Reakcijas mehānisms

Reakcijas mehānisms ķīmiskās reakcijās ir detalizēti procesi, kuru laikā ķīmiskās vielas tiek pārveidotas par citām vielām. Pašās reakcijās var būt iesaistīta atomu, molekulu, jonu, elektronu un brīvo radikāļu mijiedarbība, un tās var notikt gāzēs, šķidrumos vai cietās daļās vai pie jebkura no šiem savienojumiem.

Detalizētu reakcijas mehānismu procesu izpēte ir svarīga daudzu iemeslu dēļ, ieskaitot palīdzību, ko tas sniedz ķīmisko reakciju izpratnē un kontrolē. Daudzas ļoti nozīmīgas komerciālas reakcijas var notikt vairāk nekā vienā veidā; zināšanas par iesaistītajiem reakcijas mehānismiem var dot iespēju izvēlēties reakcijas apstākļus, kas dod priekšroku vienam ceļam pret otru, tādējādi nodrošinot maksimālo vēlamo produktu daudzumu un minimālo nevēlamo produktu daudzumu. Turklāt, pamatojoties uz reakcijas mehānismiem, dažreiz ir iespējams atrast korelācijas starp sistēmām, kas citādi nav acīmredzami saistītas. Spēja zīmēt šādas analoģijas bieži ļauj prognozēt neizmēģinātu reakciju gaitu. Visbeidzotdetalizēta informācija par reakcijas mehānismiem ļauj apvienot un izprast lielos citādā veidā nesaistītu parādību kopumus, kam ir liela nozīme ķīmijas teorijā un praksē.

Parasti ķīmiskās reakcijas, kuru mehānismi interesē ķīmiķus, notiek šķīdumā un ietver kovalento saišu pārrāvumu un pārveidošanu starp atomiem - kovalentās saites ir tās, kurās elektroni ir sadalīti starp atomiem. Interese par šīm reakcijām ir īpaši liela, jo tās ir reakcijas, kurās tiek sagatavoti tādi materiāli kā plastmasa, krāsvielas, sintētiskās šķiedras un ārstniecības līdzekļi, un tāpēc, ka lielākā daļa dzīvo sistēmu bioķīmisko reakciju ir šāda veida. Turklāt šāda veida reakcijas parasti notiek pētījumiem ērtā laikā, ne pārāk ātri, ne pārāk lēni, un apstākļos, kurus viegli manipulēt eksperimentālos nolūkos. Ir vairākas metodes, ar kuru palīdzību var izpētīt šādu reakciju mehānismus.

Ķīmiskās reakcijas ir saistītas ar izmaiņām molekulu savienošanas modeļos, tas ir, mainās atomu relatīvās pozīcijas molekulās un starp tām, kā arī mainās elektroni, kas atomus satur kopā ķīmiskās saitēs. Tāpēc reakcijas mehānismos jāiekļauj šo kustību apraksti, ņemot vērā telpiskās izmaiņas un arī laiku. Kopējo izmaiņu ceļu sauc par reakcijas gaitu, un detalizēto procesu, kurā notiek pārmaiņas, sauc par reakcijas ceļu vai ceļu.

Also important to the study of reaction mechanisms are the energy requirements of the reactions. Most reactions of mechanistic interest are activated processes—that is, processes that must have a supply of energy before they can occur. The energy is consumed in carrying the starting material of the reaction over an energy barrier. This process occurs when the starting material absorbs energy and is converted to an activated complex or transition state. The activated complex then proceeds to furnish the product of the reaction without further input of energy—often, in fact, with a release of energy. Such considerations are important to an understanding of reaction mechanisms because the actual course that any reaction follows is the one that requires the least energy of activation. This reaction course is not always the one that would seem simplest to the chemist without detailed study of the different possible mechanisms.

The study of reaction mechanisms is complicated by the reversibility of most reactions (the tendency of the reaction products to revert to the starting materials) and by the existence of competing reactions (reactions that convert the starting material to something other than the desired products). Another complicating factor is the fact that many reactions occur in stages in which intermediate products (intermediates) are formed and then converted by further reactions to the final products. In examining chemical reactions, it is useful to consider several general subjects: (1) factors that influence the course of chemical reactions, (2) energy changes involved in the course of a typical reaction, (3) factors that reveal the mechanism of a reaction, and (4) the classification of reaction mechanisms. With this information in mind, it is then possible to look briefly at some of the more important classes of reaction mechanisms. (The articles acid-base reaction, oxidation-reduction reaction, and electrochemical reaction deal with the mechanisms of reactions not described in this article.)

General considerations

Determinants of the course of reaction

The reactants

In analyzing the mechanism of a reaction, account must be taken of all the factors that influence its course. After the bulk chemical constituents have been identified by ordinary methods of structure determination and analysis, any prereaction changes involving the reactants, either individually or together, must be investigated. Thus, in the cleavage of the substance ethyl acetate by water (hydrolysis), the actual reagent that attacks the ethyl acetate molecule may be the water molecule itself, or it may be the hydroxide ion (OH―) produced from it.

The hydrolysis of ethyl acetate can be represented by the following equation:

in which the structures of the molecules are represented schematically by their structural formulas. An arrow is used to indicate the reaction, with the formulas for the starting materials on the left and those of the products on the right. In the structural formulas, the atoms of the elements are represented by their chemical symbols (C for carbon, H for hydrogen, and O for oxygen), and the numbers of the atoms in particular groups are designated by numeral subscripts. The chemical bonds of greatest interest are represented by short lines between the symbols of the atoms connected by the bonds.

Important to this reaction is an equilibrium involving the cleavage of the water molecules into positively and negatively charged particles (ions), as follows:

In this equation the numeral in front of the symbol for the water molecule indicates the number of molecules involved in the reaction. The composite arrow indicates that the reaction can proceed in either direction, starting material being converted to products and vice versa. In practice, both reactions occur together, and a balance, or equilibrium, of starting materials and products is set up. The significance of this equilibrium for the hydrolysis of ethyl acetate is that any of the three entities (water molecules, hydronium, or hydroxide ions) may be involved in the reaction, and the mechanism is not known until it is established which of these is the actual participant. This often can be established if it is possible to determine the relative amounts of the three in the reaction medium and if it can be shown that the rate of the reaction depends upon the amount (or concentration) of one of them. Under certain conditions the hydrolysis of ethyl acetate is found to involve water molecules (as shown in the equation above); in other cases, hydroxide ion is involved.