Alkenes

FIELDS OF STUDY: Organic Chemistry

ABSTRACT

The characteristic properties and reactions of alkenes are discussed. Alkenes are an infinite series of compounds containing only carbon and hydrogen atoms and at least one carbon-carbon double bond. Their variety is due to the unique electronic distribution and physical size of the carbon atom. The alkenes parallel the series of structures of the alkanes.

The Nature of the Alkenes

The alkenes are a series of compounds consisting of only carbon and hydrogen atoms, or hydrocarbons. Alkenes contain at least one double bond between two adjacent carbon atoms (C=C) as a functional group in their molecular structure and so are generally reactive materials. The structure of alkenes 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 two of the three 2p orbitals the carbon atom is able to form three equivalent sp² hybrid atomic orbitals, each containing a single electron. The fourth electron remains in the unhybridized 2p orbital. Each of the three hybrid orbitals is directed toward a vertex of a triangle, with the carbon atom nucleus at its center. Any two of the sp² hybrid orbitals ideally form an angle of 120 degrees. This angle and the physical diameter of the carbon atom are well suited to the formation of innumerable structures by linkage of the carbon atoms. The sp² orbitals of two adjacent carbon atoms readily form a covalent single bond, or sigma (σ) bond, and each carbon atom can form three such single bonds. The unhybridized p orbital on the adjacent carbon atoms are able to overlap in a side-by-side orientation to form a pi (π) bond parallel to the sigma bond. Accordingly, alkenes form a large and varied group of chemical compounds in their own right, and the reactivity of the C=C functional group enables the formation of many other compounds. In alkenes, the carbon atoms of the double bonds do not have all of the bonds that they can, and another way to describe the alkenes is as the series of unsaturated hydrocarbons. If a hydrocarbon molecule contains only carbon-carbon single bonds (C–C) or triple bonds (C≡C), it is not classed as an alkene, but as a hydrocarbon in the alkane or alkyne series, respectively.

The alkenes 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 alkene series can be expanded infinitely, at least in principle. The simplest alkene is ethene (C₂H₄), commonly known as ethylene, which has just two carbon atoms and four equivalent bonds to as many hydrogen atoms. The next are propene (C₃H₆), butene (C₄H₈), and so on. All alkenes have the general chemical formula C_nH_2n, which is the same as the cyclic alkane series. For alkenes with four or more carbon atoms (butenes or higher), there are also numerous possible isomers, chemical species that have the same molecular formula but different molecular structures. The double bond in butene can be between either the first and second carbon atoms or the second and third carbon atoms in the molecule, both of which have the chemical formula C₄H₈. In addition, the geometry of the C=C bond is fixed and rotation about a C=C bond cannot occur as it can with a C–C bond. This permits the formation of geometric isomers, in which two substituents, or side chains, can be either on the same side of the double bond or on opposite sides. Accordingly, 2-butene has the two isomeric structures

and the two compounds have both different physical properties and chemical behaviors. The same relationships exist for every C=C bond, within the structural constraints of the molecule.

With each additional carbon atom, the number of possible isomeric forms increases. Four carbon atoms can be arranged in two isomeric alkene forms. With five carbons atoms, there are six isomeric alkene structures, and with six carbon atoms there are sixteen isomeric alkene structures. There are even more isomeric forms when optical isomers are included. Optical isomers are compounds in which a saturated carbon atom has four different substituent groups bonded to the same carbon atom. They are identical in every way except the order which the bonds to substituent groups are distributed around the 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 therefore separate isomeric forms. Optical isomerism is not possible about the carbon atom of a single C=C bond, but the presence of more than one C=C bond in an alkene can satisfy the requirements for optical isomeric structures, in addition to greatly increasing the number of alkene structures.

In the cyclic hydrocarbons, the carbon atoms are bonded together in a ring structure, beginning with the three carbon atoms of cyclopropene, C₃H₄. The cyclic series follows the same order as the acyclic alkenes, with cyclobutene (C₄H₆), cyclopentene (C₅H₈), and so on. Geometric isomers of the cyclic alkenes are not physically possible to achieve for ring structures containing fewer than eight carbon atoms due to bond-angle restrictions.

