Organic Chemistry - Unit 5

Benzene Thursday, January 13, 2011 7:01 PM

Benzene was originally thought to have Kekule's structure, however the following evidence showed that that structure was not possible: - Benzene doesn't undergo electrophilic addition (as alkenes do) but electrophilic substitution substitution - X-ray diffraction data shows all the bonds in benzene are of the same length, however they are of a length between that of C-C bonds and C=C - Thermochemical analysis disagrees with benzene's st ructure:

- From IR spectra analysis:  Alkene 2952-2853 cm-1

Benzene 3050 and 750 cm-1

Due to CH of CH 2, CH3 C=C is below 1700 cm-1

Structure of Benzene

Organic Chemistry Unit 5 Page 1

Pi (π) Bonds Pi (π) bonds are formed by the sideways overlap of 2 p orbitals.

The 6 carbon atoms in benzene are all in the same plane. Each carbon has a p electron. Due to the planar structure of  benzene, the p-orbitals of benzene now overlap sideways leading to a delocalised system of 6 π electrons. This does not lead to any distinct π bonds but the overlap between the p orbitals is throughout the whole ring, above and below it. Molecules with delocalised electrons are more stable, than those whose electrons are not delocalised. In benzene this accounts for the compound being 152 kJmol -1 more stable than expected for the Kekule structure.

Physical Properties of Benzene -

It is a liquid It has a nice smell since it is volatile It is one of the most carcinogenic compounds that we have It causes leukemia.

Organic Chemistry Unit 5 Page 2

Reactions of Benzene Friday, January 14, 2011 11:02 AM

Addition reaction

When it undergoes substitution, it retains its 6π cloud of electrons, so it retains its stability.

Nitration of Benzene Electrophilic Substitution



 + 2  →  +  

nitration mixture





+ 2

↘ nitroneum ion (electrophile) 



 +   →   +  +     +   →  





+   

=  + 2  →  +  





+ 2

anic Chemi

Unit 5 Pa

At the end the 6π delocalised electron cloud is restored.

anic Chemi

Unit 5 Pa