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1
REACTION MECHANISM IN ORGANIC CHEMISTRY(II)
Group B
Dr. Akanksha Upadhyay
Assistant Professor
Department of Chemistry
Women’s College, Samastipur
2
Organic Reactions :
Organic reactions are chemical reactions which involve organic compounds.
Reaction Mechanism : The steps of an organic reaction showing the
breaking and formation of new bonds leading to the formation of product
through transitory intermediates.
In other words, In organic chemistry terms, a reaction mechanism is a
formalized description of how a reaction takes place from reactants to
products.
Reactant Intermediate
or
Transition State
Product
Most of the attacking reagents carry either a positive or a negative charge.
3
Types of Organic Reaction
The reactions in organic chemistry are mainly classified into following classes:
Organic
Reaction
1. Substitution
Reactions
2. A
dditio
n R
eactio
ns
3.
Elim
ination R
eactions
4. Rearrangement Reactions
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1. Substitution Reactions
Substitution reactions are defined as reactions in which the functional group of
one chemical compound is substituted by another group.
or
It is a reaction which involves the replacement of one atom or a molecule of a
compound with another atom or molecule.
Examples: Benzene reacted with Cl2 will produce dichlorobenzene and HCl.
This substitution reaction replaces the hydrogen atoms on the original
molecule with the Cl atom.
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Types of Substitution Reaction
Substitution reaction may be initiated by a nucleophile, electrophile or free radical.
Therefore, Substitution reactions are of three types:
1. Free-Radical Substitution Reaction
2. Nucleophilic Substitution Reaction
3. Electrophilic Substitution Reaction
1. Free-Radical Substitution Reactions
A free radical substitution reaction is initiated by free radical. A simple example of
substitution is the reaction between alkane and chlorine/bromine in the presence of UV
light (or sunlight).
Free radicals : Free radicals are atoms or groups of atoms which have a single unpaired electron
formed by homolytic fission ( studied in earlier lecture).
6
Mechanism: The mechanism for the chlorination of methane involves the following steps-
1. Initiation Step - A chlorine molecule undergoes homolytic fission in the presence of UV
light to give chlorine free radicals.
Cl2 2Cl
2. Propagation Step – A chlorine free radical attacks the methane molecule to give
methyl free radical and hydrogen chloride. Further, the methyl free radical attacks a
chlorine molecule to yield methyl chloride and chlorine radical.
CH4 + Cl CH3 + HCl
CH3 + Cl2 CH3Cl + Cl
These propagation reactions are repeated again and again.
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3. Termination Steps – These involve the formation of stable molecules by combination
of free radicals.
Cl + Cl Cl2
CH3 + Cl CH3Cl
CH3 CH3 + CH3-CH3
2. Nucleophilic Substitution Reaction
When a substitution reaction involves the attack by a nucleophile, the reaction is referred to
as SN (S stands for substitution and N for nucleophile) Nucleophilic Substitution Reaction.
Remember the role of a nucleophile by its Greek roots: Nucleo-(nucleus)-phile-(lover) – it is attracted to
the nucleus, which is positively charged! Nucleophiles are therefore negatively charged or strongly δ-.
R-X + _
OH R-OH + X_
Example:
The hydrolysis of alkyl halides by aqueos NaOH is an example of nucleophilic substitution
reaction.
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The nucleophilic substitution reactions are divided into two classes:
1. SN2 Reaction
2. SN1 Reaction
1. SN2 Reaction
The SN2 reaction is a nucleophilic substitution reaction where a bond is broken and
another is formed simultaneously or we can say that where simultaneous attack of the
nucleophile and displacement of the leaving grouptake place.
The term ‘SN2’ stands for – Substitution Nucleophilic Bimolecular.
When the rate of a nucleophilic substitution reaction depends on the concentration of
both the substrate and nucleophile,the reaction is second order reaction and termed as
SN2 reaction.
Rate∞ [Substrate][Nucleophile]
Evidently, the rate determining step include the participation of both the substrate and
the nucleophile.
