Advices For Studying Organic Chemistry

4) 1-Butene evolves the greatest amount of heat when hydrogenated, and

trans-2-butene evolves the least.

i) 1-Butene must have the greatest energy (enthalpy) and be the least stable

isomer.

ii) trans-2-Butene must have the lowest energy (enthalpy) and be the most stable

isomer.

3. Trend of stabilities: trans isomer > cis isomer

+ H 2 Pt CH 3 CH 2 CH 2 CH 2 CH 3 ∆Ho^ = −120 kJ mol-1

Pentane

CC

H

H 3 CH 2 C CH 3

H

cis-3-Pentene

CC + H 2 Pt ∆Ho^ = −115 kJ mol-1

CH 3

H 3 CH 2 C H

H

trans-3-Pentene

CH 3 CH 2 CH 2 CH 2 CH 3

Pentane

4. The greater enthalpy of cis isomers can be attributed to strain caused by the
   crowding of two alkyl groups on the same side of the double bond.

Figure 7.2 cis- and trans-Alkene isomers. The less stable cis isomer has greater
strain.

7.3B RELATIVE STABILITIES FROM HEATS OF COMBUSTION

1. When hydrogenation os isomeric alkenes does not yield the same alkane, heats of