Question

What are the best ways to determine if any residual acid is mixed in with you ester? Consider MP, IR, & UV and comment on the usefulness of each.

Answer 1

IR is best way to determine if any residual acid is mixed in with your ester, sinces an acid will show a strong broad signal in the region 3200-3600 cm-1 if it is capable of roming hydrogen bonding, a strong and sharp signal is boserved in the region 3500-3700 cm-1 if it h-bond free. C=O stretch for esters are 1735 cm-1 and two bands for C-O stretch 1000-1300 cm-1.

MP - melting point will increase due to ester hydrolyse to acid and alcohol in presence of catalytic amount of acid presence and wide range melting point is also increased dut to additional compound than pure one.

UV: For esters there are two absorption maxima π→ π^* (<200 nm) n→ π^* (~207 nm)

π electron from π of C=O

n electron from O lone pair

π^*antibonding C=O

carboxylic acids absorb very weakly around 200-215 nm, whereas ester will show absorption maxima > 250 nm.

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