Aromatic Aldehydes, Ketones and Carboxylic Acids – Derivatives of Benzene (Senior 6 Chemistry) | Ganzaa

Derivatives of Benzene

Lessons

Senior 6 · Chemistry

Derivatives of Benzene

5 lessons

Lesson 4 of 5

Aromatic Aldehydes, Ketones and Carboxylic Acids

25 min

Almonds and soft drinks

The smell of bitter almonds and of almond-flavoured cakes is benzaldehyde. The label on a bottle of soft drink or fruit juice often lists "sodium benzoate (E211)", a preservative that stops yeasts and moulds from growing. Both are benzene derivatives with a carbonyl group, C=O, joined to the ring. In S5 you studied aldehydes, ketones and carboxylic acids; their aromatic members react in the same way at the functional group, with a few important differences.

Definition

Aromatic aldehyde: a compound with a –CHO group joined directly to a benzene ring. Example: benzaldehyde, $\mathrm{C_6H_5CHO}$ .

Aromatic ketone: a compound with a C=O group joined to a benzene ring and to another carbon group. Example: phenylethanone, $\mathrm{C_6H_5COCH_3}$ .

Aromatic carboxylic acid: a compound with a –COOH group joined directly to a benzene ring. Example: benzoic acid, $\mathrm{C_6H_5COOH}$ .

Benzaldehyde and phenylethanone

Preparation. Benzaldehyde is made by the controlled oxidation of methylbenzene, or by hydrolysing (dichloromethyl)benzene from the side-chain chlorination of Lesson 2: $\mathrm{C_6H_5CHCl_2 + H_2O \rightarrow C_6H_5CHO + 2HCl}$ . Phenylethanone is made by Friedel–Crafts acylation (Unit 3).

Table 4.1: Reactions of the carbonyl group

Reagent	Benzaldehyde, $\mathrm{C_6H_5CHO}$	Phenylethanone, $\mathrm{C_6H_5COCH_3}$
2,4-dinitrophenylhydrazine (2,4-DNPH)	orange precipitate	orange precipitate
Tollens' reagent, warm	silver mirror (oxidised to benzoate)	no change
Fehling's or Benedict's solution, warm	no red precipitate: aromatic aldehydes do not reduce it	no change
iodine in NaOH(aq), warm (triiodomethane test)	no change	yellow precipitate of $\mathrm{CHI_3}$ (has a $\mathrm{CH_3CO-}$ group)
HCN (from KCN + acid)	nucleophilic addition: $\mathrm{C_6H_5CH(OH)CN}$	nucleophilic addition: $\mathrm{C_6H_5C(OH)(CH_3)CN}$
$\mathrm{NaBH_4}$	reduced to phenylmethanol, $\mathrm{C_6H_5CH_2OH}$	reduced to 1-phenylethanol

Benzaldehyde is also oxidised slowly by the air, so old bottles often contain white crystals of benzoic acid.

Benzoic acid

Benzoic acid is a white crystalline solid that melts at 122 °C. It is only slightly soluble in cold water but much more soluble in hot water, so it is purified by recrystallisation: dissolve it in the minimum of boiling water, filter while hot, and let the solution cool; pure needle-shaped crystals form and the soluble impurities stay in solution.

Acidity. Benzoic acid ( $p K_{a}$ 4.20) is a slightly stronger acid than ethanoic acid ( $p K_{a}$ 4.76), and much stronger than phenol ( $p K_{a}$ 10.0). It fizzes with sodium hydrogencarbonate, which phenol does not:

$\mathrm{C_6H_5COOH(s) + NaHCO_3(aq) \rightarrow C_6H_5COONa(aq) + H_2O(l) + CO_2(g)}$

Other reactions follow the carboxylic acids of S5:

esterification with methanol and a few drops of concentrated sulfuric acid, under reflux: $\mathrm{C_6H_5COOH + CH_3OH \rightleftharpoons C_6H_5COOCH_3 + H_2O}$ (methyl benzoate);
with phosphorus(V) chloride, the acyl chloride: $\mathrm{C_6H_5COOH + PCl_5 \rightarrow C_6H_5COCl + POCl_3 + HCl}$ ;
reduction by $\mathrm{LiAlH_4}$ in dry ether to phenylmethanol, $\mathrm{C_6H_5CH_2OH}$ ;
on the ring, nitration gives 3-nitrobenzoic acid, because –COOH deactivates the ring and directs to carbon 3 (Lesson 1).

Uses. Benzoic acid (E210) and sodium benzoate (E211) are preservatives in acidic foods and drinks such as fruit juices, soft drinks and sauces; they work best in acid, where the undissociated acid enters the microbes. Benzoates are also used in medicines, and benzaldehyde is a flavouring.

Figure 4.1: A scheme for telling apart four aromatic compounds met in this unit.

Worked example

Problem. Compounds P, Q and R are aromatic. P ( $\mathrm{C_7H_6O}$ ) gives an orange precipitate with 2,4-DNPH and a silver mirror with Tollens' reagent. Q ( $\mathrm{C_8H_8O}$ ) gives an orange precipitate with 2,4-DNPH, no silver mirror, and a yellow precipitate with iodine and sodium hydroxide. R ( $\mathrm{C_7H_6O_2}$ ) fizzes with sodium hydrogencarbonate. Identify P, Q and R.

