A2 Chemistry

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  • Created by: Em
  • Created on: 21-09-10 15:20

Benzene and Bonding

  • Benzene is a planar hexagonal molecule
  • The structure has considerable chemical stability
  • Carbon-carbon bonds in benzene are identical in length
  • Each carbon atom donates 1 electron to a pie bond
  • The pie bonds spread all over the 6 carbon atoms
  • The electrons occupy three delocalised pie orbitals
  • The pie molecular orbitals are formed by an overlap of carbon p orbitals

The reluctance of benzene to undergo addition reactions is due to the increases energetic stability that the delocalised system gives it.

Breaking the delocalised pie electron system on benzene requires a considerable input of energy.

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Benzene and Substitution reactions

Arenes such as benzene exhibit many reactions in which the delocalised system is retained - the majority of these are substitution reactions.

Groups which may directly replace a hydrogen atom on a benzene ring include halogen atoms, nitro groups and alkyl groups.

TNT is made by substituting nitro groups, -NO2, for hydrogen atoms on the benzene ring.

Benzene requires a nitrating mixture - this is a mixture of concentrated nitric acid and concentrated sulphuric acid.

The mixture is heated gently under refluz at a temp' of 50-55.

The mechanism of nitration involves electrophilic substitution.

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Benzene and Substitution reactions

The function of the sulphuric acid in the nitrating mixture is to generate an attacking species from the nitric acid. The benzene ring has a high electron charge density associated with the deocalised pie electrons. Hence an attacking reagent that is attracted by this negative charge is needed - an electrophile. The electrophile produced in the nitrating mixture is the nitryl cation, NO2+:

A electrophile must be capable of forming a new covalent bond to carbon to react.

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Formation of Halogenoarenes

Benzene undergoes electrophlic substitution reaction with chlorine and bromine.

E.g. if chlorine is bubbled through benzene in the presence of a halogen carrier, chlorobenze is formed.

The halogen carrier is usually a metallic iron such as anhydrous iron(III) chloride. The effect of the halogen carrier is to polarise the chlorine molecule so it behaves as an electrophile.

Bromine to benzene is more difficult to achieve - benzene requires more vigorous reaction conditions for the addition of a halogen such as bromine because of the chemical stability of the delocalised pie electron system.

The enthalpy of benzene is more than expected, this energy is referred to as the stabilisation energy of benzene. The extra energy is needed to overcome the delocalisation of the pie electron bonds.

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Phenols and their properties

In phenols, the -OH group is joined to a benzene ring.

Its sparingly soluble in water, the -OH group forms hydrogen bonds to water yet the benzene ring reduces the solubility because it forms only weak Van Der Waal's to other molecules.

Phenol ionises slightly in water - the O-H bond in phenol breaks to form a hydrogen ion and a negative phenoxide ion.

This bond breaking occurs more readily in a phenol molecule than water molecule because the phenoxide ion is stabilised by a partial delocalistion over the benzene ring of the negative charge on the oxygen atom.

Phenol is a weak acid, it neutralises strong bases - the acidity of phenol is due to the stabilisation of the negative charge in the phenoxide ion into the pie electron system on the benzene ring.

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Phenol reactions

Phenol reacts vigorously with sodium, due to the weak acidity of phenol.

Phenol undergoes electrophilic substitution reactions far more readily than benzene.

The hydroxyl group, -OH, raises the electron charge density of the benzene pie orbitals - enhancing the reactivity of phenol to electrophiles.

Aqueous phenol decolourises bromine water to form a white precipitate 2,4,6-tribromophenol. An iron(III) bromide catalyst is needed.

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More on Phenol

The presence in Phenol of the -OH group increases the chance of the benzene ring to electrophilic atttack.

The oxygen in the -OH group has 2 lone pairs of electrona. These can overlap with the delocalised pie electrons, partially extending delocalisation to the oxygen atom.

Overall, the pie electron density is increased so it's said the -OH group activates the benzene ring!

Phenol is used to manufacture a wide range of chemical products - first used in an antiseptic by Lister but was found to be harmful. Safer forms are now used in antiseptics and disinfectants.

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Carbonyl Compounds

Aldehydes are formed in the first stage of oxidation of primary alcohols. Ketones are the only product formed on the oxidation of secondary alcohols.

Both aldehydes and ketons contain the carbonyl group, C O.

Aldehyde - Al. Ketone - One.

The carbonyl group is polar:

The polarity enables the lower members of the homologous series of aldehydes and ketons to be completely miscible with water.

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Redox reactions - Reduction

Aldehydes and ketones can be reduced to their respective alcohols - NaBH4 (it's usual to represent this as [H] in a equation) is a suitable reducing agent.

The aldehyde or ketone is warmed with the reducing agent using water or ethanol as a solvent.

Ethanal is reduced to ethanol:

Propanone is reduced to propan-2-ol:

The reactions may also be regarded as addition of hydrogen to to the carbonyl double bond.

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Oxidation and test for presence

Aldehydes are oxidised further to carboxylic acids. The aldehyde is usually refluxed with acidified potassium dicromate(VI).

When a solution of 2,4-dinitrophenylhydrazine is added to an aldehyde or a ketone, a deep yellow or orange precipitate is formed - test is specific to an aldehyde or ketone carbonyl group.

The reaction involves an addition across the double bond followed by tge elimination of a water molecule.

Aldehyde - boil gently, reflux, acidified potassium dichromate(VI), orange solution turns green.

Tollen's reagent, aqueous solution of silver nitrate in excess ammonia, warm, silver mirror forms - the silver(I) ion is reduced to silver metal.

Propanone - mainly used for solvents

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