Topic 1.2. Heteroatom Hyperconjugation Anomeric Effect in Acyclic Molecules

Topic 1.2. Heteroatom Hyperconjugation (Anomeric Effect) in Acyclic Molecules 

It is expected that hyperconjugation would be enhanced in certain systems containing heteroatoms. If one atom with an unshared electron pair is a particularly good electron donor and another a good cr acceptor, the u contribution should be enhanced. This is represented by a charged, no-bond resonance structure. 

Heteroatom hyperconjugation can also be expressed in MO terms. The n, cr, and a orbitals are involved, as depicted below. If the A atom is the donor and C the acceptor, the MO perturbation indicates a stabilization of the A orbital by partial population of the B-C CT orbital. 

Dimethoxymethane prefers a conformation that allows alignment of an unshared pair on each oxygen (donor) with a C-0 a orbital on the other. This condition is met in the conformation labeled gauche, gauche. In contrast, the extended hydrocarbon-like anti, anti conformation does not permit this alignment. 

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