VJC 2025 J2 H2 Chemistry Hydroxy Compounds Appendix

Victoria Junior College JC2 H2 Chemistry Hydroxy Compounds – Appendix 1. Alkoxide Ions and Ethers We have learnt in this topic that alcohols undergo a redox reaction with reactive metals, such as Na, to form alkoxides and hydrogen gas. for example, 2CH3CH2OH + 2Na → 2CH3CH2O-Na+ + H2 This reaction can also be carried out with very strong bases, such as sodium amide (NaNH2) and sodium hydride (NaH): CH3CH2OH + NaNH2 → CH3CH2O-Na+ + NH3 CH3CH2OH + NaH → CH3CH2O-Na+ + H2 The alkoxide ions formed in this way can be used as bases or nucleophiles: a) As bases: Recall that alcohols are weaker acids than water. (Refer to Hydroxy Compounds Notes for more elaboration on this) – therefore, this makes alkoxide ions stronger bases than hydroxide ion. For example, elimination reactions of halogenoalkanes can also be carried out using alkoxide ions: b) As nucleophiles: Alkoxide ions can act as nucleophiles, for example, in their reactions with halogenoalkanes, to form ethers: ethanol sodium ethoxide ethoxyethane (an ether) – this functional group is not required for H2 syllabus.

The ethers formed are generally unreactive . They are commonly used as solvents in the laboratory. They are also functional group isomers of alcohols: 2. Comparing Lithium Aluminium Hydride (LiAlH4) and Sodium Borohydride (NaBH4) as Reducing Agents Both LiAlH4 and NaBH 4 are reducing agents as they contain polar metal - hydrogen bonds, where the H atoms bear a negative charge, thus behaving like a nucleophilic hydride ion, H-. (This is because H is more electronegative than both B and Al; thus when covalently bonded to B or Al, the H atom acquires a partial negative charge.) However, as Al is more electropositive than B (recall from Periodic Table that electronegativity decreases down the same group), the Al - H bond is more polar than the B – H bond. This makes Li AlH4 a stronger reducing agent than NaBH4: Functional Groups Reduced By LiAlH4 Functional Groups Reduced by NaBH4 aldehydes to 1o alcohols aldehydes to 1o alcohols ketones to 2o alcohols ketones to 2o alcohols carboxylic acids to 1o alcohols nitriles to amines amides to amines However, LiAlH4 is also much harder (and more dangerous) to handle than NaBH4. For example, Li AlH4 is incompatible with water, alcohols, halogen derivatives and esters (all of which are common solvents). It reacts vigorously with water to liberate hydrogen gas: LiAlH4 + 4H2O ⟶ LiOH + Al(OH)3 + 4H2 On the other hand, reductions using NaBH4 are usually milder and can take place in alcoholic solvents. ethoxyethane butan-1-ol