Which of the following compounds will most readily be dehydrated to give alkene under acidic condition?

When an alcohol is dehydrated under acidic conditions, the $$-OH$$ group is protonated and lost as water, forming a carbocation intermediate which then loses a proton to give an alkene. The ease of dehydration depends on (i) the stability of the carbocation intermediate or (ii) whether a special driving force (like conjugation) stabilises the product.

Let us analyse each compound:

Option A: 4-Hydroxypentan-2-one

Structure: $$CH_3 - CO - CH_2 - CH(OH) - CH_3$$

The carbonyl is at C-2 and the $$-OH$$ is at C-4. The C-4 is the $$\beta$$-carbon relative to the carbonyl group (C-2 is carbonyl, C-3 is $$\alpha$$, C-4 is $$\beta$$).

This is a $$\beta$$-hydroxy ketone. Under acidic conditions, $$\beta$$-hydroxy carbonyl compounds undergo very facile dehydration because the product is an $$\alpha,\beta$$-unsaturated ketone (a conjugated enone):

$$CH_3 - CO - CH = CH - CH_3$$

This product (pent-3-en-2-one) is stabilised by conjugation between the $$C=C$$ double bond and the $$C=O$$ group. The extended conjugation provides a strong thermodynamic driving force, making this dehydration very easy.

Option B: 3-Hydroxypentan-2-one

Structure: $$CH_3 - CO - CH(OH) - CH_2 - CH_3$$

Here the $$-OH$$ is at C-3, which is the $$\alpha$$-carbon relative to the carbonyl at C-2. This is an $$\alpha$$-hydroxy ketone. Dehydration would need to remove $$-OH$$ from C-3 and $$-H$$ from either C-2 (which is the carbonyl carbon — not possible as it has no H) or C-4. Eliminating between C-3 and C-4 gives $$CH_3-CO-C(=CH-CH_3)$$, which would place a double bond at C-3=C-4. This is not conjugated with the carbonyl group in the favourable way that the $$\beta$$-hydroxy ketone product is, so the driving force is weaker.

Option C: 1-Pentanol

Structure: $$CH_3CH_2CH_2CH_2CH_2OH$$

This is a simple primary alcohol. Dehydration of primary alcohols is the most difficult among 1°, 2°, and 3° alcohols because the resulting primary carbocation is very unstable. The reaction requires harsh conditions (high temperature, strong acid). No special stabilisation of the product occurs.

Option D: 2-Hydroxycyclopentanone

This is an $$\alpha$$-hydroxy ketone ($$-OH$$ on the carbon adjacent to the carbonyl). Dehydration would lead to a cyclopentenone, but the $$\alpha$$-hydroxy position makes it less facile than $$\beta$$-elimination because the geometry in the five-membered ring adds strain, and $$\alpha$$-elimination is less favourable than $$\beta$$-elimination.

Among all the options, 4-hydroxypentan-2-one (Option A) is a $$\beta$$-hydroxy ketone which undergoes the easiest acid-catalysed dehydration due to the formation of a conjugated $$\alpha,\beta$$-unsaturated ketone product.

The correct answer is Option A.