Aldehydes, Ketones and Carboxylic Acids

What are Aldehydes?

Aldehydes are organic compounds which have the functional group -CHO.

These carbonyl compounds consist of a central carbonyl carbon doubly bonded to an oxygen and single bonded to the R group (any alkyl group) and a hydrogen atom.

Where R stands for alkyl or aryl group.

Preparation of Aldehydes

Acid chlorides are reduced to aldehydes with hydrogen in the presence of palladium catalyst spread on barium sulfate.

This reaction is known as Rosenmund reduction and is mostly used for the preparation of aromatic aldehydes. This reaction cannot be used for the preparation of ketones and formaldehyde.

Properties of Aldehydes

• The structure of aldehydes shows a sp² hybridized central carbon which is doubly connected to oxygen and has a single bond with hydrogen.
• Small aldehydes are quite soluble in water.
• Formaldehyde and acetaldehyde are great examples of this. These two aldehydes are quite important industrially.
• Aldehydes generally exhibit a tendency to undergo oligomerization or polymerization.
• The carbonyl centre of the aldehyde has an electron-withdrawing nature. Therefore, the aldehyde group can be considered somewhat polar.

Nomenclature of Aldehydes

• Acyclic and Aliphatic Aldehydes are named after their longest carbon chain along with the “-al” suffix. For example, CH₃CH₃CH₂CHO is called butanal since it has four carbons in the chain.
• When the aldehyde functional group is added to a ring, the “-carbaldehyde” suffix must be used. For example, C₆H₁₁CHO is called cyclohexanecarbaldehyde.
• In the case of natural compounds or carboxylic acids, “-oxo” is used as a prefix to highlight the carbon which is a part of the aldehyde functional group. For example, (CHO)-CH₂COOH is called 3-oxopropanoic acid.

Some common and IUPAC names for some aldehydes are tabulated below.

Formula	Common name	IUPAC name
HCHO	Formaldehyde	Methanal
CH3CHO	Acetaldehyde	Ethanal
CH3-CH(CH3)-CHO	Isobutyraldehyde	2-Methylpropanal
CH3-CH=CH-CHO	Crotonaldehyde	2-Butenal

Uses of Aldehydes

• Formaldehyde is used as a disinfectant and as a preservative for biological specimens.
• Aldehyde is used for silvering mirrors.
• Formaldehyde is used for the production of a variety of plastic and resins.
• Benzaldehyde is used in perfumery and in the dye industry.

What are Ketones?

Ketones are organic compounds which have the functional group C=O and the structure R-(C=O)-R’.

These carbonyl compounds have carbon-containing substituents on both sides of the carbon-oxygen double bond. The carbonyl carbon of the ketone group is sp² hybridized. The structure of ketones is a trigonal planar centred around the carbonyl carbon. The bond angles of this structure approximate at 120⁰. Since the carbon-oxygen bond makes the carbonyl group polar (oxygen is more electron-withdrawing than carbon), ketones tend to be nucleophilic at the oxygen atom and electrophilic at the carbon atom.

Ketones are mass-produced industrially for their use as solvents, pharmaceuticals, and as precursors for polymers. Important ketones include methyl ethyl ketone (also called butanone), cyclohexanone, and acetone.

Preparation of Ketones

Acid chlorides on reaction with dialkyl cadmium produce ketones. Dialkyl cadmium themselves are prepared from Grignard reagents.

2R-Mg-X + CdCl₂ → R₂Cd + 2 Mg(X)Cl
2RCOCl + R₂Cd → 2R-CO-R + CdCl₂

The method is useful in a way that the mixed ketones can be prepared very conveniently.

