The structural features that make it possible to classify compounds into families are called functional groups. A functional group is a group of atoms within a molecule that has a characteristic chemical behavior. Chemically, a given functional group behaves in nearly the same way in every molecule it’s a part of. For example, compare ethylene, a plant hormone that causes fruit to ripen, with menthene, a much more complicated molecule found in peppermint oil. Both substances contain a carbon–carbon double-bond functional group, and both therefore react with Br 2 in the same way to give a product in which a Br atom has added to each of the double-bond carbons (Figure \(\PageIndex{1}\)). This example is typical: the chemistry of every organic molecule, regardless of size and complexity, is determined by the functional groups it contains. Look at Table \(\PageIndex{1}\), which lists many of the common functional groups and gives simple examples of their occurrence. Some functional groups have only carbon–carbon double or triple bonds; others have halogen atoms; and still others contain oxygen, nitrogen, or sulfur. Much of the chemistry you’ll be studying is the chemistry of these functional groups. Functional Groups with Carbon–Carbon Multiple Bonds Alkenes, alkynes, and arenes (aromatic compounds) all contain carbon–carbon multiple bonds. Alkenes have a double bond, alkynes have a triple bond, and arenes have alternating double and single bonds in a six-membered ring of carbon atoms. They look different, but because of their structural similarities, they also have chemical similarities. Name Structure* Name ending Example Alkene (double bond) NaN -ene H2C═CH2 Ethene Alkyne (triple bond) –C≡C––C≡C– -yne HC≡CH Ethyne Arene (aromatic ring) NaN NaN NaN Halide (X=F, Cl, Br, I) NaN CH3Cl Chloromethane Alcohol NaN -ol CH3OH Methanol Ether NaN ether CH3OCH3 Dimethyl ether Monophosphate NaN phosphate CH3OPO32− Methyl phosphate Diphosphate NaN diphosphate CH3OP2O63− Methyl diphosphate Amine NaN -amine CH3NH2 Methylamine Imine (Schiff base) NaN NaN NaN Nitrile –C≡N–C≡N -nitrile CH3C≡N Ethanenitrile Thiol NaN -thiol CH3SH Methanethiol Sulfide NaN sulfide CH3SCH3 Dimethyl sulfide Disulfide NaN disulfide CH3SSCH3 Dimethyl disulfide Sulfoxide NaN sulfoxide NaN Aldehyde NaN -al NaN Ketone NaN -one NaN Carboxylic acid NaN -oic acid NaN Ester NaN -oate NaN Thioester NaN -thioate NaN Amide NaN -amide NaN Acid chloride NaN -oyl chloride NaN Carboxylic acid anhydride NaN -oic anhydride NaN *The bonds whose connections aren’t specified are assumed to be attached to carbon or hydrogen atoms in the rest of the molecule. Functional Groups with Carbon Singly Bonded to an Electronegative Atom Alkyl halides (haloalkanes), alcohols, ethers, alkyl phosphates, amines, thiols, sulfides, and disulfides all have a carbon atom singly bonded to an electronegative atom—halogen, oxygen, nitrogen, or sulfur. Alkyl halides have a carbon atom bonded to halogen (–X), alcohols have a carbon atom bonded to the oxygen of a hydroxyl group (–OH), ethers have two carbon atoms bonded to the same oxygen, organophosphates have a carbon atom bonded to the oxygen of a phosphate group (–OPO 3 2 − ), amines have a carbon atom bonded to a nitrogen, thiols have a carbon atom bonded to the sulfur of an –SH group, sulfides have two carbon atoms bonded to the same sulfur, and disulfides have carbon atoms bonded to two sulfurs that are joined together. In all cases, the bonds are polar, with the carbon atom bearing a partial positive charge ( δ +) and the electronegative atom bearing a partial negative charge ( δ –). Functional Groups with a Carbon–Oxygen Double Bond (Carbonyl Groups) The carbonyl group C=O appears in several functional groups. Aldehydes have at least one H atom bonded to the C=O, ketones have two carbon atoms bonded to the C=O, carboxylic acids have an -OH group bonded to the C=O, thioesters have a sulfide-like sulfur bonded to the C=O, amides have an amine-like nitrogen bonded to the C=O, acid chlorides have a chlorine bonded to the C=O, and so on. In all of these functional groups, the carbonyl carbon atom bears a partial positive charge (\(\delta^+\)), and the oxygen atom bears a partial negative charge (\(\delta^-\)). Exercise \(\PageIndex{1}\) Use Table \(\PageIndex{1}\) to identify the functional groups in each of the following molecules: Answer Sulfide, carboxylic acid, amine Aromatic ring, carboxylic acid Ether, alcohol, aromatic ring, amide, C=C bond Exercise \(\PageIndex{2}\) Propose structures for simple molecules that contain the following functional groups: Alcohol Aromatic ring Carboxylic acid Amine Both ketone and amine Two double bonds Answer (a) (b) (c) (d) (e) (f) Exercise \(\PageIndex{3}\) Identify the functional groups in the following model of arecoline, a veterinary drug used to control worms in animals. Convert the drawing into a line-bond structure and a molecular formula (red = O, blue = N, black = C, gray = H). Answer