OCR Chemistry Notes – A2 Level

Jack Wood

1.1 Introduction to Aromatic Chemistry

Arenes

- Arenes are aromatic hydrocarbons containing one or more benzene rings.

- Benzene molecule is made up of six carbon atoms.- Which is also bonded to one hydrogen atom.- It has the molecular formula C6H6.- Anything with a benzene ring is classified as an aromatic compound.- Term ‘Aroma’ meaning fragrance, resins and plant materials give off

pleasant smells.- Aspirin used as a pain killer, has no odour and yet its structure contains

a benzene ring.- Groups attached to a benzene ring usually contain –Cl, -Br, -NO2, and

alkyl groups with CH3 or –C2H5 (ethyl).

Benzene

- Benzene is the starting material for the synthesis of many other aromatic materials.

- Benzene is classified as a carcinogen, linked to causing cancer.- Benzene is a colourless liquid, with a sweet odour and it is highly

flammable.- Natural sources of benzene include Volcanoes and Forest Fires.- Benzene is a component of crude oil, petrol and cigarette smoke.

1.2 The Structure of Benzene

Problems with the Kekule Structure

- It fails to explain the properties of benzene fully, even though It fitted the molecular C6H6.

Benzenes Low Reactivity

- Kekules model failed to explain the low chemical reactivity of Benzene.- If C=C double bonds were present then benzene would react in a similar

way to alkenes.- Each C=C bond would be expected to react with bromine water,

decolourising it.- Nor does it take place in electrophilic addition reactions expected from

the C=C bond in alkenes.

Kekules equilibrium model of benzene

- He suggested that benzene had two forms, differing only by the positions of the double bonds.

- He suggested that they were in such equilibrium that an approaching bromine molecule could not be attracted to a double bond before the structure changed. Hence Bromine could not react with the double bonds.

The Carbon- Carbon bond lengths in Benzene

- The alternating single and double bonds was represented as a symmetrical molecule. But C-C single bonds and C=C bonds have different lengths.

- Many years later it was revealed that the carbon-carbon bond lengths in Benzene are the same length: 0.139nm.

- This is between the C-C length of 0.153nm and the C=C bond length of 0.134nm.

Hydrogenation of Benzene

- The Kekule structure of Benzene contained three C=C bonds, the name for the proposed structure was cyclohexa-1,3,5-triene.

- When an alkene reacts with hydrogen, the energy change is called the enthalpy change of hydrogenation.

- When one C=C bond reacts with hydrogen, the enthalpy change of hydrogenation is -120kJ mol-1.

- When it reacts with three C=C bonds, the enthalpy change of hydrogenation is expected to be -360kJmol-1, three times that of cyclohexane.

- The actual structure of Benzene has much less energy than the proposed Kekule structure.

- The energy is known as the delocalisation energy, or resonance energy, of Benzene.

- This evidence suggests that the real structure of Benzene is more stable than a structure containing C=C bonds. It helps to explain why it is less reactive than alkenes.

1.3 The delocalised model of Benzene

The delocalised model of benzene

- Benzene is a cyclic hydrocarbon with six carbon atoms and six hydrogen atoms. The six carbon atoms are arranged in a planar hexagonal ring. Each carbon atom is bonded to two other carbon atoms and one hydrogen atom.

- The shape around each carbon atom is a bond angle of 120 degrees.- Each carbon atoms has four outer shell electrons. The three bonds in

this plane are called sigma bonds. This leaves a fourth outer shell electron in a 2p orbital above and below the plane of carbon atoms.

- The electron in a p-orbital of a carbon atom overlaps with the electrons in the p-orbitals of the carbon atoms on the other side.

- The overlap produces a system of Pi Bonds which spread over all six carbon atoms.

1.4 Benzene and its Reactions

Reactivity

- In Benzene the region of high electron density above and below the plane of carbon atoms attracts electrophiles.

- Benzene takes part in substitution reactions instead of addition reactions.

Electrophilic substitution by Nitration

- In nitration, one of the hydrogen atoms on the Benzene ring is replaced by a nitro (-NO2) group.

- It reacts with a nitrated mixture of concentrated nitric acid and concentrated sulphuric acid at a temperature of 50C, H2SO4 acts as a catalyst.

- Mixture is usually prepared in a pear shaped flask.- Nitrating mixture mixed together carefully while cooling the mixture in

a beaker of water, Benzene is then added carefully, keeping the temperature below 50C.

- Nitrobenzene is a pale yellow liquid and is an important starting material in the preparation of dyes, pesticides and pharmaceuticals e.g. paracetamol.

Nitration of Methylbenzene

- Also called toluene can be also nitrated with mixture of concentrated nitric and sulphuric acids.

- It can lead to the formation of 2,4,6-trinitromethylbenzene (trinitrotoluene, TNT) an explosive.

Halogenation of Benzene

- Benzene does not react with halogens on its own, but it does with a halogen carrier such as, FeCl3, FeBr3, AlCl3 and AlBr3. Iron metal can be used as it reacts with any halogen present to form the required Iron(III) Halide.

- This is an Electrophilic substitution reaction in which one of the hydrogen atoms is replaced by a halogen atom.

The reaction with Chlorine

- Chlorine reacts with Benzene at room temperature and pressure in the presence of AlCl3, FeCl3 or Fe to produce Chlorobenzene.

