| Abstract | L-askorbinska kiselina (vitamin C) je poznati antioksidans prisutan u voću i povrću. Svoj terapijski potencijal ostvaruje ne-enzimskom redukcijom slobodnih radikala. Moguća je njena primjena u organskim redoks reakcijama, npr. redukcija p-benzokinona. p-Benzokinon pripada skupini kinona, koji su poznati zbog svog antitumorskog, antibiotskog i fungicidnog djelovanja. Kombinacija askorbinske kiseline i kinona se razmatra zbog svoje potencijalne primjene u antitumorskoj terapiji. U ovom je diplomskom radu ispitana kinetika i mehanizam reakcije L-askorbinske kiseline i p-benzokinona radi boljeg razumijevanja reakcijskog mehanizma, ali i dodatnog opisivanja ova 2 reaktanta. Istraživanje kinetike i mehanizma ove reakcije provedeno je u čistoj vodi. Konstanta brzine reakcije L-askorbinske kiseline i p-benzokinona u vodi pri 25⁰C iznosi 415,7 ± 4,1 dm3 mol-1 s-1. Isto tako su izmjerene konstante brzine za ovu reakciju pri različitim temperaturama, u rasponu 15-40⁰C. Na temelju dobivenih rezultata prikazane su Arrheniusove i Eyringove ovisnosti konstante brzine o temperaturi iz kojih su određeni aktivacijski parametri. Arrheniusovi aktivacijski parametri iznose: Ea = 31,49 ± 1,89 kJ mol-1 i A= 1,368 ∙ 108. Eyringovi aktivacijski parametri iznose: ΔH‡ = 29,00 ± 1,89 kJ mol-1, ΔS‡= -97,53 ± 6,27 J K-1 mol-1 i ΔG‡ = 58,08 ± 2,66 kJ mol-1 pri 25⁰C. Određen je i kinetički izotopni efekt (KIE) za reakciju pri 25⁰C te iznosi 2,65 ± 0,10 i potvrđuje da u stupnju koji određuje brzinu reakcije dolazi do prijelaza protona. U konačnici je provedena termokemijska analiza reakcije te su stupnjeviti reakcijski mehanizmi (PT/ET i ET/PT) odbačeni kao mogući mehanizmi prijelaza protona i elektrona. Predloženi mehanizam je CPET. |
| Abstract (english) | L-ascorbic acid (vitamin C) is a well-known antioxidant present in fruits and vegetables. It achieves its therapeutic potential by non-enzymatic reduction of free radicals. It can be used in organic redox reactions, e.g. reduction of p-benzoquinone. p-Benzoquinone is a part of the group quinones, which are known for their antitumor, antibiotic and fungicidal activity. Combination of ascorbic acid and quinones is considered for its potential use in antitumor therapy. In this thesis, the kinetics and mechanism of the reaction of L-ascorbic acid and p-benzoquinone were examined for a better understanding of the reaction mechanism, but also for an additional description of these 2 reactants. Research of the kinetics and mechanism of this reaction was performed in pure water. The reaction rate constant of L-ascorbic acid and p-benzoquinone in water at 25⁰C is 415.7 ± 4.1 dm3 mol-1 s-1. The rate constants for this reaction were also measured at different temperatures, in the range 15-40⁰C. Based on the obtained results, the Arrhenius and Eyring plots of the rate constants against temperatures are presented, from which the activation parameters were determined. Arrhenius activation parameters are: Ea = 31.49 ± 1.89 kJ mol-1 and A= 1.368 ∙ 108. Eyring's activation parameters are: ΔH‡ = 29.00 ± 1.89 kJ mol-1, ΔS‡= -97.53 ± 6.27 J K-1 mol-1 and ΔG‡ = 58.08 ± 2.66 kJ mol-1 at 25⁰C. The kinetic isotope effect (KIE) for the reaction at 25⁰C was also determined: 2.65 ± 0.10, confirming that the proton transition occurs in the step that determines the rate of the reaction. In the end, a thermochemical analysis of the reaction was performed and stepwise reaction mechanisms (PT/ET and ET/PT) were rejected as possible mechanisms of proton and electron transition. The proposed mechanism is CPET. |
