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Introduction to Reaction Variables

The Variables of Organic Reactions

In an attempt to know why and how reactions of functional groups take place in the manner they do, the chemists try to find out just how unlike molecules and ions get relate with each other as they come together. To this end it is essential to think about the several properties and characteristics of a reaction that may be measured and/or observed as the reaction proceeds. The most general and useful of these are listed below:

1. Reactants and Reagents

A. Reactant Structure: Variations in the reactant's structure may have a marked affected on the course of a reaction, even though the functional group is not changed. So, reaction of 1-bromopropane with sodium cyanide continued easily to yield butanenitrile, where the 1-bromo-2,2-dimethylpropane fails to give any product and is recovered unchanged. In opposite, both alkyl bromides form Grignard reagents (RMgBr) on reaction with magnesium.

 

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B. Reagent Characteristics: actually minor changes in a reagent may lead to a major change in the course of a reaction. For an instance, 2-bromopropane gives a substitution reaction with sodium methylthiolate but go through predominant elimination on treatment with sodium methoxide.

 

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2. Product Selectivity

A. Regioselectivity: It is generally the case that elimination and addition reactions may in principle precede to more than one product. So 1-butene might add HBr to give either 1-bromobutane or 2-bromobutane that depends on which the carbon of the double bond receives the bromine and which the hydrogen. If the one possible product out of two or more is formed preferentially, reaction is said to be regioselective.

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Simple substitution reactions are not generally considered regioselective, because from the definition only one constitutional product is possible. Though, rearrangements are known to occur during some reactions.

B. Stereoselectivity: If the reaction products are like that stereoisomer may be formed, a reaction that yields the one stereoisomer preferentially is said to be stereoselective. In addition of bromine to cyclohexene, for an instance the cis and trans-1,2-dibromocyclohexane are both possible products of the addition. Since trans-isomer is the only isolated product this reaction is stereoselective.

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Stereospecificity: This expression is applied to cases in which stereoisomeric reactants behave differently in a given reaction. Examples of this include:

(i) Formation of the different stereoisomeric products, same as in the reaction of enantiomeric 2-bromobutane isomers with sodium methylthiolate, shown in the following picture.

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Here, the (S)-reactant produces (R)-product and (R)-reactant gives the configurationally inverted (S)-product. The (S) and (R) notations for configuration are explained in a later section of this text.

(ii) The Different rates of the reaction, as in the base-induced elimination of cis & trans-4-tert-butylcyclohexyl bromide (equation 1 below).

(iii) The Different reaction paths leading to different products, similar as in the base-induced elimination of cis & trans-2-methylcyclohexyl bromide (equation 2 below).

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The method of these substitutions and elimination reactions are discussed in the alkyl halide section of this text.

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