Page 282 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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228 Physical Organic Chemistry

1
TABLE V Energy Difference A = −RT ln([55]/ to 86 (V bend = k CCC (60 − 109.5) = 1225k CCC , which is
2
[54]) between Axial and Equatorial R 2
smaller regardless of the magnitude of k CCC ).
R A (kcal/mole)
F 0.15
Cl 0.43
Br 0.38
Torsional strain is responsible for the greater reactiv-
CN 0.17
ity of cyclopentanone (87, n = 5) toward borohydride, as
CH 3 1.74
compared to that of cyclohexanone (87, n = 6), to form the
CH 2 CH 3 1.79
alcohol (88, after protonation of the oxygen). The transi-
CH(CH 3 ) 2 2.21
tion state (89, n = 5) leading to the five-membered ring
C(CH 3 ) 3 >5.4
is destabilized by increased torsional strain along all five
COOH 1.35
C C bonds relative to the reactant, where bonds to the car-
CO − 1.92
2 bonyl carbon are not eclipsed. Transition state 89 (n = 6) is
OCH 3 0.60
stabilized by permitting all six C C bonds to be staggered,
eliminating any torsional strain. Therefore the transition
state in the six-membered ring is more stable, accounting
for the faster reaction.
a disubstituted cyclohexane like 56. Still another exam-
ple is given by the reactivity of alkyl bromides toward
substitution by hydroxide, which decreases in the order
CH 3 Br > CH 3 CH 2 Br (CH 3 ) 2 CHBr. This is due to in-

creasing destabilization of the transition state (83, 83 ,
83 ) by repulsion between OH or Br and zero, one, or two

CH 3 groups on the central carbon. J. Stereochemistry and Reactivity
How do stereoisomers differ in reactivity? The answer
lies in the energetics of stereoisomeric transition states.
Certainly enantiomeric transition states must have identi-
cal energies, since they are merely mirror images of each
other. Therefore if two enantiomers react with an achi-
ral reagent, they must react at identical rates. Moreover,
if two enantiomeric products are formed from the same
achiral reactant, they must be formed at equal rates and
therefore in equal proportion. Only a racemic product can
I. Strain Effects be formed. For example, the possible transition states for
addition of borohydride to 2-butanone are enantiomers 90
Strain effects arise from the distortion of bond or torsional

and 90 . The hydride is transferred to either of the two
angles. The strain energy V can be expressed as in Eq. (11) faces of the carbonyl group, but the two faces must be
0
or Eq. (12), where θ is the optimum angle, of minimum
equally reactive.
energy.
Like steric effects, strain effects are always destabiliz-
ing, although they can affect reactant, product, or transi-
tion state. For example, cycloheptyne (84) is less stable
than ordinary alkynes because the seven-membered ring
prohibits the 180 angles preferred by the sp-hybridized
◦
carbons of the triple bond. Another example is the un- In contrast, diastereomeric transition states have dif-
usual behavior (compared to ordinary ketones) of cyclo- ferent energies. Therefore diastereomers show unequal
propanone (85), which reacts with water to form cyclo- reactivity and they can be formed at different rates.
propanediol (86). Both of these have strain energy because For example, addition of borohydride to (R)-3-methyl-
of the 60 bond angle in a three-membered ring. How- 2-pentanone, CH 3 C( O)CH(CH 3 )C 2 H 5 , produces both
◦
ever, according to Eq. (11), the strain energy is greater for (2R,3R)- and (2S,3R)-3-methyl-2-pentanol, but in un-
2
85, where the preferred bond angle of the sp carbon is equal amounts. The reactant is chiral and there is an en-
1
2
◦
120 (V bend = k CCC (60 − 120) = 1800k CCC ), compared ergetic preference for borohydride to be transferred to
2