OTHER REACTION AND MECHANISM

Electrophilic addition :

In organic chemistry, an electrophilic addition reaction is an addition reaction where, in a chemical compound,
a π bond is broken and two new σ bonds are formed. The substrate of an electrophilic addition reaction must
have a double bond or triple bond .
Eg:-

Mechanism :
The driving force for this reaction is the formation of an electrophile X+ that forms a covalent bond with an
electron-rich unsaturated C=C bond. The positive charge on X is transferred to the carbon-carbon bond, forming
a carbocation during the formation of the C-X bond.

In step 2 of an electrophilic addition, the positively charged intermediate combines with (Y) that is electron-rich and
usually an anion to form the second covalent bond.

Step 2 is the same nucleophilic attack process found in an SN1 reaction. The exact nature of the electrophile and the nature
of the positively charged intermediate are not always clear and depend on reactants and reaction conditions.

In all asymmetric addition reactions to carbon, regioselectivity is important and often determined by Markovnikov's rule.
Organoborane compounds give anti-Markovnikov additions. Electrophilic attack to an aromatic system results in electrophilic
aromatic substitution rather than an addition reaction.

Halogen addition reaction :

A halogen addition reaction is a simple organic reaction where a halogen molecule is added to the carbon–carbon double
bond of an alkene functional group.
The general chemical formula of the halogen addition reaction is:
C=C + X₂ → X-C-C-X
(X represents the halogens bromine or chlorine, and in this case, a solvent could be CH₂Cl₂ or CCl₄).
The product is a vicinal dihalide. This type of reaction is a halogenation and an electrophilic addition.
Eg :-


Mechanism :


Addition reaction :

Addition reactions are limited to chemical compounds that have multiple bonds, such as molecules with carbon-carbon double
bonds (alkenes), or with triple bonds (alkynes). Molecules containing carbon—hetero double bonds like carbonyl (C=O) groups,
or imine (C=N) groups, can undergo addition as they too have double bond character.

An addition reaction is the opposite of an elimination reaction. For instance the hydration reaction of an alkene and the
dehydration of an alcohol are addition-elimination pairs.

There are two main types of polar addition reactions: electrophilic addition and nucleophilic addition. Two non-polar addition
reaction exists as well called free radical addition and cycloadditions. Addition reactions are also encountered in polymerizations
and called addition polymerization. Based on the number of lines and their slopes obtained by plotting alkene ionization potentials
versus logs of their relative rates of additions, Donna Nelson and her students have identified patterns in alkene addition reactions
and in their mechanisms.

Addition reactions general overview. Top to bottom: electrophilic addition to alkene, nucleophilic addition of nucleophile to carbonyl
and free radical addition of halide to alkene .

Nucleophilic addition :

In organic chemistry, a nucleophilic addition reaction is an addition reaction where in a chemical compound a pi-bond is removed by the
creation of two new sigma-bonds by the addition of a nucleophile.
Addition reactions are limited to chemical compounds that have multiple-bonded atoms:

1. molecules with carbon – hetero multiple bonds like carbonyls, imines or nitriles.
   
2. molecules with carbon – carbon double bonds or triple bonds.
   

Hydrohalogenation :

A hydrohalogenation reaction is the electrophilic addition of hydrohalic acids like hydrogen chloride or hydrogen bromide to alkenes to
yield the corresponding haloalkanes.

If the two carbon atoms at the double bond are linked to a different number of hydrogen atoms, the halogen is found preferentially at the
carbon with fewer hydrogen substituents, an observation known as Markovnikov's rule. This is due to the abstraction of a hydrogen atom by
the alkene from the acid (HX) to form the most stable carbocation(relative stability: 3⁰ > 2⁰ > 1⁰ > methyl),
as well as generating a halogen anion.
A simple example of a hydrochlorination is that of indene with hydrochloric acid gas (no solvent):


E1 mechanism :

E1 is a model to explain a particular type of chemical elimination reaction. E1 stands for unimolecular elimination and has the following specificities.

