Aromatic compounds & amines

• Describe the structure of benzene:

  C6H6- planar cyclic structure. Each carbon forms single covalent bonds to carbons on either side & to 1 hydrogen atom. Final unpaired electron on each C atom is located in p-orbital that sticks out above & below plane of ring. P-orbitals on each C atom combine to form ring of delocalised electrons. All bonds in ring are same- have same length (140pm).
• Is a double or single bond larger?

  Single (147pm).
• Why does benzene undergo electrophilic substitution?

  Benzene ring region of high electron density, so attracts electrophiles (include positively charged ions & polar molecules). Ring so stable, it doesn't undergo electrophilic addition reactions, which would destroy delocalised ring of electrons. Undergoes substitution- electrophile substituted in.
• How do you name aromatic compounds?

  Some cases, benzene main functional group & molecule named as substituted benzene ring- suffix=benzene & prefix=any other group. When benzene isn't main group molecule named as having a phenyl group attached. Suffix=other groups & prefix=phen or phenyl.
• How do you number the benzene ring?

  Groups different-start from suffix group. If no suffix- start from group alphabetically first.
• Why is benzene more stable than its theoretical compound, which has single & double bonds?

  Cyclohexene (1 double bond) has enthalpy of hydrogenation of -120KJmol-1. If benzene had 3 double bonds expected value would be -360KJmol-1. Experimental enthalpy for benzene = -208 (152 less exothermic than expected). Therefore more energy must have been put in to break bonds in benzene that theoretical compound. Benzene more stable due to delocalised ring of electrons- electron density shared over more atoms meaning energy of molecule lowered & more stable.
• Uses of nitration reactions:

  Nitro compounds can be reduced to form aromatic amines. Used to manufacture dyes & pharmaceuticals. Nitro compounds decompose violently when heated, so used as explosives e.g, TNT.
• What is a nitration reaction?

  Warm benzene with concentrated nitric acid & concentrated sulfuric acid- makes nitrobenzene. Sulfuric acid acts as catalyst-helps make nitronium ion (No2+) which is electrophile. Only want 1 No2 group added- keep temperature below 55.C.
• Products of F-C acylation reaction?

  Phenylketone & HCl. Reactants heated under reflux in non-aqueous solvent (e.g, dry ether). Acyl chloride provides electrophile & AlCl3 halogen carrier used.
• What are halogen carriers?

  Catalyst- makes electrophiles stronger by polarising it so it can attack benzene ring.
• How does the halogen carrier work?

  AlCl3 accepts lone pair electrons from acyl chloride. As lone pair pulled away, polarisation in acyl chloride increases & it forms carbocation. Makes it stronger electrophile & now can react with benzene ring.
• What is an amine?

  If 1 or more H in NH3 replaced with an organic group (e.g, alkyl or aromatic). If 1 H replaced = primary amine etc. If 4th organic group added, you get quaternary ammonium ion.
• How are quaternary ammonium salts formed?

  Quaternary ammonium ions are (+) charged, so attracted to negative ions.
• What do small amines smell like?

  Ammonia
• What do larger amines smell like?

  Fish
• What are cationic surfactants?

  Compounds partly soluble & partly insoluble in water. Quaternary ammonium salts (with at least 1 long hydrocarbon chain) used as cationic surfactants (+ charged).
• How do cationic surfactants work?

  Long hydrocarbon chain insoluble in water, but bind to non-polar substances e.g, grease, whilst positive head group soluble in water. Useful in detergents as non-polar end will bind to grease & polar head group end will dissolve in water, allowing spots of grease to mix with water & be washed away. Form induced dipole-dipole interactions with non-polar substances & hydrogen bonds with water.
• Why are cationic surfactants used in fabric conditioners & hair products?

  When hair or fabric get wet, they pick up (-) charges on their surface. (+) charged part of surfactant attracted to (-) charged surfaces & forms a coating over the surface. Coating prevents build-up of static electricity. Keeps fabric smooth & keeps hair flat.
• Why are amines weak bases?

  Accept protons. Lone pair of electrons on nitrogen atom that forms a co-ordinate bond with an H+ ion. Strength of base depends on how available nitrogen's lone pair of electrons is. Lone pair more available if its electron density is higher. More available lone pair is, more likely amine is to accept a proton & the stronger a base it will be.
• Why are primary aromatic amines weak bases?

  Benzene ring draws electrons towards itself & nitrogen's lone pair gets partially delocalised onto the ring. Electron density on nitrogen decreases- makes lone pair less available.
• Why are primary aliphatic amines strong bases?

  Alkyl groups push electrons onto attached groups. Electron density on nitrogen atom increases- lone pair more available (inductive effect).
• Strength of ammonia as a base?

  Lies between. Ammonia doesn't have aromatic group to pull lone pair of electrons away, or an alkyl group to push lone pair of electrons forward.
• What are amides?


  Derivatives of carboxylic acids-contain functional group -CONH2. Carbonyl group pulls electrons away from NH2 group, so amides behave differently from amines. Can be made by reacting acyl chlorides with ammonia.
• What are N-substituted amides?

  1 of hydrogens attached to nitrogen has been substituted with an alkyl group. Made by reacting acyl chlorides with amines.
• How can you form aliphatic amines?

  Heat halogenoalkane with excess ethanolic ammonia (nucleophilic substitution). Makes mix of primary, secondary & tertiary amines & quaternary ammonium salts as more than 1 H likely to be substituted.