Types of organic reactions

The organic compound undergoes chemical changes (transfer of groups) to attain stability. These chemical
changes are known as organic reactions. There are eight main types of organic reactions.

They are further divided into subtypes.

Types of organic reactions
1. Substitution reactions
Substitution reactions are also known as displacement reactions. In these reactions, one of the atoms or groups is
replaced by another atom or group and gives new products.
Types of substitution reactions
Substitution reactions are divided into three types based on types of reactants (substrate):
o Nucleophilic substitution reactions
o Electrophilic substitution reactions
o Free radical substitution reactions
Nucleophilic substitution reactions
In the nucleophilic substitution reactions, the attacking reagent (Nu) having lone pair of electron(s) attacks and
forms a new chemical bond by replacing the nucleofuge (leaving group). Nucleophile comes with pair of electrons
and leaving group goes away with the electron pair.
In general nucleophile attacks, form a new bond, and the leaving group leaves.

Types of nucleophilic substitution reactions

 Depending upon substrate, nucleophilic substitution reactions are classified into
SN1 reactions
SN2 reactions
SNI reactions
SNAr reaction
Electrophilic substitution reactions
In electrophilic substitution reactions, electrophiles replace the leaving group. Here electron-deficient species
(electrophile) attack an electron-rich substrate and replace the electrofuge (leaving group). In electrophilic
substitution reactions often the leaving group is the H+ ion.

Types of electrophilic substitution reactions

Depending upon the type of substrate, electrophilic substitution reactions are classified as:
Aliphatic electrophilic substitution reactions
Aromatic electrophilic substitution reactions
Free radical substitution reactions
In a free radical substitution reaction, a free radical attack a substrate and replace the leaving group. In such
reactions, attacking species and leaving species both are free radicals. They are also named chain reactions. They
proceed in the following three steps:
o Initiation
o Chain propagation
o Chain termination

2. Addition reactions
Addition reactions are given by molecules having double or triple bonds. Two or more molecules combine and
give new products.

Types of addition reactions

Addition reactions are classified into four different classes
Electrophilic addition reactions
Nucleophilic addition reactions
Free radical addition reactions
Cyclic addition reactions
Electrophilic addition reactions
In electrophilic addition reactions, an electrophile (positive species) approaches the double or triple bond. In the
first step, it converts the pi bond (π) to the sigma bonds (σ).
In the second step, an electron-rich species approaches the positive center (usually a carbocation) and give an
addition product.

Nucleophilic addition reactions

Nucleophilic addition reactions are the opposite of electrophilic addition reactions. Here a nucleophile converts the
pi bond to a sigma bond. So, the first step is the attack of electron-rich species (Nu –) on the double or triple bonds
of a carbon atom which results in the generation of carbanion. The second step is the combination of carbanion
with positive species forming an addition product.
Generally, nucleophilic addition reactions of carbonyl compounds can be broken down into the following
three steps.

 The electrophilic carbonyl carbon forms a sigma bond with the nucleophile.
 The carbon-oxygen pi bond is now broken, forming an alkoxide intermediate (the bond pair of
electrons are transferred to the oxygen atom).
 The subsequent protonation of the alkoxide yields the alcohol derivative.
The carbon-oxygen double bond is directly attacked by strong nucleophiles to give rise to the alkoxide.
However, when weak nucleophiles are used, the carbonyl group must be activated with the help of an acid
catalyst for the nucleophilic addition reaction to proceed.

The carbonyl group has a coplanar structure and its carbon is sp2 hybridized. However, the attack of the
nucleophile on the C=O group results in the breakage of the pi bond. The carbonyl carbon is now
 sp3 hybridized and forms a sigma bond with the nucleophile. The resulting alkoxide intermediate has a
tetrahedral geometry, as illustrated above.

Free radical addition reactions

In these reactions, free radical converts the pi bond to the sigma bonds. Free radical addition reaction leads to
polymerization. As discussed earlier free radical reactions are named as chain reactions and proceed in three steps.
o Initiation
o Chain propagation
o Chain termination

Cyclic addition reactions

Cyclic addition reactions are the single steps addition reactions (simultaneous addition). In these reactions, the
initial attack is not on the one-carbon of double or triple bond but both carbon atoms are simultaneously attacked.
This leads to a cyclic transition state.
 3. Elimination reactions
Elimination reactions are opposite to the addition reactions. Sigma bonds are converted into the pi bond. This
happens when two or more atoms or groups of atoms are removed to form a double or triple bond.
Types of elimination reactions
o
 𝜶 Elimination reactions
 𝜷 Elimination reactions
 𝜸 Elimination reactions
 Conjugate Elimination reactions
 Pyrolytic Elimination reactions
 Extrusion reactions
𝜶 Elimination reactions
In 𝜶 elimination reactions both the groups are removed from the same atom. This results in the formation of
carbene in the case of carbon and nitrene in the case of nitrogen.

