Organic Chemistry

The fluorine radical is the most reactive of the halogen radicals, and it reacts violently

with alkanes In contrast, the iodine radical is the least reac-

tive of the halogen radicals. In fact, it is so unreactive that it is

unable to abstract a hydrogen atom from an alkane. Consequently, it reacts with an-

other iodine radical and reforms

PROBLEM-SOLVING STRATEGY

Would chlorination or bromination of methylcyclohexane produce a greater yield of 1-halo-

1-methylcyclohexane?

To solve this kind of problem, first draw the structures of the compounds being discussed.

1-Halo-1-methylcyclohexane is a tertiary alkyl halide, so the question becomes,“Will

bromination or chlorination produce a greater yield of a tertiary alkyl halide?”Because

bromination is more selective, it will produce a greater yield of the desired compound.

Chlorination will form some of the tertiary alkyl halide, but it will also form significant

amounts of primary and secondary alkyl halides.

Now continue on to Problem 10.

PROBLEM 10

a. Would chlorination or bromination produce a greater yield of 1-halo-2,3-dimethylbutane?

b. Would chlorination or bromination produce a greater yield of 2-halo-2,3-dimethylbutane?

c. Would chlorination or bromination be a better way to make 1-halo-2,2-dimethylpropane?

9.5 Radical Substitution of Benzylic

and Allylic Hydrogens

Benzyl and allyl radicals are more stable than alkyl radicals because their unpaired

electrons are delocalized. We have seen that electron delocalization increases the sta-

bility of a molecule (Sections 7.6 and 7.8).

We know that the more stable the radical, the faster it can be formed. This means that

a hydrogen bonded to either a benzylic carbon or an allylic carbon will be preferential-

ly substituted in a halogenation reaction. As we saw in Section 9.4, bromination is more

highly regioselective than chlorination, so the percent of substitution at the benzylic or

allylic carbon is greater for bromination.

+ X 2 ∆ + HX (X = Cl or Br)

CH 2 CH 3 CHCH 3

benzylic substituted

product

X

CH 3 X CH 3

X 2

h

I 2.

(¢H°= 34 kcal>mol)

(¢H°=- 31 kcal>mol).

346 CHAPTER 9 Reactions of Alkanes • Radicals

Electron delocaliztion increases
the stability of a molecule.

3-D Molecule:

Benzyl radical

≈>>≈> >

tertiary

radical

allyl

radical

benzyl

radical

secondary

radical

primary

radical

methyl

radical

vinyl

radical

R

RC

R

CH 2 CH 2 CHCH 2 CH 2 CH

R

RC

H

H

RC

H

H

HC

H

relative stabilities of radicals

most

stable

least

stable