Generally speaking, in organic chemistry there are three types of reactions that govern most reactions. These are: addition reactions and substitution reactions in addition to elimination reactions. These reactions will be discussed sequentially one after the other. The first type of reactions in organic chemistry which I discuss here in the addition reaction.
Addition reactions in organic chemistry usually take place across double bonds such as those between two carbon atoms and on carbonyl compounds in addition to imines and nitriles. An example of a molecule which adds across double bonds in the hydrogen molecule. This reaction proceeds in a concerted manner with the release of energy.
Therefore this reaction is exothermic because it replaces pi bonds by the energetically more stable sigma bonds. Another example of an addition reaction is the addition of a halogen molecule across C-C double bonds.
This reaction proceeds in a stepwise manner, in which in the case of chlorine molecule a chloronium ion is formed by the addition of a chlorine atome followed by the attack of a chlorine ion on the carbocation. The net result os a trans addition of two chlorine atoms on the double bond.
This reaction is even more reactive and exothermic than hydrochloric acid addition. This is partly due to the formation of sigma bond from pi bond but also it is attributed to to the release of energy from the energetically high chlorine molecule due to the electron-electron repulsion in this molecule.
This is especially true for the fluorine molecule in which the small fluorine atomes and the small ionic radius of the fluorine atom makes the electron-electron repulsion in this molecule very big.
Another type of addition reaction across C-C double bonds is the addition of HCl molecule. This reaction also occurs in a stepwise manner in which the generated carbocation due to the addition of the proton is attacked by the chlorine ion. This is also an exothermic reaction as in most additions across C-C double bonds.
The second type of organic reactions that is discussed here is the substitution reaction. This is a reactioin in which an atom or group is replaced by another atom or a group.
As a specific example I will take the reaction between CH3Cl and an iodide ion. This type of reactions is referred to as SN2 reations in organic chemistry.
In this type of reactions there is a inversion of optical activity from R to S or vice versa in optically active compounds. In this reaction the iodide ion displaces the chlorine atom from the methyl group forming CH3I.
This is an equilibrium reaction in which a mixture of both compounds is present in the solution. This is so due to the ability of Cl- to also displace the iodide ion as well.
This type of displacement reaction occurs on Sp3 hybridized carbon where there is a single bond involved. It can also occur at Sp2 hybridized carbon where there is a double bond involvement at the reaction center. An example of such a reaction occurs in iodobenzene in which case the nucleophile reacts or attacks an Sp2 carbon.
An example of a nucleophile whic reacts on this reaction is Br-. There is breaking of aromaticity in this reaction. Therefore it is energetically costly due to breaking the aromaticity of the benzene ring which is a highly stabilized structure by resonance. In this reaction the aromaticity is restored after the displacement of the original halogen that was previously attached to the ring.
The last type of reactions that I discuss here is the elimination reaction. Elimination usually refers to a process in which a hydrogen atom which is usually acidic is deprotonated by a strong base. This process usually requires a strong base such as OH- or hydroxide ion.
Usually bases can function in addition to their being bases they can function as nucleophiles as well. An example is the OH- or hydroxide ion which is a mild base. It can displace chlorine in CH3Cl. In this case it functions as a nucleophile.
It can also take a proton instead in tert-butyl-chloride forming butene instead of a displacement reaction. This is so due to steric problem for the approach of the OH- to the carbon bearing the chlorine atom.