Chemistry

Identifying the Basic Types of Chemical Reactions



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We will discuss the types of chemical reactionsfrom two perspectives. Firstly, we will discuss the types of chemical reactions from a thermodynamic point of view followed by a mechanistic approach.

We will mostly discuss organic reactions, because it is my speciality.

We talk in thermodyamics about spontaneous reactions and exothermic reactions. In addition to entropically driven reactions. We will choose examples from organic chemistry to illustrate my point of view.

Spontaneity has to do with a thermodynamic function that is called free energy of Gibbs, which has an accepted mathematical symbol G. A spontaneous reaction has a negative value of G. A nonspontaneous reaction has a positive value of G. A zero value of G means that the reaction is in a state of equilibrium.

The other thermodyamic function that has to do with types of chemical reactions is the enthalpy function. An enthalpy of a molecule means the amount of energy stored in that molecule. The enthalpy of a given reaction means the difference in enthalpy of the products of the reaction to the enthalpy of the reactants.

A positive value of the reaction enthalpy means it is endothermic. This means energy is consumed during the reaction by the system. A negative value of the reaction enthalpy means it is exothermic. This means heat or energy is released during the reaction. A zero value of the reaction enthalpy means there is no input of energy necessary to drive the reaction nor is there release of energy during the reaction.

The last type of reactions based on thermodynamic functions is the entropically driven reactions. Entropy is a measure of disorder in a given reaction. A reaction is considered favored entropically if its overall entropy is negative. In which disorder has increased as a result of the reaction. An example of such a process is the dissociation of water according to the following equation:

H2O -> H+ + OH-

In this equation one starts with one molecule and ends up with two fragments. The net result is increase in disorder. Therefor this reaction is entropically favored. A reaction is considered disfavored entropically if the disorder of the over reaction is decreased. An example of such a reaction is the Diels-Alder reaction of ethylene with butadiene to form cyclohexene. In this case two fragments are replaced by one fragment. hence the order is increased and the entropy is decreased.

Now we will discuss types of reactions from a mechanistic point of view. It is important to distinguish in organic reactions before we discuss the types of the reactions between two technical terms. These are nucleophilicity and basicity. Nucleophilicity means affinity of negatively charged compounds or atoms to carbon centers bearing positive charge. Basicity on the other hand means affinity of bases to protons.

In organic reactions there is often a competition between basicity and nucleophilicity of negatively charged species. Lets take an example of this competition. OH- is both a base and a nucleophile. So its reaction can give two products. One reflects its basic character and the other reflects its nucleophilic character.

The first reaction type that we discuss here is the SN1 reaction. This type of reaction is characterized by development of positively charged carbocation and an ion that are completely separated. As an example lets take the reaction between t-butyl chloride and Br-

In this reaction which has SN1 mechanism the first step is the separation of the t-butylcarbocation from the chloride ion to form a stable tertiary carbocation. Then the Br- can bind to the carbocation to form the product. In this type of reactions one looses the optical activity of the solution if you start with optically active compound due to the racemic mixture that is obtained.

The second type of rection that we consider here is the SN2 reaction mechanism. This type of reactions is typified by no existence of carbocation intermediate and no develpment of positive charge during the reaction. In this type of reactions we get inversion of optical activity. However in contrast to SN1 reactions which looses its optical activity, in SN2 reactions the optical activity is inverted but is conserved and no racemic mixture is obtained.

An example of SN2 reaction is the rection between methylchloride and bromide ion according to the following equation:

CH3Cl + Br -> CH3Br + Cl-

The methyl group is not sufficiently stable to exist by itself so SN1 mechanism to this reaction is unlikely.

The last type of reactions that we discuss here is the elimination reaction. In this type of reactions one needs a strong base that does not have nucleophilic character. An example is as follows:

OH- + CH3-CH2-Br -> ethene

In this reaction OH- behaves like a base and abstract a hydrogen from the CH3 group to form ethene. The major product of this reaction is elimination. However substitution is observed as well but the product of substitution is the minor product.

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