Alkanes

FIELDS OF STUDY: Organic Chemistry

Abstract

The characteristic properties and reactions of alkanes are discussed. Alkanes are an infinite series of compounds containing only carbon and hydrogen atoms. Their variety is due to the unique electronic distribution and physical size of the carbon atom. The alkanes are the basic structures of all organic molecules.

The Nature of the Alkanes

The alkanes are a series of compounds consisting of only carbon and hydrogen atoms, or hydrocarbons. Because alkanes do not contain any other functional groups in their molecular structure, they are generally unreactive to materials that are not good oxidizing agents or radicals able to extract a hydrogen atom.

The structure of alkanes derives from the geometric arrangement of atomic orbitals on the carbon atom. There are four valence electrons in each carbon atom, two in the 2s orbital and one each in two of the three 2p orbitals. By combining the 2s and 2p orbitals, the carbon atom is able to form four equivalent hybrid atomic orbitals, each containing just one of the four valence electrons of the carbon atom. Each of the four orbitals is directed toward an apex of a tetrahedron, with the carbon atom nucleus at its center. Any two of these sp³ hybrid orbitals ideally form an angle of 109.5 degrees. This angle and the physical diameter of the carbon atom are ideally suited to the formation of innumerable structures by linkage of the carbon atoms in different directions. The electrons in the orbitals of two adjacent carbon atoms readily form a covalent single bond, or sigma (σ) bond, between the two atoms, and each carbon atom can form four such single bonds. Accordingly, organic compounds with structures comprising carbon atoms form the largest and most varied group of chemical compounds known. In alkanes, all of the carbon atoms have four single bonds to either other carbon atoms or to hydrogen atoms, and because they have all of the bonds that they can, they are described as saturated. Thus, another way to describe the alkanes is as the series of saturated hydrocarbons. If a hydrocarbon molecule contains a carbon-carbon C=C double or C≡C triple bond, it is not classed as an alkane, but as an unsaturated hydrocarbon in the alkene or alkyne series, respectively.

The alkanes have two parallel families of structures, one comprising acyclic molecular structures and the other comprising cyclic molecular structures. The carbon atom skeletons of the acyclic alkane series can be expanded infinitely, at least in principle. The simplest of these is methane (CH₄), having just one carbon atom and four equivalent bonds to as many hydrogen atoms. The next is ethane (C₂H₆), followed by propane (C₃H₈), butane (C₄H₁₀), and so on. All alkanes have the general chemical formula C_nH_2n+2.

There are also numerous different arrangements of the same number of carbon and hydrogen atoms, or isomers, beginning with butane. The four carbon atoms of butane can be joined in sequence (linearly) or in a T structure to form the alkane named 2-methylpropane, both of which have the chemical formula C₄H₁₀. With each additional carbon atom, the number of possible isomers multiplies. Four saturated carbon atoms can be arranged in two isomeric forms. With five carbons atoms, there are three isomers, and with six carbon atoms there are five isomers. With seven carbon atoms, the number of isomers begins to outnumber the number of carbon atoms and increase in a dramatic manner, with eight isomers or ten if the optical isomers are included.

Optical isomers are compounds in which a carbon atom is bonded to four different substituent groups—atoms or groups that can be added in place of another, also called "side chains." They are identical in every way except the order in which the bonds to the substituent groups are distributed about that central asymmetric carbon atom. While the physical properties and chemical behavior of two optical isomers are the same, they are nonetheless distinctly different structurally and are therefore separate isomeric forms. With eight carbon atoms, then, the number of isomeric forms jumps to fourteen (eighteen counting optical isomers), and with nine carbon atoms the number of possible isomeric structures numbers more than one hundred, all of which are chemically distinct compounds.

In the cyclic hydrocarbons, the carbon atoms are bonded together in a ring structure, beginning with the three carbon atoms of cyclopropane (C₃H₈). The cyclic series follows the same order as the acyclic alkanes, with cyclobutane (C₄H₈), cyclopentane (C₅H₁₀), and so on. Isomeric structures of the cyclic alkanes also begin with four carbon atoms (methylcyclopropane and cyclobutane), and optical isomers are also possible. The presence of the ring structure adds another type of isomeric structure termed a "geometric isomer." Two side chains on carbon atoms in the ring can either be on the same side or on opposite sides of the virtual plane of the ring.

Nomenclature of Alkanes

Alkanes are named according to the number of carbon atoms comprising the longest unbranched chain of carbon atoms in the molecular structure, and the positions of side chains are identified accordingly. The name sequence begins with methane, ethane, propane, and butane as common names, and then progresses through names based on the number of carbon atoms: pentane, hexane, heptane, octane, nonane, decane, undecane, and so on. Once the base chain has been determined, the next part of the alkane’s name is a number referring to the carbon atom in the base chain to which the side chain is bonded. Carbon atoms in the base chain are numbered in sequence from either end, left or right, so that the side chain branches from the carbon atom with the lowest number in the sequence. The position number is followed by a hyphen and then the name of the substituent. A six-carbon base chain that has methyl groups on the second and third carbon atoms from one end would be named 2,3-dimethylhexane and not 4,5-dimethylhexane.

