Aldehydes and Ketones part 3

Learning Objectives

• Identify differences between hemiacetals, hemiketals, acetals, and ketals.

• Predict products of hemiacetal, hemiketal, acetal, and ketal formation and their hydrolysis.

Addition Reactions of Alcohols with Aldehydes and Ketones

• Aldehydes and ketones undergo addition reactions in which an alcohol combines with the carbonyl carbon and oxygen.

Hemiacetal Formation

• Initial products of addition reactions with alcohols are known as hemiacetals.

• Hemiacetals have an –OH group and an –OR group bonded to the carbonyl carbon atom.

• In the formation: H from alcohol bonds to carbonyl-group oxygen, OR from alcohol bonds to carbonyl-group carbon.

Hemiacetal Equilibrium

• Hemiacetals can revert back to aldehydes or ketones by loss of alcohol.

• Establish an equilibrium with the original aldehyde or ketone.

• Hemiacetals are usually too unstable to be isolated, with exceptions when reacting functional groups are part of the same molecule.

• Most simple sugars exist in a cyclic hemiacetal form due to greater stability.

Acetal Formation

• In the presence of an acid catalyst, hemiacetals can be converted into acetals.

• Acetals have two etherlike groups bonded to what was the carbonyl carbon.

Hydrolysis of Acetals

• Hydrolysis is a reaction where bonds are broken; requires an acid catalyst and needs a large quantity of water to revert to aldehyde or ketone.

• Acetal hydrolysis yields the original aldehyde or ketone and alcohol.

Identifying Hemiacetals and Hemiketals

• A hemiacetal has a carbon atom with single bonds to two oxygen atoms: one –OH and one –OR.

• A hemiketal has two carbons bonded to the central carbon that also shares single bonds with oxygen atoms.

• Solutions for different compounds:

  • Compound (a): Not a hemiacetal (diol).

  • Compound (b): Cyclic hemiacetal.

  • Compound (c): Hemiketal (carbon bonded to –OH and –OR).

Identifying Acetals and Ketals

• Acetals contain single bonds to two alkoxy groups (–OR).

• Ketals are similar, differing in the type of attached groups (both carbons).

• Worked example analysis includes identifying bonds and reactants in the compounds.

Concept Map

• Distinction of organic functional groups includes:

  • Carbonyl presence denotes aldehydes and ketones.

  • Alcohols and ethers involve oxygen functionality.

  • Functional groups are categorized based on characteristics of bond types and attached groups.