Everyone about Benzene

Arenes are an AROMATIC hydrocarbon which contains one or more benzene rings.

Aromatic is a compound which contains one or more benzene rings.

Benzene contains C=C bonds

Expected to react similar to an Alkene

- decolourise bromine water (orange -> colourless)

However, it does not. It does not take part in any other electrophillic addition.
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Kekule's model explaining this (THIS IS WRONG)

He said benzene had to forms. The benzene was at such as fast equilibrium that bromine could not be attracted fast enough.

Kekule's model of benzene was symmetrical but C-C and C=C have different bond lengths so this would cause DISTORTION

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Benzene is a PLANAR MOLECULE

Carbon-Carbon bond in benzene :
- same length
- their lengths are intermediate between single and double bonds

Each carbon atom has an unpaired p-orbital electron. They overlap to form a π bonds (cloud of delocalised electrons) 

π cloud grants GREATER STABILITY to the molecule due to the p-orbital electrons spread over the whole ring

Cyclohexene + H2 ---> Cyclohexane   Enthalpy change of hydration = -120 kJ mol-1

So Benzene should be -120 x 3 = -360 kJ mol-1 THEORETICALLY

HOWEVER, it is -208 kJ mol-1

This indicates

- Bezene is MORE STABLE than the theoretical benzene, due to the cloud of delocalised π electrons

- Benzene reacts LESS READILY than the theoretical benzene

Difference of 360 - 208 = 152 kJ mol-1
- This is the delocalisation energy and represents the extra energy needed to overcome the delocalised structure to enable reactions such as hydration to occur.

Electrophiles - Electron pair acceptor

Benzene will be attacked by Electrophiles because of its delocalised π clouds causes HIGH DENSITY OF ELECTRONS, so the electrophiles are attracted to it.

To preserve the stability of benzene, it undergoes ELECTROPHILLIC SUBSTITUTION (a hydrogen is replaced by a bromine)

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Why substitution and not addition?
- In addition, the electrons from the delocalised cloud would need to bond to the atom being added
- This would distrupt the delocalised clouds and result the product to be less stable than benzene

1. Electrophile attacks the electron rich benzene molecule and an electron pair from the delocalised π cloud bond with the electrophile. This causes the delocalised ring to break and form an unstable intermediate.

2. Delocalised π cloud formed again by losing a hydrogen.

Diagram

Requirements

• Conc H2SO4 (catalyst)
• Con HNO3
  
• 50 ºC 

Mechanism:

Benzene too stable for bromine to react. So, it reacts with bromine in the presence of a HALOGEN CARRIER.

Halogen carrier can either be AlBr3 or FeBr3

Mechanism:

Cyclohexene
♦  π bond contains localised electrons
♦  High electron density above and below two carbon atoms
♦  This causes Br  (non-polar) to induce a dipole in the Br  molecule and making it polar

Benzene
♦  π bond contains delocalised electrons
♦  Benzene has a lower π-electron density than Cyclohexene
♦  Insufficient electron density to polarise Br 
♦  Br  generated by a halogen carrier

Diagram of Cyclohexene + Bromine Water

Phenol + Sodium Hydroxide

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Phenol + Sodium

• Lister was the first person to use CARBOLIC ACID (or phenol) as an antiseptic
• Success rate for amputation went from 60% to 90%
• However, it was CAUSTIC (Able to burn or corrode organic tissue) to surgeons
• Lister found a much safer and better antiseptic, BORIC ACID

Reaction of Phenol and Sodium Hydroxide:

Reaction of Phenol and Sodium:

Lone pair of electrons on the oxygen (-OH) is delocalised into the benzene ring

This causes a higher electron density within the ring and activates it

Increased electron density polarises the Br molecules

Br molecule is attracted stronger to the ring because of the increased electron density

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Increased electron density = increased reactivity with all electrophiles

Reaction of Phenol + Bromine: