Overview

Organic reactions are chemical reactions involving organic compounds. While pure hydrocarbons undergo certain limited classes of reactions, many more reactions which organic compounds undergo are largely determined by functional groups. The general theory of these reactions involves careful analysis of such properties as the electron affinity of key atoms, bond strengths and steric hindrance. These issues can determine the relative stability of metastable (or short-lived)  reactive intermediates, which often determine the path of the reaction sequence.

The basic reaction types are: addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions and redox reactions .

An example of a substitution reaction is written as:

                     Nu ⁻    +     C - X          →         C - Nu     +     X ⁻

where X is some functional group and Nu is a nucleophile.

The number of possible organic reactions is basically infinite. However, certain general patterns are observed that can be used to describe many common or useful reactions. Each reaction has a stepwise reaction mechanism that explains how it happens in sequence -- although the detailed description of steps is not always clear from a list of reactants alone.

A reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs. Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in a reaction mechanism. A mechanism describes in detail exactly what takes place at each stage of a chemical transformation.

The mechanism includes a description of which bonds are broken and in what order, which bonds are formed and in what order, and what the relative rates of the steps are. The mechanism includes a description of the transition state -- the highest energy state or conformation in the process of any chemical or physical transformation. The transition state can be considered a reaction intermediate or activated complex which is in metastable state, since both reactants and products represent lower energy configurations.

Note that only in an irreversible reaction can we assume that all reactants will indeed proceed to the formation of products. Chemical equilibrium is the state in which the concentrations of the reactants and products have no net change over time. Usually, this state results when the forward reactions proceed at the same rate as their reverse reactions.

A complete mechanism must also account for all reactants used, the function of a catalyst, stereochemistry, the qualitative composition of all products formed and the quantitative amounts of each product. A reaction mechanism must also account for the order in which molecules react. Often what appears to be a single step conversion is in fact a multi-step reaction sequence.

Physical chemistry includes the study of chemical kinetics or reaction kinetics, which emphasize the role of  reaction rates in a chemical reactions. Analyzing the influence of different reaction conditions on the reaction rate gives information about the reaction mechanism and the transition state of a chemical reaction. The law of mass action  states that the speed or rate of any chemical reaction is proportional to the quantity (or concentration) of the reacting substances.

Consider the following example:

                                                    CO  +  NO₂    →    CO₂ +  NO

It has been shown experimentally that the rate law for this reaction is given by:

                                                                     R  =   k [NO₂] ²

                                                                 R  =  Rate of reaction

                                                     k  =  Reaction (equilibrium) constant

                                                [NO₂]   =  Molar concentration of NO₂

I.E. For any given concentration of NO₂, the rate of the reaction R is determined by an equilibrium constant k which is measured in a series of carefully controlled experiments. Once k is determined, R can be calculated for any initial concentration of NO₂. 

Therefore, a possible mechanism by which this reaction takes place is:

   ( slow )                                 2 NO₂   →   NO₃   +   NO

   ( fast )                               NO₃  +  CO   →   NO₂ +  CO₂

The net reaction would then be given as initially stated:

                                                    CO  +  NO₂    →    CO₂ +  NO

Each step is called an elementary step, and each has its own rate law and molecularity. All of the elementary steps must add up to the original reaction.

There are only four types of elementary steps:

1) Addition, 2) Elimination, 3) Substitution and 4) Rearrangement.

When determining the overall rate law for a reaction, the slow step is the step that determines the reaction rate. Because the first step is the slow step, it is the rate-determining step. Because it involves the collision of 2 NO₂ molecules, it is a bimolecular reaction with a rate law of  R = k [NO₂] ². If one were to cancel out all the molecules that appear on both sides of the reaction, you would be left with the original reaction.

Note: In this particular case, the rate of reaction R depends solely on the concentration of a single reactant, and is independent of the concentration of the second reactant. The exponents determine the order of reaction and depend on the reaction mechanism. We call this a second-order reaction, as it is only dependent on the concentration of a single second-order reactant. . Another type of second-order reaction would be dependent on tow first-order reactants, such as R  =  k [A] [B]  where A and B are reactants. 

Organic reactions can be organized into several basic types. Some reactions fit into more than one category. For example, some substitution reactions follow an addition-elimination pathway. This overview isn't intended to include every single organic reaction. Rather, it is intended to cover the basic reactions.

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 Addition reactions:

Electrophilic addition, Nucleophilic addition, and Free Radical addition.

 include such reactions as:

Halogenation, Hydrohalogenation and Hydration.

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Elimination reactions (e.g. dehydration ) follow a E1 or E2 reaction mechanism

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Substitution reactions are divided into:

1. Nucleophilic aliphatic substitution - SN1 or SN2  

2. Nucleophilic aromatic substitution

3. Nucleophilic acyl substitution

4. Electrophilic aliphatic substitution 

5. Electrophilic aromatic substitution

6. Radical substitution 

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Organic redox reactions are very common.

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Rearrangement reactions are divided into:

1. 1,2-rearrangements

2. Pericyclic reactions

3. Metathesis

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In Condensation reactions a small molecule, usually water, is split off when two reactants combine in a chemical reaction. The opposite reaction, when water is consumed in a reaction, is called hydrolysis. Many Polymerization reactions are derived from organic reactions. They are divided into addition polymerizations and step-growth polymerizations.