Chemistry Chemical Reactions of Carbonyl Compounds : Due to `alpha` Hydrogen

Topics Covered :

● Reactions due to `alpha` Hydrogen
● Acidity of `alpha`-Hydrogen of Aldehydes and Ketones
● Aldol Condensation
● Cross Aldol Condensation
● Cannizzaro Reaction
● Electrophilic Substitution Reaction

Reactions due to `α`-hydrogen :

Reactions due to `color{red}(α)`-hydrogen are given as follow :

Acidity of `α`-hydrogens of aldehydes and ketones :

`=>` The aldehydes and ketones undergo a number of reactions due to the acidic nature of `color{red}(α)`-hydrogen.

`=>` The acidity of `color{red}(α)`-hydrogen atoms of carbonyl compounds is due to the strong electron withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base.

Aldol Condensation :

`=>` Aldehydes and ketones having at least one `color{red}(α)`-hydrogen undergo a reaction in the presence of dilute alkali as catalyst to form `color{red}(β)`-hydroxy aldehydes (aldol) or `color{red}(β)`-hydroxy ketones (ketol), respectively. This is known as Aldol reaction.

`color{red}(undersettext(Ethanal)(2CH_3-CHO) oversettext(dill NaOH)⇄ undersettext(3-hydroxybutanal)(CH_3- underset (underset(OH)(|))CH-CH_2-CHO) underset(-H_2O) overset(Delta)→ undersettext(Aldol condensation product)(undersettext(But-2-enal)(CH_3-CH=CH-CHO)))`

`color{red}(undersettext(Propanone)(2CH_3-CO-CH_3) overset(Ba(OH)_2)⇄ undersettext{(ketol) 4- Hydroxy-4-methylpentan-2-one}(CH_3- underset(underset(OH)(|)) overset(overset(CH_3)(|))C-CH_2COCH_3) underset(-H_2O) overset(Delta)→ undersettext{4-Methylpent -3-en-2-one (Aldol condensation product)}(CH_3- overset(overset(CH_3)(|))C=CH-CO-CH_2))`

`=>` The name aldol is derived from the names of the two functional groups, aldehyde and alcohol, present in the products.

`=>` The aldol and ketol readily lose water to give `color{red}(α, β)`-unsaturated carbonyl compounds which are aldol condensation products and the reaction is called Aldol condensation.

`=>` Though ketones give ketols (compounds containing a keto and alcohol groups), the general name aldol condensation still applies to the reactions of ketones due to their similarity with aldehydes.

Cross Aldol Condensation :

`=>` When aldol condensation is carried out between two different aldehydes and/or ketones, it is called cross aldol condensation.

`=>` If both of them contain `color{red}(α)`-hydrogen atoms, it gives a mixture of four products.

`=>` This is illustrated in fig.1 by aldol reaction of a mixture of ethanal and propanal.

`=>` Ketones can also be used as one component in the cross aldol reactions. See fig.2.

Other Reactions :

(i) `color{green}(text(Cannizzaro Reaction ))` : Aldehydes which do not have an `color{red}(α)`-hydrogen atom, undergo self oxidation and reduction (disproportionation) reaction on treatment with concentrated alkali. See fig.1.

● In this reaction, one molecule of the aldehyde is reduced to alcohol while another is oxidised to carboxylic acid salt.

(ii) `color{green}(text(Electrophilic Substitution Reaction ))` : Aromatic aldehydes and ketones undergo electrophilic substitution at the ring in which the carbonyl group acts as a deactivating and meta-directing group. See fig.2.