𝜷 Elimination reactions
In 𝜷 elimination reactions, two groups are removed from adjacent carbon atoms. One is from 𝜶 carbon and the
second is from 𝞫 carbon. This is the most common type of elimination reaction.
𝜷 elimination reactions can proceed in a single step (E2) as well as in two steps (E1).
 Types of 𝞫 Elimination reactions
𝞫 elimination reactions are subdivided into three types
o E1 elimination reactions (unimolecular)
o E2 elimination reactions (bimolecular)
o E1cB elimination reactions

𝜸 Elimination reactions
𝜸 elimination reactions make a three-membered ring structure. Here one atom is removed from 𝜶 carbon and the
other is removed from 𝜸 carbon. Three-member rings are very unstable due to angle strain.

Extrusion reactions
Extrusion reactions involve the removal of a fragment from within a chain or a ring.

Conjugate Elimination reactions

They involve the removal of atoms from the conjugate system. The conjugate system has alternate single and
double bonds.
Pyrolytic elimination reactions
As the name pyrolytic eliminations show that these elimination take place due to heating. There are several
compounds that undergo elimination on heating when no other reagent is present. They are often in the gas phase.
Such reactions are called pyrolytic elimination reactions.
 4. Rearrangement reactions
The reactions in which an atom or group of atoms moves from one atom to another atom within a molecule are
called rearrangement reactions. They result in the formation of structural isomers of original molecules.
Connectivities of groups changes within the molecule but most of the migrations are from one atom to an adjacent
atom and are called 1,2 shifts. However, there can be some migrations over a long distance.

Types of rearrangement reactions

o Nucleophilic rearrangement reactions
o Electrophilic rearrangement reactions
o Free radical rearrangement reactions
Nucleophilic rearrangement reactions
Nucleophilic rearrangement reactions are the most common rearrangement reactions and are also named as
anionotropic rearrangements. In these rearrangements, the migrating group is a nucleophile. It will move with its
electron pair.
Electrophilic rearrangement reactions
Nucleophilic rearrangement reactions are also named as cationotropic rearrangements or prototropic
rearrangements. In these rearrangements, the migrating group is an electrophile. It will move without its electron
pair.
Free radical rearrangement reactions
Free radical rearrangement reactions are not common. In these rearrangements, the migrating group is a free
radical. It will move with just one electron.
5. Condensation reactions
Condensation reactions are the type of organic reaction that involves the combination of two molecules to form a
single product by the loss of small molecules. In most cases, a water molecule is removed during condensation
reactions. In the case of water molecules, these reactions are also called dehydration reactions.
There are certain other small molecules that can be removed during condensation reactions, i.e ammonia, ethanol,
acetic acid, etc. Well known condensation reactions are:
o Aldol condensation
o Claisen condensation
o Deckman condensation
o Knoevenagel condensation
6. Pericyclic reactions
These are the type of organic reactions that requires light or heat to proceed and have a concerted mechanism. The
bond breaking and bond, formation takes place simultaneously.
These reactions have a single transition state with cyclic geometry and are also known as nonionic reactions as no
intermediate is formed during the chemical reaction. They are highly stereospecific.
 All such concerted reactions which proceed via cyclic transition states are known as pericyclic reactions.
Unaffected by polar reagents, solvent change, radical initiators, etc. Influenced by Heat or Light.
Types of pericyclic reactions
Pericyclic reactions are classified into 7 different types:
o Cycloaddition reactions
o Electrocyclic reactions
o Sigmatropic reactions
o Group transfer reactions
o Ene reactions
o Cheletropic reactions
o Dyotropic reactions
7. Polymerization reactions
These are the organic reactions that involve the formation of larger molecules called a polymer. Polymers consist
of a large number of small repeating units called monomers. These repeating units combine chemically to produce
a polymer.
Polymerization reactions are also called as chain reactions. They proceed in three steps:
o Initiation
o Chain propagation
o Chain termination
Types of polymerization reactions
These are classified into two types
o Addition polymerization
o Condensation polymerization
8. Oxidation-reduction reactions
Oxidation-reduction reactions are also called redox reactions. They involve the loss or gain of electrons as a result
of which the oxidation state of species changes. Oxidation and reduction take place simultaneously. One of the
molecules gets oxidized while the other gets reduced at the same time.
Although oxidation and reduction are concurrent, still we classify them separately depending on whether an
organic compound is reduced or oxidized.

Kinetic and Thermodynamic Control

The potential outcome of a reaction is usually influenced by two factors:

1. the relative stability of the products (i.e. thermodynamic factors)

2. the rate of product formation (i.e. kinetic factors)

The following simple reaction coordinate diagram provides a basis for the key issues about kinetic and
thermodynamic control:

Consider the case where a starting material, SM, can react in a similar manner to give two
different products, P1and P2 via different (competing) pathways represented
by green and blue curves.

Reaction 1 via pathway 1 (green) generates product 1 (P1) via transition state 1 (TS1).
This will be the faster reaction since it has a lower energy (more stable) transition state,
and therefore a lower activation barrier. Therefore, product 1, P1 is the kinetic
product (the product that forms the fastest).