Properties of Ketones

• Ketones are polar in nature due to the presence of a polar carbonyl group. Therefore they have higher boiling points than non-polar compounds.
• It cannot form any intermolecular hydrogen bond-like alcohols because there is no hydrogen attached to an oxygen atom.
• Ketones have large dipole moments compared to alcohols or ethers due to the shifting of pi electrons.
• Ketones react with hydrogen cyanide to form cyanohydrins. The reaction is normally carried out in the presence of a base, which acts as a catalyst in the absence of a base the reaction proceeds slowly.
• Most of the ketones form bisulphite addition products when it is added to sodium bisulphite.

Nomenclature of Ketones

• Ketones are named after their parent alkanes with the suffix “-anone”. The carbonyl group’s position in the ketone is denoted by a number while naming the ketone. For example, CH₃(CO)CH₃ is called 2 propanone. However, this compound is generally referred to as acetone.
• Commonly, ketones are named by writing the name of each individual alkyl group attached to the carbonyl carbon and then “ketone” as the third word of the name. For example, butanone can be written as methyl ethyl ketone.

Formula	Common name	IUPAC name
CH3-C(O)-CH3	Acetone	Propanone
CH3-C(O)-CH2-CH3	Ethylmethylketone	Butanone
CH3-CH2-C(O)-CH2-CH3	Diethyl ketone	Pentan-3-one
CH3-CH(CH3)-C(O)-CH3	Isopropyl methyl ketone	3-Methylbutan-2-one

Uses of Ketones

• Propanone is used to make polymers for example perspex.
• ketones are used as solvents and as a starting material for the synthesis of many organic compounds.
• Acetone and ethyl methyl ketone is mainly used as industrial solvents.

What is Carboxylic Acid?

Carboxylic acids are organic compounds that contain a (C=O)OH group attached to an R group (where R refers to the remaining part of the molecule).

The COOH group is commonly referred to as a carboxyl group. Carboxylic acids can generally be expressed via the formula R-COOH. Carboxylic acids have a polar nature. They can also participate in hydrogen bonding owing to their hydrogen bond accepting nature of the C=O group and the hydrogen bond donating nature of the O-H bond. They generally have higher boiling points when compared to water and tend to form stable dimers.

Carboxylic acids play an important role in the production of pharmaceuticals, food additives, solvents, and polymers. Acetic acid, adipic acid, and citric acid are a few carboxylic acids which are extremely useful industrially.

Preparation of Carboxylic Acids

Primary alcohols are readily oxidized to carboxylic acids with common oxidizing agents such as potassium permanganate in neutral acidic or alkaline media or by potassium dichromate and chromium trioxide in acidic media.

RCH₂OH → RCOOH

CH₃(CH₂)₈CH₂OH → CH₃(CH₂)₈COOH

Properties of Carboxylic Acids

• Carboxylic acids are polar compounds and can extensively enter into hydrogen bonding.
• Aromatic carboxylic acids are practically insoluble in cold water. All carboxylic acids are soluble in organic solvents like ether, alcohol, benzene, etc.
• Among organic acids, carboxylic acids are the most acidic, but they are less acidic than the mineral acids, namely nitric acid and sulphuric acid.

Nomenclature of Carboxylic Acids

• Carboxylic acids are named by adding an “-oic acid” suffix to their parent chain. For example, C₃H₇COOH is called butanoic acid.
• Even if there are other substituents present, the carboxylic acid can be considered at the first position of the parent chain, as seen in the name for 3-Chloropropanoic acid.
• The COOH group can also be called “carboxy” and used as a substituent in the name of the parent structure. For example, 2-Furoic acid is commonly referred to as 2-carboxy furan.

Formula	Common name	IUPAC name
HCOOH	Formic acid	Ethanoic acid
CH3COOH	Acetic acid	Ethanoic acid
CH3CH2COOH	Propionic acid	Propanoic acid
C6H5COOH	Benzoic acid	Benzenecarboxylic acid

To conclude – Aldehydes, Ketones and Carboxylic acids are very important carbonyl compounds industrially as well as in the laboratory. Their nomenclature can be done by following IUPAC guidelines or by following common practices.