Syntheses and mechanisms of solvolysis of benzyl bromides in trifluoroethanol

Iz odgovarajućih benzilnih alkohola bromiranjem su sintetizirani različito supstituirani benzilni bromidi. Spojevi 4-(metiltio)benzil-bromid, 4-fenoksibenzil-bromid, 4-tert-butilbenzil-bromid, 4-metilbenzil-bromid, 3,4-dimetilbenzil-bromid, 4-fenilbenzil-bromid te nesupstituirani benzil-bromid dobiveni su bromiranjem fosforovim tribromidom, dok se u sintezi 4-metoksibenzil-bromida kao sredstvo za bromiranje koristio acetil-bromid. Provjera čistoće i identifikacija produkata izvršena je nuklearnom magnetskom rezonancijom i tankoslojnom tekućinskom kromatografijom. Brzine solvolize sintetiziranih spojeva određene su konduktometrijskom metodom u čistoj otopini trifluoretanola (prethodno pročišćen destilacijom) na 25 °C. Konstante brzine manje reaktivnih supstrata određene su na višim temperaturama, nakon čega su vrijednosti ekstrapolirane na 25 °C. Logaritmi konstanti brzina solvolize pojedinih supstrata korelirani su sa σ+ vrijednostima njihovih supstituenata. Nekonzistentne rezultate davali su spojevi s metoksi i metiltio supstituentima te nesuspstituirani benzilni bromid, tako da oni nisu korišteni u daljnjoj analizi. Hammett-Brownovom korelacijskom analizom dobivena su dva pravca. Prvi pravac definiran je točkama koje pripadaju 4-fenoksibenzil-bromidu i 3,4-dimetilbenzil-bromidu, dok je drugi pravac visokog koeficijenta korelacije definiran točkama koje pripadaju preostalim spojevima (redom 4-metilbenzil-bromid, 4-tert-butilbenzil-bromid i 4-fenilbenzil-bromid). Postojanje dvaju pravaca ukazuje na činjenicu da solvoliza jedne grupe spojeva teče SN1, a druge SN2 mehanizmom. U apsolutnom iznosu, nagib (koji predstavlja reakcijsku konstantu ρ+) prvog pravca je veći od drugog i oba su padajuća (negativni predznak). To znači da se tijekom solvolize u oba slučaja smanjuje elektronska gustoća na reakcijskom centru prijelaznog stanja, no kod pravca s negativnijom reakcijskom konstantom očito dolazi do značajne promjene naboja svojstvene SN1 reakcijama – stvara se relativno stabilni karbokationski međuprodukt. Eksperimentom je, dakle, dokazano da 4-fenoksibenzil-bromid i 3,4-dimetilbenzil-bromid solvoliziraju u trifuoroetanolu SN1 mehanizmom, dok 4-metilbenzil-bromid, 4-tert-butilbenzil-bromid i 4-fenilbenzil-bromid solvoliziraju SN2 mehanizmom. Budući da se upravo tim redoslijedom smanjuje apsolutan iznos σ+ vrijednosti supstituenata, dokazana je i početna hipoteza da reakcija solvolize benzilnih bromida s jačim elektron-donorskim supstituentima teče brže.Adequate benzyl alcohols were brominated in order to synthesize differently substituted benzyl bromides. 4-(methylthio)benzyl bromide, 4-phenoxybenzyl bromide, 4-methylbenzyl bromide, 3,4-dimethylbenzyl bromide, 4-tert-butylbenzyl bromide, 4-phenylbenzyl bromide and unsubstituted benzyl bromide were synthesized using phosphorus tribromide, while bromating agent used for synthesizing 4-methoxybenzyl bromide was acetyl bromide. Identification of products and their purity were confirmed by NMR (Nuclear Magnetic Resonance) and TLC (Thin Layer Chromatography). The solvolysis rate constants of synthesized compounds were determined conductometrically in pure 2,2,2-trifluoroethanol (distilled) at 25 °C. The rate constants for less reactive benzyl bromides were determined at higher temperatures and extrapolated to 25 °C. Logarithms of solvolyis rate constants were correlated against σ+ values of their substituents. 4-Methoxybenzly bromide, 4-(methylthio)benzyl bromide and unsubstituted benzyl bromide weren’t used in further analysis due to their high reactivity (the first two compounds in the series) or inactivity (the third one). Hammett-Brown correlation analysis was used in order to construct a graph. The graph showed two intersecting lines, both with negative slopes. The first line includes data for 4-phenoxybenzyl bromide and 3,4-dimethylbenzyl bromide, while the second line (with a high correlation coefficient) includes data for (in that order) 4-methylbenzyl bromide, 4-tert-butylbenzyl bromide and 4-phenylbenzyl bromide. The existence of the two lines indicates that there are two groups of compounds, one solvolysing by the SN1 mechanism and the other solvolysing by the SN2 mechanism. The slope (i.e. the reaction constant ρ+) of the first line is higher than the slope of the other line, however, both are negative. Thus, the both lines indicate a decrease in electron density on the benzyl reaction site during solvolysis. Additionally, in the first case, the increase of the positive charge on the reaction centre of the transition state is significant, which is characteristic for SN1 reactions, i.e. formation of relatively stable carbocations. The experiment has proved that 4-phenoxybenzyl bromide and 3,4-dimethylbenzyl bromide solvolyze in 2,2,2-trifluoroethanol by the SN1 mechanism, while 4-methylbenzyl bromide, 4-tert-butylbenzyl bromide and 4-phenylbenzyl bromide solvolyze in the same solvent by the SN2 mechanism. The experiment has also proved the initial hypothesis saying that benzyl bromides with stronger electron-donating substituents (quantitavely speaking, substituents with more negative σ+ values) solvolyze faster