In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond. The simplest acyclic alkenes, with only one double bond and no other functional groups, form a homologous series of hydrocarbons with the general formula CnH2n.
The simplest alkene is ethylene (C2H4), which has the International Union of Pure and Applied Chemistry (IUPAC) name ethene. Alkenes are also called olefins (an archaic synonym, widely used in the petrochemical industry). Aromatic compounds are often drawn as cyclic alkenes, but their structure and properties are different and they are not considered to be alkenes.
Like single covalent bonds, double bonds can be described in terms of overlapping atomic orbitals, except that unlike a single bond (which consists of a single sigma bond), a carbon-carbon double bond consists of one sigma bond and one pi bond. This double bond is stronger than a single covalent bond (611 kJ/mol for C=C vs. 347 kJ/mol for C—C) and also shorter with an average bond length of 1.33 Angstroms (133 pm).
Each carbon 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 and half on the other.
Rotation about the carbon-carbon double bond is restricted because it involves breaking the pi bond, which requires a large amount of energy (264 kJ/mol in ethylene). As a consequence substituted alkenes may exist as one of two isomers called a cis isomer and a trans isomer, or alternatively (for more complex alkenes) a Z and a E isomer. For example, in cis-but-2-ene the two methyl substituents face the same side of the double bond and in trans-but-2-ene they face the opposite side; these two isomers are slightly different in their chemical and physical properties.
It is certainly not impossible to twist a double bond. In fact, a 90° twist requires an energy approximately equal to half the strength of a pi bond. The misalignment of the p orbitals is less than expected because pyramidalization takes place (See: pyramidal alkene). trans-Cyclooctene is a stable strained alkene and the orbital misalignment is only 19° with a dihedral angle of 137° (normal 120°) and a degree of pyramidalization of 18°. This explains the dipole moment of 0.8 D for this compound (cis-isomer 0.4 D) where a value of zero is expected. The trans isomer of cycloheptene is only stable at low temperatures.
As predicted by the VSEPR
model of electron
pair repulsion, the molecular geometry
of alkenes includes bond angles
about each carbon in a double bond of about 120°. The angle may vary because of steric strain
introduced by nonbonded interactions
created by functional groups
attached to the carbons of the double bond. For example, the C-C-C bond angle in propylene
The physical properties of alkenes are comparable with alkanes
. The physical state
depends on molecular mass
(gases from ethene to butene - liquids from pentene onwards). The simplest alkenes, ethylene
are gases. Linear alkenes of approximately five to sixteen carbons are liquids, and higher alkenes are waxy solids.
Alkenes are relatively stable compounds, but are more reactive than alkanes
due to the presence of a carbon-carbon pi-bond. The majority of the reactions of alkenes involve the rupture of this pi bond, forming new single bonds
Alkenes serve as a feedstock for the petrochemical industry because they can participate in a wide variety of reactions.
Alkenes react in many addition reactions
, which occur by opening up the double-bond.
- CH2=CH2 + H2 → CH3-CH3
- CH2=CH2 + Br2 → BrCH2-CH2Br
- It is also used as a quantitive test of unsaturation, expressed as the bromine number of a single compound or mixture. The reaction works because the high electron density at the double bond causes a temporary shift of electrons in the Br-Br bond causing a temporary induced dipole. This makes the Br closest to the double bond slightly positive and therefore an electrophile.
- CH3-CH=CH2 + HBr → CH3-CHBr-CH2-H
- If the two carbon atoms at the double bond are linked to a different number of hydrogen atoms, the halogen is found preferentially at the carbon with fewer hydrogen substituents (Markovnikov's rule).
- This is the reaction mechanism for hydrohalogenation:
Alkenes are oxidized
with a large number of oxidizing agents
- R1-CH=CH-R2 + O3 → R1-CHO + R2-CHO + H2O
- This reaction can be used to determine the position of a double bond in an unknown alkene.
of alkenes is an economically important reaction which yields polymers
of high industrial value, such as the plastics polyethylene
. Polymerization can either proceed via a free-radical
or an ionic mechanism.
The most common industrial synthesis of alkenes is based on cracking
. Large alkanes are broken apart at high temperatures, often in the presence of a zeolite
catalyst, to give alkenes and smaller alkanes, and the mixture of products is then separated by fractional distillation. This is mainly used for the manufacture of small alkenes (up to six carbons).
Related to this is catalytic dehydrogenation, where an alkane loses hydrogen at high temperatures to produce a corresponding alkene. This is the reverse of the catalytic hydrogenation of alkenes.
Both of these processes are endothermic, but they are driven towards the alkene at high temperatures by entropy (the TΔS portion of the equation ΔG = ΔH – TΔS dominates for high T).
Catalytic synthesis of higher α-alkenes (of the type RCH=CH2) can also be achieved by a reaction of ethylene with the organometallic compound triethylaluminium in the presence of nickel, cobalt or platinum.
One of the principal methods for alkene synthesis in the laboratory is the elimination
of alkyl halides, alcohols and similar compounds. Most common is the -elimination via the E2 or E1 mechanism, but -eliminations are also known.
The E2 mechanism provides a more reliable -elimination method than E1 for most alkene syntheses. Most E2 eliminations start with an alkyl halide or alkyl sulfonate ester (such as a tosylate or triflate). When an alkyl halide is used, the reaction is called a dehydrohalogenation. For unsymmetrical products the more substituted alkenes (those with fewer hydrogens attached to the C=C) tend to predominate (see Saytzeff's rule).Two common methods of elimination reactions are dehydrohalogenation of alkyl halides and dehydration of alcohols. A typical example is shown below; note that the H that leaves must be anti to the leaving group, even though this leads to the less stable Z-isomer.