This reaction proceeds through a backside attack by the nucleophile on the substrate. The
nucleophile approaches the given substrate at an angle of 180o to the carbon-leaving
group bond. The carbon-nucleophile bond forms and carbon-leaving group bond breaks
simultaneously through a transition state. Notice that intermediate is not formed in an
SN2 reaction, just a transition state is obtained. In the course of the reaction, the
configuration of the carbon is inverted and designated as Walden Inversion.
SN2 Reaction Mechanism: Consider the hydrolysis of methyl chloride by aqueous NaOH. The reaction mechanism is represented here-
Fig. Nucleophilic substitution by SN2 Mechanism
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Factors Affecting Rate of SN2 Reaction :
1. Nucleophilicity : Since the nucleophile is involved in the rate-determining step of
SN2 reactions, stronger nucleophiles react faster. Stronger nucleophiles are said to
have increased nucleophilicity and thus rate of reaction will increase.
2. Solvent Effect : SN2 reactions are much faster in polar aprotic solvents (e.g.
acetonitrile, dimethylsulfoxide, dimethylformamide, etc.) compared with polar protic
solvents (e.g. alcohols, water).
3. Steric Hindrance : SN2 reactions are particularly sensitive to steric factors, since
they are greatly retarded by steric hindrance (crowding) at the site of reaction. In
general, the order of reactivity of alkyl halides in SN2 reactions is:
methyl > 1° > 2°.
3° alkyl halides are so crowded that they do not generally react by an SN2
mechanism
In an SN2 reaction, the transition state has 5 groups around the central C atom. As a
consequence of the steric requirements at this center, less highly substituted systems
(i.e. more smaller H groups) will favour an SN2 reaction by making it easier to achieve the
transition state.
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2. SN1 Reaction
The SN1 reaction is a unimolecular nucleophilic substitution reaction. When the rate of
a nucleophilic substitution reaction depends only on the concentration of the alkyl halide,
hence it is first order reaction.
This reaction involves the formation of a carbocation intermediate.
SN1 Reaction Mechanism:
Consider the hydrolysis of tertiary butyl bromide as an example, the mechanism of the
SN1 reaction consists of two steps:
Step 1. Formation of Carbocation:
tert-butyl bromide Carbocation
This is the rate determining step.
The carbon-bromine bond is a polar covalent bond. The cleavage of this bond allows the removal of
the leaving group (bromide ion). When the bromide ion leaves the tertiary butyl bromide, a
carbocation intermediate is formed.
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Step 2. Attack of Nucleophile : The second step is a bond making process where the
electron rich nucleophile attack over an electron poor electrophile (carbocation).
tert-butyl alcohol
Factors SN1 Reaction SN2 Reaction
Molecularity Unimolecular Bimolecular
Kinetics First Order Second Order
Steps Two steps One step
Intermediates Carbocation No intermediate
Alkyl halide 3o > 2
o, No 1
o or CH3 CH3 > 1
o > 2
o, No 3
o
Solvent Polar protic solvent Polar aprotic solvent
Nucleophile
Weak nucleophile Strong nucleophile
Summary Of Nucleophilic Substitution Reaction
Addition Reactions
Addition Reactions are those in which atoms or group of atoms are added
to a double or triple bond without the elimination of any atom or molecules.
Addition reactions are typical of unsaturated organic compounds—
i.e., alkenes, which contain a C-C double bond, and alkynes, which
have a C-C triple bond—and aldehydes and ketones, which have a
C=O double bond.
1. Electrophilic Addition reactions
2. Nucleophilic Addition reactions
Types of Addition Reactions
These reactions may be initiated by electrophiles or nucleophiles:
Electrophilic Addition reactions
An electrophilic addition reaction is a reaction in which a substrate is initially
attacked by an electrophile, and the overall result is the addition of one or
more relatively simple molecules across a multiple bond.
The addition of HBr to ethylene is an example of electrophilic addition-
Mechanism:
Br2 gives a Br+ (electrophile) and Br-(nucleophile).
