Quick questions on Esters, esterification and saponification explained: HSC Chemistry Module 7

An ester contains the linkage $-COO-$. The general formula is $R-COO-R'$, where $R$ comes from the carboxylic acid and $R'$ comes from the alcohol.

An alcohol reacts with a carboxylic acid in the presence of a concentrated $H_2SO_4$ catalyst, heated under reflux, to give an ester and water:

Esters have a polar $C=O$ but no $O-H$, so they cannot hydrogen-bond to each other. Boiling points are lower than the parent acid and alcohol of similar molar mass, comparable to ketones. Small esters are volatile liquids with characteristic fruity smells.

Acid hydrolysis. Reflux the ester with dilute sulfuric acid; the reverse of esterification:

Fischer esterification is acid-catalysed nucleophilic acyl substitution. The acid is protonated on the carbonyl O, the alcohol O attacks the carbonyl C, proton transfers occur, water leaves, and a proton is lost from the protonated ester. You do not need the full curly-arrow mechanism for HSC, but you do need to know what each reactant contributes.

Pour the cooled mixture into saturated sodium hydrogencarbonate to neutralise unreacted acid (effervescence stops when complete). Separate the organic layer, dry over anhydrous $MgSO_4$, then distil at the boiling point of the ester.

Reflux the ester with dilute sulfuric acid; the reverse of esterification:

Reflux the ester with aqueous NaOH:

Always alkyl (from alcohol) first, then alkanoate (from acid). Do not write "ethanoate ethyl".

Acid hydrolysis and esterification are reversible. Saponification is not. Get the arrows right.

You need concentrated $H_2SO_4$, both as catalyst and dehydrator. Dilute acid does not drive the equilibrium.

Without it, the product is contaminated with unreacted carboxylic acid and traces of $H_2SO_4$.

Biodiesel is made by transesterification (swapping the alcohol on an existing ester), not by direct esterification of fatty acid plus methanol.