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Organic Chemistry

3.1 Functional Groups

Organic Chemistry3.1 Functional Groups

3.1 • Functional Groups

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 Br2 in the same way to give a product in which a Br atom has added to each of the double-bond carbons (Figure 3.2). This example is typical: the chemistry of every organic molecule, regardless of size and complexity, is determined by the functional groups it contains.

Ethylene and menthene undergo a chemical reaction with bromine, forming their respective products
Figure 3.2 The reactions of ethylene and menthene with bromine. In both molecules, the carbon–carbon double-bond functional group has a similar polarity pattern, so both molecules react with Br2 in the same way. The size and complexity of the molecules are not important.

Look at Table 3.1, 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.

The ball and stick models and general structure of alkene, alkyne and arene, respectively.
Table 3.1 Structures of Some Common Functional Groups
Name Structure* Name ending Example
Alkene (double bond) The general structure of alkene. -ene H2CCH2H2CCH2
Alkyne (triple bond) CCCC -yne HCCHHCCH
Arene (aromatic ring) The general structure of arene. None The general structure of benzene.
Halide The general structure of halide where X represents any halogen element.

(X=F, Cl, Br, I)

None CH3Cl
Alcohol The general structure of alcohol. -ol CH3OH
Ether The general structure of ether. ether CH3OCH3
Dimethyl ether
Monophosphate The general structure of monophosphate. phosphate CH3OPO32−
Methyl phosphate
Diphosphate The general structure of diphosphate. diphosphate CH3OP2O63−
Methyl diphosphate
Amine The general structure of amine featuring a lone pair on nitrogen. -amine CH3NH2
Imine (Schiff base) The general structure of imine (Schiff base). None The chemical structure of acetone imine.
Nitrile CNCN -nitrile CH3CNCH3CN
Thiol The general structure of thiol. -thiol CH3SH
Sulfide The general structure of sulfide. sulfide CH3SCH3
Dimethyl sulfide
Disulfide The general structure of disulfide. disulfide CH3SSCH3
Dimethyl disulfide
Sulfoxide The general structure of sulfoxide. sulfoxide The chemical structure of dimethyl sulfoxide where a positive and a negative charge is placed on sulphur and oxygen, respectively.
Aldehyde The general structure of aldehyde. -al The chemical structure of ethanal.
Ketone The general structure of ketone. -one The chemical structure of propanone.
Carboxylic acid The general structure of carboxylic acid. -oic acid The chemical structure of ethanoic acid.
Ester The general structure of ester. -oate The chemical structure of methyl ethanoate.
Thioester The general structure of thioester. -thioate The molecular structure of methyl ethanethioate.
Amide The general structure of amide featuring a lone pair on nitrogen. -amide The chemical structure of ethanamide.
Acid chloride The general structure of acid chloride. -oyl chloride The chemical structure of ethanoyl chloride.
Carboxylic acid anhydride The general structure of carboxylic acid anhydride. -oic anhydride The chemical structure of ethanoic anhydride.

*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 (–OPO32−), 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–).

The ball and stick models and general structure of alkyl halide, alcohol, ether, phosphate, amine, thiol, sulfide, and disulfide.

Functional Groups with a Carbon–Oxygen Double Bond (Carbonyl Groups)

The carbonyl group, C=OC=O (pronounced car-bo-neel) is common to many of the families listed in Table 3.1. Carbonyl groups are present in a majority of organic compounds and in practically all biological molecules. These compounds therefore behave similarly in many respects but differ depending on the identity of the other atoms bonded to the carbonyl-group carbon. Aldehydes have at least one hydrogen bonded to the C=OC=O, ketones have two carbons bonded to the C=OC=O, carboxylic acids have an –OH group bonded to the C=OC=O, esters have an ether-like oxygen bonded to the C=OC=O, thioesters have a sulfide-like sulfur bonded to the C=OC=O, amides have an amine-like nitrogen bonded to the C=OC=O, acid chlorides have a chlorine bonded to the C=OC=O, and so on. In all these functional groups, the carbonyl carbon atom bears a partial positive charge (δ+), and the oxygen bears a partial negative charge (δ–).

Eight general structures of acetone, aldehyde, ketone, carboxylic acid, ester, thioester, amide, and acid chloride. A ball and stick model of acetone is also shown.
Problem 3-1
Use Table 3.1 to identify the functional groups in each of the following molecules:
The chemical structure of methionine which is an amino acid.
The chemical structure of ibuprofen which is a pain reliever.
The chemical structure of capsaicin which is the pungent substance in chili peppers.
Problem 3-2
Propose structures for simple molecules that contain the following functional groups:
Aromatic ring
Carboxylic acid
Both ketone and amine
Two double bonds
Problem 3-3

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).

The ball and stick model of arecoline where grey, black, blue and red spheres represent hydrogen, carbon, nitrogen, and oxygen, respectively.
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