Nomenclature of Alkenes

The alkenes 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 ethylene, propene, and butene as common names, and then progresses through names based on the number of carbon atoms: pentene, hexene, heptene, octene, nonene, decene, and so on. When the base chain has been determined and has three or more carbon atoms, the position of the bond is indicated by the lower of the two position numbers of the carbon atoms in the bond. The position number is inserted into the name of the primary chain immediately before the –ene suffix, which identifies the compound as an alkene. (Some naming conventions place the position number before the name of the primary chain.) If no position number is given, it is assumed that the bond is between the first and second carbon atoms. A seven-carbon chain with a C=C bond between the second and third carbon atoms from one end would thus be named hept-2-ene (or 2-heptene), not hept-3-ene or hept-5-ene. The order of the side chains is determined next. The numerical positions of the side chains are assigned as determined by the position assigned to the C=C bond. For example, a six-carbon chain that has methyl groups (−CH₃) on the second and third carbon atoms from the end that has the C=C bond would be named 2,3-dimethylhexene. When different side chains are present on a hydrocarbon molecule, they are assigned by the priority of their size, again to attain the lowest position numbers, and named alphabetically. The relative positions of side chains are indicated in the formal name of the compound using the identifiers cis- for side chains on the same side of the double bond and trans- when the side chains are on opposite sides of the double bond.

Organic 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

is much more easily and clearly understood than

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

Cyclic alkenes are named in a similar manner. The base 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 carbon atom of the C=C bond, and other substituents are assigned so as to attain the lowest set of position numbers. A cyclooctene ring bearing a methyl substituent on the third carbon atom from the beginning of the C=C bond and an ethyl substituent on another carbon atom two positions farther around the ring would be named 5-ethyl-3-methylcyclooctene. Cyclic alkene structures are slightly harder to draw due to the number of carbon atoms in the ring and the geometric constraints imposed by the C=C system, but the structural drawings are even more informative than they are for alkane structures.

Formation of Alkenes

The simple alkenes do not occur naturally in appreciable quantities. They are instead produced by dehydration reactions that eliminate the components of water molecules from adjacent carbon atoms in alcohols. Ethanol, for example, can be dehydrated to produce ethylene and water, according to the equation

H₃C–CH₂–OH → H₂C=CH₂ + H₂O

Other alkenes are produced in quantity by similar methods. However, the processes are not selective and will generally produce mixtures of alkene isomers rather than single compounds. The processes used are reversible under the conditions used, requiring that the product water and alkenes must be separated from each other as soon as possible. This is usually not a significant problem due to the typically broad difference in boiling points of water and the lower alkenes. In laboratory procedures, specific methods are used to generate C=C bonds in a particular molecular structure. Controlled dehydration reactions can be used to eliminate water molecules from a molecule containing a suitable –OH group. Dehydrohalogenation can be used to eliminate the hydrogen halide molecules when a suitable –X substituent (X = chlorine, bromine, or iodine) is present. Another method is dehalogenation, which eliminates X₂ from molecules containing two halogen atoms on adjacent carbon atoms. Many other specific synthetic methods can also be used in which a leaving group is eliminated from one carbon atom and another substituent from an adjacent atom. These methods are effective when the leaving group and its companion can form a very stable compound that does not react with the C=C bond.

Reactions of Alkenes

Alkenes are reactive compounds. They readily undergo reduction reactions such as hydrogenation, in which a hydrogen atom is bonded to each of the two carbon atoms of the C=C bond. This is typically carried out using a metal catalyst. Alkenes are also readily and rapidly oxidized by good oxidizing agents, such as ozone (O₃) and permanganate ion (MnO₄⁻). Electrophilic addition reactions such as halogenation and hydrohalogenation are also facile reactions with alkenes. The C=C bond forms a flattened region in a molecular structure that exposes the electron-rich region, facilitating bond formation with an electrophile (a species attracted to electrons). Simple alkenes are thus able to readily undergo polymerization reactions with ease. Ethylene and stryrene, for example, readily accept an electrophile and subsequently enter into a chain reaction that produces polyethylene and polystyrene, respectively. As hydrocarbons, alkenes are highly combustible, and the extra energy of the C=C bond increases the amount of energy released in combustion and other oxidation reactions. Their reactivity precludes their use as solvents, except when the presence of several C=C bonds in a cyclic structure, as in benzene, makes the compound extremely stable.

PRINCIPAL TERMS

• dehydration reaction: a chemical reaction in which hydrogen and oxygen atoms are removed from the reactants and combine to form water.
• double bond: a type of chemical bond in which two adjacent atoms are connected by four bonding electrons rather than two.
• functional group: a specific group of atoms with a characteristic structure and corresponding chemical behavior within a molecule.
• hydrogenation: a chemical reaction in which two hydrogen atoms, usually in the form of molecular hydrogen (H₂), are bonded to another molecule, almost always as a result of catalysis.
• 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.
• unsaturated: describes an organic compound in which carbon atoms are attached to other atoms via double or triple bonds, preventing the compound from containing the maximum possible number of hydrogen atoms.

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