Nucleophilic Addition reactions
When an addition reaction involves the initial attack by a nucleophile, the
reaction is referred to as nucleophilic addition reaction.
Aldehydes and ketones which contain carbon-oxygen double bonds undergo
such reactions.
Reactivity of aldehydes and ketones: Aldehyde and ketones demonstrate
polar nature: Since, oxygen is more electronegative than carbon, so electron
density is higher on the oxygen side of the bond and lower on the carbon side.
Recall that bond polarity can be depicted with a dipole arrow, or by showing the
oxygen as holding a partial negative charge and the carbonyl carbon a partial
positive charge.
Fig. Structure of Carbonyl group
Carbon becomes more electrophilic
Therefore, C-centre behaves as an electrophilic target for attack by an electron-rich
nucleophilic group.
Relative Reactivity of Carbonyl Compounds to Nucleophilic Addition
Aldehydes are more reactive and readily undergo nucleophilic addition
reactions in comparison to ketones. In the case of ketones, two large substituents are
present in the structure of ketones which causes steric hindrance when the
nucleophile approaches the carbonyl carbon.However, aldehydes contain one
substituent and thus the steric hindrance to the approaching nucleophile is less.
Moreover, electronically aldehydes demonstrate better reactivity than ketone. This is
because ketones contain two alkyl groups (+I effect) which decrease the
electrophilicity of carbonyl carbon atom more than aldehydes.
Fig. Nucleophilic Addition Reaction
Elimination Reaction
Elimination reaction is a type of reaction which is mainly used to convert saturated
compounds (organic compounds which contain single carbon-carbon bonds) to
unsaturated compounds (compounds containing double or triple carbon-carbon
bonds).
Besides, it is an important method for the synthesis of alkenes.
The elimination reaction consists of three fundamental steps:
1. Proton removal.
2. C-C 𝜋 bond is formed. 3. There is a breakage in the bond of the leaving group.
Elimination reactions can occur mostly by two mechanisms:
1. E1 Elimination reaction
2. E2 Elimination reaction
where E is referred to as elimination and the number represent the molecularity.
1. E1 Reaction
.
E1 mechanism is also known as unimolecular elimination. There are usually two steps involved – ionization and deprotonation.
During ionization, there is a formation of carbocation as an intermediate. In
deprotonation, a proton is lost by the carbocation.
This happens in the presence of a base which further leads to the formation of a
pi-bond in the molecule.
In E1, the reaction rate is also proportional to the concentration of the substance
to be transformed.
It exhibits first-order kinetics.
The initial step is the formation of a carbocation intermediate through the loss of
the leaving group. This slow step becomes the rate-determining step
The rate of the E1 reaction is;
Rate = k[RX].
Major and minor product is decided on the basis of Saytzeff’s rule.
Saytzeff’s rule:
Saytzeff or Zaitsev Rule states that the more substituted alkene will be the
major product. So by looking at the number of alkyl groups attached to the
alkene, the degree of substitution and hence major and minor products can
be determined.
2. E2 Reaction
E2 reaction is bimolecular one-step elimination mechanism.
Here, the carbon-hydrogen and carbon-halogen bonds mostly break off in the
presence of base to form a new double bond. It exhibits second-order kinetics.
.
In general, more substituted alkenes are more stable, and as a result,1-butene is minor
product and 2-butene is major product (this is the regiochemical aspect of the outcome,
and is often referred to as Zaitsev’s rule). In addition, trans–alkenes are generally
more stable than cis-alkenes, so we can predict that more of the trans product will form
compared to the cis product (stereochemical aspect).
Example: The elimination products of 2-chloropentane
Minor product
Major product
However, certain other eliminations favor the least substituted alkene as the predominant
product, due to steric factors. Such a product is known as the Hoffmann product, and
it is usually the opposite of the product predicted by Zaitsev’s Rule.
Example:
Here, less substituted alkene is major product rather than more substituted
alkene in both the cases.
This is because of the bulky nature of quaternary ammonium salt (steric
hindrance).Hence, hoffmann product dominates over saytzeff product.