Chapter 27 - Aromatic Chemistry

Question 1

What is the nature of the bonding in a benzene ring?

Answer 1

Instead of having a sequence of double bonds, instead all of the carbons only have two carbon single bonds and one hydrogen bonds. All of the electrons which would have been hekd in double bonds are instead held in delocalised p orbitals both above and below the benzene ring

Question 2

What properties does this give benzene?

Answer 2

It makes it into a planar molecule due to equal repulsion from both above and below the molecule. It also makes very dense areas of electrons, making benzene rings susceptible to attack by electrophiles

Question 3

What proof is there that the structure of cyclohexa-1,3,5-triene is benzene, and not formed of double bonds?

Answer 3

Due to the theoretical existence of double bonds, it should easily undergo addition reactions but it does not, due to the fact that there are no double bonds.
The bond length is also different to what it should be in a cyclohexene, because all of the bonds are the length between carbon single and carbon double bonds, as opposed to having three of each length
The proof usually comes in the form of thermochemical evidence, because cyclohexene has a hydrogenation enthalpy change of 120 Kj, which suggests that in the triene it would be 360 Kj to hydrogenate all three double bonds, however it is 208 due to this increased stability afforded by the delocalised electrons

Question 4

What is nitration, and how is a nitronium ion produced from nitric acid? (in either 1 or 2 equations)

Answer 4

Nitration is the addition of the electrophile NO2+ to a benzene ring. The NO2+ group is formed by:
H2SO4 + HNO3 -> H2NO3+ + HSO4-
H2NO3+ -> H2O + NO2+

Question 5

What is the mechanism for electrophilic addition of a nitro group?

Answer 5

The delocalised ring will donate an electron and break to form a bond with NO2+, in the process breaking to become a “horseshoe” shape and become positively charged. To reform the delocalised ring, the hydrogen bond is broken on the same carbon, and is donated to the ring to reform it, now with a substituted electrophile and a H+ ion. This H+ ion will regenerate the sulfuric acid (it is a catalyst)

Question 6

What are nitrobenzenes used for?

Answer 6

Explosives and the manufacture of amines

Question 7

Which reaction with a catalyst allows acid chlorides to become electrophiles capable of reacting with benzene?

Answer 7

RCOCl + AlCl3 -> RCO+ + AlCl4-
Once the electrophilic addition reaction is complete and the H+ ion has been formed, it will regenerate AlCl3 by forming hydrochloric acid