Question

Write structural formulas and names of four possible aldol condensation products from propanol and butanal. In each case, indicate which acts as nucleophile and which as electrophile.

Answer 1

Aldol condensation is a reaction in which α-hydrogen atom in aldehyde undergoes a reversible self-addition to give $\beta$ -hydroxy aldehyde as a condensation product in presence of dilute alkali. In every case the addition occurs in such a way that the α carbon of the first carbonyl compound gets attached to the carbonyl carbon of the second compound.
$\text{C}{{\text{H}}_{3}}\text{CHO}+\text{C}{{\text{H}}_{3}}\text{CHO}\to \text{C}{{\text{H}}_{3}}\text{CH}\left( \text{OH} \right)\text{C}{{\text{H}}_{2}}\text{CHO}$

Complete solution step by step
Two molecules of propanal react with each other in presence of dilute $NaOH$ , one acts as nucleophile and other acts as electrophile.
$2\text{C}{{\text{H}}_{3}}\text{C}{{\text{H}}_{_{2}}}\text{CHO}\to \text{C}{{\text{H}}_{3}}\text{C}{{\text{H}}_{2}}\text{CH}\left( \text{OH} \right)\text{CH}\left( C{{H}_{3}} \right)\text{CHO}$
Propanal 3-hydroxy-2-methylpentanal
Two molecules of butanal react with each other in presence of dilute $NaOH$ , one act as nucleophile and other acts as electrophile.
$\text{2C}{{\text{H}}_{3}}\text{C}{{\text{H}}_{2}}\text{ C}{{\text{H}}_{2}}\text{CHO}\to \text{C}{{\text{H}}_{3}}\text{C}{{\text{H}}_{2}}\text{ C}{{\text{H}}_{2}}\text{CH(OH) CH}\left( \text{CH2C}{{\text{H}}_{3}} \right)\text{CHO}$
Butanal 2-Ethyl-3-hydroxyhexanal
One molecule of propanal reacts with one molecule of butanal in which propanal acts as nucleophile and butanal acts as an electrophile.
$\text{C}{{\text{H}}_{3}}\text{C}{{\text{H}}_{2}}\text{ C}{{\text{H}}_{2}}\text{CHO}+\text{C}{{\text{H}}_{3}}\text{C}{{\text{H}}_{2}}\text{ CHO}\to \text{C}{{\text{H}}_{3}}\text{C}{{\text{H}}_{2}}\text{C}{{\text{H}}_{2}}\text{CH}\left( \text{OH} \right)\text{CH}\left( \text{C}{{\text{H}}_{3}} \right)\text{CHO}$
Butanal Propanal 3-Hydroxy-2-methylpentanal $\text{CH3CH2CH(OH)CH(CH2C}$
(Electrophile) (Nucleophile)
One molecule of butanal reacts with propanal in which butanal acts as a nucleophile and propanal acts as electrophile.
$\text{C}{{\text{H}}_{3}}\text{C}{{\text{H}}_{2}}\text{CHO}+\text{C}{{\text{H}}_{3}}\text{C}{{\text{H}}_{2}}\text{C}{{\text{H}}_{2}}\text{CHO}\to \text{C}{{\text{H}}_{3}}\text{C}{{\text{H}}_{2}}\text{CH}\left( \text{OH} \right)\text{CH}\left( \text{C}{{\text{H}}_{2}}\text{C}{{\text{H}}_{3}} \right)\text{CHO}$
Propanal Butanal 2-Ethyl-3-hydroxyhexanal
(Electrophile) (Nucleophile)

Additional Information
Aldol condensation also takes place in ketones containing α-hydrogen atoms and the final product formed will be $\beta$ -hydroxy ketone. Generally, aldol condensation involves the nucleophilic attack on carbonyl to produce aldehyde of $\beta$ -hydroxy or ketone. The nucleophile is generally an enolate of an aldehyde or ketone attacking ketone molecule or another aldehyde. The condensation can catalyse either by acidic or basic solution.

Note
If an aldehyde does not contain an α-hydrogen atom then self-condensation reactions do not occur. In self-condensation reaction the carbonyl group chemical compound behaves both like electrophile and nucleophile. Electrophiles are electron deficient in nature so they are attracted towards electron rich species whereas nucleophiles are the ones which donate electron pairs for forming a chemical bond.