Substituent alkanes switch the -ane suffix for -yl, so that methane becomes methyl (−CH₃), propane becomes propyl (−CH₂CH₂CH₃), butane becomes butyl (−CH₂CH₂CH₂CH₃), and so on. Some substituent alkanes are structured in branched chains; these add the prefix iso-, as in isopropyl (−CH₃CHCH₃, with the second carbon in the side chain linked to the base chain). Isobutyl has −CH₂ linked to the base chain and to a central carbon, which is also linked to a hydrogen and two methyls. A prefix such as di- is added to specify the number of substituents of each type. So a dimethylhexane is a hexane with two methyl side chains. When different side chains are present on a hydrocarbon molecule, they are assigned by the priority of their size, again to attain the lowest numbers, and listed alphabetically.

Alkane structures are typically represented by structural formulas, line drawings that depict the angles between the carbon atoms in such a way that every angle vertex and line end represents a single carbon atom. Such line drawings allow chemists to communicate a large amount of chemical information in a very small space. This is a much more convenient and readily understood representation than strings of Cs connected by lines, and makes the nature and identity of the side chains immediately apparent. For example, the structure

much more clearly understood than

and can be easily assigned the proper name 4-isopropyl-2,7-dimethylundecane, according to the rules of nomenclature prescribed by the International Union of Pure and Applied Chemistry (IUPAC).

Cyclic alkanes are named in a similar manner. The basic name reflects the number of carbon atoms in the largest ring structure in the molecule. The first position in the ring structure is assigned to the carbon atom bearing the first substituent, and other substituents are assigned so as to attain the lowest set of position numbers. A cyclohexane ring bearing a methyl substituent on one carbon atom and an ethyl substituent on another carbon atom would be named 1-ethyl-3-methylcyclohexane, but not 1-methyl-3-ethylcyclohexane or any other combination of substituent positions. Cyclic alkane structures are slightly harder to draw if there are more than six carbon atoms in the ring, but they are even more readily understood than acyclic alkane structures, especially when cyclic structures are fused together.

Formation of Alkanes

The simple alkanes—methane, ethane, propane, and butane—occur naturally in natural gas, although the quantities of propane and butane are comparatively quite low. These are formed in the same processes that alter the molecular structures of organic materials in the formation of petroleum deposits. Alkanes higher than butane are typically obtained from "cracking," the industrial-scale fractional distillation of crude petroleum and its products. The range of products obtained from the process includes light petroleum ethers that contain essentially all isomers of pentane, hexane, heptane, and octane, as well as gasoline, kerosene, diesel and fuel oils, lubricating oils and greases, waxes, and tars. The residue from the process is normally used to pave highways. The compounds are never obtained from this process as single compounds. Specific alkanes can be prepared by synthetic reactions involving molecules with various functional groups that can undergo substitution, addition, and reduction reactions. However, since alkanes are not themselves reactive, they have little use other than as structural components of other molecules.

Reactions of Alkanes

Alkanes are unreactive compounds with acids and bases. They do not undergo reductions, but are readily and rapidly oxidized by good oxidizing agents, such as molecular oxygen (O₂) and perchloric acid (HClO₄). Hence, all hydrocarbons are highly combustible, and reaction with perchloric acid is violent. Due to their nonpolar character and inherent stability or unreactiveness, alkanes typically see use as solvents, though their ability to undergo oxidation to carbon dioxide and water also makes them very useful in quantities as combustion fuels.

Combustion with molecular oxygen occurs by a "radical" process involving electrically neutral single atoms rather than ions. In combustion, electrons in the bonding orbitals of molecules such as oxygen acquire sufficient energy to allow them to transfer to a higher-energy "antibonding" orbital. When this happens, the bond between the two atoms simply ceases to exist, and the individual atoms each have an unpaired electron. This makes such an atom highly reactive. An oxygen atom, for example, is able to abstract a hydrogen atom from an alkane molecule, which begins a radical chain reaction mechanism that ultimately produces water molecules and carbon dioxide molecules. Alkanes essentially undergo no other types of reactions.

PRINCIPAL TERMS

• functional group: a specific group of atoms with a characteristic structure and corresponding chemical behavior within a molecule.
• hydrocarbon: an organic compound composed solely of carbon and hydrogen atoms.
• isomer: one of two or more chemical species that have the same molecular formula but different molecular structures.
• orbital: a specific region of space about the nucleus of an atom in which electrons of a given energy level are most likely to be found.
• saturated: describes an organic compound in which carbon atoms are attached to other atoms via single bonds only, allowing the compound to contain the maximum possible number of hydrogen atoms.
• single bond: a type of chemical bond in which two adjacent atoms are connected by a single pair of electrons via the direct overlap of their atomic orbitals.

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