Pericyclic Level - 1 - Free MCQ Practice Test with solutions, Chemistry

[8 + 2] cycloaddition

Basicity is inversely proportional to resonance of lone pair electrons. Benzylamine is more basic. The electron pairs do not involve in resonance in benzylamine. In other amines, there is delocalization of lone pair of electron on N atom on the ring. In acetanilide, the delocalization of lone pair of electrons on N atom is due to adjacent CO group.
Hence option (B) is the answer.

The Claisen rearrangement is a powerful carbon–carbon bond-forming chemical reaction discovered by Rainer Ludwig Claisen. The heating of an allyl vinyl ether will initiate a [3,3]-sigmatropic rearrangement to give a γ,δ-unsaturated carbonyl.

The Diels-Alder reaction is a chemical reaction between a conjugated diene and a dienophile to form a six-membered ring. A crucial requirement for the diene is that it must be able to adopt an s-cis conformation. This means the two double bonds must be on the same side of the single bond that connects them.

Let's analyze the options:

• a) 2-methyl-1,3-butadiene (isoprene): This is an open-chain diene. The single bond between the two double bonds can freely rotate, allowing it to easily adopt the required s-cis conformation. Thus, it can undergo a Diels-Alder reaction.

• b) 1-methylene-2-cyclohexene: In this molecule, the conjugated diene system is held in a rigid s-trans conformation by the ring structure. The exocyclic double bond (the one outside the ring) cannot rotate into an s-cis position relative to the endocyclic double bond (the one inside the ring). Since it is "locked" in the s-trans form, it cannot undergo a Diels-Alder reaction.

• c) 1,3-cyclohexadiene: The two double bonds are part of the same ring and are permanently locked in the s-cis conformation. This makes it an excellent diene for Diels-Alder reactions.

• d) (2E,4E)-hexa-2,4-diene: Like molecule (a), this is an open-chain diene. It is shown in the s-trans conformation, but it can freely rotate around the central single bond to achieve the reactive s-cis conformation. Therefore, it can undergo a Diels-Alder reaction.

Therefore, the diene that cannot undergo a Diels-Alder reaction is b) because it is locked in an s-trans conformation.