See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: The dipole moment of a carbonyl compound depends on the electronegativity difference across the C=O bond and the contribution of resonance structures that place charge separation (C(+)-O(-) ionic form). A higher contribution of the ionic resonance structure increases the dipole moment. Step 1 – Identify the key factor: For cyclic ketones, ring strain and aromaticity of the transition state affect how much the C=O bond acquires single-bond (ionic) character through resonance. The more the ionic resonance form (C(+)=O(-)) contributes, the larger the dipole moment. Step 2 – Cyclopropenone analysis (option C): Cyclopropenone (cycloprop-2-en-1-one) is a three-membered ring with a C=C and a C=O. When the C=O donates electron density back, it generates a cyclopropenyl cation (2pi Huckel aromatic, highly stabilized) in the ring. This means the ionic resonance structure C(+)-O(-) is extremely stabilized by the aromatic cyclopropenyl cation character of the ring. This dramatically increases the contribution of the charge-separated form, giving cyclopropenone an exceptionally large dipole moment (~4.4 D, experimentally among the highest for small ketones). Step 3 – Compare with other options: - (a) 2-methylcyclobutenone: some conjugation but no special aromatic stabilization of the ionic form. - (b) Cyclopentanone: normal saturated cyclic ketone, typical dipole moment (~3.3 D), no special resonance enhancement. - (d) Cyclohex-2-en-1-one: conjugated enone, but the ionic resonance form does not gain aromatic stabilization; dipole moment is moderate. Step 4 – Conclusion: The aromatic stabilization of the cyclopropenyl cation in the resonance structure of cyclopropenone makes its ionic C(+)-O(-) form far more stable than in any of the other options, resulting in the maximum dipole moment. Therefore, the correct answer is C.