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AS/A-Level Chemistry - Benzene

Benzene

· A-level Chemistry,Organic Chemistry,benzene,functional group,Aromatic rings

Let's look into Benzene in A-Level Chemistry to see how it is different from other hydrocarbon!

Benzene 

C6H6– first discovered by Michael Faraday in 1825 is colourless, sweet smelling, highly flammable.

Found naturally in crude oil, petrol and cigarette smoke.

 Made of hexagonal ring of 6 carbons with each carbon bonded to 2 carbons and 1 hydrogen

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The R group is H in benzene.

Kekulé Model

1865 Friedrich August Kekulé suggested that benzene is a 6 carbon ring with alternating double and single bonds between the carbons.

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Evidence to disprove the Kekulé Model 

Lack of reactivity of benzene –benzene does not decolourise bromine under normal conditions. It also does not undergo electrophilic addition. 

Lengths of carbon-carbon bonds –using x-ray diffraction it was found all bonds were 0.139 nm the length between a double bond 0.134nm and a single 0.153 nm

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Hydrogenation enthalpies –if benzene had the Kekulé model it would have the hydrogenation enthalpy x3 of cyclohexene. However, it is only -208 kJ mol-1 not -306kJ mol-1, suggesting that Benzene is more stable.

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Delocalisation

The 6 carbons are arranged in a hexagoanl planar structure. Each carbon forms 3 bonds, leaving one delocalised electron in the p orbital. The six p orbitals delocalise to form an extended pi system above and below the plane of the ring. This gives the molecule stability. 

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Benzene derivatives

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Nitration of Benzene 

Benzene + nitric acid (sulfuric acid catalyst at 50°) creates nitrobenzene and water 

H is replaced with NO2 

Nitric acid and sulfuric acid react to for NO2+electrophile

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Then the H+ and HSO4 react to form H2SO4

Halogenation of Benzene 

Halogens do not react with benzene unless a halogen carrier is present e.g. AlCl3 

Bromination of Benzene 

Room temperature in the presence of a halogen carrier on hydrogen on benzene is replaced with Br 

Electrophilic substitution

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Alkylation of Benzene 

H replaced with alkyl group 

Benzene + haloalkane (in the presence of halogen carrier) creates and alkylbenzene plus an acid 

Sometimes called a Friedel-Crafts Reaction. 

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Acylation Reactions 

Benzene + acyl chloride (in the presence of AlCl3) ---> aromatic ketone +HCl

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Drafted by Eunice (Chemistry)

References

https://www.cliffsnotes.com/study-guides/chemistry/organic-chemistry-ii/reactions-of-aromatic-compounds/friedelcrafts-alkylation-reaction

https://en.wikipedia.org/wiki/Acylation

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