Alkene


In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as α-olefins.
The International Union of Pure and Applied Chemistry recommends using the name "alkene" only for acyclic hydrocarbons with just one double bond; alkadiene, alkatriene, etc., or polyene for acyclic hydrocarbons with two or more double bonds; cycloalkene, cycloalkadiene, etc. for cyclic ones; and "olefin" for the general class – cyclic or acyclic, with one or more double bonds.
Acyclic alkenes, with only one double bond and no other functional groups form a homologous series of hydrocarbons with the general formula with n being a >1 natural number. When n is four or more, isomers are possible, distinguished by the position and conformation of the double bond.
Alkenes are generally colorless non-polar compounds, somewhat similar to alkanes but more reactive. The first few members of the series are gases or liquids at room temperature. The simplest alkene, ethylene is the organic compound produced on the largest scale industrially.
Aromatic compounds are often drawn as cyclic alkenes, however their structure and properties are sufficiently distinct that they are not classified as alkenes or olefins. Hydrocarbons with two overlapping double bonds are called allenes—the simplest such compound is itself called allene—and those with three or more overlapping bonds are called cumulenes.

Structure and bonding

Bonding

A carbon–carbon double bond consists of a sigma bond and a pi bond. This double bond is stronger than a single covalent bond, but not twice as strong. Double bonds are shorter than single bonds with an average bond length of 1.33 Å vs 1.53 Å for a typical C-C single bond.
Each carbon atom of the double bond uses its three sp2 hybrid orbitals to form sigma bonds to three atoms. The unhybridized 2p atomic orbitals, which lie perpendicular to the plane created by the axes of the three sp2 hybrid orbitals, combine to form the pi bond. This bond lies outside the main C–C axis, with half of the bond on one side of the molecule and a half on the other. With a strength of 65 kcal/mol, the pi bond is significantly weaker than the sigma bond.
Rotation about the carbon–carbon double bond is restricted because it incurs an energetic cost to break the alignment of the p orbitals on the two carbon atoms. Consequently cis or trans isomers interconvert so slowly that they can be freely handled at ambient conditions without isomerization. More complex alkenes may be named with the E–''Z'' notation for molecules with three or four different substituents. For example, of the isomers of butene, the two methyl groups of -but-2-ene appear on the same side of the double bond, and in -but-2-ene the methyl groups appear on opposite sides. These two isomers of butene have distinct properties.

Shape

As predicted by the VSEPR model of electron pair repulsion, the molecular geometry of alkenes includes bond angles about each carbon atom in a double bond of about 120°. The angle may vary because of steric strain introduced by nonbonded interactions between functional groups attached to the carbon atoms of the double bond. For example, the C–C–C bond angle in propylene is 123.9°.
For bridged alkenes, Bredt's rule states that a double bond cannot occur at the bridgehead of a bridged ring system unless the rings are large enough. Following Fawcett and defining S as the total number of non-bridgehead atoms in the rings, bicyclic systems require S ≥ 7 for stability and tricyclic systems require S ≥ 11.

Isomerism

In organic chemistry, the prefixes cis- and trans- are used to describe the positions of functional groups attached to carbon atoms joined by a double bond. In Latin, cis and trans mean "on this side of" and "on the other side of" respectively. Therefore, if the functional groups are both on the same side of the carbon chain, the bond is said to have cis- configuration, otherwise, the bond is said to have trans- configuration.
For there to be cis- and trans- configurations, there must be a carbon chain, or at least one functional group attached to each carbon is the same for both. E- and Z- configuration can be used instead in a more general case where all four functional groups attached to carbon atoms in a double bond are different. E- and Z- are abbreviations of German words zusammen and entgegen. In E- and Z-isomerism, each functional group is assigned a priority based on the Cahn–Ingold–Prelog priority rules. If the two groups with higher priority are on the same side of the double bond, the bond is assigned Z- configuration, otherwise, the bond is assigned E- configuration. Cis- and trans- configurations do not have a fixed relationship between E- and Z-configurations.

