Chemistry - A Molecular Science

Chapter 13 Organic Chemistry

carbon that is next to the carboxyl carbon. In

almost all biologically relevant amino acids,

the carbon to which the amine is attached is a stereocenter, although only one of the two enantiomers is biologically active.

13.5

INTRODUCTION TO

ORGANIC REACTIONS

Organic chemists design molecules with sp

ecific sizes, shapes, and functionalities and

then develop procedures to make them.

Procedures for making almost any organic

molecule can be developed because so many organic reactions proceed by a few well understood

mechanisms.

The mechanism of a reaction is the series of elementary steps

through which the reaction proceeds from reactants to products.

Organic reaction

mechanisms are based on the ideas used to draw resonance forms and are guided by the principles of the octet rule and Lewis acid-base theory. In this section, we introduce a few fundamental organic reactions and their mech

anisms. These are not new concepts; rather,

they are the application of the fundamentals

presented in Chapter 12 to a new field.

ADDITION REACTIONS OF ALKENES Hydrogen halides

add across

the double bond of alkenes. This is called an

addition

reaction

because two reactants combine to produce one

product. The reaction of an alkene

with a hydrogen halide is shown in Figure 13.24. The

π electrons in a double bond are

Lewis basic, so they attack the proton of the strongly acidic H-X molecule to form a C-H bond (Figure 13.24a). The H-X bond breaks as the C-H bond forms, and the loss of the

π^

electrons produces a carbocation at the other carbon in the double bond. The carbocation is strongly acidic, so it is attacked by X

1- (Figure 13.24b) to produce the product shown in

Figure 13.24c. The positive charge on a carbocation is stabilized by electron donating alkyl groups, so the carbocation usually forms on the carbon atom with the most alkyl groups (fewest hydrogen atoms) bound to it. FORMATION OF ESTERS FROM ALCOHOLS AND CARBOXYLIC ACIDS An ester is the product of the reaction between a carboxylic acid and an alcohol in a process called

esterification

. Water is the other product of the reaction. Reactions like

this, in which two reactants combin

e to produce two products, are called

condensation

reactions

. The net reaction is shown in Figure 13.25.

C C

OOHHC 3

HNH^2

C C

OO

H^3

C

HNH^3

(a)

(b)

Figure 13.23 Alanine a) Alanine is an amino acid. b) At pH ~ 7, alanine exists in a charge-separated form because the basic amine gains a proton and the carboxylic acid loses a proton. H

CH

3

CH

3

H X

HH

CH

3

CH

3

X

H

H

H

H

H

CH

3 CH

3

(a)

(b)

(c)

X

Figure 13.24 Hydrohalogenation of an alkene

O

RO

HO

R'

+

O

ROR'

OH

+

carboxylic acid + alcohol ester + water

H

H

Figure 13.25 Esterification The water molecule that is produced is highlighted in red.

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