- Chlorobenzene is used as a solvent and in the production of pesticides.

The reaction with Bromine

- Benzene reacts with bromine in the same way that it reacts with chlorine. This time the halogen carrier catalyst needed is AlBr3, FeBr3 or Fe. The organic product is Bromobenzene, a compound used in the production of pharmaceuticals.

1.5 Substitution reactions of Benzene

General mechanism of Electrophilic substitution

- Electrophilic substitution reactions occur in benzene because of the delocalised ring of electrons above and below the plane of carbon atoms.

- The electron dense ring attacks an electrophiles, it then accepts a pair of pi electrons from the delocalised ring to form a covalent bond.

- The delocalised pi electron cloud has been disrupted and the intermediate is less stable than benzene.

- The unstable intermediate immediately loses the hydrogen as an H+ ion. The delocalised ring of electrons reforms and stability is restored.

Nitration

- The electrophile is the nitryl cation or nitronium ion and has the formula NO2+

- The equation: HNO3 + H2SO4 NO2+ + HSO4- + H2O- The NO2+ electrophile then reacts with benzene. The mechanism shown

in book.- Finally the H+ reacts with the HSO4- from the first step to reform

H2SO4 so the sulphuric acid is acting as a catalyst.- H+ + HSO40 H2SO4

Halogenation

- Benzene is too stable to react with Bromine on its own, Br2, however it does react in the presence of a carrier such as FeBr3. The halogen carrier generates the Bromonium ion, Br+, which is a more powerful electrophile than Br2.

- Equation: Br2 + FeBr3 Br+ + FeBr4-- Finally the H+ reacts with FeBr4- from the first step to form the other

product of the overall reaction, HBr, and to reform FeBr3. So the Iron (III) bromide is acting as a catalyst.

Different Halogen Carriers

- For bromination the halogen carrier is AlBr3, FeBr3 or Fe.- FOR Chlorination, Alcl3, FeCl3 or Fe.- You can use Fe in both as it forms the halide Iron (III) halide.

1.6 The reactivity of alkenes and Benzene

Cyclohexene and Bromine Water

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1.17 Amines

Something fishy about amines?

- Amines are all derivatives of ammonia, hydrogen atoms from the ammonia molecule are replaced one at a time by hydrocarbon chains (shown by R), primary, secondary and tertiary amines.

- Amphetamine is a complex amine used to treat daytime drowsiness and chronic fatigue syndrome.

- Phenylephrine is an amine commonly used as decongestant, in many cold and flu medicines, with paracetamol.

- Adrenaline is fight of flight amine helping the body to deal with sudden stress.

- Amines are also known for their unpleasant smells, said to be a fishy smell.

Naming Amines

- A primary amine is one with only one carbon chain attached to the nitrogen.

- Four carbons in the alkyl chain, given the name butyl.- A secondary amine is one with two carbon chains attached to the

nitrogen.- In Fig 5. Contains three carbons in longest chain, so it is –Propyl.- Suffix added is –amine.- Shorter alkyl chain has one carbon attached to Nitrogen so it is N-

methyl- So the compound is N-methylpropylamine.

Basicity in amines

- Amines are weak bases (so is ammonia)- Base is defined as proton acceptor. Each has a lone pair of electrons on

nitrogen atom and they can accept a proton H+.- When base accepts a proton, a dative covalent bond froms between the

lone pair of the nitrogen atom and the proton.

Base Reactions of amines

- Amines = base + acid salt- Ethylamine reacts with hydrochloric acid to form an alkylammonium

salt: CH3CH2NH2(aq)+HCl(aq)CH3CH2NH3+Cl-(aq), this forms ethylammonium chloride.

1.18 Amines and their Reactions

Preparation of primary aliphatic amines

- Aliphatic amines can be prepared by warming halogenoalkanes gently with an excess of ammonia, using ethanol as a solvent. The formation of propylamine by the reaction of 1-chloropropane with ammonia.

- As a nucleophile, ammonia has a lone pair of electrons and attacks the delta positive carbon atom in the polar carbon-halogen bond. The product propylamine, has a lone pair of electrons, this attacks another molecule of 1 chloropropane, causing further substitution.

Preparing aromatic amines

- Nitrobenzene and other nitroarenes can be reduced with a mixture of tin and concentrated hydrochloric acid, heated under reflux, followed by the neutralisation of the hydrochloric acid, aromatic amines are formed as products in these reactions.

Synthesis of dyes form phenylamine

- Two steps in industrial preparation of dyestuffs:- Diazotisation - Coupling Reactions

Diazotisation

- Responsible for the formation of Diazonium ion. When a mixture of phenylamine and nitrous acid is kept below 10C a Diazonium salt is formed.

- Nitrous acid, HNO2, generated in reaction mixture, by reacting Sodium Nitrite, NaNO2, and excess Hydrochloric acid, HCl.

- Cold nitrous acid then reacts with an aromatic amine to form Diazonium salt.

Coupling

- A coupling reaction happens when the Diazonium salt, benzenediazonium chloride, is reacted with a phenol or aromatic compound such as amine, under alkaline conditions.

- Two benzene rings are linked together as an azo functional group -N=N-- The product is brightly coloured that is used as an azo dye.