HardChemistryHigh School (11–12)Organic Compounds with Functional Groups › Aldehydes, Ketones and Carboxylic AcidsConceptual
Four acids are given: A, 4-chlorobutanoic acid, B, 2-chlorobutanoic acid, C, butanoic acid, and D…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Four acids are given: A, 4-chlorobutanoic acid, B, 2-chlorobutanoic acid, C, butanoic acid, and D, 3-chlorobutanoic acid. Arrange them in order of increasing acid strength. Name the effect that causes the order, and say why the order of A, D and B follows the position of the chlorine atom. Then say which is stronger, 4-nitrobenzoic acid or benzoic acid.
Need a reminder? The key ideaShow
. For a weak acid HA at concentration , , so , and . The approximation holds only when the acid is ionised by a few per cent. Withdrawing groups make an acid stronger, and the effect falls with distance. In a titration, moles of base .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The effect. The chlorine atom pulls electrons through the chain, the effect. It stabilises the carboxylate ion.
Step 2. Distance. The pull falls off quickly with the number of bonds between the chlorine and the carboxyl group. So the order of increasing strength is C, A, D, B: butanoic acid, 4-chloro, 3-chloro, 2-chloro.
Step 3. The nitro compound. A nitro group on the ring withdraws electrons. 4-Nitrobenzoic acid is stronger than benzoic acid.
Check. The pKa values are about , , and , so the order is the right way round.
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