Everyone about Benzene

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  • Created by: Amin
  • Created on: 22-02-13 12:27

What is an Arene?

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

Benzene C6H6 (http://4.bp.blogspot.com/-vecA1Re0u9g/TdyYy1RYuBI/AAAAAAAAAEs/lNFSyzi1cwQ/s200/benzene.jpg)

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

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Benzene's Low Reactivity

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)

(http://www.kentchemistry.com/images/links/organic/32a.gif)He said benzene had to forms. The benzene was at such as fast equilibrium that bromine could not be attracted fast enough.

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Benzene's Bond Length

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

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Benzene's Hydrogenation

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.

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Benzene and Electrophillic Substitution

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

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General Mechanism of ELECTROPHILLIC SUBSTITUTION

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

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Electrophillic Substitution - Nitration

Requirements

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

Mechanism:

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Electrophillic Substitution - Halogenation (bromin

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:

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Bromonation of Alkenes and Arenes

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

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Reaction of Phenols to form Salts

Phenol + Sodium Hydroxide

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Phenol + Sodium

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Phenol as an early Antiseptic

  • 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
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Phenol + Sodium Hydroxide

Reaction of Phenol and Sodium Hydroxide:

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Phenol + Sodium

Reaction of Phenol and Sodium:

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Ease of Bromination of Phenol

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

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Phenol + Bromine

Reaction of Phenol + Bromine:

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