RCHM 203eaction

CHM 203

LECTURE NOTES

ELIMINATION REACTION

An elimination reaction is a reaction in which atoms are removed as molecules or

compounds. Elimination is generally catalyzed by a metal, an acid or base.

Elimination reactions often compete with substitution reactions. In this reaction, a

substrate (typically an alkyl halide) eliminates one equivalent (unit) of acid to form
an alkene. Two possible mechanisms are available for this elimination reaction
– E1 and E2 mechanisms.

What is Elimination Reaction?

Elimination reaction is a type of reaction that is mainly used to transform
saturated compounds (organic compounds which contain single carbon-carbon
bonds) to unsaturated compounds (compounds which feature double or triple
carbon-carbon bonds).

Besides, it is an important method for the preparation of alkenes.

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.

Important Methods of Elimination Reaction

Normally, elimination reactions are distinguished by the kind of atoms or groups of
atoms that leave the molecule. Due to this, there are two main methods involved in
this type of reaction;

 Dehydration
 Dehydrohalogenation
In the dehydration method, there is the elimination of a water molecule mostly
from compounds such as alcohol. Sometimes, this method is also called a Beta
elimination reaction where the leaving group and H are placed at neighbouring
carbon atoms. On the other hand, in dehydrohalogenation, there is a removal of a
hydrogen atom and a halogen atom.

Some other common types of elimination reactions are α-elimination and γ- and
δ-elimination.

Mechanism Of Elimination Reaction

The elimination reaction consists of three fundamental events, and they are;

1. Proton removal.
2. Formation of C-C pi bond.
3. Removal of the leaving group.
Depending on the reaction kinetics, elimination reactions can occur mostly by two
mechanisms namely E1 or E2 where E is referred to as elimination and the number
represents the molecularity.

E1 Reaction
  In the E1 mechanism which 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 E1 mechanism shares the features of the SN1 reaction. 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 for the whole reaction.

E2 Reaction

 In an E2 mechanism which refers to bimolecular elimination is basically a

one-step mechanism.
 Here, the carbon-hydrogen and carbon-halogen bonds mostly break off to
form a new double bond.
 However, in the E2 mechanism, a base is part of the rate-determining step
and it has a huge influence on the mechanism.
 The reaction rate is mostly proportional to the concentrations of both
the eliminating agent and the substrate.
 It exhibits second-order kinetics.
The E2 mechanism can generally be represented as below. In the below-mentioned
representation, B stands for base and X stands for halogen.

The rate of the E2 reaction is

Rate = k[RX][Base]

So the reaction rate depends on both the substrate (RX) and the base involved. In
the elimination reaction, the major product formed is the most stable alkene.

Elimination Reaction Example

One of the common examples of elimination reaction is the dehydration of alcohol.
Here the process takes place in the presence of a base such as an ethoxide ion
(C2H5O–). It can be represented as;

“E2 and E1 reactions differ significantly in the nature of the transition states
that determine the regiochemistry of the product”. The E2 pathway involves a
transition state leading from starting material directly to the product. The product
forming step of an E1 reaction is more exothermic than that of an E2 reaction.
Thus, the E1 reaction has a relatively early transition state, closely resembling the
carbocation formed in the rate-determining step.

Elimination reactions

Elimination reactions are important transformations in organic chemistry where

atoms or groups are removed from a molecule to generate a double bond or a π
bond. This lecture will focus on two main types of elimination reactions: �1E1
(unimolecular elimination) and �2E2 (bimolecular elimination). Understanding
the mechanisms and factors influencing these reactions is crucial for synthetic
design.

Unimolecular elimination, denoted as �1E1, is a type of elimination reaction in

organic chemistry. It involves a two-step mechanism where a carbocation
intermediate is formed before the elimination of a leaving group to produce a
double bond or a π bond. This lecture will explore the �1E1 mechanism, factors
influencing the reaction, and provide examples with reaction equations.

