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  2 CH 3 CH 2 CH ± ± ±0.5  H r (298)= 2( ±0.5) - (-125.5±0.7) - (83.7 ±0.4)  H r (298)= (-0.2 ±1.3) kJ mol -1

3CH 3 CH 2 CH 2 CH 3  3 CH 3 CH 3 + cyclohexane ± ± ±0.8  H r (298)= ( ±0.8) + 3(-83.7 ±0.4) -3 ( ±0.7 )  H r (298)= (2.0 ± 2.5) kJ mol -1

2CH 3 CH 2 CH 2 CH 3  2 CH 3 CH 3 + cyclobutane ± ± ±0.6  H r (298) = (28.4±0.6) +2(-83.7±0.4)-2(-125.5±0.7 )  H r (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,

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

Heats of Hydrogenation of Alkenes / kJ mol -1 H 2 + CH 2 =CH 2  CH 3 CH 3 ;  H r (298) = H 2 + CH 3 CH=CH 2  CH 3 CH 2 CH 3 ;  H r (298) = H 2 + trans CH 3 CH=CHCH 3  CH 3 CH 2 CH 2 CH 3 ;  H r (298) = H 2 + cis CH 3 CH=CHCH 3  CH 3 CH 2 CH 2 CH 3 ;  H r (298) = H 2 + CH 3 C=CHCH 3  CH 3 CHCH 2 CH 3 ; | | CH 3 CH 3  H r (298) = H 2 + CH 3 C=CCH 3  CH 3 CHCHCH 3 ; | | | | CH 3 CH 3 CH 3 CH 3  H r (298) =

Isodesmic Reactions / kJ mol -1 CH 2 =CH 2 + CH 3 CH 3  CH 3 CH 2 CH 2 CH 3 ; 52.3± ± ±0.7  H r (298) = ±0.7-(-83.7±0.4 )-(52.3±0.4) = -94.1±0.9 CH 3 CH=CH 2 + 2CH 3 CH 3  CH 3 CH 2 CH 3 + CH 3 CHCH ± ± ±0.5 | ±0.7 CH 3  H r (298) = (-134.3±0.7)-(104.6±0.5)-2(-83.7±0.4)-(20.1±0.8)  H r (298) = (-91.6±1.2)

Isodesmic Reactions trans CH 3 CH=CHCH 3 +2CH 3 CH 3  2CH 3 CHCH 3 ; -11.4± ±0.4 | CH ±0.7  H r (298) = 2(-134.3±0.7) - 2(-83.7±0.4)-(-11.4 ±1.0)  H r (298) = (-89.9±1.3) kJ mol -1 cis CH 3 CH=CHCH 3 +2CH 3 CH 3  2CH 3 CHCH 3 ; | CH 3  H r (298) = 2(-134.3±0.7) - 2(-83.7±0.4)- (-7.1±1.0)  H r (298) = (-94.1±1.3) kJ mol -1

CH 3 C=CHCH 3 +2CH 3 CH 3  CH 3 CHCH 3 + CH 3 | -83.7±0.4 | | CH 3 CH 3 CH 3 CCH ± ±0.7 | CH ±0.8  H r (298) = (-168.2±0.8)-(134.3±0.7)-[-2(83.7±0.4)+(-41.8±1.1]  H r (298) = (-93.3±1.7) kJ mol -1

2CH 3 CH 3 + CH 3 C = CCH 3  2 C(CH 3 ) 4 | | -83.7±0.4 H 3 C CH ± ±1.1  H r (298) = 2(-168.1±0.8) - [(-68.2±1.1)+2(-83.7±0.4)]  H r (298) = (-100.6±3.9) kJ mol -1

Summary Hydrogenation  a Isodesmic  a ethylene ±0.9 propene ± ±1.5 cis 2-butene ± ±1.6 trans 2-butene ± ±1.6 2-methylbutene ± ±1.9 2,3-dimethylbutene ± ±4.0 a relative to ethylene

CH 2 =CHCH 2 CH 2 CH 3 + 3CH 3 CH 3  CH 3 CH 2 CH 3 + CH 3 CHCH ± ± ±0.5 CH 3 + CH 3 CH 2 CH 2 CH 3 ) ± ±0.7  H r (298)=(-134.2±0.7)+(-104.6±0.6)+(-125.5±0.7)-[3(-83.7±0.4)] +(-21.3±1.0)]  H r (298)= (- 91.9±1.9) kJ mol -1 CH 3 CH=CHCH 2 CH CH 3 CH 3  CH 3 CH 2 CH 3 +2 CH 3 CHCH 3 cis -27.6±1.0 CH 3 trans -31.9±1.1 cis  H r (298)=2(-134.2±0.7)+(-104.6±0.5)-[(-27.6±1.0)+3(-83.7±0.4)]  H r (298)= (-94.3±2.2) kJ mol -1 trans  H r (298)=2(-134.2±0.7)+(-104.6±0.5)-[(-31.9±1.1)+3(-83.7±0.4)]  H r (298)= (-90.0± 2.2) kJ mol -1

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

What’s the strain energy of cyclopropane? + CH 3 CH 3  CH 3 CH 2 CH 2 CH 2 CH ± ±0.9 Cyclopropane  H f (298)= 53.3±0.6  H r (298)= (-146.9±0.9) - (-83.7±0.4) -(53.3±0.6)  H r (298)= ±1.2  H s (298)= kJ mol -1

What’s the strain energy of spiropentane?  H f (298)= 185.2±0.8 CH3 + 4CH 3 CH 3  2 CH 3 CH 2 CH 2 CH 3 + CH 3 CCH 3 CH ± ± ±0.8  H r (298)= (-168.1±0.8) + 2(-125.5±0.7) - 4(-83.7±0.4) -(185.2±0.8)  H r (298)= (-269.5±2.4) kJ mol -1  H s (298)= (116.5) = 36.5 kJ mol -1 cyclopropane  H s (298)= ± 1.2