Nucleofugality of ethyl-carbonates in aqueous acetonitrile solutions

Izmjerene su brzine solvolize sintetiziranih spojeva: 4,4'-dimetoksibenzhidril-etil-karbonata, 4-metoksibenzhidril-etil-karbonata, 4-metoksi-4'-fenoksibenzhidril-etil-karbonata i 4-metoksi-4'-metilbenzhidril-etil-karbonata u 60%-tnoj i 80%-tnoj vodenoj otopini acetonitrila. Pomoću izračunatih brzina solvolize konstruiran je graf ovisnosti log k o Ef vrijednostima koji je pokazao izvrsnu linearnu korelaciju. Primjenom jednadžbe pravca, log k (25 °C) = sf (Nf + Ef), izračunati su parametri nukleofugalnosti etil-karbonata, i iznose: Nf = –2,85 i sf = 0,99 u 80%-tnom acetonitrilu, te Nf = –2,54 i sf = 0,90 u 60%-tnom acetonitrilu. Usporedbom vrijednosti izmjerenih u dvije vrste otapala uočeno je da se povećanjem udjela vode u otapalu, odnosno povećanje polarnosti otapala povećava reaktivnost izlazne skupine tj. nukleofugalnost, a smanjuje vrijednost sf, što je u skladu s prethodno dobivenim rezultatima za metil- i fenil-karbonate. Prema reaktivnosti u vodenim otopinama acetonitrila, etilkarbonat se smjestio među manje reaktivne izlazne skupine pri čemu je tosilatna najreaktivnija (Nf/sf =7,97/0,82 u 60% acetonitrilu), a 2,2-dimetilpropanoatna (Nf/sf = –5,32/1,02 u 60% acetonitrilu) najmanje reaktivna izlazna skupina.The rate constants of solvolysis have been determined for synthesized compounds: 4,4'-dimethoxybenzhydryl ethyl carbonate, 4-methoxybenzhydryl ethyl carbonate, 4-methoxy-4'-phenoxybenzhydryl ethyl carbonate and 4-methoxy-4'-methylbenzhydryl ethyl carbonate in 80 % aqueous acetonitrile and 60 % aqueous acetonitrile. The observed and calculated rate constants of solvolysis have been used for plotting log k vs. Ef - value graph which shows great linear correlation. The equation log k (25 °C) = sf (Nf + Ef) was used to calculate Nf and sf values in 80 % aqueous acetonitrile and 60 % aqueous acetonitrile. Obtained values are Nf = –2,85 and sf = 0,99 in 80 % aqueous acetonitrile and Nf = –2,54 and sf = 0,90 in 60 % aqueous acetonitrile. By comparing this values it can be seen that Nf value decreases as the water content in the solution increases, ie. the more polar solvent is, leaving group is more reactive, while sf value decreases, which is in correspondence with previously known results for methyl and phenyl carbonates. According to reactivity in aqueous acetonitrile solutions, ethyl carbonate is less reactive leaving group, on the scale on which tosylate group is the most reactive, Nf/sf = 7,97/0,82 in 60 % aqueous acetonitrile and pivalate group is the least reactive Nf/sf = –5,32/1,02 in 60 % aqueous acetonitrile