Key Concepts:

1. Elimination Reactions Overview:

 Elimination reactions involve the removal of atoms or groups from a

molecule to form a double bond or a π bond.

 �1E1 is a unimolecular elimination, meaning it proceeds through a

single molecular entity.

2. Mechanism of �1E1 Elimination:

 Steps:

1. Formation of a carbocation intermediate.

2. Deprotonation to generate the double bond.

 General Reaction Equation: R-CH3→HeatR-CH2+

+H2→BaseR=CH2+H+R-CH3HeatR-CH2++H2BaseR=CH2+H+
Factors Influencing E1 Elimination:

1. Substrate Structure:

 More substituted carbocations are favored due to increased stability.

2. Stability of Carbocation:

 The rate of E1 reactions depends on the stability of the carbocation

intermediate.

3. Leaving Group Ability:

 Good leaving groups facilitate the formation of carbocations.

Example of E1 Elimination:

Dehydration of Alcohols:

 Reaction Equation: R-OH→HeatR-CH2++H2O→BaseR=CH2+H+R-

OHHeatR-CH2++H2OBaseR=CH2+H+

 In this example, heating an alcohol (R-OHR-OH) leads to the formation of a

carbocation intermediate, which then undergoes deprotonation to form an
alkene.

Conclusion:

The E1 mechanism is a key pathway for elimination reactions, particularly in

processes involving the formation of carbocation intermediates. Understanding the
factors influencing E1 reactions is crucial for predicting reaction outcomes and
designing synthetic routes in organic chemistry.

Bimolecular Elimination (E2) in Organic Chemistry

Introduction:

Bimolecular elimination, denoted as E2, is a type of elimination reaction in organic

chemistry. It involves a one-step concerted mechanism where both the removal of
a leaving group and the abstraction of a proton occur simultaneously. This lecture
will delve into the E2 mechanism, influencing factors, and provide examples with
reaction equations.

Key Concepts:

1. Elimination Reactions Overview:

 Elimination reactions involve the removal of atoms or groups from a

molecule to form a double bond or a π bond.

 E2 is a bimolecular elimination, occurring in a single, concerted step

involving two molecular entities.

2. Mechanism of E2 Elimination:

 Steps:

1. Simultaneous deprotonation and elimination.

2. Requires an anti-coplanar arrangement of the leaving group and

the hydrogen being removed.

 General Reaction Equation: R-CH3CH2X+Base→E2R-

CH=CH2+HXR-CH3CH2X+BaseE2R-CH=CH2+HX

Factors Influencing E2 Elimination:

1. Base Strength:

 Strong bases favor E2 reactions, facilitating deprotonation and

elimination.

2. Steric Hindrance:

 Bulky substituents can hinder the elimination process, making anti-

coplanar arrangements less accessible.

3. Substrate Structure:

 More substituted double bonds are favored due to increased stability.

Example of E2 Elimination:

Dehydrohalogenation:

 Reaction Equation: R-CH2CH2X+Base→E2R-CH=CH2+HXR-CH2CH2

X+BaseE2R-CH=CH2+HX

 In this example, a halide-substituted alkane undergoes deprotonation by a

strong base, leading to the formation of an alkene and the expulsion of a
hydrogen halide.

Conclusion:

The E2 mechanism is a crucial pathway in elimination reactions, offering a direct

and concerted route to the formation of double bonds. Understanding the factors
influencing E2 reactions is essential for predicting reactivity and designing
efficient synthetic routes in organic chemistry.

Aliphatic Electrophilic Substitution Reaction Mechanism

Aliphatic substitution reactions play a crucial role in organic chemistry, allowing

us to modify and synthesize a variety of compounds. In this lecture, we will focus
on electrophilic aliphatic substitution reactions, where an electrophile attacks the
carbon atoms in an aliphatic compound. These reactions are fundamental to the
field of organic synthesis and provide versatile tools for constructing complex
molecules.

