Functional Groups

FIELDS OF STUDY: Organic Chemistry

ABSTRACT

Functional groups are combinations of atoms that influence the reactivity of organic molecules. Although the number of different organic molecules is immense, the number of different functional groups is quite small and their behavior is consistent from compound to compound.

The Nature of Functional Groups

In organic chemistry, the term "functional group" refers to a specific combination of atoms within a molecular structure that governs the chemical behavior of the molecule. Functional groups exhibit characteristic chemical behavior regardless of the molecule they are found in. Accordingly, although the number of different organic molecules is nearly infinite, the chemical properties and reactions of any organic compound are highly predictable, based on the typical chemical behavior of its component functional groups. This is not a hard and fast rule, as the overall chemical behavior of any particular compound depends on several other factors as well, such as the three-dimensional shape of the molecule and the electronic effects exerted by other groups within the molecule. The environment in which a compound exists, its solvent interactions, and the physical state of the compound are other factors that affect its specific chemical behaviors.

The simplest organic compounds, or carbon-containing molecules, are pure hydrocarbons consisting of only hydrogen and carbon atoms. When all of the carbon atoms in a hydrocarbon compound are bonded to the maximum possible number of atoms, forming four single bonds each, the hydrocarbon compound is said to be saturated. Saturated hydrocarbons, or alkanes, are effectively unreactive compounds under most conditions, though alkanes can be highly reactive in combustion, halogenation, and dehydrogenation reactions.

Hydrocarbon derivatives are hydrocarbon compounds that have additional functional groups in the place of one or more hydrogen atoms. The presence of a functional group, no matter how simple, in a hydrocarbon molecule alters the chemical properties of the molecule in specific ways, according to the nature of the functional group. Through the addition of functional groups, it is possible to convert the simplest hydrocarbon, methane (CH₄), into even the largest and most complex organic molecules imaginable.

Nomenclature of Functional Groups

The nomenclature of an organic compound is based on the characteristic functional group it contains. Because organic molecules can contain any number of functional groups, determining the best systematic name for a compound can be difficult. The International Union of Pure and Applied Chemistry (IUPAC) maintains a set of standardized rules for the nomenclature of chemical compounds, based on the type and position of their functional groups, in order to ensure that the systematic name of a compound accurately reflects its unique molecular structure. Within that system of nomenclature, the essential functional group of a molecular structure is indicated by the suffix of the compound name. For example, the names of all ketones end in "-one," those of all aldehydes end in "-al," those of all alcohols end in "-ol," and so on. The list is quite extensive and is updated as required.

Essential Functional Groups

Active Hydrocarbons. Hydrocarbons come in two basic types: saturated and unsaturated. The electronic structure of the carbon atom permits it to hybridize its 2s and 2p atomic orbitals into either three equivalent sp² orbitals or four equivalent sp³ orbitals, which allows it to form two, three, or four bonds of equal lengths. Carbon atoms that have four single bonds to other atoms have saturated their ability to form bonds, and hydrocarbons in which all carbon atoms are saturated are generally the most unreactive. Organic compounds that contain at least one double (C=C) or triple (C≡C) carbon-carbon bond are considered to be unsaturated. These double and triple bonds are reactive sites that define the alkene and alkyne series of hydrocarbon compounds, respectively. The reactive sites of unsaturated hydrocarbons most commonly undergo addition reactions.

Halogen-Containing Compounds. Although alkanes are generally unreactive, they can undergo substitution reactions, primarily through the replacement of a hydrogen atom with a halogen atom (group 17). Removal of a hydrogen atom from an alkane creates an alkyl functional group, which has the general molecular formula C_nH_2n+1; in addition, an alkane in which more than one hydrogen atom has been replaced is also considered to be an alkyl group. An alkane that has been substituted with one or more halogen atoms is known as an alkyl halide or a haloalkane. Alkenes and alkynes often undergo addition reactions whereby a halogen atom joins the molecular structure of an unsaturated hydrocarbon at the C=C or C≡C bond.

