Question

A compound A with a molecular formula C3H8O is oxidised with acidified potassium dichromate(VI) to form a liquid B. B reacts with hydrogen cyanide to form a compound C that contains 3 carbon atoms. What are the structures of A B and C?

Answer 1

Well, this is what I have... If `"A"` is `"H"_3"C"-("CH"_2)_2-"OH"`, then `"B"` is `"H"_3"C"-"CH"_2-"COOH"` and `"C"` `=` `"B"` because `"HCN"` doesn't effectively react with carboxylic acids.

Likely the intention is the secondary alcohol for `"A"`, acetone for `"B"`, and the cyanohydrin for `"C"`, even though `"C"` has 4 carbons.

---

There are three isomers of `"C"_3"H"_8"O"` (`"A"`):

1. `"H"_3"C"-"O"-"CH"_2"CH"_3`
2. `"H"_3"C"-("CH"_2)_2-"OH"`
3. `"H"_3"C"-("C"-"OH")-"CH"_3`

The ether does not react with `"H"_2"Cr"_2"O"_7`, while the alcohols get oxidized. The `1^@` alcohol should go all the way to propanoic acid (`"B"`), and the `2^@` alcohol would stop at acetone (`"B"`); `"H"_2"Cr"_2"O"_7` is a very good oxidizing agent.

In reacting with `"HCN"`, I would expect the carbonyl oxygen to get protonated, followed by nucleophilic attack of the now-electrophilic carbonyl carbon.

That, however, should not happen with carboxylic acids. I showed the intermediate to hopefully make it clear.

It looks like `"B"` must be the carboxylic acid in order to leave `"C"` with only `3` carbons. Otherwise there must be one carbon added from `"CN"^(-)`, necessarily giving `4`.

Answer 2

Considering that the final product `C` contains `3` carbon atoms in its longest carbon chain, I present here a possible answer.

The starting material `A` has got molecular formula `C_3H_8O`.
It follows general molecular formula `C_nH_(2n+2)O` of saturated ether or alcohol. But compound `A` is oxidized by acidified `K_2Cr_2O_7` to a liquid `B` which further reacts with `HCN` to form `C`. So the compound `A` must be an alcohol as ether is not oxidisable under given condition,

The possible structural formula of saturated alcohols having molecular formula `C_3H_8O` are.

(1) `CH_2CH_2CH_2OH`,prpoan-1-ol
and
(2) `CH_2CH(OH)CH_3`,prpoan-2-ol

For compound (1)

(1) `CH_2CH_2CH_2OH`,prpoan-1-ol which being a primary alcohol is first oxidized to propanal `CH_3CH_2CHO` and finally to propanoic acid `CH_3CH_2COOH` which does not react with `HCN` used here as final reactant. Hence `A` should not be prpoan-1-ol

`CH_2CH_2CH_2OHstackrel(color(red)([O]))->CH_3CH_2CHOstackrel(color(red)([O]))->CH_3CH_2COOH`

For compound (2)

(2) `CH_2CH(OH)CH_3`,prpoan-2-ol which being a secondary alcohol is oxidized to propanone `CH_3COCH_3` and its further oxidation is difficult . This reacts with `HCN` used here as final reactant to form 2-hydroxy-2-methylpropanenitrile. Hence `A` should be prpoan-2-ol

`CH_2CH(OH)CH_3stackrel(color(red)([O]))->CH_3COCH_3stackrel(color(red)([HCN]))->CH_3CH(OH)(CN)CH_3`