Organic chemistry is comparatively simple to learn because most organic chemical reactions follow a single pattern. In chemistry a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.
There are tens of thousands of different organic reactions. Hundreds of them are so well known that they are named after the chemist or chemists that invented them and are called name reactions. A given organic reaction can occur on any one of a million different organic substrates. In order to predict the reaction of a molecule one has never encountered before one needs to understand the mechanism of important organic reactions.
Addition ReactionBack to Top
In an addition reaction two reactant molecules combine to form a product containing the atoms of both reactants. The following are the two examples of addition reaction one is hydration (addition of water) and catalytic hydrogenation (addition of two hydrogen atoms) of alkenes.
In the hydration reaction, water and cyclohexene combine to produce cyclohexanol. In the catalytic hydrogenation reaction, hydrogen is added to cyclohexene to form cyclohexane. Some addition reactions require the presence of a catalyst a substance that does not appear in the product.
Elimination ReactionBack to Top
Elimination reaction in which two groups are removed from a molecule neither been replaced by another group are the reverse of addition reactions. Usually they involve the loss of substituents from vicinal atoms resulting in the formation of a double or triple bond. Most commonly a proton is lost from one carbon whereas a nucleophile is lost from the adjacent carbon; these two carbon atoms are usually referred to as $\beta$- and $\alpha$-carbons respectively.
This type of elimination reaction is known as $\beta$-elimination or a 1,2-elimination reaction. The most familiar examples of $\beta$-elimination reactions include dehydrohalogenation of alkyl halides, dehydration of alcohols etc.
Specific examples are given below.
Substitution ReactionBack to Top
In a substitution reaction one atom or group of atoms in a molecule is replaced by another. One type of substitution reaction, the nucleophilic substitution reaction occurs when a nucleophile replaces another group. For example, a hydrogen atom in cyclohexane is replaced by a bromine atom when the alkane is exposed to Br2 in the presence of light or heat.
In the products the bromine is substituted at the position previously occupied by a hydrogen in cyclohexane and hydrogen takes the place of one of the two bromine atoms in molecular bromine.
A sub-field of organic chemistry known as synthetic organic chemistry deals with multi step conversions of one material into a final product that is often considerably more complicated than is the starting material.
Oxidation Reduction ReactionsBack to Top
Reduction of an organic molecule usually corresponds to increasing its hydrogen content or to decreasing its oxygen content. For example, converting a carboxylic acid to an aldehyde is a reduction because the oxygen content is decreased.
In this example, the symbol [H] is used to indicate that the reduction of the organic compound has taken place.
The opposite of reduction is oxidation. Thus increasing the oxygen content of an organic molecule or decreasing its hydrogen content is an oxidation of the organic molecule. In this type of reaction the symbol [O] is used to represent that the organic molecule has been oxidized.
Oxidation of an organic compound may be more broadly defined as a reaction that increases its content of any element more electronagative than carbon.Oxidation of an organic compound may be more broadly defined as a reaction that increases its content of any element more electronagative than carbon.
Rearrangement ReactionBack to Top
The term "rearrangement" is used to describe two different types of organic chemical reactions. A rearrangement may involve the one step migration of an H atom or of a larger molecular fragment within a relatively short lived intermediate. On the other hand a rearrangement may be a multi step reaction that includes the migration of an H atom or of a larger molecular fragment as one of its steps.
In a rearrangement reaction a group moves from one atom to another in the same molecule. Most are migrations from an atom to an adjacent one but some are over longer distances.
The migrating group (W) may move with its electron pair, without its electron pair or with just one electron. The atom A is called the migration origin and B is called migration terminus.
Some of the rearrangement reactions are listed below.
- Cope rearrangement
- Claisen rearrangement
- Baeyer-villiger rearrangement
- Baker-venkataraman rearrangement
- Beckman rearrangement
- Brook rearrangement