Organic Chemistry

PROBLEM-SOLVING STRATEGY

A carbeneis an unusual carbon-containing species. It has a carbon with a lone pair of

electrons and an empty orbital. The empty orbital makes the carbene highly reactive. The

simplest carbene, methylene is generated by heating diazomethane. Propose a

mechanism for the following reaction:

The information provided is all you need to write a mechanism. First, because you know

the structure of methylene, you can see that it can be generated by breaking the C N

bond of diazomethane. Second, because methylene has an empty orbital, it is an elec-

trophile and, therefore, will react with ethene (a nucleophile). Now, the question is, What

nucleophile reacts with the other carbon of the alkene? Because you know that cyclo-

propane is the product of the reaction, you also know that the nucleophile must be the lone-

pair electrons of methylene.

(Note:Diazomethane is a gas that must be handled with great care because it is both ex-

plosive and toxic.)

Now continue on to Problem 25.

PROBLEM 25

Propose a mechanism for the following reaction:

4.10 Addition of Radicals • The Relative Stabilities

of Radicals

The addition of HBr to 1-butene forms 2-bromobutane. But what if you wanted to
synthesize 1-bromobutane? The formation of 1-bromobutane requires the anti-
Markovnikov addition of HBr. If an alkyl peroxide (ROOR) is added to the reaction
mixture, the product of the addition reaction will be the desired 1-bromobutane. Thus,
the presence of a peroxide causes the anti-Markovnikovaddition of HBr.

A peroxide reverses the order of addition because it changes the mechanism of the
reaction in a way that causes to be the electrophile. Markovnikov’s rule is not fol-
lowed because it applies only when the electrophile is a hydrogen. The general rule—
that the electrophile adds to the sp^2 carbon bonded to the greater number of

Br–

Br

CH 3 CH 2 CH

1-butene 2-bromobutane

CH 2 + HBr CH 3 CH 2 CHCH 3

CH 3 CH 2 CH

1-butene 1-bromobutane

CH 2 HBr CH 3 CH 2 CH 2 CH 2 Br

peroxide

+

CCH 2 CHCH 2 OH

CH 3

H 3 C

H 3 C O

CH 2

CH 3 CH 3

H 2 SO 4

CH 2 N 2 +

−

NN

+

CH 2

H 2 CCH 2

sp^2

¬

CH 2

−

NN+ 2

diazomethane

N

+

H 2 CCH 2

∆

(≠CH 2 ),

Section 4.10 Addition of Radicals • The Relative Stabilities of Radicals 167