FACTOID # 20: Statistically, Delaware bears more cost of the US Military than any other state.
 
 Home   Encyclopedia   Statistics   States A-Z   Flags   Maps   FAQ   About 
   
 
WHAT'S NEW
RELATED ARTICLES
People who viewed "Alkenes" also viewed:
 

SEARCH ALL

FACTS & STATISTICS    Advanced view

Search encyclopedia, statistics and forums:

 

 

(* = Graphable)

 

 


Encyclopedia > Alkenes
Alkenes
Image:ethene.png
Properties
General formula CnH2n


An alkene is one of the three classes of unsaturated hydrocarbons that contain at least one carbon-carbon double bond and have the general molecular formula of CnH2n (the other two being alkynes and arenes).


The simplest alkene is C2H4, which has the common name "ethylene" and the IUPAC name "ethene".



Contents

Structure of Alkenes

Shape of Alkenes

As predicted by the VSEPR model of electron pair replusion (see covalent bond), the bond angles about each carbon in a double bond are about 120, although the angle may be larger because of strain introduced by nonbonded interactions created by groups attached to the carbons of the double bond. For example, the C-C-C bond angle in propene (propylene) is 123.9.


See also: molecular geometry


Molecular Geometry Carbon-Carbon Double Bond

Like single covalent bonds, double bonds can be described in terms of overlapping atomic orbitals, except that unlike a single bond (which consist of a single sigma bond), a carbon-carbon double bond consists of one sigma bond and one pi bond.


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.


Because it requires a large amount of energy to break a pi bond (264 kJ/mol in ethylene), rotation about the carbon-carbon double bond is very difficult and therefore severely restricted.


See also: molecular geometry


Physical properties



Chemical properties

Alkenes are relatively stable compounds, but are more reactive than alkanes.




Reactions

Synthesis

  1. The most common industrial synthesis path for alkenes is cracking of petroleum.
  2. Alkenes can be synthesized from alcohols via an elimination reaction that removes one water molecule:
    H3C-CH2-OH + H2SO4 → H3C-CH2-O-SO3H + H2O → H2C=CH2 + H2SO4
  3. Catalytic synthesis of higher α-alkenes can be achieved by a reaction of ethene with triethylaluminium, an organometallic compound in the presence of nickel, cobalt or platinum.

Addition reactions

Catalytic addition of hydrogen

Catalytic hydrogenation of alkenes produce the corresponding alkanes. The reaction is carried out under pressure in the presence of a metallic catalyst. Common industrial catalysts are based on platinum, nickel or palladium, for laboratory syntheses, Raney's nickel is often employed. This is an alloy of nickel and aluminium.


This is the catalytic hydrogenation of ethylene to yield ethane:


CH2=CH2 + H2 → CH3-CH3


Electrophilic addition

Most addition reactions to alkenes follow the mechanism of electrophilic addition.

  1. Halogenation: Addition of elementary bromine or chlorine to alkenes yield vicinal Dibromo- and dichloroalkenes, respectively. The decoloration of a solution of bromine in water is an analytical test for the presence of alkenes:
    CH2=CH2 + Br2 → BrCH2-CH2Br
  2. Hydrohalogenation: Addition of hydrohalic acids like HCl or HBr to alkenes yield the corresponding haloalkanes.
    CH3-CH=CH2 + HBr → CH3-CHBr-CH3
    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 less hydrogen substituents (Markovnikov's rule).

Oxidation

  1. In the presence of oxygen, alkenes burn with a bright flame to carbon dioxide and water.
  2. Catalytic oxidation with oxygen or the reaction with percarboxylic acids yields epoxides
  3. Reaction with ozone leads to the breaking of the double bond, yielding two aldehydes or ketones
    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.

Polymerisation

Polymerization of alkenes is an economically important reaction which yields polymers of high industrial value, such as the plastics polyethylene and polypropylene. Polymerization can either proceed via a free_radical or an ionic mechanism. For detail regarding the reaction mechanisms, see the polymerization article.


Nomenclature of Alkenes

IUPAC Names

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:

  1. Number the longest carbon chain the 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

CH3CH2CH2CH2CH==CH2
6  5  4  3  2   1

1-Hexene

      CH3
      |
CH3CH2CH2CH2CH==CH2
6  5  4  3  2   1

4-Methyl-1-hexene

      CH3
      |
CH3CH2CH2CH2CH==CH2
6  5  4  3  |2   1
            CH2CH3

2-Ethyl-4-methyl-1-hexene

Common Names

Despite the precision and universal acceptance of the IUPAC naming system, some alkenes are known almost exclusively by their common names:

CH2=CH2 CH3CH=CH2 CH3C(CH3)=CH2
IUPAC name: Ethene Propene 2-Methylpropene
Common name: Ethylene Propylene Isobutylene

See also:





  Results from FactBites:
 
alkene – FREE alkene Information | Encyclopedia.com: Facts, Pictures, Information! (915 words)
Two alkenes, 1-butene and 2-butene, are related to butane; these two compounds, which differ in the location of the double bond in their molecules, are structural isomers.
Many of the reactions in which alkenes take part involve the cleavage of half the carbon-carbon double bond and subsequent formation of two single bonds, one to each of the adjacent carbon atoms.
Alkenes are made by the dehydration of alcohols, and are used as...
alkene. The Columbia Encyclopedia, Sixth Edition. 2001-05 (282 words)
Two alkenes, 1-butene and 2-butene, are related to butane; these two compounds, which differ in the location of the double bond in their molecules, are structural isomers.
The alkenes as a group are sometimes called the ethylene series.
Many of the reactions in which alkenes take part involve the cleavage of half the carbon-carbon double bond and subsequent formation of two single bonds, one to each of the adjacent carbon atoms.
  More results at FactBites »

 
 

COMMENTARY     


Share your thoughts, questions and commentary here
Your name
Your comments

Want to know more?
Search encyclopedia, statistics and forums:

 


Press Releases |  Feeds | Contact
The Wikipedia article included on this page is licensed under the GFDL.
Images may be subject to relevant owners' copyright.
All other elements are (c) copyright NationMaster.com 2003-5. All Rights Reserved.
Usage implies agreement with terms, 1022, m