Organic Chemistry

122 CHAPTER 3 Alkenes• Thermodynamics and Kinetics

To further reduce the need to memorize, one needs to understand whya functional

group reacts the way it does. It is not sufficient to know that a compound with a

carbon–carbon double bond reacts with HBr to form a product in which the H and Br

atoms have taken the place of the bond; we need to understand whythe compound

reacts with HBr. In each chapter that discusses the reactivity of a particular functional

group, we will see how the nature of the functional group allows us to predict the kind

of reactions it will undergo. Then, when you are confronted with a reaction you have

never seen before, knowledge of how the structure of the molecule affects its reactivi-

ty will help you predict the products of the reaction.

In essence, organic chemistry is about the interaction between electron-rich atoms or

molecules and electron-deficient atoms or molecules. It is these forces of attraction that

make chemical reactions happen. From this follows a very important rule that determines

the reactivity of organic compounds:Electron-rich atoms or molecules are attracted to

electron-deficient atoms or molecules.Each time you study a new functional group, re-

member that the reactions it undergoes can be explained by this very simple rule.

Therefore, to understand how a functional group reacts, you must first learn to rec-

ognize electron-deficient and electron-rich atoms and molecules. An electron-deficient

atom or molecule is called an electrophile. An electrophile can have an atom that can

accept a pair of electrons, or it can have an atom with an unpaired electron and, there-

fore, is in need of an electron to complete its octet. Thus, an electrophile looks for

electrons. Literally,“electrophile”means “electron loving”(phileis the Greek suffix

for “loving”).

An electron-rich atom or molecule is called a nucleophile. A nucleophile has a pair

of electrons it can share. Some nucleophiles are neutral and some are negatively

charged. Because a nucleophile has electrons to share and an electrophile is seeking

electrons, it should not be surprising that they attract each other. Thus, the preceding

rule can be restated as a nucleophile reacts with an electrophile.

Because an electrophile accepts a pair of electrons, it is sometimes called a Lewis

acid. Because a nucleophile has a pair of electrons to share, it is sometimes called a

Lewis base(Section 1.21).

these are nucleophiles because they

have a pair of electrons to share

HO − Cl − CH 3 NH 2 H 2 O

these are electrophiles because they

can accept a pair of electrons

H+ CH 3 CH 2 BH 3

this is an electrophile because

it is seeking an electron

Br

+

p

H

H H

H

ethene

cholesterol

C C

HO HO

H

Br

HCCH

Br H

H H

HBr

HBr

+

+

Electron-rich atoms or molecules are

attracted to electron-deficient atoms or

molecules.

A nucleophile reacts with an

electrophile.

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