See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Concept: Gem-diols (hydrates of ketones/aldehydes) are formed by addition of water to a carbonyl compound. Their stability is governed by the equilibrium between the gem-diol and the corresponding carbonyl compound (ketone). A gem-diol is MORE stable when the corresponding carbonyl compound is LESS stable, i.e., when there is greater relief of ring strain upon going from the planar sp2 carbonyl carbon to the tetrahedral sp3 carbon in the gem-diol. Step 1 - Identify the corresponding carbonyl compounds: - Gem-diol (I) corresponds to cyclopropanone - Gem-diol (II) corresponds to cyclobutanone - Gem-diol (III) corresponds to cyclopentanone Step 2 - Ring strain considerations: The carbonyl carbon in a ketone is sp2 hybridized, requiring bond angles of ~120°. When the carbon is part of a small ring, forcing the ring angles to accommodate sp2 geometry creates additional angle strain. The gem-diol carbon is sp3 hybridized (~109.5°), which is closer to the natural small-ring angles for very strained rings. Step 3 - Cyclopropanone vs cyclopropane-1,1-diol: Cyclopropane has internal angles of ~60°. An sp2 carbon (120° preferred) in a 3-membered ring experiences enormous angle strain. Converting to sp3 (109.5° preferred) in the gem-diol relieves considerable strain. Therefore cyclopropanone strongly favors the gem-diol form. Cyclopropanone is known to exist predominantly as its gem-diol in water. Step 4 - Cyclobutanone vs cyclobutane-1,1-diol: Cyclobutane has internal angles of ~90°. An sp2 carbon in a 4-membered ring also experiences significant angle strain (though less than in 3-membered ring). The gem-diol (sp3) is more stable than for a typical unstrained ketone, but less so than cyclopropanone's gem-diol. Step 5 - Cyclopentanone vs cyclopentane-1,1-diol: Cyclopentane has internal angles of ~108°, which is very close to the ideal sp3 angle (109.5°) and reasonably close to sp2 (120°). The cyclopentanone carbonyl carbon experiences little additional ring strain compared to an open-chain ketone. Therefore the gem-diol of cyclopentanone is the least stabilized relative to its carbonyl form among the three. Step 6 - Order of gem-diol stability: More ring strain in the carbonyl form → greater relief upon forming the gem-diol → more stable gem-diol. Cyclopropanone (most strained sp2) > Cyclobutanone (moderately strained sp2) > Cyclopentanone (least strained sp2) Therefore: gem-diol (I) > gem-diol (II) > gem-diol (III) Why other options fail: - (b) III > II > I: Reverses the trend; cyclopentane-1,1-diol would be least stable, not most. - (c) I > III > II: Incorrectly places cyclopentyl gem-diol above cyclobutyl gem-diol; cyclobutanone is more strained than cyclopentanone. - (d) III > I > II: Incorrect ordering with no ring-strain basis. Therefore, the correct answer is A.