Alkene
In
The International Union of Pure and Applied Chemistry (IUPAC) 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.[2][3][4]
Acyclic alkenes, with only one double bond and no other functional groups (also known as mono-enes) form a homologous series of hydrocarbons with the general formula CnH2n with n being a >1 natural number (which is two hydrogens less than the corresponding alkane). When n is four or more, isomers are possible, distinguished by the position and conformation of the double bond.
Alkenes are generally colorless
Structural 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:[6]
- C2H4: ethylene only
- C3H6: propylene only
- C4H8: 3 isomers: 2-butene, and isobutylene
- C5H10: 5 isomers: 1-pentene, 2-pentene, 2-methyl-1-butene, 3-methyl-1-butene, 2-methyl-2-butene
- C6H12: 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 (from C5). The number of potential isomers increases rapidly with additional carbon atoms.
Structure and bonding
Bonding
A carbon–carbon double bond consists of a
Each carbon atom of the double bond uses its three sp2
Rotation about the carbon–carbon double bond is restricted because it incurs an energetic cost to break the alignment of the
Shape
As predicted by the
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.[8] Following Fawcett and defining S as the total number of non-bridgehead atoms in the rings,[9] bicyclic systems require S ≥ 7 for stability[8] and tricyclic systems require S ≥ 11.[10]
Isomerism
In
-
structure of cis-2-butene
-
structure of trans-2-butene
-
(E)-But-2-ene
-
(Z)-But-2-ene
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 (together) and entgegen (opposite). 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 (i.e. the two groups with higher priority are on the opposite side of the double bond), the bond is assigned E- configuration. Cis- and trans- configurations do not have a fixed relationship with E- and Z-configurations.
Physical properties
Many of the physical properties of alkenes and
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.[11]
Boiling and melting points
Below is a list of the boiling and melting points of various alkenes with the corresponding alkane and alkyne analogues.[12][13]
Number of carbons |
Alkane | Alkene | Alkyne | |
---|---|---|---|---|
2 | Name | ethane | ethylene | acetylene |
Melting point | −183 | −169 | −80.7 | |
Boiling point | −89 | −104 | −84.7 | |
3 | Name | propane | propylene | propyne |
Melting point | −190 | −185 | −102.7 | |
Boiling point | −42 | −47 | −23.2 | |
4 | Name | butane | 1-butene | 1-butyne |
Melting point | −138 | −185.3 | −125.7 | |
Boiling point | −0.5 | −6.2 | 8.0 | |
5 | Name | pentane | 1-pentene | 1-pentyne |
Melting point | −130 | −165.2 | −90.0 | |
Boiling point | 36 | 29.9 | 40.1 |
Infrared spectroscopy
The stretching of C=C bond will give an
NMR spectroscopy
In 1H
In their 13C NMR spectra of alkenes, double bonds also deshield the carbons, making them have low field shift. C=C double bonds usually have chemical shift of about 100–170 ppm.[15]
Combustion
Like most other hydrocarbons, alkenes combust to give carbon dioxide and water.
The combustion of alkenes release less energy than burning same
Number of carbons |
Substance | Type | Formula | Hcø (kJ/mol) |
---|---|---|---|---|
2 | ethane | saturated | C2H6 | −1559.7 |
ethylene | unsaturated | C2H4 | −1410.8 | |
acetylene | unsaturated | C2H2 | −1300.8 | |
3 | propane | saturated | CH3CH2CH3 | −2219.2 |
propene
|
unsaturated | CH3CH=CH2 | −2058.1 | |
propyne | unsaturated | CH3C≡CH | −1938.7 | |
4 | butane | saturated | CH3CH2CH2CH3 | −2876.5 |
1-butene
|
unsaturated | CH2=CH−CH2CH3 | −2716.8 | |
1-butyne
|
unsaturated | CH≡C-CH2CH3 | −2596.6 |
Reactions
Alkenes are relatively stable compounds, but are more reactive than
Addition to the unsaturated bonds
Hydrogenation involves the addition of H2 resulting in an alkane. The equation of hydrogenation of ethylene to form ethane is:
- H2C=CH2 + H2→H3C−CH3
Hydrogenation reactions usually require
Similar to hydrogen, halogens added to double bonds.