Alkenes can be synthesized from alcohols via dehydration, in which case water is lost via the E1 mechanism. For example, the dehydration of ethanol produces ethene:
- CH3CH2OH + H2SO4 → H2C=CH2 + H3O+ + HSO4−
An alcohol may also be converted to a better leaving group (e.g., xanthate), so as to allow a milder syn-elimination such as the Chugaev elimination and the Grieco elimination. Related reactions include eliminations by β-haloethers (the Boord olefin synthesis) and esters (ester pyrolysis).
Alkenes can be prepared indirectly from alkyl amines. The amine or ammonia is not a suitable leaving group, so the amine is first either alkylated (as in the Hofmann elimination) or oxidized to an amine oxide (the Cope reaction) to render a smooth elimination possible. Hofmann elimination is unusual in that the less substituted (non-Saytseff) alkene is usually the major product. The Cope reaction is a syn-elimination that occurs at or below 150 °C, for example:
Alkenes are generated from α-halo sulfones in the Ramberg-Bäcklund reaction, via a three-membered ring sulfone intermediate.
Synthesis from carbonyl compounds
Another important method for alkene synthesis involves construction of a new carbon-carbon double bond by coupling of a carbonyl compound (such as an aldehyde
) to a carbanion
equivalent. Such reactions are sometimes called olefinations
. The most well-known of these methods is the Wittig reaction, but other related methods are known.
The Wittig reaction involves reaction of an aldehyde or ketone with a Wittig reagent (or phosphorane) of the type Ph3P=CHR to produce an alkene and Ph3P=O. The Wittig reagent is itself prepared easily from triphenylphosphine and an alkyl halide. The reaction is quite general and many functional groups are tolerated, even esters, as in this example:
Related to the Wittig reaction is the Peterson olefination. This uses a less accessible silicon-based reagent in place of the phosphorane, but it allows for the selection of E or Z products. If an E-product is desired, another alternative is the Julia olefination, which uses the carbanion generated from a phenyl sulfone. The Takai olefination based on an organochromium intermediate also delivers E-products. A titanium compound, Tebbe's reagent, is useful for the synthesis of methylene compounds; in this case, even esters and amides react.
A pair of carbonyl compounds can also be reductively coupled together (with reduction) to generate an alkene. Symmetrical alkenes can be prepared from a single aldehyde or ketone coupling with itself, using Ti metal reduction (the McMurry reaction). If two different ketones are to be coupled, a more complex, indirect method such as the Barton-Kellogg reaction may be used.
A single ketone can also be converted to the corresponding alkene via its tosylhydrazone, using sodium methoxide (the Bamford-Stevens reaction) or an alkyllithium (the Shapiro reaction).
Alkenes can be prepared by exchange with other alkenes, in a reaction known as olefin metathesis
. Frequently loss of ethene gas is used to drive the reaction towards a desired product. In many cases, a mixture of geometric isomers is obtained, but the reaction tolerates many functional groups. The method is particularly effective for the preparation of cyclic alkenes, as in this synthesis of muscone
Use of palladium-catalyzed coupling reactions
, most notably those catalyzed by palladium
compounds, have become popular for the synthesis of alkenes. The Heck reaction
provides a method for coupling an aryl halide to an alkene, for example in the synthesis of the pharmaceutical naproxen
Other couplings, such as the Stille, Suzuki and Negishi involve the reaction of an alkenyl, allyl or aryl halide (or triflate) with an alkenyl, alkyl (not for Stille) or aryl derivative of a metal or metalloid. For example, Suzuki coupling has been used on a citronellal derivative for the synthesis of caparratriene, a natural product which is highly active against leukemia:
Reduction of alkynes
is a useful method for the stereoselective
synthesis of disubstituted alkenes. If the cis
-alkene is desired, hydrogenation
in the presence of Lindlar's catalyst
is commonly used, though hydroboration followed by hydrolysis provides an alternative approach. Reduction of the alkyne by sodium
metal in liquid ammonia
gives the trans
For the preparation multisubstituted alkenes, carbometalation of alkynes can give rise to a large variety of alkene derivatives.
Rearrangements and related reactions
Alkenes can be synthesized from other alkenes via rearrangement reactions
. Besides olefin metathesis
), a large number of pericyclic reactions
can be used such as the ene reaction
and the Cope rearrangement
In the Diels-Alder reaction, a cyclohexene derivative is prepared from a diene and a reactive or electron-deficient alkene.
To form the root of the IUPAC
names for alkenes, simply 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
In higher alkenes, where isomers exist that differ in location of the double bond, the following numbering system is used:
- Number the longest carbon chain that contains the double bond in the direction that gives the carbon atoms of the double bond the lowest possible numbers.
- Indicate the location of the double bond by the location of its first carbon
- Name branched or substituted alkenes in a manner similar to alkanes.
- Number the carbon atoms, locate and name substituent groups, locate the double bond, and name the main chain
The Cis-Trans notation
In the specific case of disubstituted alkenes where the two carbons have one substituent each, Cis-trans
notation may be used. If both substituents are on the same side of the bond, it's defined as (cis-). If the substituents are on either side of the bond, it's defined as (trans-).
The E,Z notation
When an alkene has more than one substituent (especially necessary with 3 or 4 substituents), the double bond geometry is described using the labels E
. These labels come from the German words "entgegen" meaning "opposite" and "zusammen" meaning "together". Alkenes with the higher priority groups (as determined by CIP rules
) on the same side of the double bond have these groups together and are designated Z
. Alkenes with the higher priority groups on opposite sides are designated E
. A mnemonic to remember this: Z notation has the higher priority groups on "ze zame zide".
Groups containing C=C double bonds
IUPAC recognizes two names for hydrocarbon groups containing carbon-carbon double bonds, the vinyl
group and the allyl