Organic Chemistry

716 CHAPTER 17 Carbonyl Compounds I

NERVE IMPULSES, PARALYSIS,

AND INSECTICIDES

After a nerve impulse is transmitted between cells,

acetylcholine must be rapidly hydrolyzed to enable the recipient

cell to receive another impulse.

CH 3 OCH 2 CH 2 NCH 3

CH 3

CH 3

+

CH 3

CH 3

+

H 2 O

acetylcholinesterase

+

CH 3

+ HOCH 2 CH 2 NCH 3

O−

C

O

C

O

Acetylcholinesterase, the enzyme that catalyzes this hydrolysis,

has a group that is necessary for its catalytic activity.

Diisopropyl fluorophosphate (DFP), a military nerve gas, in-

hibits acetylcholinesterase by reacting with the CH 2 OHgroup.

CH 2 OH

When the enzyme is inhibited, paralysis occurs because the

nerve impulses cannot be transmitted properly. DFP is extreme-

ly toxic: Its (the lethal dose for 50% of the test animals) is

only of body weight.0.5 mg>kg

LD 50

enzyme CH 2 OH + FPO

OCH(CH 3 ) 2

DFP

OCH(CH 3 ) 2

OCH(CH 3 ) 2

OCH(CH 3 ) 2

enzyme CH 2 O P O + HF

active inactive

Malathion and parathion, compounds related to DFP, are used

as insecticides. The LD 50 of malathion is 2800 mg>kg.Para-

thion is more toxic, with an LD 50 of 2 mg>kg.

parathion

O 2 NO

malathion

CHS P

OCH 3

CH 3 CH 2 OC OCH 3

S P

OCH 2 CH 3

OCH 2 CH 3

S

O

CH 3 CH 2 OCCH 2

O

17.21 Dicarboxylic Acids and Their Derivatives

The structures of some common dicarboxylic acids and their values are shown in

Table 17.2.

Although the two carboxyl groups of a dicarboxylic acid are identical, the two

values are different because the protons are lost one at a time and therefore leave from

different species. The first proton is lost from a neutral molecule, whereas the second

proton is lost from a negatively charged ion.

A COOH group withdraws electrons (more strongly than does an H) and therefore

increases the stability of the conjugate base that is formed when the first COOH group

loses a proton—thereby increasing its acidity. The pKavalues of the dicarboxylic acids

pKa

pKa

HO

pKa1 = 2.86

CH 2 OH

pKa2 = 5.70

O−

+ HH+ + +

−OCH

2 O

−

C

O

C

O

HO CH 2

C

O

C

O

C

O

C

O