- H2C=CH2 + Br2→H2CBr−CH2Br
Halonium ions are intermediates. These reactions do not require catalysts.
Bromine test is used to test the saturation of hydrocarbons.[17] The bromine test can also be used as an indication of the degree of unsaturation for unsaturated hydrocarbons. Bromine number is defined as gram of bromine able to react with 100g of product.[18] Similar as hydrogenation, the halogenation of bromine is also depend on the number of π bond. A higher bromine number indicates higher degree of unsaturation.
The π bonds of alkenes hydrocarbons are also susceptible to
- H2C=CH2 + H2O→H3C-CH2OH
Hydrohalogenation involves addition of H−X to unsaturated hydrocarbons. This reaction results in new C−H and C−X σ bonds. The formation of the intermediate carbocation is selective and follows Markovnikov's rule. The hydrohalogenation of alkene will result in haloalkane. The reaction equation of HBr addition to ethylene is:
- H2C=CH2 + HBr → H3C−CH2Br
Cycloaddition
Alkenes add to
Oxidation
Alkenes react with percarboxylic acids and even hydrogen peroxide to yield epoxides:
- RCH=CH2 + RCO3H → RCHOCH2 + RCO2H
For ethylene, the
- C2H4 + 1/2 O2 → C2H4O
Alkenes react with ozone, leading to the scission of the double bond. The process is called ozonolysis. Often the reaction procedure includes a mild reductant, such as dimethylsulfide (SMe2):
- RCH=CHR' + O3 + SMe2 → RCHO + R'CHO + O=SMe2
- R2C=CHR' + O3 → R2CHO + R'CHO + O=SMe2
When treated with a hot concentrated, acidified solution of KMnO4, alkenes are cleaved to form ketones and/or carboxylic acids. The stoichiometry of the reaction is sensitive to conditions. This reaction and the ozonolysis can be used to determine the position of a double bond in an unknown alkene.
The oxidation can be stopped at the vicinal diol rather than full cleavage of the alkene by using osmium tetroxide or other oxidants:
This reaction is called dihydroxylation.
In the presence of an appropriate
Polymerization
Terminal alkenes are precursors to
Allylic substitution
The presence of a C=C π bond in unsaturated hydrocarbons weakens the dissociation energy of the
Metathesis
Alkenes undergo olefin metathesis, which cleaves and interchanges the substituents of the alkene. A related reaction is ethenolysis:[23]
Metal complexation
In
Reaction overview
Reaction name | Product | Comment |
---|---|---|
Hydrogenation | alkanes | addition of hydrogen |
Hydroalkenylation | alkenes | hydrometalation / insertion / beta-elimination by metal catalyst |
Halogen addition reaction | 1,2-dihalide | electrophilic addition of halogens |
Hydrohalogenation (Markovnikov) | haloalkanes | addition of hydrohalic acids |
Anti-Markovnikov hydrohalogenation | haloalkanes | free radicals mediated addition of hydrohalic acids |
Hydroamination | amines | addition of N−H bond across C−C double bond |
Hydroformylation | aldehydes | industrial process, addition of CO and H2 |
Hydrocarboxylation and Koch reaction
|
carboxylic acid | industrial process, addition of CO and H2O. |
Carboalkoxylation | ester | industrial process, addition of CO and alcohol. |
alkylation | ester | industrial process: alkene alkylating carboxylic acid with silicotungstic acid the catalyst. |
Sharpless bishydroxylation
|
diols | oxidation, reagent: osmium tetroxide, chiral ligand |
Woodward cis-hydroxylation | diols | oxidation, reagents: iodine, silver acetate |
Ozonolysis | aldehydes or ketones | reagent: ozone |
Olefin metathesis | alkenes | two alkenes rearrange to form two new alkenes |
Diels–Alder reaction | cyclohexenes | cycloaddition with a diene |
Pauson–Khand reaction | cyclopentenones | cycloaddition with an alkyne and CO |
Hydroboration–oxidation
|
alcohols | reagents: borane, then a peroxide |
Oxymercuration-reduction
|
alcohols | electrophilic addition of mercuric acetate, then reduction |
