Syntheses and mechanism of solvolysis of benzyl chlorides

Ovaj diplomski rad baziran je na sintezama para-supstituiranih benzilnih klorida iz parasupstituiranih benzilnih alkohola. Sintetizirani su sljedeći spojevi: 4-fenilbenzilni klorid i 4- fenoksibenzilni klorid pomoću klorovodične kiseline, a 4-metilbenzilni klorid, 4-metoksibenzilni klorid, 4-(metiltio)benzilni klorid i 4-tert-butilbenzilni klorid pomoću tionil-klorida. Navedeni spojevi su nakon sintetiziranja pročišćeni te su im identitet i čistoća potvrđeni nuklearnom magnetskom rezonancijom. Konduktometrijskim mjerenjem određene su konstante brzine solvolize na 60 °C u 80 %-tnoj vodenoj otopini etanola prethodno sintetiziranih različito supstituiranih benzilnih klorida kako bi se mogla prikazati Hammett-Brownova korelacija. U tu svrhu izračunati su logaritmi konstanti brzine solvolize (log k) koji su korelirani sa σ+ vrijednostima kako bi se utvrdio mehanizam nukleofilne supstitucije, SN1 ili SN2, kojim se odvija solvoliza sintetiziranih supstrata. Grafičkim su prikazom ovisnosti log k o σ+ vrijednosti dobivena dva pravca koja ukazuju na dva mehanizma reakcije, SN1 i SN2 mehanizam nukleofilne supstitucije. 4-metoksibenzilni klorid, 4-(metiltio)benzilni klorid i 4-fenoksibenzilni klorid solvatiziraju SN1 mehanizmom, dok 4-tert-butilbenzilni klorid i 4-fenilbenzilni klorid reagiraju s otapalom SN2 mehanizmom. Budući da se spoj 4-metilbenzilni klorid nalazi na sjecištu dvaju pravaca, za taj spoj će se izvršiti dodatna ispitivanja kako bi se utvrdio točan mehanizam reakcije, iako se može pretpostaviti da njegova reakcija s otapalom teče SN2 mehanizmom. Provedeni su i kvantno-mehanički izračuni. Na IEFPCM -wB97XD/6-311+G(3df, 3pd) razini teorije optimizirana je geometrija sljedećih supstrata: 4-metilbenzilnog klorida i 4-fenilbenzilnog klorida te SN2 prijelaznih stanja reakcije supstrata s vodom. Na optimiziranim geometrijama proveden je frekvencijski račun uz standardne uvjete (25 °C) kako bi se dobile slobodne energije supstrata i prijelaznih stanja. Izračunate su slobodne energije aktivacije za SN2 reakciju između supstrata i vode te konačno pripadajuće konstante brzine. U relativnim iznosima, teoretske i eksperimentalne reaktivnosti supstrata su u skladu.This thesis is based on the syntheses of para-substitued benzyl chlorides from para-substitued benzyl alcohols. The following compounds have been synthesized: 4-phenylbenzyl chloride and 4- phenoxybenzyl chloride using hydrochloric acid, and 4-methylbenzyl chloride, 4-methoxybenzyl chloride, 4-methyltiobenzyl chloride and 4-tert-butylbenzyl chloride using thionyl chloride. The above mentioned compounds were purified after the synthesis and their identity and purity were confirmed by nuclear magnetic resonance. Conductometric measurements were used to determine solvolysis rate constants at 60 °C in 80 % aqueous ethanol previously synthesised para-substituted benzyl chlorides in order to show Hammett-Brown correlation. For this purpose, logarithms of solvolysis rate constants were correlated with σ+ values in order to determine the nucleophilic substitution mechanism through which the solvolysis of these substrates proceeded. A correlation of log k versus σ+ values yielded two lines which represented two reaction mechanisms, SN1 and SN2 mechanism. Solvolysis reactions of 4-methoxybenzyl chloride, 4-methyltiobenzyl chloride and 4-phenoxybenzyl chloride proceed through SN1 mechanism, while 4-tert-butylbenzyl chloride and 4-phenylbenzyl chloride react with solvent through SN2 mechanism. Due to the fact that a point for 4-methylbenzyl chloride is situated at the intersection of the two lines, for this compound will be conducted an additional investigation in order to determine the exact reaction mechanism, however, it can be assumed that its solvolysis proceeds through the SN2 mechanism. In addition, quantum-mechanical calculations were carried out. Using the IEFPCM-wB97XD/6-311+G(3df, 3pd) level of theory, geometries of 4-methylbenzyl chloride and 4-phenylbenzyl chloride, as well as those of the corresponding SN2 transition structures were optimized. A frequency calculations were further performed on the optimized geometries using standard conditions in order to estimate free energies of substrates and transition structures. Free energies of activation for the SN2 reactions of the substrates with water were calculated, as well as first-order rate constants. The results reveal that relative theoretical and experimental reactivities of the substrates are consistent