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Definitions

Definitions. Aliphatic Aromatic Reflux Primary amine Base Substitution Nucleophile Reduction. Amines and their reactions. Describe the preparation of aliphatic amines by substitution of halogenoalkanes . Describe the preparation of aromatic amines by reduction of nitroarenes .

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Definitions

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  1. Definitions • Aliphatic • Aromatic • Reflux • Primary amine • Base • Substitution • Nucleophile • Reduction

  2. Amines and their reactions Describe the preparation of aliphatic amines by substitution of halogenoalkanes. Describe the preparation of aromatic amines by reduction of nitroarenes. Describe the synthesis of an azo dye by diazotisation and coupling. State the use of these reactions in the formation of dyestuffs.

  3. Preparation of primary aliphatic amines • Prepared by warming halogenoalkanes gently with an excess of ammonia, using ethanol as the solvent • For example: • CH3CH2CH2Cl + NH3 CH3CH2CH2NH2 + HCl • What is the name of the compound formed? • Propylamine • What type of reaction is this? • Nucleophilic substitution

  4. Further ammonia reacts with the hydrogen chloride formed: • NH3 + HCl NH4+Cl- • Describe (with diagrams) how the ammonia acts as a nucleophile to the carbon-halogen bond. • The product, propylamine, also has a lone pair of electrons. This can also attack another molecule of 1-chloropropane, causing further substitution. • Can you write the equation for this reaction? • CH3CH2CH2Cl + CH3CH2CH2NH2 (CH3CH2CH2)2NH2 + HCl • To prepare a primary amine you need to use an excess of ammonia to minimise this further substitution

  5. Preparing aromatic amines • Nitrobenzene and other nitroarenes can be reduced using a mixture of tin and concentrated hydrochloric acid, heated under reflux and then neutralisation of the excess hydrochloric acid. • Aromatic amines are formed as products • [H] is the simplified symbol for the reducing agent

  6. Find out… • Who was William Henry Perkin? • What is aniline? • What did he synthesise and why is this important?

  7. Synthesis of dyes from Phenylamine • There are 2 steps you need to be aware of: • Diazotisation • Coupling reactions • Diazotisation is responsible for forming the diazonium ion (R – N+ÐN) • When phenylamine is reacted with nitrous acid (HNO2) and kept below 10°C a diazonium salt is formed. • The nitrous acid can be generated in situ via: • NaNO2 + HCl HNO2 + NaCl • The cold nitrous acid then reacts with an aromatic amine to form a diazonium salt. Can you write the equation for the reaction using phenylamine as the amine? Further reading

  8. Coupling • The positive charge on the diazonium ion means that this is a strong electrophile. • Therefore we might expect it to attack other benzene rings, particularly ones with electron-donating groups such as an –OH group. • This is an example of a coupling reaction. • It takes place under alkaline conditions and forms brightly coloured compounds that are used as azo dyes. Further reading

  9. Questions • Complete the questions on page 39 of your text book • Complete question 12 on page 45

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