Prins reaction | 1,3-diols | electrophilic addition with aldehyde or ketone |
Paterno–Büchi reaction
|
oxetanes | photochemical reaction with aldehyde or ketone |
Epoxidation
|
epoxide | electrophilic addition of a peroxide |
Cyclopropanation | cyclopropanes | addition of carbenes or carbenoids |
Hydroacylation | ketones | oxidative addition / reductive elimination by metal catalyst |
Hydrophosphination | phosphines |
Synthesis
Industrial methods
Alkenes are produced by hydrocarbon cracking. Raw materials are mostly natural-gas condensate components (principally ethane and propane) in the US and Mideast and naphtha in Europe and Asia. Alkanes are broken apart at high temperatures, often in the presence of a zeolite catalyst, to produce a mixture of primarily aliphatic alkenes and lower molecular weight alkanes. The mixture is feedstock and temperature dependent, and separated by fractional distillation. This is mainly used for the manufacture of small alkenes (up to six carbons).[25]
Related to this is catalytic
This process is also known as reforming. Both processes are endothermic and are driven towards the alkene at high temperatures by entropy.
.Elimination reactions
One of the principal methods for alkene synthesis in the laboratory is the elimination reaction of alkyl halides, alcohols, and similar compounds. Most common is the β-elimination via the E2 or E1 mechanism.[26] A commercially significant example is the production of vinyl chloride.
The E2 mechanism provides a more reliable β-elimination method than E1 for most alkene syntheses. Most E2 eliminations start with an alkyl halide or alkyl sulfonate ester (such as a
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 ethylene:
- CH3CH2OH → H2C=CH2 + H2O
An alcohol may also be converted to a better leaving group (e.g.,
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. The Cope reaction is a syn-elimination that occurs at or below 150 °C, for example:[28]
The Hofmann elimination is unusual in that the less substituted (non-
Alkenes are generated from α-halosulfones in the Ramberg–Bäcklund reaction, via a three-membered ring sulfone intermediate.
Synthesis from carbonyl compounds
Another important class of methods for alkene synthesis involves construction of a new carbon–carbon double bond by coupling or condensation of a carbonyl compound (such as an aldehyde or ketone) to a carbanion or its equivalent. Pre-eminent is the aldol condensation. Knoevenagel condensations are a related class of reactions that convert carbonyls into alkenes.Well-known methods are called olefinations. The Wittig reaction is illustrative, but other related methods are known, including the Horner–Wadsworth–Emmons reaction.
The Wittig reaction involves reaction of an aldehyde or ketone with a
Related to the Wittig reaction is the
A pair of ketones or aldehydes can be deoxygenated to generate an alkene. Symmetrical alkenes can be prepared from a single aldehyde or ketone coupling with itself, using titanium metal reduction (the McMurry reaction). If different ketones are to be coupled, a more complicated method is required, such as the Barton–Kellogg reaction.
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).
Synthesis from alkenes
The formation of longer alkenes via the step-wise polymerisation of smaller ones is appealing, as
Olefin metathesis is also used commercially for the interconversion of ethylene and 2-butene to propylene. Rhenium- and molybdenum-containing heterogeneous catalysis are used in this process:[30]
- CH2=CH2 + CH3CH=CHCH3 → 2 CH2=CHCH3
Transition metal catalyzed hydrovinylation is another important alkene synthesis process starting from alkene itself.[31] It involves the addition of a hydrogen and a vinyl group (or an alkenyl group) across a double bond.
