| Abstract | Tema ovog rada je proučavanje sinteze i mehanizma solvolize različito supstituiranih benzilnih
bromida u 80 %-tnom metanolu. U tu svrhu sintetizirani su 4-metoksibenzil-bromid uz pomoć acetilbromida, a 4-(metiltio)benzil-bromid, 4-fenoksibenzil-bromid, 3,4-dimetilbenzil-bromid, 4-
metilbenzil-bromid, 4-tert-butilbenzil-bromid, 4-fenilbenzil-bromid i 3,5-dimetilbenzil-bromid uz
pomoć fosforovog tribromida. Čistoća ovih spojeva utvrđena je tankoslojnom tekućinskom
kromatografijom (TLC) i nuklearnom magnetskom rezonancijom (NMR).
Konstante brzine solvolize navedenih benzil-bromida utvrđene su konduktometrijski u 80 %-
tnom metanolu pri 25 °C. Spoj 3,5-dimetilbenzil-bromid sporije je solvolizirao pri 25 °C, pa su
njegove konstante brzine solvolize utvrđene pri višim temperaturama i potom ekstrapolirane na 25
°C pomoću Eyringove jednadžbe. Napravljena je korelacijska analiza logaritama konstanti brzine
solvolize i σ + - vrijednosti odgovarajućih supstituenata kako bi dobili Hammett-Brownov graf.
Rezultat su dva pravca različitih nagiba, pri čemu je prvi definiran točkama koje pripadaju 4-
metoksibenzil-bromidu, 4-(metiltio)benzil-bromidu i 4-fenoksibenzil-bromidu. Drugi graf definiran
je točkama koje pripadaju 3,4-dimetilbenzil-bromidu, 4-metilbenzil-bromidu, 4-tert-butilbenzilbromidu, 4-fenilbenzil-bromidu i 3,5-dimetilbenzil-bromidu. Nagibi pravaca označuju iznos reakcijske konstante ρ + koja nam govori o promjeni naboja na reakcijskom centru. U slučaju strmijeg pravca visoka negativna vrijednost reakcijske konstante (ρ + = -6,79) sugerira na nastanak stabilnog
karbokationa koji je karakterističan za SN1 reakciju. Manje strmi pravac ima vrijednost reakcijske
konstante ρ + = - 2,49, što sugerira na SN2 mehanizam reakcije.
Eksperimentom je dokazano da spojevi 4-metoksibenzil-bromid, 4-(metiltio)benzil-bromid i
4-fenoksibenzil-bromid solvoliziraju SN1 mehanizmom, a 3,4-dimetilbenzil-bromid, 4-metilbenzilbromid, 4-tert-butilbenzil-bromid, 4-fenilbenzil-bromid i 3,5-dimetilbenzil-bromid solvoliziraju
SN2 mehanizmom. |
| Abstract (english) | The topic of this thesis is the study of the synthesis and solvolysis mechanisms of variously
substituted benzyl bromides in 80 % methanol. For this purpose, 4-methoxybenzyl bromide was
synthesized using acetyl bromide, while 4-(methylthio)benzyl bromide, 4-phenoxybenzyl bromide,
3,4-dimethylbenzyl bromide, 4-methylbenzyl bromide, 4-tert-butylbenzyl bromide, 4-phenylbenzyl
bromide, and 3,5-dimethylbenzyl bromide were synthesized using phosphorus tribromide. The
purity of these compounds was confirmed using thin-layer chromatography (TLC) and nuclear
magnetic resonance (NMR).
The rate constants of solvolysis for the benzyl bromides were determined conductometrically
in 80 % methanol at 25 °C. The solvolysis of 3,5-dimethylbenzyl bromide was slower at 25 °C, so
its rate constants were measured at higher temperatures and then extrapolated to 25 °C using the
Eyring equation. A correlation analysis of the solvolysis rate constants and σ+ values of the
corresponding substituents was conducted to create a Hammett-Brown plot. The result is two lines
with different slopes: one defined by 4-methoxybenzyl bromide, 4-(methylthio)benzyl bromide, and
4-phenoxybenzyl bromide, and the other by 3,4-dimethylbenzyl bromide, 4-methylbenzyl bromide,
4-tert-butylbenzyl bromide, 4-phenylbenzyl bromide, and 3,5-dimethylbenzyl bromide. The slopes
indicate the reaction constant ρ+, which provides insight into the change in charge at the reaction
center. The steeper line, with a highly negative ρ+ value (-6.79), suggests the formation of a stable
carbocation typical of an SN1 reaction, while the less steep line, with ρ+ = -2.49, points to an SN2
mechanism.
The experiment confirmed that 4-methoxybenzyl bromide, 4-(methylthio)benzyl bromide,
and 4-phenoxybenzyl bromide undergo solvolysis via the SN1 mechanism, whereas 3,4-
dimethylbenzyl bromide, 4-methylbenzyl bromide, 4-tert-butylbenzyl bromide, 4-phenylbenzyl
bromide, and 3,5-dimethylbenzyl bromide undergo solvolysis via the SN2 mechanism. |