1. It is a two-step process of elimination: ionization and deprotonation.
   1. Ionization: the carbon-halogen bond breaks to give a carbocation intermediate.
   2. Deprotonation of the carbocation.
2. E1 typically takes place with tertiary alkyl halides, but is possible with some secondary alkyl halides.
3. The reaction rate is influenced only by the concentration of the alkyl halide because carbocation formation is the slowest step aka rate-determining step.
   Therefore first-order kinetics apply (unimolecular).
4. Reaction usually occurs in complete absence of base or presence of only a weak base (acidic conditions and high temperature).
5. E1 reactions are in competition with SN1 reactions because they share a common carbocationic intermediate.
6. A secondary deuterium isotope effect of slightly larger than 1 (commonly 1 - 1.5) is observed.



E2 mechanism :

E2 stands for bimolecular elimination. The reaction involves a one-step mechanism in which carbon-hydrogen and carbon-halogen bonds break to form a double bond. C=C Pi bond.
The specifics of the reaction are as follows:

1. E2 is a one-step process of elimination with a single transition state.
2. Typically undergone by primary substituted alkyl halides, but is possible with some secondary alkyl halides.
3. The reaction rate, influenced by both the alkyl halide and the base (bimolecular), is second order.
4. Because E2 mechanism results in formation of a pi bond, the two leaving groups (often a hydrogen and a halogen) need to be antiperiplanar.
   An antiperiplanar transition state has staggered conformation with lower energy than a synperiplanar transition state which is in eclipsed
   conformation with higher energy. The reaction mechanism involving staggered conformation is more favorable for E2 reactions (unlike E1 reactions).
5. E2 typically uses a strong base, it needs a chemical strong enough to pull off a weakly acidic hydrogen.
6. In order for the pi bond to be created, the hybridization of carbons need to be lowered from sp³ to sp².
7. The C-H bond is weakened in the rate determining step and therefore a primary deuterium isotope effect much larger than 1 (commonly 2-6) is observed.
8. E2 competes with the SN₂ reaction mechanism.

Elimination is favored over substitution when - -> steric hindrance increases
-> basicity increases
-> temperature increases
-> the steric bulk of the base increases (such as in Potassium tert-butoxide)
-> the nucleophile is poor

E1cB-elimination reaction :

The E1cB elimination reaction is a type of elimination reaction which occurs under basic conditions, where a particularly poor leaving group (such as -OH or -OR)
and an acidic hydrogen eliminate to form an additional bond. E1cB is a two-step process. First, a base abstracts the most acidic proton to generate a stabilized anion.
The lone pair of elections on the anion then moves to the neighboring atom, thus expelling the leaving group and forming double or triple bond. The name of the
mechanism - E1cB - stands for Elimination Unimolecular conjugate Base. Elimination refers to the fact that the mechanism is an elimination reaction and will lose two
substituents. Unimolecular refers to how this reaction only involves one molecular entity. Finally, conjugate base refers to the formation of the carbanion intermediate,
which is the conjugate base of the starting material.



SN1 reaction :

The SN1 reaction is a substitution reaction in organic chemistry. "SN" stands for nucleophilic substitution and the "1" represents the fact that the rate-determining
step is unimolecular. Thus, the rate equation is often shown as having first-order dependence on electrophile and zero-order dependence on nucleophile. This relationship
holds for situations where the amount of nucleophile is much greater than that of the carbocation intermediate. Instead, the rate equation may be more accurately described
using steady-state kinetics. The reaction involves a carbocation intermediate and is commonly seen in reactions of secondary or tertiary alkyl halides under strongly basic
conditions or, under strongly acidic conditions, with secondary or tertiary alcohols. With primary alkyl halides, the alternative SN2 reaction occurs. In inorganic chemistry,
the SN1 reaction is often known as the dissociative mechanism.




SN2 reaction :

The SN2 reaction is a type of reaction mechanism that is common in organic chemistry. In this mechanism, one bond is broken and one bond is formed synchronously, i.e.,
in one step. SN2 is a kind of nucleophilic substitution reaction mechanism. Since two reacting species are involved in the slow (rate determining) step, this leads to
the term substitution nucleophilic (bi-molecular) or SN2, the other major kind is SN1.Many other more specialized mechanisms describe substitution reactions.