Aromatic Chemistry

Question 1

Suggest why sodium benzoate is soluble in cold water but benzoic acid is insoluble in cold water

Answer 1

Sodium benzoate soluble because it is ionic. Benzoic acid insoluble because: despite the polarity of the COOH group / ability of COOH to form H bonds, the benzene ring is non-polar

Question 2

After solid benzoic acid has been filtered off from another reaction, it can be purified. Describe the method to purify the benzoic acid

Answer 2

1. Dissolve crude product in hot solvent/water of minimum volume
2. Filter (hot to remove insoluble impurities)
3. Cool to recrystallise
4. Filter under reduced pressure / with Buchner/Hirsch apparatus
5. Wash (with cold solvent) and dry

Question 3

In an experiment, a student used 0.040 mol of ethyl benzoate and obtained 5.12 g of benzoic acid. Calculate the percentage yield of benzoic acid. Suggest why the yield is not 100%

Answer 3

5.12 / 122 (= 0.042 mol)
(0.042/0.04) x 100 = 105 %

OR

Calculation that shows 0.04 mol of benzoic = 4.88 g so
% yield = (5.12/4.88) x100 = 105%

Suggestion - Product not dried / impurities present in product

Question 4

Which compound is formed when phenyl benzenecarboxylate is hydrolysed under box acidic conditions?

A. C6H5CH2OH
B. C6H5CHO
C. C6H5COCH3
D. C6H5COOH

Answer 4

C6H5COOH