Oxygen-Containing Compounds. The addition of oxygen to a hydrocarbon molecule forms the basic structure of a number of different classes of compounds. Alcohols are organic compounds than contain the hydroxyl (−OH) functional group bonded to a saturated carbon atom. Phenols feature the hydroxyl group bonded to an aromatic hydrocarbon, which is a hydrocarbon whose carbon atoms form a ring with alternating double and single bonds, distributed in such a way that all bonds are of equal length and strength.

A functional group derived from an aromatic hydrocarbon is known as an aryl group. An alkoxyl, or alkoxy group, is an oxygen atom bonded to an alkyl or aryl group. Another class of oxygen-containing organic compounds is the ethers, characterized by the presence of an ether group, which is similar to an alkoxy group but has a second alkyl or aryl group bonded to the oxygen atom.

Perhaps the most important oxygen-containing compounds are those in which the oxygen atom shares a double bond with a carbon atom, forming a carbonyl group (C=O). The carbonyl group is the fundamental feature of aldehydes, ketones, and all of the derivatives of the carboxylic acids, including acid anhydrides, acid chlorides, esters, amides, and their variations. A carboxylic acid is defined by the presence of a carboxyl group (−COOH), which is a carbonyl group that is attached via the carbon atom to a hydroxyl group. Similarly, an acyl group consists of a carbonyl whose carbon atom is bonded to an alkyl or aryl group, while in an ester group, the carbonyl carbon is bonded to both an alkoxy group and an alkyl or aryl group.

Nitrogen-Containing Compounds. Nitrogen atoms can be incorporated into hydrocarbon molecules as primary, secondary, or tertiary amines. Amines are derivatives of ammonia (NH₃) in which one or more nitrogen-hydrogen bonds have been replaced by nitrogen-carbon bonds; the primary, secondary, and tertiary labels indicate whether one, two, or three of the hydrogen atoms have been replaced. The nitrogen atom can also form double bonds (C=N) and triple bonds (C≡N) with carbon, forming the basis for the imines and the nitriles, respectively.

Sulfur-Containing Compounds. Sulfur forms several different types of hydrocarbon-based compounds. Compounds containing a sulfur atom between carbon and hydrogen (C−S−H) are named thiols. Compounds with a sulfur atom between two carbon atoms (C−S−C) are called sulfides, while those with two sulfur atoms between two carbon atoms (C−S−S−C) are named disulfides. The sulfur atoms in the sulfide and disulfide compounds can also be bonded to a number of oxygen atoms, giving rise to sulfoxides, sulfones, thiosulfoxides, thiosulfones, sulfinates, sulfonates, thiosulfinates, and thiosulfonates.

Phosphorus-Containing Compounds. Phosphorus is not as versatile as sulfur and does not form as many different compounds with hydrocarbon moieties. The most common of the phosphorus-containing compounds are the phosphines, in which three carbon atoms are bonded to one phosphorus atom.

Reactions of Functional Groups

Chemists can deduce the presence of a functional group within a compound based on the compound’s reaction when exposed to certain reagents and the electronegativity of the atoms within the molecule. The carbonyl group, for example, is slightly polarized by the difference in electronegativity between the oxygen and carbon atoms. Accordingly, a nucleophilic atom introduced to the carbonyl group will generally join the carbon atom. The same feature also makes hydrogen atoms on carbon atoms adjacent to the carbonyl group somewhat acidic, so that a strong base can remove them to form the corresponding anion. Because functional groups undergo the same types of reactions regardless of the compound they are in, they are an important tool for distinguishing similar compounds from one another.

PRINCIPAL TERMS

• derivative: a compound that is obtained by subjecting a similar parent compound to one or more chemical reactions that target certain functional groups, leaving the basic molecular structure unaltered.
• IUPAC: the International Union of Pure and Applied Chemistry, an organization that establishes international standards and practices for chemistry.
• moiety: a specific portion of a molecular structure.
• nomenclature: a system of specific names or terms and the rules for devising or applying them; in chemistry, refers mainly to the system of names for chemical compounds as established by the International Union of Pure and Applied Chemistry (IUPAC).
• organic chemistry: the study of the chemical identities, behaviors, and reactions of carbon-based compounds and materials.

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