Isodesmic Reactions

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Isodesmic Reactions
Isodesmic Reactions chemical changes in which
there is a net formal retention of the number of
bonds (groups) but a change in their formal
relation to each other. Consider the following
reaction CH3CH2CH2CH3 CH3CH3 ? 2 CH3CH2CH3
-125.50.7 -83.70.4
-104.70.5 ?Hr(298) 2(-104.7 0.5) -
(-125.50.7) - (83.7 0.4) ?Hr(298) (-0.2
1.3) kJ mol-1
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3CH3CH2CH2CH3 ? 3 CH3CH3 cyclohexane
-125.50.7 -83.70.4 -123.4 0.8
?Hr(298) (-123.4 0.8) 3(-83.7 0.4) -3
(-125.5 0.7 ) ?Hr(298) (2.0 2.5) kJ mol-1
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2CH3CH2CH2CH3 ? 2 CH3CH3 cyclobutane
-125.5 0.7 -83.70.4 28.40.6 ?Hr(298)
(28.40.6) 2(-83.70.4)-2(-125.50.7 ) ?Hr(298)
(1121.7) kJ mol-1 strain energy Please read
Fuchs, R. The Evaluation of Strain and
Stabilization in Molecules Using Isodesmic
Reactions, J. Chem Educ. 1984, 61, 133-136.
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List the following olefins in terms of their
stability 1. Ethylene 2. Propene 3.
Tetramethylethylene 4. Trimethylethylene 5. cis
2-butene 6. trans-butene
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Heats of Hydrogenation of Alkenes / kJ mol-1
H2 CH2CH2 ? CH3CH3 ?Hr(298)
-136.4 H2 CH3CHCH2 ? CH3CH2CH3 ?Hr(298)
-125.1 H2 trans CH3CHCHCH3 ? CH3CH2CH2CH3
?Hr(298) -114.6 H2 cis CH3CHCHCH3 ?
CH3CH2CH2CH3 ?Hr(298) -118.4 H2
CH3CCHCH3 ? CH3CHCH2CH3
CH3
CH3 ?Hr(298) -112.5 H2
CH3CCCH3 ? CH3CHCHCH3
CH3CH3
CH3CH3 ?Hr(298) -111.3
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Isodesmic Reactions / kJ mol-1 CH2CH2 CH3CH3 ?
CH3CH2CH2CH3 52.30.4 -83.70.4
-125.50.7 ?Hr(298) -125.50.7-(-83.70.4
)-(52.30.4) -94.10.9 CH3CHCH2 2CH3CH3 ?
CH3CH2CH3 CH3CHCH3 20.10.8 -83.70.4
-104.60.5 -134.3 0.7
CH3 ?Hr(298) (-134.30.7)-(104.60.5)-2(-83.70
.4)-(20.10.8) ?Hr(298) (-91.61.2)
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Isodesmic Reactions trans CH3CHCHCH3 2CH3CH3 ?
2CH3CHCH3 -11.41.0
-83.70.4
CH3 -134.30.7 ?Hr(298)
2(-134.30.7) - 2(-83.70.4)-(-11.4
1.0) ?Hr(298) (-89.91.3) kJ mol-1 cis
CH3CHCHCH3 2CH3CH3 ? 2CH3CHCH3


CH3 ?Hr(298)
2(-134.30.7) - 2(-83.70.4)- (-7.11.0) ?Hr(298)
(-94.11.3) kJ mol-1
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CH3CCHCH3 2CH3CH3 ? CH3CHCH3 CH3
-83.70.4
CH3
CH3 CH3CCH3 -41.8 1.1
-134.30.7
CH3 -168.20.8 ?Hr(298)
(-168.20.8)-(134.30.7)--2(83.70.4)(-41.81.
1 ?Hr(298) (-93.31.7) kJ mol-1
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2CH3CH3 CH3C CCH3 ? 2 C(CH3 )4

-83.70.4 H3C CH3
-168.10.8 -68.21.1
?Hr(298) 2(-168.10.8) - (-68.21.1)2(-83.70
.4) ?Hr(298) (-100.63.9) kJ mol-1
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Summary
Hydrogenation ?a Isodesmic ?a
ethylene -134.6 -94.10.9
propene -125.1 -9.5 -91.61.2
-2.51.5 cis 2-butene -114.6 -20.0
-94.11.3 0.01.6 trans 2-butene
-118.4 -16.2 -89.91.3 -4.21.6 2-methylbuten
e -112.5 -22.1 -93.31.7
-0.81.9 2,3-dimethylbutene -111.3 -23.3
-100.83.9 6.74.0 arelative to ethylene
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CH2CHCH2CH2CH3 3CH3CH3 ? CH3CH2CH3
CH3CHCH3 -21.31.0 -83.70.4
-104.60.5 CH3
CH3CH2CH2CH3 )
-134.20.7 -125.50.7
?Hr(298)(-134.20.7)(-104.60.6)(-125.50
.7)-3(-83.70.4) (-21.31.0) ?Hr(298)
(- 91.91.9) kJ mol-1 CH3CHCHCH2CH3 3
CH3CH3 ? CH3CH2CH32 CH3CHCH3 cis -27.61.0
CH3 trans -31.91.1 cis ?Hr(2
98)2(-134.20.7)(-104.60.5)-(-27.61.0)3(-83.
70.4) ?Hr(298) (-94.32.2) kJ mol-1
trans ?Hr(298)2(-134.20.7)(-104.60.5)-(-31.
91.1)3(-83.70.4) ?Hr(298) (-90.0 2.2) kJ
mol-1
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Aromaticity
82.60.7 -123.40.8
-5.00.7
?Hr(298) 3 (-5.00.7) - 2(123.40.8)-(82.60.7) ?
Hr(298) (149.23.3) kJ mol-1
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Cyclopropane ?Hf(298) 53.30.6
Whats the strain energy of cyclopropane?
CH3CH3 ? CH3CH2CH2CH2CH3 -83.70.4
-146.90.9
?Hr(298) (-146.90.9) - (-83.70.4)
-(53.30.6) ?Hr(298) -116.51.2 ?Hs(298) 116.5
kJ mol-1
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cyclopropane ?Hs(298) 116.5 1.2
Whats the strain energy of spiropentane?
?Hf(298) 185.20.8 CH3
4CH3CH3 ? 2 CH3CH2CH2CH3 CH3CCH3
CH3 -83.70.4 -125.50.7
-168.10.8 ?Hr(298) (-168.10.8) 2(-125.50.7)
- 4(-83.70.4) -(185.20.8) ?Hr(298)
(-269.52.4) kJ mol-1 ??Hs(298) 269.5 -
2(116.5) 36.5 kJ mol-1