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Answer the following.
An organic compound [A] with molecular formula is obtained when compound [B] reacts with red…
An organic compound [A] with molecular formula $\text{C}_2\text{Cl}_3\text{O}_2\text{H}$ is obtained when compound [B] reacts with red phosphorous and $\text{Cl}_2$. The organic compound [B] can be obtained on the reaction of methyl magnesium bromide with carbon dioxide followed by acid hydrolysis. Upon partial reduction, compound [B] forms compound [C] with molecular formula $\text{C}_2\text{H}_4\text{O}$, which gives yellow precipitate of iodoform on heating with iodine in presence of sodium hydroxide. Compound [C] also reacts with dilute $\text{NaOH}$ to form compound [D].
(a)[1.25]
Identify the compounds [A], [B], [C] and [D].
(b)[1.25]
Write down the reaction for the formation of [A] from [B]. What is the reaction called?
(c)[1.25]
Write the chemical test to distinguish between compound [C] and acetone.
(d)[1.25]
Which will be more acidic, compound [A] or [B]? Why?
Answer
Answer (b)
Official answer keyEquation: $\mathrm{CH_{3}COOH} + 3\mathrm{Cl_{2}} \xrightarrow{\text{red}\text{ }\text{P}} \mathrm{CCl_{3}COOH} + 3\mathrm{HCl}$
Reaction: Hell-Volhard-Zelinsky reaction
Product: trichloroacetic acid
The reaction is called the Hell-Volhard-Zelinsky (HVZ) reaction.
Answer (c)
Official answer key| Test | Ethanal [C] | Acetone |
|---|---|---|
| Tollens' reagent | Silver mirror | No silver mirror |
| Fehling's solution | Red precipitate of $\mathrm{Cu_2O}$ | No red precipitate |
| Schiff's reagent | Pink colour restored | No colour |
Answer (d)
Official answer keyCompound [A], trichloroacetic acid, is more acidic. The three Cl atoms have a strong electron withdrawing ($-I$) effect, which disperses the negative charge and stabilises the carboxylate anion, so the acid is stronger. In acetic acid [B] there is no $-I$ effect (the $+I$ effect of $\mathrm{CH_3}$ destabilises the anion), so it is less acidic.
From ISC 2027 Specimen Chemistry Paper 1, question 19(i).
Check your working with the Distinguish two organic compounds: the chemical tests that tell two organic compounds apart, with what each one shows.