Advices For Studying Organic Chemistry

2 H• + 2 Cl• 2 H–Cl

10.2B HOMOLYTIC BOND DISSOCIATION ENERGIES AND THE RELATIVE

STABILITIES OF RADICALS:

1. Bond dissociation energies can be used to eatimate the relative stabilities of
   radicals.

1. 
   

   ∆H° for 1° and 2° C–H bonds of propane:
   CH 3 CH 2 CH 2 –H (CH 3 ) 2 CH–H
   (∆H° = 410 kJ mol–1) (∆H° = 395 kJ mol–1)

   
   


2. 
   

   ∆H° for the reactions:
   CH 3 CH 2 CH 2 –H CH 3 CH 2 CH 2 • + H• ∆H° = + 410 kJ mol–1
   Propyl radical
   (a 1° radical)
   (CH 3 ) 2 CH–H (CH 3 ) 2 CH• + H• ∆H° = + 395 kJ mol–1
   Isopropyl radical
   (a 2° radical)

   
   

3) These two reactions both begin with the same alkane (propane), and they both

produce an alkyl radical and a hydrogen atom.

4) They differ in the amount of energy required and in the type of carbon radical

produced.

2. Alkyl radicals are classified as being 1°, 2°, or 3° on the basis of the carbon atom
   that has the unpaired electron.

1. More energy must be supplied to produce a 1° alkyl radical (the propyl radical)
   from propane than is required to produce a 2° carbon radical (the isopropyl
   radical) from the same compound ⇒ 1° radical has greater potential energy
   ⇒ 2° radical is the more stable radical.

3. Comparison of the tert-butyl radical (a 3° radical) and the isobutyl radical (a 1°
   radical) relative to isobutene:

1. 3° radical is more than the 1° radical by 29 kJ mol–1.