3.2: Functional Groups

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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)		-ene	H₂C═CH₂ Ethene
Alkyne (triple bond)	$– C ≡ C –$	-yne	$HC ≡$ CH Ethyne
Arene (aromatic ring)		None
Halide	(X=F, Cl, Br, I)	None	CH₃Cl Chloromethane
Alcohol		-ol	CH₃OH Methanol
Ether		ether	CH₃OCH₃ Dimethyl ether
Monophosphate		phosphate	CH₃OPO₃²⁻ Methyl phosphate
Diphosphate		diphosphate	CH₃OP₂O₆³⁻ Methyl diphosphate
Amine		-amine	CH₃NH₂ Methylamine
Imine (Schiff base)		None
Nitrile	$– C ≡ N$	-nitrile	CH3C≡N Ethanenitrile
Thiol		-thiol	CH₃SH Methanethiol
Sulfide		sulfide	CH₃SCH₃ Dimethyl sulfide
Disulfide		disulfide	CH₃SSCH₃ Dimethyl disulfide
Sulfoxide		sulfoxide
Aldehyde		-al
Ketone		-one
Carboxylic acid		-oic acid
Ester		-oate
Thioester		-thioate
Amide		-amide
Acid chloride		-oyl chloride
Carboxylic acid anhydride		-oic 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=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 ($\delta^-$).

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.

Exercise $\PageIndex{1}$

Use Table 3.1 to identify the functional groups in each of the following molecules:

(a) The chemical structure of methionine which is an amino acid. (b) The chemical structure of ibuprofen which is a pain reliever.

(c) The chemical structure of capsaicin which is the pungent substance in chili peppers.

Answer

(a) Sulfide, carboxylic acid, amine

(b) Aromatic ring, carboxylic acid

(c) Ether, alcohol, aromatic ring, amide, C=C bond

Exercise $\PageIndex{2}$

Propose structures for simple molecules that contain the following functional groups:

(a) Alcohol (b) Aromatic ring (c) Carboxylic acid (d) Amine (e) Both ketone and amine (f) 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).

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

Answer

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Functional Groups with Carbon–Carbon Multiple Bonds

Functional Groups with Carbon Singly Bonded to an Electronegative Atom

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

Exercise \(\PageIndex{1}\)

Exercise \(\PageIndex{2}\)

Exercise \(\PageIndex{3}\)