Introduction to Organic Reaction

Meaning of Organic Reaction Mechanism

A chemical equation indicates the reactants and the final products of the reaction, it rarely indicates how the
reaction proceeds. The steps of an organic reaction depicting the breaking and making of new bonds of
carbon atoms in the reactant called substrate leading to the formation of the final products through transistory
intermediates are often referred to as its mechanism. In brief, the organic reaction is the detailed step-by-step
know-how of a chemical reaction that takes place.

Substrate Intermediate Produçts

(Transistory)

Organic reaction mechanism may also be defined as the description of the path followed by the reactants as
they are transformed into products.

Electrophile
The reactant CH3Br is an alkyl halide. The C-X bond (X: F, Cl and Br) in alkyl halide is polar because
halogen is more electronegative than carbon, and as a result carbon has a partial positive charge and halogen
has a partial negative charge.

Because of the partial positive charge on carbon, the carbon atom in C-X bond is electron-deficient, and it is
going to seek electron-rich reagent to connect with. Such electron-deficient species is called an electrophile
(phile is the Greek suffix means “love”), means the species that loves electrons. The electron-deficient
species are usually electrophiles. Other electrophile examples include positive charged ions and atom with
incomplete octet, for example: H+, CH3 +, BH3, BeF2, AlCl3.

Nucleophile
The hydroxide, OH – , is another reactant in above reaction. It is shown clearly with the Lewis structure of
OH – that the oxygen atom has three lone pair electrons and is negatively charged, so it is an electron-rich
species with high electron density.
An electron-rich species is called a nucleophile (“nucleo” comes from nucleus, that means positive charge),
that is the reagent seeking positively charged or electron-poor species to react with. OH – is the nucleophile
for above reaction. Generally, any species with the electron pair available for sharing could be nucleophile.
Nucleophile can be either negatively charged (Nu:– ), or neutral (Nu:), for example: OR – , H2O, ROH,
NH3, RNH2, RCOO– are all possible nucleophiles.

Leaving Group
To ensure the above substitution occurs, another critical factor is that the Br must leave together with the
electron pairs in C-Br bond, and the bromide, Br-, is called the leaving group. The leaving group (LG) leaves
with the bonding pair of electrons, and is replaced by the nucleophile in the substitution reaction. Without a
proper leaving group, even nucleophile is attracted to electrophile, the substitution reaction still cannot move
forward.

Leaving group can be negatively charged or neutral. Applying the three key terms, the above substitution
reaction can be summarized as: the nucleophile displaces the leaving group in a substrate, so such reaction is
called nucleophilic substitution reaction. Nucleophilic substitution reaction could therefore be shown in a
more general way:
1. Addition Reactions
Addition reactions are the characteristics of unsaturated compounds with no loss of any simple molecules.
Conversely, it can be said that the compounds, which can undergo addition reactions, are called unsaturated
compounds and this state of molecules or atoms is commonly referred to as unsaturation. The phenomenon
that causes the addition reaction is called the Electromeric effect.
Different Types of Addition Reaction
The different types of addition reactions are:
 Nucleophilic addition reaction
 Electrophilic addition reaction
 Free radical addition reaction

Nucleophilic addition reaction

Nucleophiles are the species that donate a pair of electrons to form a covalent bond, they are usually referred
to as electron-rich species. Nucleophilic addition reactions usually occur in electrophilic unsaturated
compounds. Ex: Aldehydes and Ketones

Mechanism
In the carbonyl compounds, the electron pairs in the pi bond are more attracted towards the oxygen, making
the carbon electrophilic. In the presence of a nucleophile, the pi bond is shifted towards the oxygen atom and
the carbon atom becomes the electrophilic centre. As a result of nucleophilic addition, an intermediate
tetrahedral product is formed which on hydrogenation gives the end product.

Electrophilic addition reaction (Markovnikov addition reaction)

In an electrophilic addition reaction, the electrophile gets attacked by the pi electrons of the unsaturated
species and in the next step, the nucleophile attacks the electron-deficient species. Ex: Alkenes and Alkynes

 In the first step, the pi electrons of the alkenes attack the electron-deficient hydrogen ion.
 After the electrophilic addition, an intermediate carbocation is formed. Carbocation formation is a
slow step and the most stable carbocation will undergo nucleophilic addition.
 In the second step, the most stable carbocation undergoes nucleophilic addition by the bromide ion.

Free radical addition reaction (Anti-Markovnikov addition reaction.)

Anti-Markovnikov’s rule states that when unsymmetrical alkynes or alkenes react with HX (HBr) in the
presence of peroxide the free radical will be formed on the carbon with less hydrogen bonded to it.
Mechanism
Initiation: In the presence of sunlight the peroxide (solvent) molecules undergo homolytic fission and form
free radicals. The reagent HBr reacts with the solvent radicals and forms Bromine radicals.

