Bruice - Chimica Organica

310 Special Topic II

In the second step of the reaction, Br

is the nucleophile; its HOMO is a filled nonbonding

orbital. The

carbocation is the electrophile; the LUMO is carbon’s empty

orbital (Figure 9).

In Section 9.1 of the text, we use MO theory to explain why the alkyl halide in an S

2 reaction undergoes

back-side attack, resulting in inversion of configuration. The filled nonbonding orbital of the nucleophile is the

HOMO. The LUMO of the electrophilic alkyl halide is the

antibonding MO, which has its largest lobe at the

back of the carbon in the C—Br bond, so this is where overlap is best. (See Figure 9.1 on page 395 of the text.)

III. Molecular Orbital Theory and Delocalized Electrons

Molecular orbital theory is useful to describe compounds with delocalized electrons. For example, in Section 6.2,

we saw that tertiary carbocations are more stable than secondary carbocations, which are more stable than primary

carbocations, because the electrons in a filled

bond are delocalized by overlapping the empty

orbital of the

positively charged carbon. (See Figure 6.1 on page 238 of the text.) This kind of electron delocalization is known

as hyperconjugation; it is one more example of the stabilization that results when a filled orbital overlaps an

empty orbital.

Molecular orbital theory avoids having to use contributing resonance structures, because the electrons in the

most stable MO are delocalized over the entire molecule. For example, in Section 8.8, we saw that 1,3-butadiene

is more stable than 1,4-pentadiene, because the

electrons in 1,3-butadiene are delocalized over four

carbons,

whereas the intervening methylene group in 1,4-pentadiene prevents the

orbitals of C-2 and C-4 from overlapping.

Thus, the

electrons of 1,4-pentadiene are localized, causing its molecular orbitals to have the same energy as

those of ethene—another compound with localized electrons. (See page 337 of the text.)

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