Organic Compounds: Aromatic Scents From C-O Bonds

Aroma compounds are chemical compounds that have a smell or odour. They are often referred to as odorants, fragrances or flavourings. Aroma compounds can be found in various foods, such as fruits, wine, spices, floral scents, perfumes, and essential oils.

Organic compounds are those that contain carbon (C) and hydrogen (H) atoms. The few carbon-containing compounds not classified as organic include carbides, carbonates, and cyanides. Organic compounds are termed alkenes if they contain a carbon-carbon double bond. The presence of a carbon-oxygen double bond is known as a carbonyl group.

Aldehydes and ketones are two closely related carbonyl-based functional groups that react in very similar ways. In a ketone, the carbon atom of a carbonyl is bonded to two other carbons. In an aldehyde, the carbonyl carbon is bonded on one side to a hydrogen, and on the other side to a carbon. Aldehydes and ketones are known for their sweet and sometimes pungent odours.

The hydroxyl group (OH) is the functional group of alcohols. Alcohols are derived from alkanes by replacing one or more hydrogen atoms with an OH group. Alcohols can be considered derivatives of water (H2O). The hydroxyl group of alcohols and phenols is responsible for an interesting variety of physical and chemical properties.

Aromatic compounds or arenes usually refer to organic compounds with a chemistry typified by benzene and are cyclically conjugated. The word aromatic originates from the past grouping of molecules based on odour, before their general chemical properties were understood. The current definition of aromatic compounds does not have any relation to their odour.

Aldehydes and Ketones

The difference between aldehydes and ketones lies in the placement of the carbonyl group within the molecule. In an aldehyde, the carbonyl group is attached to a carbon atom at the end of a carbon chain, and has at least one hydrogen atom attached to it. The general formula for an aldehyde is -CHO, where R may be a hydrogen atom or any length of the carbon chain.

Ketones, on the other hand, have the carbonyl group attached to a carbon atom within the carbon chain. The carbonyl group in a ketone has two hydrocarbon groups attached to it, and no hydrogen atom. The general formula for a ketone is RC(=O)R’, where R and R’ represent carbon-containing substituents.

Some common aldehydes include formaldehyde, acetaldehyde, and benzaldehyde. Formaldehyde, also known as methanal, was once used as a biological preservative but has since been replaced due to its carcinogenic properties. It is now used in the production of resins and plastics. Acetaldehyde is used in the production of acetic acid and pyridine derivatives, while benzaldehyde is used in perfumes, cosmetics, and food flavoring.

Acetone, or propanone, is the simplest and most common ketone. It is a common organic solvent used in nail polish removers, paint thinners, and chemical peeling. Other important ketones include methyl ethyl ketone (butanone), which is used in the production of textiles, plastics, and varnishes, and cyclohexanone, which is used in the production of nylon.

Alcohols and Phenols

Phenols, on the other hand, are compounds where the -OH group is attached directly to an aromatic ring and are represented as ArOH in chemical equations. Phenols differ from alcohols in their slight acidity in water. They are widely used as antiseptics and disinfectants, such as the compound called phenol, which was the first widely used antiseptic. However, phenol is toxic and can cause severe burns and systemic poisoning.

Both alcohols and phenols have distinct roles in various chemical reactions. Alcohols can undergo dehydration reactions to form alkenes or ethers, and they can also be oxidized to form aldehydes, ketones, or carboxylic acids. Primary and secondary alcohols can be oxidized, while tertiary alcohols cannot. Phenols, due to their aromatic ring, exhibit different reactivity patterns compared to alcohols.

In terms of acidity, phenols are more acidic than alcohols but less acidic than carboxylic acids. The increased acidity of phenols is due to the resonance and polarization effects of the aromatic ring, which stabilizes the conjugate base, the phenoxide ion. This enhanced stability allows phenols to be deprotonated by weaker bases, such as hydroxide, whereas alcohols require stronger bases for deprotonation.

Carboxylic Acids and Their Derivatives

Carboxylic acids are named as such because they can donate a hydrogen to produce a carboxylate ion. They are weak acids and react with bases to form salts. They have strong, often pungent, and even rancid odours. For example, the smell of vinegar is due to ethanoic acid (acetic acid).

The derivatives of carboxylic acids include carboxylates (deprotonated carboxylic acids), amides, esters, thioesters, acyl phosphates, acid chlorides, and acid anhydrides. Esters are perhaps the most well-known derivative, as they are responsible for the aroma of many fruits and flowers. They can be synthesised from carboxylic acids using a reaction called Fischer esterification.

Amides, on the other hand, are the 'peptide bonds' that link amino acids together in proteins. Thioesters are found in acetyl-CoA, an important molecule in metabolism.

Alkanes

• Primary (1°): The carbon atom with the OH group is attached to one other carbon atom. Its general formula is RCH2OH.
• Secondary (2°): The carbon atom with the OH group is attached to two other carbon atoms. Its general formula is R2CHOH.
• Tertiary (3°): The carbon atom with the OH group is attached to three other carbon atoms. Its general formula is R3COH.

Alkenes and Alkynes

The physical properties of alkenes and alkynes are quite similar to those of alkanes. Like alkanes, alkenes and alkynes are non-polar, immiscible in water, less dense than water, generally soluble in organic solvents, and do not conduct electricity. The melting and boiling points of alkenes and alkynes are determined by the regularity of their packing and the surface area of interaction.

Frequently asked questions