Alkene




 

In hydrocarbons with the general formula CnH2n. [2]

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 Aromatic compounds are often drawn as cyclic alkenes, but their structure and properties are different and they are not considered to be alkenes.[2]

Structure

Bonding

Like single 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 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 and chemical 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 cycloheptene is only stable at low temperatures.

Shape

As predicted by the propylene is 123.9°.

Physical properties

The physical properties of alkenes are comparable with butylene are gases. Linear alkenes of approximately five to sixteen carbons are liquids, and higher alkenes are waxy solids.

Chemical properties

Alkenes are relatively stable compounds, but are more reactive than single bonds.

Alkenes serve as a feedstock for the petrochemical industry because they can participate in a wide variety of reactions.

Addition reactions

Alkenes react in many addition reactions, which occur by opening up the double-bond.

  • Catalytic addition of hydrogen: ethane:
CH2=CH2 + H2 → CH3-CH3
  • carbonyl group.
  • vicinal dibromo- and dichloroalkanes, respectively. The decoloration of a solution of bromine in water is an analytical test for the presence of alkenes:
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:

Oxidation

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.

Polymerization

radical or an ionic mechanism.

Synthesis

Industrial methods

The most common industrial synthesis of alkenes is based on 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).[1]

Related to this is catalytic dehydrogenation, where an alkane loses hydrogen at high temperatures to produce a corresponding alkene. [1] 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).

platinum.

Elimination reactions

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, [4] 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 Saytzeff's rule). 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.[5]

Alkenes can be synthesized from alcohols via ethanol produces ethene:

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 ester pyrolysis).

Alkenes can be prepared indirectly from alkyl Saytseff) alkene is usually the major product. The Cope reaction is a syn-elimination that occurs at or below 150 °C, for example:[6]

Alkenes are generated from α-halo 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 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 triphenylphosphine and an alkyl halide. The reaction is quite general and many functional groups are tolerated, even esters, as in this example:[7]

Related to the Wittig reaction is the 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 Barton-Kellogg reaction may be used.

A single ketone can also be converted to the corresponding alkene via its tosylhydrazone, using sodium methoxide (the Shapiro reaction).

Olefin metathesis

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

naproxen:

Other couplings, such as the metalloid. For example, Suzuki coupling has been used on a citronellal derivative for the synthesis of capparatriene, a natural product which is highly active against leukemia:[9]

From alkynes

Reduction of ammonia gives the trans-alkene.[10]

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 Cope rearrangement.

In the Diels-Alder reaction, a cyclohexene derivative is prepared from a diene and a reactive or electron-deficient alkene.

Nomenclature

IUPAC Names

To form the root of 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:

  1. 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.
  2. Indicate the location of the double bond by the location of its first carbon
  3. Name branched or substituted alkenes in a manner similar to alkanes.
  4. Number the carbon atoms, locate and name substituent groups, locate the double bond, and name the main chain
 


The Cis-Trans notation

Main article: Cis-trans isomerism

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

Main article: 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 and Z. 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 allyl group. .[2]

See also

References

  1. ^ a b c d Wade, L.G. (Sixth Ed., 2006). Organic Chemistry. Pearson Prentice Hall, 279. 
  2. ^ a b c Moss, G. P.; Smith, P. A. S. (1995). "Glossary of Class Names of Organic Compounds and Reactive Intermediates Based on Structure (IUPAC Recommendations 1995)". Pure and Applied Chemistry 67: 1307-1375.
  3. ^ Barrows, Susan E.; Eberlein, Thomas H. (2005). "Understanding Rotation about a C=C Double Bond". J. Chem. Educ. 82: 1329.
  4. ^ Saunders, W. H. (1964). in Patai, Saul: The Chemistry of Alkenes. Wiley Interscience, 149-150. 
  5. ^ Cram, D.J.; Greene, F.D.; Depuy, C.H. (1956). "Studies in Stereochemistry. XXV. Eclipsing Effects in the E2 Reaction1". Journal of the American Chemical Society 78 (4): 790-796. Retrieved on 2008-01-02.
  6. ^ Bach, R.D.; Andrzejewski, D.; Dusold, L.R. (1973). "Mechanism of the Cope elimination". J. Org. Chem. 38: 1742-3.
  7. ^ Snider, Barry B.; Gao, X.; Matsuo, Y. (2006). "Synthesis of ent-Thallusin". Org. Lett. 8: 2123-6. doi:10.1021/ol0605777.
  8. ^ Zweifel, George S.; Nantz, Michael H. (2007). Modern Organic Synthesis: An Introduction. New York: W. H. Freeman & Co., 322-339. 
  9. ^ Vyvyan, J.R.; Peterson, E.A.; Stephan, M.L. (1999). "An expedient total synthesis of (+/-)-caparratriene". Tetrahedron Letters 40 (27): 4947-4949. Retrieved on 2008-01-02.
  10. ^ Zweifel, George S.; Nantz, Michael H. (2007). Modern Organic Synthesis: An Introduction. New York: W. H. Freeman & Co., 366. 
v • d • e
Alkenes
Ethylene ( C2H4 ) • Butylene ( C4H8 ) • Pentylene ( C5H10 ) • Hexylene ( C6H12 )
v • d • e
Thiol
 
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