Page 281 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Physical Organic Chemistry 227
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TABLE IV Some C H Bond-Dissociation and in (CH 3 ) 3 C there are nine, paralleling the increased
Energies (BDE)
stabilization.
Similarly, the solvolysis of tert-butyl chloride is faster
BDE
Entry Bond (kcal/mole) than that of isopropyl chloride. The transition state for the
former is given in Eq. (29). That for the latter is given as
1 CH 3 H 104 follows:
2 CH 3 CH 2 H 98
δ− ‡
3 (CH 3 ) 2 CH H 95 (CH 3 ) 2 CH Cl → [(CH 3 ) 2 CH δ+ ··· Cl ]
4 (CH 3 ) 3 C H 92
−
→ (CH 3 ) 2 CH + Cl → etc. (31)
+
5 CH 2 CHCH 2 H 89
However, those structures are incomplete. The transition
state from isopropyl chloride has six additional hypercon-
hydrogen. For entry 5 that radical (75) is stabilized by an jugative resonance forms, corresponding to delocalization
additional resonance form (76) which is not available for of electrons in six adjacent C H bonds. That from tert-
CH 3 ·. According to the rules of resonance, 76 is not just butyl chloride has nine such forms, which confer greater
75 flipped left-for-right, but it expresses the delocalization stability. In terms of Fig. 12, the change from isopropyl to
of the pi electrons in the double bond. tert-butyl represents a stabilization of the transition state
and therefore a faster reaction.
Another example is cyclopentadiene (pK a 16), which
is remarkably acidic for a hydrocarbon. This can be at-
tributed to electron delocalization in the conjugate base
For the ethyl radical of entry 2 (77), resonance is less ob- (79, C’s, H’s, and lone pairs omitted), where there are

vious, but three additional resonance forms are 78, 78 , and five equivalent resonance forms, since the formal nega-

78 . According to the rules of resonance, these do not im- tive charge can be at any carbon. However, cyclohepta-
ply that hydrogens are removed. They symbolize only the triene does not show any unusual acidity compared to
delocalization of electrons from the adjacent C H bonds other polyenes, even though its conjugate base (80) has
to relieve the nonoctet character of the radical center at the seven equivalent resonance forms. This contrast is the
expense of a hydrogen which loses its octet. In contrast consequence of the aromaticity of 79, owing to the six
to the delocalization of pi electrons in 75–76, electrons pi electrons delocalized around the five-membered ring,
in 77–78 are delocalized from C H sigma bonds. This whereas 80 is antiaromatic, with eight pi electrons.
phenomenon is often called hyperconjugation. Similarly,
for entries 3 and 4 there are hyperconjugative resonance
forms that stabilize the radical that results from hydrogen
removal. For the 2-propyl radical, (CH 3 ) 2 CH·, of entry 3,
there are a total of six additional forms, since electrons in
each of the six adjacent C H bonds can be delocalized.
H. Steric Effects
For entry 4 there are a total of nine additional forms. Thus
the number of resonance forms for the radical increases A steric effect arises from the sheer bulk of substituents.
and consequently the bond-dissociation energy decreases Just as two helium atoms repel, any two substituents re-
from entry 1 to 2 to 3 to 4. pel whenever their orbitals overlap. In contrast to res-
onance, which is always stabilizing, steric repulsion is
always destabilizing, although it can destabilize reactant,
product, or transition state.
A simple example is the destabilization of o-xylene (81,
R = CH 3 ) or o-di-tert-butylbenzene [81, R = C(CH 3 ) 3 ]
The heterolytic bond-dissociation energies of R H to by 0.3 and 22.3 kcal/mole, respectively, relative to their
−
R + H (or of R Cl to R + Cl ) also decrease along meta isomers (82). Another is the destabilization of ax-
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+
−
the series R = CH 3 > CH 3 CH 2 > (CH 3 ) 2 CH > (CH 3 ) 3 C. ial conformer 55. A quantitative measure of the effec-
Again this is due to hyperconjugation, which delocal- tive bulk of a substituent R is the free-energy differ-
izes electrons in the adjacent C H bonds and relieves the ence, −RT ln([55]/[54]), often called the A value of
nonoctet character of the carbocation at the expense of a the substituent. Some representative values are listed in
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hydrogen which loses its octet. In CH 3 CH there are three Table V. These values can be used to estimate the posi-
2
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additional resonance forms, in (CH 3 ) 2 CH there are six, tion of the equilibrium between the two conformers of