Test: Reaction Mechanism Level - 5 - Chemistry MCQ

The given reaction involves cyclisation using sodium hydride (NaH) as a base. The mechanism can be understood as follows:

1. Deprotonation by NaH: Sodium hydride (NaH) is a strong base and will deprotonate the hydroxyl (OH) group, forming an alkoxide ion (RO-).

2. Nucleophilic Substitution: The alkoxide ion will act as a nucleophile and will attack the electrophilic carbon that is attached to the bromine (Br). This will lead to the displacement of the bromine atom and formation of a cyclic ether.

Analyzing the structure, the most probable cyclisation would be a 5-membered ring formation due to the stability and strain factors.

Let's consider the provided options:

• Option A: This forms a linear structure with no ring formation, which is not consistent with the given reagents and conditions.
• Option B: This forms a 3-membered ring, which is highly strained and less likely to form under these conditions.
• Option C: This forms a 4-membered ring where the oxygen is part of the ring and the hydroxyl group is on an adjacent carbon, which fits the expected cyclisation.
• Option D: This also forms a 4-membered ring but with the hydroxyl group on a different position than Option B, which is not consistent with the given starting material.

Based on the analysis, Option C is the most probable product formed in this cyclisation reaction.

The stability of the given carbocations can be compared by the hyperconjugation effect. Since, hyperconjugation refers to the delocalization of electrons with the participation of bonds of primarily σ−character. For transfer of σ−electrons, we have to break either C−H,C−D or C−T bond. since, atomic mass increases from H to D and to T. Hence, bond strength also increases as C−H<C−D<C−T. So, C−H bond breaks easily. So, (i) has a higher hyperconjugation tendency and it is more stable than the other two. Hence, the order of stability is (i)>(ii)>(iii).

Explanation:
DIBAL-H (Di-isobutyl aluminium hydride) is a reducing agent with the preparation of aldehydes. Using DIBAL - H, Lactones are reduced directly to aldehydes.

Correct answer:

c) 

Elimination reactions by the E2 mechanism are facilitated when the nucleofuge and H on adjacent C atoms can achieve an antiperiplanar relationship. This is only possible in chair trans-1,2-dibromocyclohexane when both Br atoms are axial. The Br at C1 can then be eliminated with a trans axial H on C6 from this (unfavourable) conformation, and the Br at C2 can be eliminated with a trans axial H on C3. Either way, the product is 3-bromocyclohexene which can then undergo a further E2 elimination to give cyclohexa-1,3-diene.