrophilic

In the aldehyde structure the carbonyl carbon becomes the center for nucleophilic attack

center for nucleophilic attack

Is acetaldehyde an electrophile?

Acetaldehyde (CH3CHO) acts as electrophile but can also be a nucleophile depending on the nature of reaction

carbonyl compound(acetaldehyde) as electrophile

Here the carbonyl carbon acts as an electrophile and undergoes nucleophilic addition reactions

Ex Acetaldehyde as electrophile

addition of HCN on acetaldehyde
mechanism of addition of HCN

Acetaldehyde as nucleophile;

When the hydrogen atom of the carbon alpha to carbonyl carbon is removed the resulting are the carbanions and are resonance stabilized. They act as nucleophiles.

carbonyl compound as nucleophile

Can aldehydes react with electrophiles?

Yes, aldehydes can react with electrophiles due to the presence of the electronegative oxygen atom which acts as a nucleophile.

Reaction with H+

Usually, the reactivity of aldehydes involves the attack of nucleophiles towards the electrophilic carbonyl carbon but when an electrophilic acid like H+ is present in the reaction then it tends to react with the electronegative oxygen atom.

As H+ has no electrons and is electron deficient ready to accept electron and hence is considered an electrophile.

Can aldehydes undergo electrophilic substitution?

Aldehyde is a deactivating group and undergoes electrophilic substitution by facilitating the ring towards electrophilic attack on meta position

resonance of benzaldehyde

From the resonance structure we come to know that ortho and para position are electron deficient and cannot be attacked by an electrophile in turn making the electrophile attack at meta position and undergo substitution as the possibility of finding electrons is more in meta position

for example,

nitration of benzaldehyde

Do aldehydes undergo electrophilic addition?

Due to the presence of an electronegative oxygen atom in the aldehyde structure, the electrophilic addition takes place in the oxygen atom

When H+ forms bond with the oxygen atom a positive charge is obtained on oxygen atom making it unstable and hence attracts electron from the bonded adjacent carbon atom intensifying the positive charge on carbonyl carbon.

Are aldehydes more electrophilic than ketones?

Aldehyde compounds (RCHO) are more electrophilic than the ketone compounds(RCOR’) due to the difference in the groups attached to carbonyl structure.

In aliphatic carbonyl compounds the alkyl group attached to it releases electrons by +I effect increasing the electron density at the carbonyl carbon making it less electrophilic

As we proceed from left to the right the +I effect increases and reactivity decreases the reason is +I effect will destabilize the transition state formed in the reaction by intensifying the negative charge developed on oxygen atom.

In aliphatic aldehydes only one alkyl group is present while 2 alkyl groups are present in ketone making aldehyde more electrophilic.

Aldehydes and nucleophilic substitution

The carbonyl compound of aldehyde (RCHO) does not undergo nucleophilic substitution reaction it is because the hydrogen and the R group present in aldehyde are considered to be poor leaving groups.

When the  nucleophile attacks the carbonyl carbon of aldehyde only just the pi electron clouds are pushed towards the more electronegative oxygen atom making it suitable for nucleophilic addition reaction

attack of nucleophile on aldehyde

What is an electrophilic carbon?

If a carbon in a compound is short of electrons, then these carbons tend to attract electrons pairs from electron rich centres then such a carbon is said to be a electrophilic carbon.

Electrophilic carbons are usually categorized into two types on the basis of its hybridization i.e., sp2electrophile carbons and sp3 electrophile carbons.

The sp3 carbon electrophiles have a good leaving group so when a nucleophile attacks the electrophile it forms a bond with the carbon and substitutes the leaving group

The reason why they have a leaving group is because carbon can form only four bonds and all the bonds are used up so for it to form a new bond with the nucleophile one group must leave.

substitution of Br by a nucleophile

Sp3 carbocation is formed in the reaction which is highly reactive. They have a sextet configuration and hence is electron deficient and is ready to receive electrons.

Sp2 carbon electrophiles are usually found in polar bonds where one atom is more electronegative than the other sp2bonded atom.

E.g.; carbonyl compound bonds

the oxygen atom is more electronegative than the carbonyl carbon which is sp2 hybridized and making it electrophilic.

addition of Grignard reagent on formaldehyde
mechanism of addition of Grignard reagent

What makes carbon more electrophilic?

The carbon atom should be more electron deficient or have more positive charge in it for it to be more electrophilic.

For instance, if we take a carbonyl compound and if in a reaction suppose an acid proton like H+ is added then it will catalyze the nucleophilic attack on carbonyl carbon.

This acidic proton will combine with the electron rich oxygen atom and intensify the electron deficiency on carbonyl carbon. Prior this protonation develops a positive charge on oxygen atom and that helps the pi electron to move towards the oxygen atom.

Conclusion

Through this article we have learnt about the electrophilic nature of aldehyde compounds like its reactivity with other electrophiles or whether it undergoes electrophilic substitution or electrophilic addition reactions and also about its relation with nucleophilic addition reaction. with the help of the electrophilic nature of aldehyde and ketone we have also come to know that aldehydes are more electrophilic than ketones.

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