Organic Chemistry

124 CHAPTER 3 Alkenes• Thermodynamics and Kinetics

additionof 1 mole of HBr to 1 mole of the alkene. The reaction, therefore, is called an

addition reaction. Because the first step of the reaction involves the addition of an elec-

trophile to the alkene, the reaction is more precisely called an electrophilic addi-

tion reaction. Electrophilic addition reactions are the characteristic reactions of alkenes.

At this point, you may think that it would be easier just to memorize the fact that

2-bromobutane is the product of the reaction, without trying to understand the mecha-

nism that explains why 2-bromobutane is the product. Keep in mind, however, that the

number of reactions you encounter is going to increase substantially, and it will be im-

possible to memorize them all. If you strive to understand the mechanism of each

reaction, the unifying principles of organic chemistry will become apparent, making

mastery of the material much easier and a lot more fun.

PROBLEM 11

Which of the following are electrophiles, and which are nucleophiles?

+

H− AlCl 3 CH 3 O− CH 3 CCHCH 3 CHCH 3 NH 3

(H+)

It will be helpful to do the exercise on

drawing curved arrows in the Study

Guide/Solution Manual (Special

Topic III).

A FEW WORDS ABOUT CURVED ARROWS

1. Make certain that the arrows are drawn in the direction of the electron flow and never against the flow. This means
   that an arrow will always be drawn away from a negative charge and/or toward a positive charge.


2. Curved arrows are drawn to indicate the movement of electrons. Never use a curved arrow to indicate the movement of an atom.
   For example, you can’t use an arrow as a lasso to remove the proton, as shown here:


3. The arrow starts at the electron source. It does not start at an atom. In the following example, the arrow starts at the electrons of
   the bond, not at a carbon atom:

+

CH 3 CH CHCH 3 + H Br CH 3 CH CHCH 3 + Br−

H

+

CH 3 CH CHCH 3 + H Br CH 3 CH CHCH 3 + Br−

H

incorrect

correct

p

O

CH 3 CCH 3

H

+ H+

+

O

CH 3 CCH 3

O

CH 3 CCH 3 CH 3 CCH 3

H

+ H+

+

O

correct incorrect

CH 3 C

CH 3

+

O−

Br CH 3 C

CH 3

O

Br−

CH 3 +O

H

HHCH 3 O + H+

correct

correct

CH 3 C

CH 3

+

O−

Br CH 3 C

CH 3

O

Br−

CH 3 +O

H

HHCH 3 O + H+

incorrect

incorrect

BRUI03-109_140r4 24-03-2003 11:53 AM Page 124