functional group, in organic chemistry, group of atoms within a molecule that is responsible for certain properties of the molecule and reactions in which it takes part. Organic compounds are frequently classified according to the functional group or groups they contain. For example, methanol, ethanol, and isopropanol are all classified as alcohols since each contains a functional hydroxyl group.

In molecules, any of numerous combinations of atoms that undergo characteristic chemical reactions themselves and in many cases influence the reactivity of the rest of the molecule. Organic compounds are often classified according to the functional groups they contain. Common functional groups include hydroxyl (singlehorzbondOH), in alcohols and phenols; carboxyl (singlehorzbondCOOH), in carboxylic acids; carbonyl (singlehorzbondCdoublehorzbondO), in aldehydes, ketones, amides, carboxylic acids, esters, and quinones; and nitro (singlehorzbondNO₂) and amino (singlehorzbondNH₂), in certain organic nitrogen compounds.

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