Propagation: The bromine radical generated in the initiation process will react with the alkene to attain
stability. The radical is attracted by the pi electrons of the alkene. The stability of the intermediate radical
formed will be determined by the Anti-Markovnikov’s rule.
Termination: The reaction ceases only when any of the two reactants is finished.

Substitution reactions

Replacement of an atom or group by any other atom or group is known as substitution reaction. Attack of
nucleophile at saturated carbon atom bearing a substituent, known as leaving group, result in substitution
reaction. The group that is displaced (leaving group) carries it’s bonded pair of electrons. The new bond
is formed between nucleophile and the carbon using the electron supplied by the nucleophilic reactant. In
general, an aliphatic substitution reaction may be depicted as follows:

There are 2 types of substitution reactions:

 Nucleophilic substitution
 Electrophilic substitution
Nucleophilic substitution
The nucleophilic substitution reaction is a type of organic reaction in which a nucleophile displaces a leaving
group from a carbon atom. The nucleophile is typically a negatively charged atom or molecule, while the
leaving group is typically a positive atom or molecule.
SN1 mechanism

Step 1 (Formation of a tert-butyl carbocation by separation of a leaving group (a bromide anion) from the
carbon atom): This step is slow and reversible.

Step 2 (Nucleophilic attack): The carbocation reacts with the nucleophile. If the nucleophile is a neutral
molecule (i.e. a solvent) a third step is required to complete the reaction. When the solvent is water, the
intermediate is an oxonium ion. This reaction step is fast.

Step 3 (Deprotonation): Removal of a proton on the protonated nucleophile by water acting as a base
forming the alcohol and a hydronium ion. This reaction step is fast.

SN2 mechanism

In this reaction, the Br in the reactant methyl bromide (CH3Br) is replaced by the OH group, and the
methanol (CH3OH) is produced as the major product, together with bromide Br-, the side product.
SN2 mechanism involves two electron pair transfers that occur at the same time, nucleophile attacking (red
arrow) and leave group leaving (blue arrow). The nucleophile OH – approaches the electrophilic carbon from
the back side, the side that is opposite to the direction that leaving group Br leaves. With the nucleophile OH
– getting closer, the Br start to leave as well. The new C—OH bond formation and the old C—Br bond
breaking occur at the same time.

Electrophilic substitution
Electrophilic substitution reactions occur when an electrophile removes a functional group in a molecule,
typically but not always a hydrogen atom. Electrophilic aromatic substitution processes are frequently adding
functional groups to benzene rings and are characteristic of aromatic molecules. Some aliphatic molecules
can also be electrophilically substituted.

Electrophilic substitution reaction on aromatic compounds

An electrophilic substitution reaction involving aromatic compounds occurs when the aromatic ring itself is
substituted or displaced by an electrophile. The aromaticity of molecules is preserved in such reactions.
Chemical reactions including sulfonation, Friedel-Crafts reactions, and nitration in aromatic compounds are
examples of electrophilic aromatic substitution reactions.

Ex :Nitration of Benzene
Treatment of benzene with a mixture of HNO3+ H2SO4 brings about nitration giving Nitrobenzene. The
species that bring Nitration NO2+ (electrophilic nitronium ion). H2SO4 hepls to protonate to give H3O+ and
NO2+ ions.
(a)Generation of electrophile:

In ‘aromatic compounds’, an electrophile replaces an atom attached towards the aromatic ring, which is
commonly hydrogen. Friedel-Crafts reactions, and nitration

Elimination Reactions
An elimination reaction is a type of chemical reaction where several atoms either in pairs or groups are
removed from a molecule. The removal usually takes place due to the action of acids and bases or the
action of metals. It can also happen through the process of heating at high temperatures. Elimination
reactions are distinguished by the kind of atoms or groups of atoms that leave the molecule.
Dehydration and
Dehydrohalogenation
E1 reaction is also known as elimination unimolecular reaction. This reaction is particularly common in
secondary and tertiary alkyl halides in the absence of a strong base. For example, when 2-chloro-2-
methylpropane is treated with aqueous ethanol, 2-methyl propene is formed.
Step 1: When tert. Butyl chloride heated with alc.KOH undergoes elimination leading to the formation of a
carbonium ion
Step 2 : Loss of the proton from the neighbouring carbon having + ion to form 2 methyl propene.

E2 reaction is also known as elimination bimolecular reaction. This reaction occurs when an alkyl halide is
treated with a strong base such as hydroxide ion (OH-) and forms a carbon-carbon double bond. Example:
Ethyl chloride when boiled with KOH yields ethane here the process involves the breaking of the 2 sigma
bond and the formation of the pi bond. E2 reaction is initiated by base.

In E2 reactions the leaving group (Cl) leaves the substrate with a pair of electrons (Cl-),
leaving a vacant p-orbital on α carbon atom of the substrate. On the other hand, an
electrophile usually a proton, leaves the substrate as a cation, whereby, the β-carbon atom of
the substrate contains a pair of electrons in the p- orbitals. The p orbitals of α and β carbon
lying in the same plane undergo sideways overlap, resulting in the formation of π bond