8.7 Structure and Properties of Aldehydes, Ketones, Carboxylic Acids, and Esters

General Structure and the Carbonyl Group

• The four functional groups discussed in this section are aldehydes, ketones, carboxylic acids, and esters.

• Every one of these four functional groups possesses a carbonyl carbon.

• Verbatim Definition of Carbonyl Group: A functional group consisting of a carbon atom doubly bonded to an oxygen atom ($C=O$).

• The specific type of functional group formed is determined by the connectivity—specifically, what atoms or groups are attached to the left and the right of the carbonyl carbon.

Aldehydes and Ketones: Definitions and Distinctions

• Aldehydes:

  • A carbonyl group that is bonded to at least one hydrogen atom.

  • General Structure: $R-CHO$, where the carbonyl carbon is attached to an $R$ branch (carbon chain) and at least one hydrogen.

  • Variation: If the carbonyl carbon is bonded to two hydrogens ($H-CHO$), it is still classified as an aldehyde functional group.

  • Example: An aldehyde where the $R$ group is a methyl group ($CH_3$), with the carbonyl carbon bonded to one hydrogen.

• Ketones:

  • A carbonyl group that is bonded to two other carbon atoms.

  • General Structure: $R-CO-R'$, where the carbonyl group is "nestled" between two carbon chains.

  • Example: Butanone:

  • A ketone with a base of four carbons.

  • The carbonyl carbon is located between a methyl group ($CH_3$) on the left side and an ethyl group ($CH_2CH_3$) on the right side.

• Instructional Note: Students are expected to look at a chemical structure and differentiate between various carbonyl functional groups, similar to differentiating between alcohols and ethers.

Identification Examples: Aldehydes and Ketones

• Compound A: The carbonyl carbon is not bonded to two other carbons but is bonded to hydrogens. Classification: Aldehyde.

• Compound B: Featured in skeletal structure form. The carbonyl carbon is bonded to a phenyl group on the left side but is bonded to a hydrogen on the right side. Classification: Aldehyde.

• Compound C: The carbonyl carbon is located between two methyl groups. It is bonded to two carbons rather than a hydrogen. Classification: Ketone.

• Compound D: Shown in skeletal structure. The carbonyl carbon is bonded to two other carbons. Classification: Ketone.

• Compound E: The carbonyl group is located at the end of the chain, and the carbonyl carbon is bonded to a hydrogen. Classification: Aldehyde.

• Compound F: The carbonyl carbon is positioned between two phenyl groups. Because it is bonded to two other carbons. Classification: Ketone.

Carboxylic Acids and Esters: The Carboxyl Group

• The Carboxyl Group:

  • Consists of a carbonyl carbon ($C=O$) with an additional oxygen atom attached to that same carbon through a single bond ($C-O$).

  • The general structure involves a carbonyl carbon attached to another oxygen ($R-CO-O-$).

  • The difference between a carboxylic acid and an ester depends entirely on what is attached to that singly bonded oxygen atom.

• Carboxylic Acids:

  • Occur when the second oxygen atom (the one singly bonded to the carbon) is bonded to a hydrogen (a proton).

  • General Structure: $R-COOH$.

  • Properties: These compounds tend to be weak acids.

• Esters:

  • Occur when the second oxygen atom (the one singly bonded to the carbonyl carbon) is attached to a carbon chain ($R$ group).

  • General Structure: $R-COOR'$.

Real-World Examples and Applications

Carboxylic Acids

• Acetic Acid: Commonly known as vinegar.

• Arachitic Acid: A chemical component found in cocoa butter.

• Butyric Acid: A compound found in butter. Note that butyric acid makes up approximately 3\% \t\text{to} \t4\% of the mass of butter.

Esters

• Esters are primarily utilized as flavor additives and for producing specific scents.

• Ethylbuterate: A popular flavor enhancer used in many citrus juices and pineapple juice.

• Benzyl Acetate: Utilized as a peach flavorizer.

• Methyl Salicylate: A popular compound used to create the taste and/or smell of wintergreen.

Identification Examples: Carboxylic Acids and Esters

• Compound A: In this skeletal structure, the carbonyl carbon is singly bonded to an oxygen, which is followed by a carbon chain. Classification: Ester.

• Compound B: The carboxyl group is followed by a hydrogen on the singly bonded oxygen. Classification: Carboxylic Acid.

• Compound C: The carboxyl group is followed by a hydrogen on the singly bonded oxygen. Classification: Carboxylic Acid.

• Compound D: The singly bonded oxygen in the carboxyl group is followed by an ethyl group. Classification: Ester.

Summary of Classifications

• Aldehyde: Carbonyl group ($C=O$) bonded to at least one $H$.

• Ketone: Carbonyl group ($C=O$) bonded to two carbons.

• Carboxylic Acid: Carboxyl group where the singly bonded oxygen is attached to a hydrogen ($H$).

• Ester: Carboxyl group where the singly bonded oxygen is attached to a carbon chain ($R$).