Chapter 19: Aldehydes and Ketones

When treated with a strong oxidizing agent, primary alcohols

are oxidized to carboxylic acids. Formation of an aldehyde

requires a mild oxidizing agent, such as the oxidizing agents

shown above, that will not further oxidize the resulting

aldehyde.

Ozonolysis will cleave a C=C double bond. If either carbon

atom bears a hydrogen atom, an aldehyde will be formed.

Hydroboration-oxidation results in an anti-Markovnikov

addition of water across a 7 bond, followed by

tautomerization of the resulting enol to form an aldehyde.

A variety of strong or mild oxidizing agents can be used to

oxidize secondary alcohols. The resulting ketone does not

undergo further oxidation.

tetrasubstituted alkenes are cleaved to form ketones

This procedure results in a Markovnikov addition of water across

the pi bond, followed by tautomerization to form a methyl ketone.

Aromatic rings that are not too strongly deactivated will react

with an acyl halide in the presence of a Lewis acid to produce

an aryl ketone.

a strong nucleophile means the reaction is under

basic conditions

Acidic conditions are required in order for a weak nucleophile

to attack a carbonyl carbon

In the presence of water, a ketone/aldehyde is in equilibrium

with its hydrate

Under acidic conditions, one ketone/aldehyde reacts with

two alcohols to form an acetal

For most simple aldehydes, the acetal is favored at equilibrium

For most ketones, the acetal is not favored at equilibrium

• Acetal formation is reversible, and can be controlled by

adding/removing water:

• To favor acetal formation, water is removed from the reaction

• To convert an acetal back into the ketone/aldehyde, water is

added to the acetal, with H+ catalyst:

First, protect the ketone as a cyclic acetal

Then we can reduce the ester, and deprotect the ketone

A cyclic hemiacetal is possible when a compound contains both

the carbonyl group and the hydroxy group

• Acetals are hydrolyzed with aqueous acid to yield a ketone (or

aldehyde) and two equivalents of alcohol:

• Simply the reverse of acetal formation

Acetals will only react with water under acidic condition

Under acidic conditions, aldehyde/ketone reacts with a 1º

amine to form an imine

• The reaction requires acidic conditions to work

Under acidic conditions, aldehyde/ketone reacts with a 2º

amine to form an enamine

• The reaction requires acidic conditions to work; the mechanism

  is identical to imine formation, except for the last step

Wolff-Kishner reduction is a two-step synthesis, converting a

ketone to an alkane

• First step is imine formation between the ketone and hydrazine

(which is like a primary amine)

• Second reaction is like an elimination

Hydrolysis of imines and enamines undergoes a very similar

mechanism under acidic conditions

Hydrolysis of imines and enamines undergoes a very similar

mechanism under acidic conditions

Ketones/aldehydes react with thiols virtually the same way

they react with alcohols

Ketones/aldehydes react with thiols virtually the same way

they react with alcohols

reductions are carried out under basic conditions

This reaction works better under

basic conditions, and so a catalytic

amount of base is usually used

To achieve basic conditions, KCN is usually added along with

HCN.

Installation of a cyano group is advantageous because it can be

converted to other functional groups

The Wittig Reaction is an extremely important reaction in organic chemistry, like Grignard reactions, in that the carbon skeleton is modified

• Ketone/aldehyde is converted to an alkene, with the formation of a new C=C double bond

The Wittig Reaction is an extremely important reaction in organic chemistry, like Grignard reactions, in that the carbon skeleton is modified

• Ketone/aldehyde is converted to an alkene, with the formation of a new C=C double bond

AND, the Wittig rxn is stereoselective:

• Using a Wittig derived from a simple alkyl halide, the (Z) alkene is favored

AND, the Wittig rxn is stereoselective:

• But if the Wittig rgt contains an electron-withdrawing group,

the (E) alkene is the major product:

AND, the Wittig rxn is stereoselective:

• But if the Wittig rgt contains an electron-withdrawing group,

the (E) alkene is the major product:

Horner-Wadsworth-Emmons (HWE) reaction employs a

reagent similar to a stabilized Wittig reagent, and also yields

E-alkenes as the major product

Baeyer-Villiger Oxidation - An oxygen is inserted into an

aldehyde/ketone between a carbonyl carbon and neighboring

alkyl group