From alkynes
Reduction of
For the preparation multisubstituted alkenes, carbometalation of alkynes can give rise to a large variety of alkene derivatives.
Alkenes can be synthesized from other alkenes via rearrangement reactions. Besides olefin metathesis (described above), many 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.
Application
Unsaturated hydrocarbons are widely used to produce plastics, medicines, and other useful materials.
Name | Structure | Use |
---|---|---|
Ethylene |
| |
1,3-butadiene |
| |
vinyl chloride |
| |
styrene |
|
Natural occurrence
Alkenes are pervasive in nature. Plants are the main natural source of alkenes in the form of
-
Limonene, a monoterpene.
-
taxol, an anticancer agent.
IUPAC 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.[2] Olefins comprise a larger collection of cyclic and acyclic alkenes as well as dienes and polyenes.[3]
To form the root of the
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:
- 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:
- Number the carbons in that chain starting from the end that is closest to the double bond.
- Define the location k of the double bond as being the number of its first carbon.
- Name the side groups (other than hydrogen) according to the appropriate rules.
- Define the position of each side group as the number of the chain carbon it is attached to.
- Write the position and name of each side group.
- 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 (e.g. "2-pentene"), rather than before the suffix ("pent-2-ene").
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: (H
3C)
3C–CH
2–CH
3 is "2,2-dimethyl pentane", whereas (H
3C)
3C–CH=CH
2 is "3,3-dimethyl 1-pentene".
More complex rules apply for polyenes and cycloalkenes.[4]
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- (from Latin "on this side of") or trans- ("across", "on the other side of") before the name, respectively; as in cis-2-pentene or trans-2-butene.
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
Groups containing C=C double bonds
IUPAC recognizes two names for hydrocarbon groups containing carbon–carbon double bonds, the
See also
- Alpha-olefin
- Annulene
- Aromatic hydrocarbon("Arene")
- Dendralene
- Nitroalkene
- Radialene
Nomenclature links
- Rule A-3. Unsaturated Compounds and Univalent Radicals IUPAC Blue Book.
- Rule A-4. Bivalent and Multivalent Radicals IUPAC Blue Book.
- Rules A-11.3, A-11.4, A-11.5 Unsaturated monocyclic hydrocarbons and substituents IUPAC Blue Book.
- Rule A-23. Hydrogenated Compounds of Fused Polycyclic Hydrocarbons IUPAC Blue Book.
References
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- ^ ISBN 978-0-07-462083-0.
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- ^ Nguyen, Trung; Clark, Jim (23 April 2019). "Physical Properties of Alkenes". Chemistry LibreTexts. Retrieved 27 May 2019.
- ^ Ophardt, Charles (2003). "Boiling Points and Structures of Hydrocarbons". Virtual Chembook. Retrieved 27 May 2019.
- ^ Hanson, John. "Overview of Chemical Shifts in H-NMR". ups.edu. Retrieved 5 May 2019.
- ^ a b "Nuclear Magnetic Resonance (NMR) of Alkenes". Chemistry LibreTexts. 23 April 2019. Retrieved 5 May 2019.
- ^ "Organic Compounds: Physical and Thermochemical Data". ucdsb.on.ca. Retrieved 5 May 2019.
- ISBN 0-471-59748-1.
- ^ "Bromine Number". Hach company. Retrieved 5 May 2019.
- ^ Clark, Jim (November 2007). "The Mechanism for the Acid Catalysed Hydration of Ethene". Chemguide. Retrieved 6 May 2019.
- ISBN 978-0-471-72091-1
- PMID 28084040.
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- ^ ISBN 978-1-938787-15-7.
- ^ Toreki, Rob (31 March 2015). "Alkene Complexes". Organometallic HyperTextbook. Retrieved 29 May 2019.
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