Naslov Sinteze i mehanizmi etanolize derivata benzilnih bromida Naslov (engleski) Syntheses and mechanisms of ethanolysis of benzyl bromide derivatives Autor Armin Šahić Mentor Mirela Matić (mentor)Član povjerenstva Mirela Matić (predsjednik povjerenstva)Član povjerenstva Bernard Denegri (član povjerenstva)Član povjerenstva Jasna Jablan (član povjerenstva)Ustanova koja je dodijelila Sveučilište u Zagrebu Datum i država obrane 2022-04-12, Hrvatska Znanstveno / umjetničko PRIRODNE ZNANOSTI Sažetak 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.
Sažetak (engleski) 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.
Ključne riječi
nukleofilna supstitucija
benzil-bromidi
solvoliza
Hammett-Brownova jednadžba
Ključne riječi (engleski)
nucleophilic substitution
benzyl bromides
solvolysis
Hammett-Brown equation
Jezik hrvatski URN:NBN urn:nbn:hr:163:901961 Studijski program Naziv: Farmacija Vrsta resursa Tekst Način izrade datoteke Izvorno digitalna Prava pristupa Otvoreni pristup Uvjeti korištenja Datum i vrijeme pohrane 2022-05-26 09:16:42