Reaction 2 via pathway 2 (blue) generates product 2 (P2) via transition state 2 (TS2).
 P2 is the more stable product since P2 is at a lower energy than P1. Therefore, P2 is
the thermodynamic product (the more stable product).

We now need to consider how the outcome of this situation changes with the competing reactions of the
starting material as we alter the reaction temperature and therefore the average energy of the molecules
changes.

1. At low temperature, the average energy of the molecules is low and more molecules have enough
sufficient energy cross activation energy EA1 than EA2. Therefore the reaction preferentially proceeds along
the green path to P1. The reaction is not reversible since the molecules lack sufficient energy to reverse
to SM, i.e. it is irreversible, so the product ratio of the reaction is dictated by the rates of formation
of P1 and P2, k1: k2.

2. At some slightly higher temperature, reaction 1 will become

reversible when sufficient molecules have enough energy to cross the
reverse reaction barrier for reaction 1, while reaction 2 remains irreversible. So
although P1 may form initially, over time it will revert to SM and react
to give the more stable P2.

3. At high temperature, both reaction 1 and 2 are reversible and the

product ratio of the reaction is dictated by the equilibrium
constants for P1 and P2, K1 : K2.

Summary

At low temperature, the reaction is under kinetic control (rate, irreversible conditions) and the major
product is that from the fastest reaction.

At high temperature, the reaction is under thermodynamic control (equilibrium, reversible conditions) and
the major product is the more stable system

The

Hammond Postulate Chemists are often very

interested in the structures of the transition states in a reaction's mechanism. In particular, the transition state for a
mechanism's rate determining step directly determines the energy of activation barrier and thereby the rate for the
overall reaction. Understanding the structure of a transition state allows chemists to consider structural features which
might stabilize or destabilize the transition state causing a corresponding change in the rate of reaction. However,
transition state structures cannot be directly observed because they are highly unstable activated complexes which
instantly convert to a more stable species. In order to gain some insight into the structure of particular transition state,
chemists often invoke the Hammond postulate, which states that a transition state resembles the structure of the
nearest stable species (reactant, intermediate or product).
For an exergonic reaction, the transition state is closer in
energy to the reactants. Therefore, the structure of the
transition state can assumed to resemble the reactants
more than the products. Shown below is a hypothetical
exergonic reaction between reactant compounds A and B
to form the product AB. The Hammond postulate would
theorize that the distance between A and B in the
transition state would be relatively large thus resembling
the reactants where A and B are two isolated species.

For an endergonic reaction, the transition state is

closer in energy to the product. Therefore, the
structure of the transition state can assumed to
resemble the products more than the reactants. In the
hypothetical endergonic reaction shown below,
reactant compounds C and D react to form the
product CD. The Hammond postulate would predict
that the distance between C and D in the transition
state would be relatively small thus resembling the
products where C and D are bonded together as a
single product CD.

The Hammond Postulate and Electrophilic Addition

By applying the Hammond postulate and other

ideas cultivated in this chapter the reason why
electrophilic additions tend to follow
Markovnikov's rule. When the energy diagram of
an electrophilic addition was discussed
in Section 7.2, it was noted the first step of the
mechanism was the rate determining step. The
first step of the mechanism also is endergonic
and results in the formation of a carbocation
intermediate.

The Hammond postulate suggests that the

transition state structure for the first step of the mechanism resembles that of the carbocation intermediate
because they are the closest in energy. A transitions state, seen below, is typically drawn as a theoretical
structure part way between the reactants and the product. For this transition state the pi bonds and the H-Br
bond are in the process of being broken and are represented with a dashed line. The C-H bond is in the
process of being formed so it also represented with a dashed line. The bromine is shown with a partial
negative charge (sigma-) because it is becoming a bromide ion (Br -) which has a full negative charge. Most
importantly, the carbon is in the process of becoming a carbocation so it is shown to have a partial positive
change (sigma+).
Because the Hammond postulate predicts
this transitions state closely resembles the
carbocation intermediate, the partial
positive charge can said to closely resemble
the full positive charge of the carbocation.
Consequently, any structural feature that
stabilized the carbocation intermediate will
also stabilize the transition state. The partial
positive charge of the transition state is
stabilized by adjacent alkyl groups
thorough inductive effects and
hyperconjugation much like the carbocation
intermediate. Adding more alkyl
substituents to the partially positive charged carbon stabilizes the transition state, causing it to become lower
in energy. This in turn, decreases the energy of activation and increases the rate of the reaction. In short,
during an electrophilic addition, the double bond carbon with the most alkyl substituents will for a
carbocation intermediate and therefore its C-X bond faster than the double bond carbon with feweralkyl
substituents. These effects cause electrophillic additions to follow Markovnikov's rule and place the halogen
(X) group on the more substituted carbon of asymmetrically alkyl substituted double bond.