Aromatic Chemistry (3) Comparing bromination of benzene and ethene

Aromatic Chemistry (3) Bromination of Benzene and Alkenes compared

Specification extracts:

OCR

Electrophilic substitution 

6.1.1.(f) and (g) 

—the explanation of the relative resistance to bromination of benzene, compared with alkenes, in terms of the delocalised electron density of the π-system in benzene compared with the localised electron density of the π-bond in alkenes.
—the interpretation of unfamiliar electrophilic substitution reactions of aromatic compounds, including prediction of mechanisms.

Aromatic Chemistry (3) Bromination of Benzene and Alkenes compared

In this third post on aromatic chemistry, I’m comparing the reaction mechanism of the electrophilic substitution of benzene with that of the electrophilic addition of bromine to an alkene such as propene (C₃H₆).  

I’m also going to discuss the general mechanism for the electrophilic substitution of benzene and other aromatic compounds such as phenol.

Comparing the mechanisms for the bromination of benzene (C₆H₆) and propene (C₃H₆).

1. The benzene problem

Benzene is a relatively unreactive molecule.  The reason for its unreactivity lies in its distinctive electronic structure.

The 6 carbon ring is bonded in two ways.  There is the end-on overlap of s orbitals to form a series of six sigma bonds between the carbon atoms.  However that does not account for all the electrons in the molecule.  6 electrons remain unbonded in sigma bonds.  There is one delocalised electron per carbon atom.  These in each carbon atom occupy a half filled p orbital.  The genius of the molecule is that these six half filled p orbitals combine in a side-on overlap move to form a ring shaped molecular orbital that resides above and below the plane of the sigma bond ring.

You can see how that forms in the illustration below:

So for a species to access the benzene molecule and add or substitute itself in the molecule it has to at least disrupt this ring shaped molecular orbital and that requires considerable energy around 150kJmol^-1 to be precise.  Hence there is considerable resistance to reaction.

The energy profile for the bromination of benzene reaction is found below:

In an alkene of course there is only one double bond in which the π bond is exposed above and below the plane of the single sigma bond.  

The energy required to break this bond is much less than that required to disrupt benzene’s π system.  Alkenes tend to undergo addition reactions fairly quickly and easily with reagents like bromine and hydrogen bromide.  

The reaction energy profile for the action of hydrogen bromide on an alkene is shown below.

Bromination of benzene on the other hand requires a raised temperature and the use of a halogen carrier to generate the electrophile the bromonium ion Br+.

If the benzene ring is already substituted with an electron rich group such as —OH or —NH₂ then these extra electrons enable the ring to be more susceptible to electrophilic substitution since they become incorporated in the π system.  The result being that the conditions for nitration of phenol are much less severe than they are for benzene as we can see illustrated in the two reactions below.