| Abstract | Različito supstituirani benzil-bromidi su sintetizirani bromiranjem odgovarajućih
benzilnih alkohola. Spojevi 3,4-dimetilbenzil-bromid, 3,5-dimetilbenzil-bromid, 4-
fenoksibenzil-bromid, 4-(metiltio)benzil-bromid, 4-tert-butilbenzil-bromid, 4-metilbenzilbromid i 4-fenilbenzil-bromid sintetizirani su uz fosforov tribromid, a 4-metoksibenzil-bromid
uz acetil-bromid. Kako bi utvrdili čistoću spojeva provedene su nuklearna magnetska rezonancija (NMR) i
tankoslojna tekućinska kromatografija (TLC). Konduktometrijski su određene konstante
brzine solvolize sintetiziranih benzil-bromida u etanolu na 25 °C. Spojevima koji se zbog
manje reaktivnosti nisu mogli mjeriti na 25 °C su konstante brzine solvolize izmjerene na
povišenim temperaturama i naknadno ekstrapolirane na 25 °C. Logaritmi izmjerenih konstanti
brzina solvolize korelirani su sa σ + – vrijednostima odgovarajućih supstituenata prema
Hammett-Brownovoj korelacijskoj analizi. Dobivena su dva pravca različitih nagiba pri čemu
prvi pravac čine točke koje pripadaju 4-metoksibenzil-bromidu i 4-(metiltio)benzil-bromidu, a
drugi je definiran točkama koje pripadaju ostalim spojevima (redom 4-fenoksibenzil-bromid,
3,4-dimetilbenzil-bromid, 4-metilbenzil-bromid, 4-tert-butilbenzil-bromid, 4-fenilbenzilbromid, 3,5-dimetilbenzil-bromid). Svaki pravac predstavlja zaseban mehanizam nukleofilne
supstitucije u solvolizi ove serije bromida. Iznos reakcijske konstante ρ + (nagib pravaca) daje
nam prikaz promjene naboja na reakcijskom centru benzila. Nagibi oba pravca negativnog su
predznaka što upućuje na nastanak pozitivnog naboja na reakcijskom centru. Negativnija
vrijednost ρ + benzila lijevog pravca (ρ1 + = −6,30) upućuje na nastajanje stabilnog karbokationa
u prvom stupnju SN1 reakcije. Manje negativan nagib desnog pravca (ρ2 + = −2,34) sugerira
SN2 mehanizam pripadajućih benzil-bromida. Dakle, eksperimentom je dokazano da 4-metoksibenzil-bromid i 4-(metiltio)benzil-bromid u etanolu solvoliziraju SN1 mehanizmom, dok 4-fenoksibenzil-bromid, 3,4-dimetilbenzilbromid, 4-metilbenzil-bromid, 4-tert-butilbenzil-bromid, 4-fenilbenzil-bromid i 3,5- dimetilbenzil-bromid solvoliziraju SN2 mehanizmom. Budući da 4-metoksibenzil-bromid ima najveći apsolutan iznos σ + vrijednosti supstituenta i najveću konstantu brzine reakcije, dokazana je početna pretpostavka da reakcija solvolize spojeva s jačim elektron-donorskim supstituentima teče brže. |
| Abstract (english) | Differently substituted benzyl bromides were synthesized by bromination from adequate
benzyl alcohols. Compounds 3,4-dimethylbenzyl bromide, 3,5-dimethylbenzyl bromide, 4-
phenoxybenzyl bromide, 4-(methylthio)benzyl bromide, 4-tert-butylbenzyl bromide, 4-
methylbenzyl bromide and 4-phenylbenzyl bromide were synthesized with phosphorus
tribromide, while 4-methoxybenzyl bromide was synthesized with acetyl bromide.
Nuclear magnetic resonance (NMR) and Thin-layer chromatography (TLC) were carried
out to determine the purity of the compounds. The solvolysis rate constants of synthesized
benzyl bromides were determined conductometrically in ethanol at 25 °C. Solvolysis reaction
constants for compounds which couldn`t be measured at 25 °C because od their lesser
reactivity were determined at higher temperatures and later extrapolated to 25 °C. The
measured values of the logarithms of reaction constants of solvolysis were correlated with the
σ + -values of adequate substituents according to the Hammett-Brown correlation analysis. Two
plots with different slopes were obtained where the first plot includes data for 4-metoxybenzyl
bromide and 4-(methylthio)benzyl bromide, while the second plot is defined by the data which
belongs to other compounds (4-phenoxybenzyl bromide, 3,4-dimethylbenzyl bromide, 4-
methylbenzyl bromide, 4-tert-butylbenzyl bromide, 4-phenylbenzyl bromide and 3,5-
dimethylbenzyl bromide). Each plot represents a separate mechanism of nucleophilic
substitution in solvolysis of this bromide series. The value of the reaction constant ρ
+ (the slope of the plots) gives us a display of the charge change on the benzyl reaction site. The
slopes of both plots have negative value which indicates a formation of a positive charge on
the reaction center. The more negative value of ρ + for benzyls on the left plot (ρ1
+ = −6,30) indicates the formation of a stable carbocation in the first step of an SN1 reaction. Lesser
negative slope of the right plot (ρ2 + = −2,34) suggests an SN2 mechanism of belonging benzyl
bromides. Therefore, the experiment has proven that 4-methoxybenzyl bromide and 4-
(methylthio)benzyl bromide solvolyze in ethanol by SN1 mechanism, while 4-phenoxybenzyl
bromide, 3,4-dimethylbenzyl bromide, 4-methylbenzyl bromide, 4-tert- butylbenzyl bromide,
4-phenylbenzyl bromide and 3,5-dimethylbenzyl bromide solvolyze by SN2 mechanism. Since
4-methoxybenzyl bromide has the highest absolute substituent σ + value and the highest
reaction rate constant, the inital assumption that the solvolysis reaction of compounds with
stronger electron-donating substitutents happens faster, was proven. |
