Advanced Higher Chemistry Unit 3 Alcohols

1. Advanced Higher Chemistry Unit 3 Alcohols

2. Alcohols • Hydroxyl (-OH) functional group. • Uses of alcohols • Recreational use • Industrial solvent (methylated spirit - 90% ethanol / 10 % methanol) • Laboratory solvent (~95% ethanol) • Antiseptic (propan-2-ol) • Antifreeze (ethylene glycol) • Baking (glycerol) • Explosives (glycerol)

3. Hydroxyl group is strongly polar resulting in hydrogen bonding. • Compared with molecules of similar masses and shape alcohols have • Higher melting points • Higher boiling points • Higher solubility in water

4. Comparison of similar molecules Ethanol hydrogen bonding with water

5. As chain length increases solubility in water decreases due to • Non-polar nature of alkyl group. • Polarity of O-H becoming less significant. ∞- alcohol completely miscible with water

6. Synthesis of alcohols • Fermentation of sugars (ethanol only). • Hydration of alkenes (see ‘Alkenes - Acid Catalysed Addition of Water’) • Hydrolysis of halogenalkanes (see ‘Halogenalkanes - Nucleophilic Substitution’)

7. In industry the far most important method for large scale production of alcohols, except methanol, is the acid-catalysed hydration of alkenes. • The alcohol is produced under pressure using a solid supported phosphoric acid catalyst in the presence of water.

8. Methanol is produced industrially from synthesis gas ( a mixture of carbon monoxide and hydrogen) which in turn is produced by the steam reforming of natural gas. CO + 2H2 CH3OH • The synthesis gas is reacted at high temperature and low pressure over a copper-based catalyst to produce methanol.

9. Bonding in Alcohols • All bonds in alcohols are sigma bonds (see Bonding in Alkanes).

10. Reactions of alcohols • Given the similarities in the structures of the molecules, some of the reactions of alcohols are analogous to some reactions of water. • Phosphorus(V) chloride is easily hydrolysed by water, producing white fumes of HCl PCl5 + H2O  POCl3 + 2HCl If ethanol is used instead of water, a similar reaction occurs with the production of chloroethane: PCl5 + C2H5OH  POCl3 + HCl + C2H5Cl This method can be used to prepare specific halogenoalkanes.

11. Sodium metal will react with water to give hydrogen gas. In this reaction water behaves as an acid by losing hydrogen ions: 2Na + 2H2O  2NaOH + H2 When ethanol is used: 2Na + 2C2H5OH  2NaOC2H5 + H2 Although the mechanism is probably different, the outcome is similar. Ethanol molecules lose hydrogen ions to produce ethoxide ions and hydrogen gas is formed. • The resultant solution of sodium ethoxide in ethanol (-OC2H5) is very useful as an organic base in situations where the absence of water is essential.

12. Reactions of Alcohols • Oxidation • Primary alcohols  aldehydes • Secondary alcohols  ketones • Tertiary alcohols  Degradation

13. Synthesis of alkenes • Catalysed by conc. H2SO4 or phosphoric acid. • E1 mechanism. • Mechanism favoured by high temp and high acid concentration. • Mechanism is the exact opposite of that proposed for the acid-catalysed hydration of alkenes.

14. Formation of esters • Reaction is reversible and slow to reach equilibrium • Conc. sulphuric acid is used as a dehydrating agent to remove water and hence, move equilibrium to the right-hand side. It also serves as a catalyst for the reaction.

15. SOCl2 or PCl3 or PCl5 acid chloride • Alternative formation of esters Step 1 involves the conversion of the carboxylic acid into an acid chloride by reaction with thionyl chloride (SOCl2), phosphorus(III) chloride or phosphorus(V) chloride.

16. In step 2 the acid chloride reacts rapidly with the alcohol to form the ester. Both these steps are irreversible and occur faster than the direct reaction between the carboxylic acid and the alcohol. The presence of a chlorine atom bonded to the carbon atom of the carbonyl group makes the carbon susceptible to nucleophilic attack by the alcohol.