JEE Advanced 2025 · Paper 1 · Chemistry Q4

← JEE Advanced 2025 · Paper 1

ChemistrySingle correct+3 / -1Question 36 of 48

Consider the depicted hydrogen (H) in the hydrocarbons given below. The most acidic hydrogen (H) is

A fulvene: a cyclopentadiene ring with an exocyclic =CH2 at C1; ring double bonds C2=C3 and C4=C5. An isopropyl group is on C4; the depicted H is the isopropyl C-H.

6,6-Dimethylfulvene with a methyl on ring C3: the ring carbon C1 carries an exocyclic C=C(CH3)CH3; ring double bonds C2=C3 and C4=C5. The depicted H is on one methyl of the exocyclic carbon.

A fulvene with an exocyclic =CH2 at C1 and an isopropyl group on the adjacent ring carbon C2 (ring double bonds C2=C3 and C4=C5). The depicted H is the isopropyl C-H.

A cyclopentene ring with two exocyclic double bonds: =CH2 at C1 and =C(CH3)CH3 at C3; ring double bond C4=C5 and an sp3 CH2 at C2 between the two exocyclic carbons. The depicted H is on one methyl of the =C(CH3)2 group.

Answer and worked solution

Official answer: B

The most acidic H is the one whose conjugate base is most stable.

(B). Removing the depicted methyl H gives $\ce{CH2^-}-C(\ce{CH3})=$ C1 of the fulvene. The charge moves onto C1, which already has two ring double bonds. All five ring carbons then share 6 π electrons, forming an aromatic cyclopentadienyl anion. This is the large stabilisation.

(A) and (C). The carbanion conjugates with the fulvene, but every resonance form keeps an exocyclic $\ce{C=C}$ on a ring carbon. The ring never becomes an aromatic 6π anion.

(D). The ring contains an sp³ $\ce{CH2}$ , which breaks cyclic conjugation, so no aromatic anion can form.

The most acidic H is the one in (B).

Topics:Acidity and basicity Aromaticity Reactive intermediates

← Previous question Next question →

JEE Advanced 2025 · Paper 1, Chemistry question 4