Question

The minimum number of Carbon atoms in Ketones which will show chain isomerism is:
A.Seven
B.Four
C.Six
D.Five

Answer 1

Hint: The components of chain isomerism isomers display differently branched structures. Think about the structures of ketones with least number of carbon atoms and then proceed.

Complete step by step answer:
A ketone is a functional group with the structure RC(=O)R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond). The simplest ketone is acetone (R = R' = methyl), with the formula \[C{H_3}C\left( O \right)C{H_3}\]
 let's try from the least possible number of C atoms for ketones.
3 C atoms.
Look at the smallest ketone with 3 carbon atoms; the carbons on either side of the carbon bonded with oxygen cannot be arranged in any other way. Putting both carbons on one side of the carbonyl group(c=o grp) turns the compound into an aldehyde.
2. 4 C atoms
Look at the ketone with carbon atoms; the carbons on either side of the carbon bonded with oxygen cannot be arranged in any other way. Putting both carbons on one side of the carbonyl group(c=o grp) turns the compound into an aldehyde. So, it will not show chain isomerism.
3. 5 C atoms
With pentanone, we can shift the carbonyl group either on 2nd or 3rd C atoms. This will give us either Pentan-2-one or Pentan-3-one.
So, the correct option is (D)

Note:
The Ketone with the minimum number of carbon is Propanone which contains 3 carbon atoms. The carbons on either side of the carbon bonded with oxygen cannot be arranged in any other way. Putting both carbons on one side of the carbonyl group(c=o group) turns the compound into an aldehyde.