Science

The Role Of Functional Groups In Shaping Scent and Flavour

Krisha Nathan

09 Mar 2025 — 5 min read

You may have experienced the sweet smell of vanilla or the refreshingly sharp smell of citrus. But have you thought about why they smell so different? and what is the chemistry behind this phenomena?

This is where functional groups of molecules come in—Functional groups can be thought of as 'personality traits' that shape how different substances smell! This article will explore how different functional groups affect the fragrance or odour of various substances.

Introduction; what are functional groups?

The fundamental definition is that functional groups are a group of atoms within a molecule that have similar chemical properties when it appears in various compounds.

Take the image below for example,

These molecules are alcohols. In this image, there are Carbon atoms covalently bonded to other Hydrogen atoms. But the interesting thing to note is the '-OH' group at the end of each molecule. These '-OH' groups are known as Hydroxyl groups, which is the functional group of alcohols.

These groups are what play a role in the reactivity of molecules and thereby are of interest. Simple hydrocarbons (with only Carbon & Hydrogen atoms) are non-reactive and usually thought of as 'lacking a functional group'.

The Most Common Functional Groups ⌬

The image above consists of 14 different classes of compounds. The atom groups that are highlighted are the functional groups of these classes of compounds.

A few examples,

Alkanes consist of only C-C single bonds (apart from bonds with hydrogen).

Alkenes consist of C=C double bonds (apart from bonds with hydrogen).

and Alkynes consist of C☰C triple bonds (apart from bonds with hydrogen).

A Carboxylic acid has the functional group -COOH, where the Carbon is doubly bonded to an Oxygen (C=O) and singly bonded to another -OH group. (refer to image above for clarity)

The last four classes of compounds, aldehydes, ketones, esters, carboxylic acids and amides may seem slightly similar as they all consist of the C=O bond as part of their functional group.

However this doesn't mean they have the same reactivity as it also depends on what the carbonyl group (C=O) is attached to!

Scents, Smell, Science 🧪⋆°✰

Class of Compound : Aromatic rings / Arenes ⌬

Known as aromatic rings or arenes, are compounds that consist of a benzene ring or have benzene-like properties.

Benzene is the simplest ring structured hydrocarbon or aromatic compound.

An example of an aromatic compound is;

Vanillin🍦☁️₊ ⊹

As you might've guessed, vanillin is the major component of vanilla. It consists of a benzene ring, a hydroxyl group (-OH), a carbonyl (aldehyde) group (-CHO) and an ether group (R'-O-R), the R stands for Residual group and just means it is connected to something, for example a hydrocarbon chain.

Vanillin has a sweet, warm smell like vanilla and a pleasant taste. It appears as a colourless substance in its pure form but may appear white to pale yellow in its crystalline structure.

However, it is sensitive to oxidation in the presence of light or air. This means that it can lose its aroma over time.

It is commonly used as follows;

In the food industry, it is added to cookies, cakes, ice-cream, chocolate etc. Vanillin is also much much cheaper than Natural Vanilla beans which is another reason why it is so frequently used. (natural Vanilla contains hundreds of aromatic compounds apart from Vanillin)
Perfumery and the cosmetic industry use vanillin to add vanilla scents to create lotions, soaps and perfumes.
Pharmaceutical industries sometimes use vanillin to mask unpleasant odors, especially in liquid medicine.

Other examples of substances (containing benzene) that have a pleasant / aromatic smell are

Toluene has a sharp/sweet odour and is often used as a solvent.

Eugenol (from its ending ol, we can determine that it has an -OH group, found in alcohols), has an 'autumn' vibe to it, and has a clove-like, warm scent. It is found in several types of essential oils such as clove, cinnamon and nutmeg.

Cinnamaldehyde (It has the -CHO group, found in aldehydes), is the main component of cinnamon and found in the bark of cinnamon trees and other Cinnamomum plants.

Although it is important to note that not all compounds with a benzene ring are aromatic or have a pleasant fragrance.

Class of compounds - Carboxylic acids (-COOH)

Carboxylic acids are molecules that consist of the a Carbon doubly bonded to an Oxygen (C=O) known as a ketone and the same Carbon is singly bonded to a hydroxyl group (-OH)

An example of a carboxylic acid is;

Formic acid / Methanoic acid 🐜 ⟡✶

Formic acid is the simplest carboxylic acid and in its pure form, it is a colourless liquid with a pungent odor, causing blisters in the skin. It was initially isolated from a certain type of ant, and was named after formica, meaning 'ant' in Latin.

The carboxylic acid group has a variety of applications which is as follows;

1) One of the most commonly used carboxylic acids is vinegar or ethanoic acid. Many foods are preserved in vinegar or pickled, to help eliminate microorganisms.

2) It is also used in the production of detergents, plastics, dyes and perfumes!

3) Oleic acid is a fatty acid that helps in reducing cholesterol or lowering inflammation; improving heart conditions.

Fish oil is a common source of *Oleic acid* (Source)

Carboxylic acids are often described as pungent and rancid/rotten however, this doesn't necessarily apply to all carboxylic acids.

For example, some exceptions are ;

Lactic acid; mild, sour and pleasant smell (used in skin care products and yoghurt)

Malic acid; slightly sour, found in apples and other fruit-flavored products.

Conclusion

By understanding the functional groups and structures in certain molecules, we are able to appreciate the complex science behind scents and odors!

Now that we know that functional groups play a major role in influencing a substance's scent/odour, we must also remember there are other factors apart from function groups that influence this.

For example,