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Alkenes, alkynes, arenes

Alkanes have all single bonded carbons -

What about C2H4,  Cannot satisfy the octet rule with only single bonds.  Need a double bond:

Alkenes: Hydrocarbons with carbon-carbon double bonds are called alkenes.

Naming alkenes follows from naming of alkanes, but we need to specify the position of the double bond and change "ane" to "ene". 

The parent chain in an alkyl substituted alkene is the longest chain that contains the double bond.

Number the longest chain containing the double bond (parent chain) so that the double bond gets the lowest possible number.  The double bond gets priority over any alkyl substituents

Structure (hydrogens omitted)

Name

C=C

ethene

C=C-C

propene

C=C-C-C

1-butene

C-C=C-C

2-butene



cyclobutene

2-methyl-2-pentene

3,4-dimethyl-2-pentene

Bonding in alkenes - sp2 hybrid orbitals

We used sp3 hybrid orbitals (see a realistic picture of one sp3 orbital and of all three on one carbon) to explain bonding in alkanes.  How can we explain bonding in alkenes?

sp2 hydrid orbitals (see one sp2 hybrid orbital) - two 2p orbitals mix with the 2s orbital to form three sp2 hybrid orbitals all in the same plane and pointing to the corners of a triangle.  The angle between the sp2 hybrid orbitals is 120o.

The sp2 hydrid orbitals are available to make sigma (s) bonds whereas the left over pz orbital is available to make a pi (p) bond as shown for ethene (C2H4) as shown below:

 

 

A carbon-carbon double bond consists of a sigma bond from the overlap of sp2 hybrid orbitals and a pi bond from the overlap of pz orbitals.  

Double bonds are not free to rotate.  

Rotation about the carbon-carbon single bond in ethane occurs freely because rotating the carbons does not affect the overlap of the sp3 hybrid orbitals that make up the bond.  

 

In ethene, however, rotation can only occur if the pi bond breaks.  Hence, the pi bond prevents the double bond from rotating freely.

 

 

cis-trans isomers - geometric isomers

We have talked about structural isomers - compounds that have the same molecular formula but different molecular structures (different connectivity of atoms)

Because double bonds cannot rotate it raises the possibility for another type of isomer:

geometric isomers - organic structures that differ from one and other only in the geometry of the molecule and not in the linkage of atoms.

Consider for instance, 2-hexene.  We can draw two pictures for 2-hexene

When the substituents are on the same side of the double bond the isomer is termed cis

cis-2-butene

When the substituents are on different sides of the double bond the isomer is termed trans

trans-2-butene

Hence, for many alkenes, the complete name requires specifying whether it is the cis or trans isomer.  Again, it is only possible to distinguish between these isomers because there is not free rotation about double bonds.  

 

Alkynes: hydrocarbons with carbon-carbon triple bonds.

For instance C2H2

Naming is the same as with alkenes, but where we know use the suffix yne.

Structure (hydrogens omitted) (drawn out in class)

Name

 

ethyne

 

propyne

 

1-butyne

 

2-butyne

 

4-methyl-2-octyne

Bonding in alkynes - sp hybrid orbitals

sp hydrid orbitals (see one sp hybrid) - one of the 2p orbitals mixes with the 2s orbital to form two sp hybrid orbitals pointing in opposite directions.  The angle between them is 180o.  

The sp hydrid orbitals form sigma bonds, the pz orbitals form two pi bonds.  

 

A carbon-carbon triple bond consists of one sigma bond and two pi bonds.  

As with carbon-carbon double bonds, carbon-carbon triple bonds are not free to rotate, but the concept of cis-trans isomers about a triple bond does not make sense.

Arenes - 

The hydrocarbons we have dealt with to date are called aliphatic compounds and only contain carbon-carbon single, double or triple bonds

Arenes are a class of compounds based on the parent molecule benzene C6H6.  Because of the special bonding in benzene, arenes are generally much less reactive than aliphatic compounds.

Bonding in arenes

Dot structure for benzene:

Three of the electrons on each carbon are taken up in sigma bonds.  This leaves one electron left to form a pi bond (and hence partner with a sigma bond to form a double bond)

There are two ways we can form the pi bonds.  The are termed resonance structures and neither is the best representation of bonding in benzene.  A double bond is shorter than a single bond.  Hence, these structures imply three bonds in benzene are shorter than the other three.  This is not the case, the c-c double bonds are all the same length in benzene.  

A better view of bonding in benzene is an average of the two pictures above (such an average is called a resonance hybrid).  

The p-orbitals on each carbon not involved in the sp2 hybrid form a pi-bonding system that extends over the entire carbon framework rather than being localized to a single bond.  The pi-bonding electrons (see a picture of the pi-bonding in benzene) in benzene are said to be delocalized and it is this delocalization that gives benzene and related compounds their low chemical reactivity.  We represent the delocalized character of benzene by inscribing a circle.  

Note that the benzene molecule is flat unlike the "boat" and "chair" conformations of cyclohexane.

Because of the special bonding in benzene, it is not classified as an alkene and it is much less reactive than most alkenes. 

 

Note: 

Hybrid orbital and benzene rasmol files generated by:

W. R. Salzman
Department of Chemistry
University of Arizona

http://www.chem.arizona.edu/~salzmanr/orbitals.html