| Abstract | Sintetizirani su spojevi 4-fenoksibenzil-bromid, 4-tert-butilbenzil-bromid, 4-metilbenzil-bromid i 4-fenilbenzil-bromid uz reagens za bromiranje fosforov tribromid, a 4-(metiltio)benzil-bromid i 4-metoksibenzil-bromid uz acetil-bromid. Brzine solvolize sintetiziranih benzil-bromida određene su konduktometrijski u 80 %-tnoj vodenoj otopini etanola na 25 °C. Logaritmi izmjerenih solvolitičkih konstanti brzine korelirani su sa σ+-vrijednostima prema Hammett-Brownovoj korelacijskoj analizi. Analizom su dobivena dva pravca s različitim negativnim nagibima te visokim koeficijentima korelacije, što je potvrdilo validnost jednadžbe: log(𝑘X𝑘H⁄)= 𝜌+× 𝜎+X. No, kako su dobivena dva pravca, to odstupanje od linearnosti objašnjeno je postojanjem dva mehanizma nukleofilne supstitucije u solvolizi ove serije bromida. Kako se mijenja priroda supstituenta benzil-bromida, mijenja se i reaktivnost pojedinog supstrata solvolize, kao i sam mehanizam. Vrijednost reakcijske konstante ρ+ daje uvid u promjenu naboja na reakcijskom centru benzila, dok negativna vrijednost upućuje na nastanak i delokalizaciju pozitivnog naboja u karbokationskom fragmentu prijelaznog stanja, što je karakteristika SN1 mehanizma kod solvolize. Negativniji ρ+ (ρ1+ = −6,81) za bromide lijevog pravca (4-metoksibenzil-bromid, 4-(metiltio)benzil-bromid i 4-fenoksibenzil-bromid) govori da solvolizom nastaju prijelazna stanja sa znatnom količinom pozitivnog naboja na benzilnom reakcijskom centru, što je preduvjet za nastanak karbokationa, tj. odvijanje reakcije SN1 mehanizmom. S obzirom na navedeno, može se zaključiti da 4-metilbenzil-bromid, 4-tert-butilbenzil-bromid i 4-fenilbenzil-bromid s nukleofilom 80 %-tnim etanolom reagiraju SN2 mehanizmom budući da je nagib pripadajućeg pravca iz Hammett-Brownove korelacijske analize manjeg negativnog iznosa (ρ2+ = −2,26).
4-metoksibenzil-bromid s najvećom konstantom brzine reakcije te s negativnom σ+-vrijednosti pokazuje najjači elektron-donorski efekt supstituenta kojim stabilizira pozitivan naboj na benzilnom dijelu prijelaznog stanja SN1 reakcije solvolize, a samim time i pozitivan naboj slobodnog karbokationa koji nastaje u prvom stupnju SN1 reakcije.
Reakcijski mehanizam solvolize proučen je i programskim paketom Gaussian 16. Optimizirane su geometrije supstrata 4-metilbenzil-bromida i 4-fenilbenzil-bromida i njihovih SN2 prijelaznih stanja u reakciji s vodom te su provedeni frekvencijski računi na M08HX/6-311+G(3df,3pd) razini teorije, uz korištenje IEFPCM solvatacijskog modela za vodu. Iz dobivenih podataka, pomoću Eyringove jednadžbe, izračunate su konstante brzine prvog reda. Vrijednosti konstanti brzine ukazuju da je reaktivniji spoj 4-metilbenzil-bromid, kao što su pokazala i eksperimentalna mjerenja. |
| Abstract (english) | The following compounds were synthesized with a brominating agent phosphorus tribromide: 4-phenoxybenzyl bromide, 4-tert-butylbenzyl bromide, 4-methylbenzyl bromide and 4-phenylbenzyl bromide, while 4-(methylthio)benzyl bromide and 4-methoxybenzyl bromide were synthesized with acetyl bromide. The rates of solvolysis of the synthesized compounds were determined conductometricly in 80 % aq. ethanol at 25 °C. The measured values of the logarithms of the rate constants of solvolysis were correlated with the σ+-values of the substituents according to the Hammett-Brown correlation analysis. Plots, with different negative slopes and the high degree of correlation were obtained, which confirmed the validity of the equation: log(𝑘X𝑘H⁄)= 𝜌+× 𝜎+X. However, with two plots, this deviation from the linearity was explained by two different mechanisms of nucleophilic substituion of benzyl bromides. Changing the nature of the substituent, the reactivity and the mechanism were also changed. The magnitude of the reaction parameter ρ+ gives an insight into the charge change on the benzyl reaction site. Its negative value refers to the delocalisation of a carbocation's positive charge, which is a charateristic of the SN1 mechanism. The more negative value of ρ+ (ρ1+ = −6,81) for bromides on the left plot (4-methoxybenzyl bromide, 4-(methylthio)benzyl bromide and 4-phenoxybenzyl bromide) than for ones on the right plot indicates that in the transition state of the first ones a lot of positive charge is generated. That is necessary for the formation of a carbocation in SN1 mechanism. Therefore, 4-methylbenzyl bromide, 4-tert-butylbenzyl bromide and 4-phenylbenzyl bromide with 80 % aq. ethanol react by SN2 mechanism (ρ2+ = −2,26).
4-Methoxybenzyl bromide, with the highest value of the rate constant and a negative σ+-value, indicates the highest electron donating effect so it stabilizes the positive charge of the SN1 carbocation.
The reaction mechanism of solvolysis was also studied with software package Gaussian 16. The geometry optimization was made for the substrates 4-methylbenzyl bromide, 4-phenylbenzyl bromide and their transition states in SN2 reaction with water. Then frequency calculations were performed at the M08HX/6-311+G(3df,3pd) level of theory using IEFPCM solvent model for water. The rate constants were measured from the data obtained with the Eyring equation. The values of the rate constants indicate 4-methylbenzyl bromide is more reactive than 4-phenylbenzyl bromide, as it was shown by experimental measurements. |