Isomerism

Alkenes having four or more carbon atoms can form diverse structural isomers. Most alkenes are also isomers of cycloalkanes. Acyclic alkene structural isomers with only one double bond follow:
  • : ethylene only
  • : propylene only
  • : 3 isomers: 1-butene, 2-butene, and isobutylene
  • : 5 isomers: 1-pentene, 2-pentene, 2-methyl-1-butene, 3-methyl-1-butene, 2-methyl-2-butene
  • : 13 isomers: 1-hexene, 2-hexene, 3-hexene, 2-methyl-1-pentene, 3-methyl-1-pentene, 4-methyl-1-pentene, 2-methyl-2-pentene, 3-methyl-2-pentene, 4-methyl-2-pentene, 2,3-dimethyl-1-butene, 3,3-dimethyl-1-butene, 2,3-dimethyl-2-butene, 2-ethyl-1-butene
Many of these molecules exhibit cis–''trans'' isomerism. There may also be chiral carbon atoms particularly within the larger molecules. The number of potential isomers increases rapidly with additional carbon atoms.

Nomenclature

Although the nomenclature is not followed widely, according to IUPAC, an alkene is an acyclic hydrocarbon with just one double bond between carbon atoms. Olefins comprise a larger collection of cyclic and acyclic alkenes as well as dienes and polyenes.
To form the root of the IUPAC names for straight-chain alkenes, change the -an- infix of the parent to -en-. For example, CH3-CH3 is the alkane ethANe. The name of CH2=CH2 is therefore ethENe.
For straight-chain alkenes with 4 or more carbon atoms, that name does not completely identify the compound. For those cases, and for branched acyclic alkenes, the following rules apply:
  1. Find the longest carbon chain in the molecule. If that chain does not contain the double bond, name the compound according to the alkane naming rules. Otherwise:
  2. Number the carbons in that chain starting from the end that is closest to the double bond.
  3. Define the location k of the double bond as being the number of its first carbon.
  4. Name the side groups according to the appropriate rules.
  5. Define the position of each side group as the number of the chain carbon it is attached to.
  6. Write the position and name of each side group.
  7. Write the names of the alkane with the same chain, replacing the "-ane" suffix by "k-ene".
The position of the double bond is often inserted before the name of the chain, rather than before the suffix.
The positions need not be indicated if they are unique. Note that the double bond may imply a different chain numbering than that used for the corresponding alkane: C–– is "2,2-dimethyl pentane", whereas C–= is "3,3-dimethyl 1-pentene".
More complex rules apply for polyenes and cycloalkenes.
Image:Alkene nomenclature.svg|550px|center|thumb|Naming substituted hex-1-enes

''Cis''–''trans'' isomerism

If the double bond of an acyclic mono-ene is not the first bond of the chain, the name as constructed above still does not completely identify the compound, because of cis–''trans isomerism. Then one must specify whether the two single C–C bonds adjacent to the double bond are on the same side of its plane, or on opposite sides. For monoalkenes, the configuration is often indicated by the prefixes cis- or trans- before the name, respectively; as in cis-2-pentene or trans-2-butene.
Image:Cis-trans example.svg|thumb|300px|center|The difference between cis- and trans- isomers
More generally, cis–trans isomerism will exist if each of the two carbons of in the double bond has two different atoms or groups attached to it. Accounting for these cases, the IUPAC recommends the more general E–Z notation, instead of the cis and trans prefixes. This notation considers the group with highest CIP priority in each of the two carbons. If these two groups are on opposite sides of the double bond's plane, the configuration is labeled E'' ; if they are on the same side, it is labeled Z. This labeling may be taught with mnemonic "Z means 'on ze zame zide'".
Image:EZalkenes2.png|400px|center|thumb|The difference between E and Z isomers

Groups containing C=C double bonds

IUPAC recognizes two names for hydrocarbon groups containing carbon–carbon double bonds, the vinyl group and the allyl group.
Image:AlkenylGroups.png|200px|center

Physical properties

Many of the physical properties of alkenes and alkanes are similar: they are colorless, nonpolar, and combustible. The physical state depends on molecular mass: like the corresponding saturated hydrocarbons, the simplest alkenes are gases at room temperature. Linear alkenes of approximately five to nineteen carbon atoms are liquids, and higher alkenes are waxy solids. The melting point of the solids also increases with increase in molecular mass.
Alkenes generally have stronger smells than their corresponding alkanes. Ethylene has a sweet and musty odor. Strained alkenes, in particular, like norbornene and trans-cyclooctene are known to have strong, unpleasant odors, a fact consistent with the stronger π complexes they form with metal ions including copper.