272 CHAPTER 7 Electron Delocalization and Resonance• More About Molecular Orbital Theoryδδ δδ δδresonance contributorsresonance hybridresonance contributorsresonance hybridCH 3 CH CH
resonance contributorsCH 3 CH
resonance hybridresonance contributorsCH CH 2resonance hybridCH 2δδ δδ δδδδδδδCH 2 CH 2 CH 2 CH 2CH 2 CH 3 CH CH CH 2CH 2Figure 7.5N
Resonance structures for an allylic
radical and for the benzyl radical.electrons on nitrogen can be shared with the adjacent carbon (since the carbon–
carbon bond can be broken):In contrast, all the electrons in the compound on the right are localized. The lone-pair
electrons on nitrogen cannot be shared with the adjacent carbon because carbon can-
not form five bonds. The octet rule requires that second-row elements be surrounded by
no more than eight electrons, so hybridized carbons cannot accept electrons.
Because an hybridized carbon has a bond that can break, has a positive charge, or
has an unpaired electron, it can accept electrons without violating the octet rule.
The carbocation shown on the left in the next example has delocalized electrons
because the electrons can move into the empty porbital of the adjacent carbon
(Section 1.10). We know that this carbon has an empty porbital since it has a positive
charge.The electrons in the carbocation on the right are localized because the electrons
cannot move. The carbon they would move to is hybridized, and hybridized
carbons cannot accept electrons.
The next example shows a ketone with delocalized electrons (left) and a ketone
with only localized electrons (right):CHOCH 3 C CHCH 3 CH 3 C CH CHCH 3+O−
OCH 3 C CH 2 CH CHCH 3
delocalized electrons localized electronsan sp^3 hybridized carbon
cannot accept electronssp^3 sp^3pdelocalized electrons localized electronsCH 2 CH CHCH 3 CH 2 CH CHCH 3+ + +
CH 2 CH CH 2 CHCH 3an sp^3 hybridized carbon
cannot accept electronsp sp^2sp^2 psp^3sp^3localized electronsan sp^3 hybridized carbon
cannot accept electronsdelocalized electronsCH 3 CH CH NHCH 3 CH 3 CH CH NHCH 3− +
CH 3 CH CH CH 2 NH 2psp^2Tutorial:
Localized and delocalized
electronsBRUI07-263_297r4 21-03-2003 11:31 AM Page 272