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Molecularity, on the other hand, is deduced from the mechanism of an elementary reaction, and is used only in context of an elementary reaction. It is the number of molecules taking part in this reaction. This difference can be illustrated on the reaction between nitric oxide and hydrogen: [11]
An example of a simple chain reaction is the thermal decomposition of acetaldehyde (CH 3 CHO) to methane (CH 4) and carbon monoxide (CO). The experimental reaction order is 3/2, [4] which can be explained by a Rice-Herzfeld mechanism. [5] This reaction mechanism for acetaldehyde has 4 steps with rate equations for each step :
An example of a reaction taking place with an S N 1 reaction mechanism is the hydrolysis of tert-butyl bromide forming tert-butanol: This S N 1 reaction takes place in three steps: Formation of a tert -butyl carbocation by separation of a leaving group (a bromide anion) from the carbon atom: this step is slow.
An example of a substitution reaction taking place by a so-called borderline mechanism as originally studied by Hughes and Ingold [6] is the reaction of 1-phenylethyl chloride with sodium methoxide in methanol. The reaction rate is found to the sum of S N 1 and S N 2 components with 61% (3,5 M, 70 °C) taking place by the latter.
In practice, a reaction is assumed to be elementary if no reaction intermediates have been detected or need to be postulated to describe the reaction on a molecular scale. [1] An apparently elementary reaction may be in fact a stepwise reaction, i.e. a complicated sequence of chemical reactions, with reaction intermediates of variable lifetimes.
Another example is the unimolecular nucleophilic substitution (S N 1) reaction in organic chemistry, where it is the first, rate-determining step that is unimolecular. A specific case is the basic hydrolysis of tert-butyl bromide (t-C 4 H 9 Br) by aqueous sodium hydroxide. The mechanism has two steps (where R denotes the tert-butyl radical t-C ...
where A and B are reactants C is a product a, b, and c are stoichiometric coefficients,. the reaction rate is often found to have the form: = [] [] Here is the reaction rate constant that depends on temperature, and [A] and [B] are the molar concentrations of substances A and B in moles per unit volume of solution, assuming the reaction is taking place throughout the volume of the ...
Examples of associative mechanisms are commonly found in the chemistry of 16e square planar metal complexes, e.g. Vaska's complex and tetrachloroplatinate. The rate law is governed by the Eigen–Wilkins Mechanism. Dissociative substitution resembles the S N 1 mechanism in organic chemistry.