Aldehydes, Ketones, and Carboxylic Acids: Classification and Introduction and Classification

Definition of Carbonyl Group: The carbonyl group consists of a carbon-oxygen double bond, represented as >C=O. This is one of the most important functional groups in organic chemistry.
Components of the Carbonyl Group:
- Carbonyl Carbon: The carbon atom involved in the double bond.
- Carbonyl Oxygen: The oxygen atom involved in the double bond.
General Classification based on the Carbonyl Group:
- Aldehydes: In these compounds, the carbonyl carbon is bonded to at least one hydrogen atom, along with an alkyl or aryl group. The functional group is $-CHO$ , known as the formyl group or aldehydic carbonyl group.
- Ketones: In these compounds, the carbonyl carbon is bonded to two alkyl or aryl groups. These groups can be identical ( $R = R'$ ) or different ( $R \neq R'$ ). The functional group is the ketonic carbonyl group, represented as >C=O.
- Carboxylic Acids: The functional group is $-COOH$ , known as the carboxyl group. It consists of a hydroxyl group ( $-OH$ ) bonded to a carbonyl group (>C=O). Carboxylic acids are distinct from aldehydes and ketones because of this $-OH$ group.

Aldehydes are classified based on the nature of the carbon skeleton bonded to the >C=O group into aliphatic and aromatic categories.
Aliphatic Aldehydes:
- Definition: Compounds where the $-CHO$ group (formyl group) is attached directly to an $sp^3$ hybridized carbon atom (a saturated carbon atom).
- Exception: Formaldehyde ( $H-CHO$ ) is classified as an aliphatic aldehyde even though the $-CHO$ group is not attached to any carbon atom.
- General Formula: $R-CHO$ (where $R = H$ or an alkyl group).
- Examples:
  - Acetaldehyde: $CH_3 - CHO$
  - Propionaldehyde: $CH_3 - CH_2 - CHO$
Aromatic Aldehydes:
- Definition: Compounds in which the $-CHO$ group is attached directly to an aromatic ring.
- Examples:
  - Benzaldehyde
  - Salicylaldehyde (contains an $-OH$ group ortho to the $-CHO$ group)
  - p-Nitrobenzaldehyde (contains a $-NO_2$ group para to the $-CHO$ group)

Ketones are classified into aliphatic and aromatic ketones.
Aliphatic Ketones:
- Definition: Compounds in which the >C=O group is attached to two alkyl groups.
- General Formula: $R-C(=O)-R'$ (where $R$ and $R'$ are alkyl groups, which may be identical or different).
- Sub-classification of Aliphatic Ketones:
  1. Simple or Symmetrical Ketones: Ketones where both alkyl groups bonded to the carbonyl carbon are identical.
    - Example: Dimethyl ketone (Acetone) ( $H_3C-C(=O)-CH_3$ )
    - Example: Diethyl ketone ( $H_5C_2-C(=O)-C_2H_5$ )
  2. Mixed or Unsymmetrical Ketones: Ketones where the two alkyl groups bonded to the carbonyl carbon are different.
    - Example: Ethyl methyl ketone ( $H_5C_2-C(=O)-CH_3$ )
    - Example: Ethyl n-propyl ketone ( $H_5C_2-C(=O)-CH_2-CH_2-CH_3$ )
Aromatic Ketones:
- Definition: Compounds in which the >C=O group is attached to either two aryl groups or to one aryl and one alkyl group.
- Examples:
  - Benzophenone (Diphenyl ketone): Both groups are aryl (phenyl).
  - Acetophenone (Methyl phenyl ketone): One group is aryl (phenyl) and one is alkyl (methyl).

Carboxylic acids are classified as aliphatic or aromatic based on the carbon skeleton attached to the $-COOH$ group.
Aliphatic Carboxylic Acids (Fatty Acids):
- Definition: Organic compounds where the carboxyl ( $-COOH$ ) group is bonded to an alkyl group.
- Exception: Formic acid ( $H-COOH$ ) is classified as an aliphatic carboxylic acid despite the $-COOH$ group not being attached to a carbon.
- General Formula: $R-COOH$ (where $R = H$ or an alkyl group).
- Numerical Classification (based on the number of $-COOH$ groups):
  - Monocarboxylic Acids: Contain one $-COOH$ group. Example: Propionic acid ( $H_3C-CH_2-COOH$ ).
  - Dicarboxylic Acids: Contain two $-COOH$ groups. Example: Oxalic acid ( $COOH-COOH$ ).
  - Tricarboxylic Acids: Contain three $-COOH$ groups. Example: Citric acid ( $CH_2(COOH)-C(OH)(COOH)-CH_2(COOH)$ ).
- Additional Examples: Acetic acid ( $H_3C-COOH$ ).
Aromatic Carboxylic Acids:
- Definition: Compounds in which one or more carboxyl groups ( $-COOH$ ) are attached directly to the aromatic ring.
- Examples:
  - Benzoic acid
  - Phthalic acid (benzene ring with two ortho $-COOH$ groups)
  - Acetyl salicylic acid (Aspirin)
Side-chain Aromatic Acids:
- Definition: Aromatic compounds where the $-COOH$ group is not attached directly to the ring but to a side chain.
- Example: Phenyl acetic acid ( $C_6H_5-CH_2-COOH$ ).

Natural Distribution: Carboxylic acids are found in plants and animals.
- L-lactic acid is present in curd.
- Citric acid is found in citrus fruits like lemons.
- Acetic acid is the key ingredient in vinegar.
Flavors and Odors of Aldehydes and Ketones:
- Vanillin: Provides vanilla flavor.
- Benzaldehyde: Provides bitter almond flavor.
- Cinnamaldehyde: Provides cinnamon flavor.
- Camphor: Known for camphor fragrance.
- Butyraldehyde: Used in margarine and food for its buttery odor.
- Acetophenone: Has the smell of pistachio and is used in ice cream.
- Muscone: Has a musky aroma and is used in perfumes.
- Butane-2,3-dione: Responsible for the butter flavor in popcorn.
Biological Importance: Many important biological compounds contain the carbonyl moiety, specifically the male and female sex hormones:
- Progesterone: Female sex hormone.
- Testosterone: Male sex hormone.

Historical/Trivial names:
- Aldehyde names are often derived from the corresponding trivial names of carboxylic acids.
- The ending "-ic acid" of the carboxylic acid is replaced with the ending "-aldehyde".
Substituent Labeling in Trivial System:
- The position of substituents is indicated using Greek letters ( $\alpha, \beta, \gamma$ , etc.).
- The carbon atom adjacent to the carbonyl carbon is labeled as the $\alpha$ -carbon.
- The carbon atom next to the $\alpha$ -carbon is labeled as the $\beta$ -carbon, and so on.
Naming Systems: Aldehydes and carboxylic acids are named using two systems: Trivial and IUPAC.

Recall Exercise: Draw structures and classify based on $C-O$ single bonds vs. $C=O$ double bonds:
- Ethyl alcohol
- Acetaldehyde
- o-Nitrophenol
- Diethyl ether
- Isopropyl alcohol
- Acetone
Classification Task (Ketones): Classify as simple or mixed ketones:
- Benzophenone (Simple)
- Acetophenone (Mixed)
- Acetone (Simple)
- Butanone (Mixed)
Classification Task (Aldehydes): Classify the following as aliphatic or aromatic:
- Isobutyraldehyde (Aliphatic)
- Phenylacetaldehyde (Note: The transcript implies differentiating based on direct attachment to the ring; phenylacetaldehyde is a side-chain aromatic aldehyde).