aqa a-level chemistry amines Questions and Answers

aqa a-level chemistry amines Questions and Answers What are the four structures of amines? 1. Primary (1°): RNH2 2. Secondary (2°): R2NH 3. Tertiary (3°): R3N 4. Quaternary (4°): R4N+X-. Why is ETHANOL's (alcohol) boiling point higher than ETHYLAMINE's (amide) despite the similar size and Mr? Both have hydrogen bonding BUT the H bonding in ethanol is STRONGER than in ethylamine as the O-H bond is more POLAR than the N-H bond and has potential for more extensive H bonding. What are the 2 ways to prepare PRIMARY aliphatic amines? 1. Nucleophilic substitution of halogenoalkanes by ammonia. 2. Reduction of nitriles. What is the nucleophilic substitution mechanism with ammonia to produce primary aliphatic amines? Ammonia first acts as a nucleophile then as a base. - Further substitution is possible, forming 2°, 3° and 4° salts. - Inhibited by XS NH3 reacting with halogenoalkane before amine has a chance. How are nitriles reduced to produce primary aliphatic amines? The nitrile formed from the nucleophilic substitution mechanism is reduced with 4[H] -- LiAlH4 or H2/Ni at 150°C. What are the pros and cons of reducing nitriles to prepare amines? + further substitution not possible. + only one product. + method exists to extend chain length. - only produces primary amines with formula RCH2NH2. Why can't aromatic compounds undergo substitution to produce primary aromatic amines? Resists attack by nucleophiles as it is repelled by delocalised charge density. Lone pairs delocalise into the ring. How are aromatic nitro compounds reduced to primary aromatic amines? - With 6[H], involving 3 stages: 1. Reflux with tin and conc. HCl, making amine salt. 2. Add conc NaOH to free the amine. 3. Steam-distill to remove aromatic amine from mixture. What are aromatic amines produced used for? Used in the manufacture of dyes. How are amines BRONSTED-LOWRY BASES? The lone pair of electrons on the N atom mean they are LONE PAIR ACCEPTORS. React with acids to form salts like: CH3NH2 + HCl -- CH3NH3+Cl- (methyl ammonium chloride). Why is the amine salt used in drugs? It is more soluble in body fluids than the amine. How is the pKa value related to the base properties of amines? The pKa value refers to the CONJUGATE ACIDS of each base. The higher the pKa value, the stronger the base and weaker the conjugate acid. Amines are WEAK bases - they have LOW pKa VALUES. What is the difference between base strength of ammonia, primary aliphatic and primary aromatic amines? 1° aliphatic amine ammonia 1° aromatic amine. Why are aliphatic amines STRONGER bases than ammonia? Lone pair of electrons on N atom more AVAILABLE to accept protons due to the INDUCTIVE EFFECT of alkyl groups. Charge is induced towards lone pair on N atom due to polar C-H bonds. Thus, lone pair is more available to attack a proton. Why are aromatic amines WEAKER bases than aliphatic amines? Lone pair of electrons on N atom is LESS available to accept protons since they overlap with delocalised electrons in ring, so less available to attack a proton. What are the uses of quaternary ammonium salts? Used as CATIONIC SURFACTANTS in fabric softeners and hair conditioners. At least ONE of the 4 groups needs to be a LONG-CHAIN ALKYL GROUP. How do cationic surfactants work? 1. During washing, -ve charge builds on clothing and hair, giving harsh feel. 2. Cationic surfactants 'neutralise' these charges. 3. Non-polar (insoluble) long chains stick out from fabric and form a lubricating surface, so easier to iron.