Key Concepts:

1. Electrophile Definition:

 An electrophile is a species with an electron deficiency, seeking

electrons in a chemical reaction. In electrophilic aliphatic substitution,
an electrophile reacts with an aliphatic compound, replacing a
hydrogen atom.

2. Mechanism of Electrophilic Aliphatic Substitution:

 The general mechanism involves the following steps:

 1. Generation of the electrophile.

2. Attack of the electrophile on the aliphatic compound.

3. Formation of an intermediate.

4. Deprotonation to regenerate stability.

Examples of Electrophilic Aliphatic Substitution Reactions:

1. Halogenation:

 Reaction Equation: RH+X2→UV lightRX+HXRH+X2UV light

RX+HX

 In this reaction, a hydrogen atom in an alkane (RHRH) is replaced by

a halogen (XX) in the presence of ultraviolet (UV) light.

2. Nitration:

 Reaction Equation: RH+HNO3→H₂SO₄RNO2+H2ORH+HNO3

H₂SO₄RNO2+H2O

 Nitration involves the substitution of a hydrogen atom in an alkane

with a nitro group (NO2NO2) in the presence of nitric acid (HNO₃)
and sulfuric acid (H₂SO₄).

3. Sulfonation:

 Reaction Equation: RH+H₂SO₄→RSO₃H+H2ORH+H₂SO₄

RSO₃H+H2O

 Sulfonation replaces a hydrogen atom in an alkane with a sulfonic

acid group (RSO₃HRSO₃H) in the presence of sulfuric acid.

4. Friedel-Crafts Alkylation:

 Reaction Equation: R’H+AlCl3→R’R′++AlCl4−+H+R’H+AlCl3R’R′

++AlCl4−+H+
  Alkylation of aromatic compounds involves the substitution of a
hydrogen atom with an alkyl group in the presence of a Lewis acid
catalyst such as aluminum chloride.

5. Friedel-Crafts Acylation:

 Reaction Equation: R’H+Acyl-X+AlCl3→R’C(O)R′++AlCl4−

+H+R’H+Acyl-X+AlCl3R’C(O)R′++AlCl4−+H+

 Acylation replaces a hydrogen atom in an aromatic compound with an

acyl group (C(O)R′C(O)R′) in the presence of a Lewis acid catalyst.

Nucleophilic Addition at Unsaturated Carbon Atoms

Introduction:

Nucleophilic addition reactions are fundamental processes in organic chemistry

where a nucleophile reacts with a molecule containing a multiple bond
(unsaturation). In this lecture, we will explore nucleophilic addition at unsaturated
carbon atoms, focusing on reactions with alkenes and alkynes. The mechanisms,
key concepts, and examples with chemical equations will be discussed.

Key Concepts:

1. Nucleophilic Addition Overview:

 Nucleophiles, which are electron-rich species, attack electrophiles,

often resulting in the addition of a nucleophile across a double or
triple bond.

2. Mechanism of Nucleophilic Addition:

 The general mechanism involves the nucleophile attacking the

electrophile, resulting in the formation of a new bond and the
displacement of a leaving group.

3. Examples of Nucleophilic Addition:

a. Addition to Alkenes:
  Reaction Equation: R-CH=CH2+Nu−→R-CH(Nu)-CH2R-CH=CH2
+Nu−R-CH(Nu)-CH2

 In this case, the nucleophile (��−Nu−) attacks the carbon-carbon

double bond, leading to the addition of the nucleophile across the
double bond.

b. Addition to Alkynes:

 Reaction Equation: R-C≡C-R’+Nu−→R-C(Nu)≡C-R’R-C≡C-

R’+Nu−R-C(Nu)≡C-R’

 Here, the nucleophile (��−Nu−) attacks the carbon-carbon triple

bond, resulting in the addition of the nucleophile across the triple
bond.

