Question

Polar solvent favours keto while non-polar solvent favours enol form, which has maximum enol content in $CC{{l}_{4}}$ ?
A.$C{{H}_{3}}-C(=O)-C{{H}_{2}}-C(=O)-C{{H}_{3}}.$
B.$C{{H}_{3}}-C(=O)-C{{H}_{2}}-C(=O)-H.$
C.$C{{H}_{3}}-C(=O)-C{{H}_{2}}-COO{{C}_{2}}{{H}_{5}}.$
D.$C{{H}_{3}}-C(=O)-C{{H}_{2}}C{{H}_{3}}.$

Answer 1

Hint: We know that the extent of enol content is explained based on orifice hydrogen and intermolecular hydrogen bonding. In keto form alpha-hydrogen shifts to oxygen forming a hydroxyl group. The newly formed structure is known as enol. The amount of enol in keto-enol form depends upon how stable the enol form is.

Complete answer:
The functional group having double-bonded oxygen atoms and two bonds with hydrogen or alkyl group is known as the keto group and the group having hydroxyl group attached with double-bonded carbon atoms is known as the enol form. The conversion of keto to enol is known as ketone-enol tautomerism. The shifting of alpha-hydrogen of keto to double-bonded oxygen gives enol form. Generally, the ketone compounds exhibit the conversion of keto form to enol form through keto-enol tautomerism. To exhibit keto-enol tautomerism the carbonyl compound should contain alpha hydrogen atoms in it.
In keto form alpha-hydrogen shifts to oxygen forming a hydroxyl group. The newly formed structure is known as enol. The amount of enol in keto-enol form depends upon how stable the enol form is. Keto and enol form a compound are isomers. These isomers are called tautomerism and isomerization is known as tautomerism. Tautomers are isomers of a compound which differ in portion of protons and electrons. When a reaction involves a simple intermolecular proton, transfer is called tautomerism. In the following example $C{{H}_{3}}-C(=O)-C{{H}_{2}}-C(=O)-C{{H}_{3}}$ enol content is maximum due to intermolecular h-bonding and resonance stabilization. IUPAC name is \[pentane-2,4-dione.\] Enol content is high in carbon with electron withdrawing group \[>\] carbonyl carbon \[>\] number of atoms of hydrogen. Keto is electrophilic and enol is nucleophilic.
Therefore, the correct answer is option A.

Note:
Remember that keto form is more stable than the enol form because the carbon-oxygen double bond is more stable than the carbon-carbon double bond. If enol is present in more amounts than keto form, then there will be some additional stability factor in enol form such as hydrogen bonding.