Dehydrobromination is an elimination reaction where a hydrogen halide (HBr) is removed from an alkyl halide using a base (like alcoholic KOH), forming an alkene. The possible alkenes depend on the number of distinct β-hydrogens available for elimination.

Step 1: Identify the structure. The compound is 3-bromo-3-cyclopentylhexane. The parent chain is hexane with a bromine and a cyclopentyl group both attached to carbon 3. So, carbon 3 is tertiary and chiral.

Step 2: Locate β-carbons. β-carbons are adjacent to the carbon bearing the bromine (C3). So, C2 and C4 are β-carbons.

Step 3: Count β-hydrogens. Each β-carbon has hydrogens that can be eliminated.

• C2 (CH₂ group in hexane chain) has 2 identical hydrogens.
• C4 (CH₂ group in hexane chain) has 2 identical hydrogens.
• The cyclopentyl ring attached to C3 has β-carbons too. The carbon adjacent to the point of attachment in cyclopentyl (a CH₂ group) has 2 hydrogens. But note: due to symmetry of cyclopentyl, all β-carbons are equivalent.

Step 4: Determine distinct elimination products.

• Elimination involving H from C2 gives alkene between C2-C3.
• Elimination involving H from C4 gives alkene between C3-C4.
• Elimination involving H from cyclopentyl β-carbon gives alkene between C3 and cyclopentyl carbon. But since the cyclopentyl ring is symmetric, this gives one alkene.

So, total 3 alkenes are possible.

Step 5: Check for stereoisomers. The alkene formed with cyclopentyl might show E/Z isomerism? But in this case, the alkene is trisubstituted and the groups are different, so it can have E and Z isomers. However, the question asks for "number of alkenes", which typically includes stereoisomers if they are distinct.

For alkene between C3 and cyclopentyl carbon: one side has hexyl chain (C4-C5-C6), the other side has cyclopentyl. So, it is unsymmetrical and E/Z isomers are possible. Therefore, this elimination gives 2 alkenes (E and Z).

The other two alkenes (C2-C3 and C3-C4) are disubstituted and do not have stereoisomers.

So, total alkenes = 2 (from C2 and C4) + 2 (E and Z from cyclopentyl) = 4.

Final Answer: 4 alkenes are possible.

Related Topics

Elimination Reactions: These are reactions where a small molecule is removed from a substrate. Dehydrohalogenation is a common elimination reaction where HX is removed from an alkyl halide to form an alkene. The most common mechanism is E2, which is bimolecular and concerted.

Zaitsev's Rule: In elimination reactions, the more substituted alkene is generally the major product. However, the question is about the total number of possible alkenes, not the major one.

Formulae

General reaction for dehydrohalogenation: $R-\mathrm{CH}2-\mathrm{CH}2-X+\mathrm{KOH}(\mathrm{alc})\to R-\mathrm{CH}=\mathrm{CH}2+\mathrm{KX}+H2O$

Number of alkenes depends on the number of unique β-hydrogens and the possibility of stereoisomers in the resulting alkene.