Factors Influencing Nucleophilic Addition:

1. Nature of Nucleophile:

 The nucleophile's strength and nucleophilicity influence the rate and

outcome of the reaction.

2. Substrate Structure:

 The type of multiple bond and its reactivity play a role in the
selectivity of nucleophilic addition.

3. Steric Hindrance:

 Bulky substituents can hinder the approach of the nucleophile,

affecting the reaction rate and regioselectivity.

Examples of Nucleophilic Addition Reactions:

1. Hydroboration of Alkenes:

 Reaction Equation: R-CH=CH2+BH3→R-CH2CH2BR-CH=CH2

+BH3R-CH2CH2B
  Boron adds across the double bond, followed by oxidation to give an
anti-Markovnikov alcohol.

2. Hydrogenation of Alkynes:

 Reaction Equation: R-C≡C-R’+H2→PtR-CH2CH2−�′R-C≡C-

R’+H2PtR-CH2CH2−R′

 Hydrogen adds across the triple bond, resulting in the saturation of the
alkyne.

Conclusion:

Nucleophilic addition at unsaturated carbon atoms is a versatile strategy in organic

synthesis, enabling the construction of complex molecules. Understanding the
mechanisms and factors influencing these reactions is crucial for designing and
optimizing synthetic routes in organic chemistry.

Electrophilic Addition in Organic Chemistry

Introduction:

Electrophilic addition reactions are fundamental processes in organic chemistry

wherein an electrophile reacts with a nucleophile, resulting in the addition of the
electrophile to a multiple bond. This lecture will explore the mechanisms, key
concepts, and examples of electrophilic addition reactions, focusing on reactions
with alkenes and alkynes.

Key Concepts:

1. Electrophilic Addition Overview:

 Electrophilic addition involves the addition of an electrophile to a

double or triple bond, typically resulting in the formation of a new
single bond.

2. Mechanism of Electrophilic Addition:

  The general mechanism involves the electrophile attacking the
multiple bond, forming a new bond and leading to the addition of the
electrophile.

3. Examples of Electrophilic Addition:

a. Addition to Alkenes:

 Reaction Equation: R-CH=CH2+X2→R-CHX-CH2�R-CH=CH2

+X2R-CHX-CH2X

 In this example, a halogen (X2) acts as the electrophile, adding across

the carbon-carbon double bond.

b. Addition to Alkynes:

 Reaction Equation: R-C≡C-R’+HCl→R-CH(Cl)-CH2−�′R-C≡C-

R’+HClR-CH(Cl)-CH2−R′

 Here, hydrogen chloride (HCl) acts as the electrophile, adding across

the carbon-carbon triple bond.

Factors Influencing Electrophilic Addition:

1. Nature of Electrophile:

 The electrophile's nature and reactivity influence the rate and outcome
of the reaction.

2. Substrate Structure:

 The type of multiple bond and its reactivity play a role in the
selectivity of electrophilic addition.

3. Steric Hindrance:

 Bulky substituents can hinder the approach of the electrophile,

affecting the reaction rate and regioselectivity.

Examples of Electrophilic Addition Reactions:

 1. Hydrohalogenation of Alkenes:

 Reaction Equation: R-CH=CH2+HBr→R-CH(Br)-CH2R-CH=CH2

+HBrR-CH(Br)-CH2

 Bromine adds across the double bond in an anti-Markovnikov manner.

2. Hydration of Alkenes:

 Reaction Equation: R-CH=CH2+H2O→HgSO4/H2SO4R-CH(OH)-

CH2R-CH=CH2+H2OHgSO4/H2SO4R-CH(OH)-CH2

 Water adds across the double bond, forming an alcohol in accordance

with Markovnikov's rule.

Conclusion:

Electrophilic addition reactions are fundamental in the construction of various

organic compounds. Understanding the mechanisms and factors influencing these
reactions is crucial for predicting reactivity and designing synthetic routes in
organic chemistry.