Organic Chemistry

Section 18.7 Reactions of Aldehydes and Ketones with Oxygen Nucleophiles 755

PRESERVING BIOLOGICAL

SPECIMENS

A 37% solution of formaldehyde in water is known

as formalin—commonly used in the past to preserve biological

specimens. Because formaldehyde is an eye and skin irritant, it

has been replaced in most biology laboratories by other preserva-

tives. One preservative frequently used is a solution of 2 to 5%

phenol in ethanol with added antimicrobial agents.

If the amount of hydrate formed from the reaction of water with a ketone is too
small to detect, how do we know that the reaction has even occurred? We can prove
that it occurs by adding the ketone to water and isolating the ketone after
equilibrium has been established. Finding that the label has been incorporated into the
ketone indicates that hydration has occurred.

PROBLEM 24

Trichloroacetaldehyde has such a large equilibrium constant for its reaction with water that

the reaction is essentially irreversible. Therefore, chloral hydrate, the product of the reac-

tion, is one of the few hydrates that can be isolated. Chloral hydrate is a sedative that can

be lethal. A cocktail laced with it is commonly known—in detective novels, at least—as a

“Mickey Finn.”Explain the favorable equilibrium constant.

PROBLEM 25

Which of the following ketones has the largest equilibrium constant for the addition of water?

Addition of Alcohol
The product formed when one equivalent of an alcohol adds to an aldehyde is called a
hemiacetal. The product formed when a second equivalent of alcohol is added is
called an acetal. Like water, an alcohol is a poor nucleophile, so an acid catalyst is re-
quired for the reaction to take place at a reasonable rate.

OH

+ H 2 O

trichloroacetaldehyde

Cl 3 CCH

OH

chloral hydrate

Cl 3 C H

O

C

(^18) O-labeled

• 
  


• 
  


• 
  


• 
  


• 
  

  H 2 O
  CH 3 CH 3
  CH 3 C CH 3
  OH
  H
  OH OH
  O
  OH
  CH 3 CH 3
  H 2 O
  H 2 O
  C OH
  H
  H
  OH

  
  


• 
  

  H OH
  H

  
  


• 
  

  O
  H H
  OH
  CH 3 C CH 3
  OH
  18
  (^1818)
  18
  18 18
  C
  CH 3 CH 3
  C
  CH 3 CH 3
  C
  CH 3 CH 3
  C
  O
  CH 3 O C OCH 3
  O
  O 2 NCNO 2
  O
  C
  O