Solution. P is a carbonyl compound (2,4-DNPH) that is an aldehyde (Tollens'): with seven carbons it is benzaldehyde, $\mathrm{C_6H_5CHO}$ . Q is a ketone (2,4-DNPH positive, Tollens' negative) containing $\mathrm{CH_3CO-}$ (triiodomethane): phenylethanone, $\mathrm{C_6H_5COCH_3}$ . R is a carboxylic acid: benzoic acid, $\mathrm{C_6H_5COOH}$ .

Worked example

Problem. 12.2 g of benzoic acid is refluxed with excess methanol and concentrated sulfuric acid, giving 10.9 g of methyl benzoate. Calculate the percentage yield, and give one reason it is below 100%. ( $M_{r}$ : benzoic acid 122.1, methyl benzoate 136.1)

Solution.

$n = \frac{12.2}{122.1} = 0.09992\,\mathrm{mol} \qquad \text{theoretical mass} = 0.09992 \times 136.1 = 13.60\,\mathrm{g}$

$\text{yield} = \frac{10.9}{13.60} \times 100 = 80.1\,\%$

Answer: 80.1%. Esterification is reversible, so the reaction does not go to completion; some product is also lost during separation and distillation.

Warning

Exam trap. Benzaldehyde gives a silver mirror with Tollens' reagent but no red precipitate with Fehling's or Benedict's solution. Do not use Fehling's solution to prove that an aromatic compound is an aldehyde. Also take care in the laboratory: benzaldehyde is harmful if swallowed, and 2,4-DNPH is toxic, and the dry solid is flammable and can explode, so it is kept damp.

Tip

Work through tests from the most general to the most specific: acid (NaHCO₃) → carbonyl (2,4-DNPH) → aldehyde (Tollens') → methyl ketone (iodine and NaOH). Figure 4.1 is this order drawn out.

chemistry edb: Using Tollens' Reagent to Test for Aldehydes (3 min). Watch how the reagent is made and how the silver mirror forms on the inside of the test tube.

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Practice

Name and draw the structures of benzaldehyde, phenylethanone and benzoic acid.
Give one test that distinguishes benzaldehyde from phenylethanone, with the result for each.
Explain why Fehling's solution should not be used to show that benzaldehyde is an aldehyde.
Write the equation for the reaction of benzoic acid with sodium hydrogencarbonate. How does this reaction distinguish benzoic acid from phenol?
Describe how impure benzoic acid is purified by recrystallisation, and explain why this works.
Name the main product when benzoic acid is nitrated, and explain the position of the nitro group.
An old bottle of benzaldehyde contains white crystals. Suggest what they are, and write an equation (use [O]) for their formation.
Sodium benzoate works as a preservative in acidic fruit juice but much less well in neutral foods. Suggest why, using the equilibrium $\mathrm{C_6H_5COOH \rightleftharpoons C_6H_5COO^- + H^+}$ .

Show answersHide answers

Benzaldehyde, $\mathrm{C_6H_5CHO}$ : a ring with –CHO. Phenylethanone, $\mathrm{C_6H_5COCH_3}$ : a ring with –COCH₃. Benzoic acid, $\mathrm{C_6H_5COOH}$ : a ring with –COOH.
Tollens' reagent, warm: benzaldehyde gives a silver mirror; phenylethanone gives none. (Or iodine and NaOH: phenylethanone gives a yellow precipitate; benzaldehyde does not.)
Aromatic aldehydes do not reduce Fehling's solution, so benzaldehyde gives a negative result even though it is an aldehyde.
$\mathrm{C_6H_5COOH + NaHCO_3 \rightarrow C_6H_5COONa + H_2O + CO_2}$ . Benzoic acid fizzes ( $\mathrm{CO_2}$ ); phenol, a weaker acid than carbonic acid, does not.
Dissolve it in the minimum volume of boiling water, filter hot to remove insoluble impurities, cool slowly, filter off the crystals and dry them. Benzoic acid is much more soluble in hot water than in cold, so most of it crystallises on cooling, while the small amounts of soluble impurities stay in solution.
3-Nitrobenzoic acid. The –COOH group withdraws electrons and deactivates the ring, especially at carbons 2, 4 and 6, so the electrophile attacks carbon 3.
Benzoic acid, formed by oxidation in air: $\mathrm{C_6H_5CHO + [O] \rightarrow C_6H_5COOH}$ .
In acid, the equilibrium lies to the left, so most of the preservative is present as undissociated benzoic acid, which can pass into microbial cells and act. In neutral food it is mostly benzoate ions, which enter cells much less easily.

Sources and credits

Image: Benzoic-acid-crystals.jpg by Unknown61173, via Wikimedia Commons, dedicated to the public domain under CC0 1.0.
Video: Using Tollens' Reagent to Test for Aldehydes (Silver Mirror Test), chemistry edb, embedded from YouTube.
The text, flowchart, tables, worked examples and Practice items were written and drawn by Ganzaa.

1Almonds and soft drinks

2Benzaldehyde and phenylethanone

3Benzoic acid

Almonds and soft drinks

Benzaldehyde and phenylethanone

Benzoic acid