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?

2 Answers
Apr 4, 2018

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.

Apr 24, 2018

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

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