Aroma Chemicals refer to chemical substances that impart odor and they are often highly volatile. Along with Essential Oils, aroma chemicals form the backbone of ingredients used when formulating fragrances.
Aroma chemicals can be alcohols, esters, aldehydes, terpenes, ketones etc. They can be produced synthetically or by processes that conform to a definition of natural:
Natural aroma chemicals are obtained from various plant parts including, flowers, fruits, peels, leaves, barks, seeds, woods, roots, and resinous exudates. Extraction methods such as distillation, steam distillation, and mechanical separation or cold pressing are various ways to obtain pure plant extracts used for producing natural aroma chemicals. A natural aroma chemical is further refined and perfected when the raw material goes through a process of fermentation and isolation. Fermentation enhances the aroma, texture, and overall appearance, while isolation separates the constituents of the plant extract and purifies them.
 Aroma chemicals produced by isolation or by fermentation are easily understood as natural.
 Aroma chemicals that are produced in chemical reactors where the starting materials are synthetic, and undergo substantial chemical transformation, are synthetic.
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Since synthetic aroma chemicals are developed in a laboratory, it is much easier to be certain about the aromatic profile and chemical constituents that create the fragrance, thus providing an example of how the finished aroma will smell like.
An example of a natural aroma chemical would be citral produced by fractional distillation from lemongrass oil or oil Litsea Cubeba. An example of a synthetic aroma chemical would be Anethole produced by a series of complex chemical reactions from crude sulfate turpentine.
Natural aroma chemicals are also extracted from essential oils. For example from Geranium oil we can extract geraniol, citronellol, citral, linalool, limonene, isomenthone, citronellyl formate, geranyl formate, epi-γ-eudesmol.