Non-aqueous media Chapter 9

9.1 Introduction Some reagents may react with H 2 O (e.g. the alkali metals) and non-polar molecules are insoluble in water. solvents other than water

 protic solvents (e.g. HF, H 2 SO 4, MeOH);  aprotic solvents (e.g. N 2 O 4, BrF 3 );  coordinating solvents (e.g. MeCN, Et 2 O, Me 2 CO).  A protic solvent undergoes self-ionization to provide protons which are solvated.  If it undergoes self ionization, an aprotic solvent does so without the formation of protons.  A protic solvent undergoes self-ionization to provide protons which are solvated.  If it undergoes self ionization, an aprotic solvent does so without the formation of protons. Using non-aqueous solvents are limited many are highly reactive.

9.2 Relative permittivity In a vacuum, the Coulombic potential energy of a system of two unit electronic charges is given by equation If a material is placed between the charges, the force is reduced by an amount that depends upon the relative permittivity of the material  r.

Ion–solvent interactions are favoured (e.g. to facilitate the dissolution of an ionic salt) by using a solvent with a large dipole moment, but for maximum effect, the solvent molecule should also be small

9.4 Acid--base behaviour in non-aqueous solvents Strengths of acids and bases if HCl is dissolved in acetic acid, the extent of ionization is far less than in water and HCl behaves as a weak acid. Levelling and differentiating effects in a basic solvent, all acids are strong. The solvent is said to exhibit a leveling effect on the acid

In an acidic solvent (e.g. MeCO 2 H, H 2 SO 4 ), ionization of bases is facilitated; most acids are relatively weak under these conditions, and some even ionize as bases. In acetic acid the extent of ionization of the three hydrogen halides varies along the series: HI > HBr > HCl. Thus, acetic acid exerts a differentiating effect on the acidic behaviour of HCl, HBr and HI, whereas water does not.

‘Acids’ in acidic solvents Acids and bases: a solvent-oriented definition A Brønsted acid is a proton donor, and a Brønsted base accepts protons. in a liquid ammonia solution, an acid may be described as a substance that produces NH 4 + ions, while a base produces NH 2 

In a self-ionizing solvent, an acid is a substance that produces the cation characteristic of the solvent, and a base is a substance that produces the anion characteristic of the solvent. Liquid dinitrogen tetraoxide, N 2 O 4, undergoes the self ionization Nitrosyl salts such as [NO][ClO 4 ] behave as acids, and metal nitrates (e.g. NaNO 3 ) behave as bases.

9.6 Liquid ammonia Liquid NH 3 is an ideal solvent for reactions requiring a strong base, since the amide ion is strongly basic.

9.7 Liquid hydrogen fluoride It can be handled in polytetrafluoroethene (PTFE) containers, or, if absolutely free of water, in Cu or Monel metal (a nickel alloy) equipment.

9.9 Superacids Extremely potent acids, capable of protonating even hydrocarbons, are termed superacids and include mixtures of HF and SbF 5 and HSO 3 F and SbF 5 is called magic acid (one of the strongest acids known)

In superacidic media, hydrocarbons act as bases