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Isodesmic Reactions

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: CH 3 CH 2 CH 2 CH 3 + CH 3 CH 3 ï‚® 2 CH 3 CH 2 CH 3

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Isodesmic Reactions

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  1. 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 -125.5±0.7 -83.7±0.4 -104.7±0.5 Hr(298)= 2(-104.7 ±0.5) - (-125.5±0.7) - (83.7 ±0.4) Hr(298)= (-0.2 ±1.3) kJ mol-1

  2. 3CH3CH2CH2CH3 3 CH3CH3 + cyclohexane -125.5±0.7-83.7±0.4-123.4 ±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

  3. 2CH3CH2CH2CH3 2 CH3CH3 + cyclobutane -125.5 ±0.7-83.7±0.428.4±0.6 Hr(298) = (28.4±0.6) +2(-83.7±0.4)-2(-125.5±0.7) Hr(298) = (112±1.7) 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.

  4. List the following olefins in terms of their stability 1. Ethylene 2. Propene 3. Tetramethylethylene 4. Trimethylethylene 5. cis 2-butene 6. trans-butene

  5. Heats of Hydrogenation of Alkenes / kJ mol-1 H2 + CH2=CH2  CH3CH3; Hr(298) = -136.4 H2 + CH3CH=CH2  CH3CH2CH3; Hr(298) = -125.1 H2 + trans CH3CH=CHCH3 CH3CH2CH2CH3; Hr(298) = -114.6 H2 + cis CH3CH=CHCH3 CH3CH2CH2CH3; Hr(298) = -118.4 H2 + CH3C=CHCH3 CH3CHCH2CH3; | | CH3 CH3 Hr(298) = -112.5 H2 + CH3C=CCH3 CH3CHCHCH3; | | | | CH3CH3 CH3CH3 Hr(298) = -111.3

  6. Isodesmic Reactions / kJ mol-1 CH2=CH2 + CH3CH3 CH3CH2CH2CH3; 52.3±0.4 -83.7±0.4 -125.5±0.7 Hr(298) = -125.5±0.7-(-83.7±0.4 )-(52.3±0.4) = -94.1±0.9 CH3CH=CH2 + 2CH3CH3 CH3CH2CH3 + CH3CHCH3 20.1±0.8 -83.7±0.4 -104.6±0.5 | -134.3 ±0.7 CH3 Hr(298) = (-134.3±0.7)-(104.6±0.5)-2(-83.7±0.4)-(20.1±0.8) Hr(298) = (-91.6±1.2)

  7. Isodesmic Reactions trans CH3CH=CHCH3+2CH3CH3 2CH3CHCH3; -11.4±1.0 -83.7±0.4 | CH3 -134.3±0.7 Hr(298) = 2(-134.3±0.7) - 2(-83.7±0.4)-(-11.4 ±1.0) Hr(298) = (-89.9±1.3)kJ mol-1 cis CH3CH=CHCH3+2CH3CH3 2CH3CHCH3; | CH3 Hr(298) = 2(-134.3±0.7) - 2(-83.7±0.4)- (-7.1±1.0) Hr(298) = (-94.1±1.3)kJ mol-1

  8. CH3C=CHCH3+2CH3CH3 CH3CHCH3 + CH3 | -83.7±0.4 | | CH3 CH3 CH3CCH3 -41.8 ±1.1 -134.3±0.7 | CH3 -168.2±0.8 Hr(298) = (-168.2±0.8)-(134.3±0.7)-[-2(83.7±0.4)+(-41.8±1.1] Hr(298) = (-93.3±1.7) kJ mol-1

  9. 2CH3CH3 + CH3C = CCH3 2 C(CH3 )4 | | -83.7±0.4H3CCH3 -168.1±0.8 -68.2±1.1 Hr(298) = 2(-168.1±0.8) - [(-68.2±1.1)+2(-83.7±0.4)] Hr(298) = (-100.6±3.9) kJ mol-1

  10. Summary Hydrogenation a Isodesmic a ethylene -134.6 -94.1±0.9 propene -125.1 -9.5 -91.6±1.2 -2.5±1.5 cis 2-butene -114.6 -20.0 -94.1±1.3 0.0±1.6 trans 2-butene -118.4 -16.2 -89.9±1.3 -4.2±1.6 2-methylbutene -112.5 -22.1 -93.3±1.7 -0.8±1.9 2,3-dimethylbutene -111.3 -23.3 -100.8±3.9 +6.7±4.0 arelativeto ethylene

  11. CH2=CHCH2CH2CH3 + 3CH3CH3 CH3CH2CH3 +CH3CHCH3 -21.3±1.0 -83.7±0.4 -104.6±0.5 CH3 + CH3CH2CH2CH3) -134.2±0.7 -125.5±0.7 Hr(298)=(-134.2±0.7)+(-104.6±0.6)+(-125.5±0.7)-[3(-83.7±0.4)] +(-21.3±1.0)] Hr(298)= (- 91.9±1.9) kJ mol-1 CH3CH=CHCH2CH3 + 3 CH3CH3 CH3CH2CH3+2 CH3CHCH3 cis -27.6±1.0 CH3 trans -31.9±1.1 cis Hr(298)=2(-134.2±0.7)+(-104.6±0.5)-[(-27.6±1.0)+3(-83.7±0.4)] Hr(298)= (-94.3±2.2) kJ mol-1 trans Hr(298)=2(-134.2±0.7)+(-104.6±0.5)-[(-31.9±1.1)+3(-83.7±0.4)] Hr(298)= (-90.0± 2.2) kJ mol-1

  12. Aromaticity 82.6±0.7 -123.4±0.8 -5.0±0.7 Hr(298)= 3 (-5.0±0.7) - 2(123.4±0.8)-(82.6±0.7) Hr(298)= (149.2±3.3) kJ mol-1

  13. Cyclopropane Hf(298)= 53.3±0.6 What’s the strain energy of cyclopropane? + CH3CH3  CH3CH2CH2CH2CH3 -83.7±0.4 -146.9±0.9 Hr(298)= (-146.9±0.9) - (-83.7±0.4) -(53.3±0.6) Hr(298)= -116.5±1.2 Hs(298)= 116.5 kJ mol-1

  14. cyclopropane Hs(298)= 116.5 ±1.2 What’s the strain energy of spiropentane? Hf(298)= 185.2±0.8 CH3 + 4CH3CH3 2 CH3CH2CH2CH3 +CH3CCH3 CH3 -83.7±0.4 -125.5±0.7-168.1±0.8 Hr(298)= (-168.1±0.8) + 2(-125.5±0.7) - 4(-83.7±0.4) -(185.2±0.8) Hr(298)= (-269.5±2.4) kJ mol-1 Hs(298)= 269.5 - 2(116.5) = 36.5 kJ mol-1

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