Advices For Studying Organic Chemistry

2. Comparison of the heat of hydrogenation of 1,3-butadiene and 1-butene:
   ∆H° (kJ mol–1)
   2 CH 2 =CHCH 2 CH 3 + 2 H 2 2 CH 3 CH 2 CH 2 CH 3 (2 × –127) = –254
   1-Butene
   CH 2 =CHCH=CH 2 + 2 H 2 2 CH 3 CH 2 CH 2 CH 3 = –239
   1,3-Butadiene Difference 15 kJ mol–1
   
   
   1. Hydrogenation of 1,3-butadiene liberates 15 kJ mol–1 less than expected ⇒
      conjugation imparts some extra stability to the conjugated system.
   
   
   
   

Figure 13.6 Heats of hydrogenation of 2 mol of 1-butene and 1 mol of
1,3-butadiene.

3. Assessment of the conjugation stabilization of trans-1,3-pentadiene:
   CH 2 =CHCH 2 CH 3
   1-Pentene^ ∆H° = –126 kJ mol

–1

CC

H

CH 3 CH 2

CH 3

H

trans-2-Pentene

∆H° = –115 kJ mol

–1

Sum = –241 kJ mol–1

CC

H

CH

CH 3

H 2 C H

trans-1,3-Pentadiene

∆H° = –226 kJ mol

–1

Diffference = –15 kJ mol–1