Question

5. Consider these BF: affinities for various sulfoxides (in all of them sulfur atom serves as electron pair donor to the boron center): (6 points) BF, Affinity Lewis base Formula (kJ/mol 90.34 Diphenylsulfoxide Ph2SO 97.37 Methyl phenyl sulfoxide PhSoMe 105.34 Dimethyl sulfoxide Me,SO Draw the structures of each sulfoxide and its BF, complex. Then, rationalize this affinity trend using inductive and resonance arguments for electronic structures of the sulfoxides involved.

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Answer 1

Structure of sulphoxide Structure of attesty adduct with BF3 (1) Phaso, ph 1 mm Ph (2) Phsome, ho B - Me TD Measo F 1 Me -$: B - F Me MO 2 This is a Lewis acid-base reaction. Here sulphoxide acts as base and BF3 acts as auid. Lewis barse character is totally depends on the availaleality of lone pair electrons on the base (here sulphur) tere affnity value of sulphoxide towords BF3 in KJ/mole determines ease of formation of the adduct. Move the affinity more will be tendency of formation of adduct and more will be ene be energy released in terms of afinity in KJ mole. In Phaso, the lone pair on the s-atom ing in conjugation with the benzene rings (by two benzene rings). so the avialability of none pairs of electron is less for the adduct formation, Hence the affinity in terms of energy CKJ /mol) is less C90.34 KJ/mok). In shsome the delocalisation of lone pair take place via one sing and CHg group unoreases some electron density on s atom via t I effect. Hence the affiniter is greater than Phaso (97,37 kJ/mole out in Measo, there is no benzene ring. Hence detocalisation of lone pair con not take pplace. Again two - che groups increases the electron density by off effect. Hence affinity is highest ( 105.34 kJ/moles Affinity (KJ smole) & Phaso <Ph some < Measo.