poc – 1 unit – 2 alkenes...

TOPIC NAME : ALKENES

USEFUL FOR :

GPAT NIPER

PHARMACEUTICAL ORGANIC CHEMISTRY – 1 SUBJECT CODE (BP202T)

CONTENT:

ELECTROPHILIC ADDITION REACTION

FREE RADICAL ADDITION REACTION

ELIMINATION REACTION

DESTINATION PHARMAGENS

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About alkenes

• Alkenes are the compound containing double bond. This double bond in their structure is functional group of alkenes.

• Functional group is nothing but the part of the molecule which is responsible for the characteristic reaction of that substrate

Characteristic reactions of alkenes

There are two types of characteristic reactions for alkenes

• Type-1: Reaction takes place at double bond; Here double bond is destroyed

– Electrophilic Addition Reaction

– Free Radical Addition Reaction

• Type-2: Reaction takes place at position other than double bond and may or may not be conjugated with the double bond

– Allylic Rearrangement



Electrophilic Addition Reaction- And Structure of Alkenes

• Considering the structure of Alkenes, Out of two bond one is sigma bond and other one is pi bond.

• Sigma bond is comparatively strong and electrons participating in this sigma bond are strongly held by Carbon Nuclei

• Pi Bond is comparatively weak bond and electrons participating in this bond are loosely held by Carbon Nuclei

• Now becoz of loosely held pi-electron pair or pi-electrons , Alkenes serve as source of electrons. Thus acting as Lewis Base

• Now Electron Deficient species can attack the pi-electron source i.e Alkenes.

• These Electron deficient species are nothing but Electrophiles.

• Electrophiles undergo addition reaction with alkenes. “EAR”

Electrophilic Addition Reaction & Elimination Reaction- A Comparison

• Addition Reaction are those where double bond is broken While Elimination Are those where double bond is formed

• In Case of eliminaion reaction Nucleophile (Strong Base) is recquired While in Electrophilc addition Reaction Electrophile is

recquired (Strong Acid)



• In Case of elimination reaction Nucleophile (Strong Base) is required While in Electrophilic

addition Reaction Electrophile is required (Strong Acid)

• Since Alkenes are weak bases we need strong acid as electrophilic reagents, which can

donate proton readily

– Addition oh HX

– Addition of H2SO4

• Exception; Hydration (addition of water) and addition of alkanes (strong acidic environment

used)

Electrophilic Addition Reaction & Elimination Reaction



Electrophilic Addition Reaction

Addition of HX (Hydrogen Halides)

Addition of HX occurs across the double bond, this is carried out in anhydrous condition and acetic acid is recquired as solvent Since Alkenes are non polar substrates and HX is polar reactant, so to dissolve both we need mid polar solvent where acetic acid found good. Addition of HX follows Markovnikovs rules (while HBr in presence of Peroxide forms antimarkovnikovs product, known as peroxide effect)



Electrophilic Addition Reaction Addition of HX (Hdrogen Halides) - Mechanism

Step-1 (Proton addition and Formation of carbocation)

Step-2 (Conjugate Base addition)



Electrophilic Addition Reaction Addition of HX (Hdrogen Halides) - Mechanism

Step-1 (Proton addition and Formation of carbocation)

Here H-Br bond breaks Heterolytically, where proton is generated Addition of this proton governed by Markovnikovs rule so as to generate more stable carbocation.




Addition of HX (Hdrogen Halides) –follows Markovnikovs Rule

In the first step of reaction mechanism, H+ addition occurs on that C-atom of double bond which

holds more number of H-atom

allowed Not-allowed

1,2-Methyl shift

Stability of Carbocation 3° > 2° >1°




Addition of HX (Hydrogen Halides) – Evidences

Follows Second order kinetics

Reactivity governed by markovnikovs rule

Strong Acid Required

Allows rearrangement if possible



Free Radical addition Reaction

This reaction occurs in presence of peroxide compound where H-Br breaks homolytically, and free

radical are generated. These free radical are then added over C=C. Hence known as Free Radical

Addition Reaction.



These are the reaction where loss of atom or functional group occurs. One being Hydrogen atom and

other Functional Group.

Elimination Reaction

C-atom from which Hydrogen atom is lost is designated as α-c-atom and c-atom from which other

Functional group is lost is known as β-c-atom.

On the basis of Location of loss of atoms or functional group there are two types of elimination

reaction α-elimination (1,1-elimination) & β-elimination (1,2-elimination)

β-elimination (1,2-elimination)

On the basis of molecularity / kinetics it is classified into three types

- E2 elimination

- E1 elimination

- E1cb eliminaiton



β-elimination (1,2-elimination) - E2 elimination

It is bimolecular elimination reaction which proceeds via single step In this usually strong base

(Nucleophile/electron rich) is required as catalyst This reaction doesn’t allows the rearrangement at transition state

and also no intermediate formed in this reaction.

In this single step E2 elimination reaction, Following events occurs:

1) Since alkane are acting as very weak acids here (Here they are loosing protons) We will need very

strong base which can not only readily accept proton but also possess ability to abstract

covalently bonded proton. So first, attack of base occurs and it abstracts the proton without is

bonding electrons.

2) Now the bonding electrons are left on that α-c-atom. This electron pair now utilized for

formation of pi-bond with the adjacent β-c-atom. Here heat of bond formation is released as a

result of pi—bond formation.




It is bimolecular elimination reaction which proceeds via single step In this usually strong base

(Nucleophile/electron rich) is required as catalyst This reaction doesn’t allows the rearrangement at transition state

and also no intermediate formed in this reaction.

In this single step E2 elimination reaction,

Following events occurs:

3) Now the heat of bond formation is released as a result of pi—bond formation is utilized as

Heat of dissociation for breaking C-X bond. As the pi-bond formation occurs, Halogen atom starts

departing itself from the carbon atom.



Dehydrohalogenationof RX (Alkyl Halides) - KINETICS

Follows Second order kinetics – why?

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It is bimolecular elimination reaction which proceeds via single step Since the rate of reaction depends on both concentration of Conc of substrate as well as conc of Strong base it follows second order kinetics

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Governed by SAYTZEFF Rule El

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Elimination Reaction- Evidences • Follows Second Order Kinetics • Strong Base Required • Don’t rearrangement • Shows isotope effect • Shows element effect El

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It is unimolecular elimination reaction which proceeds via three step In this usually strong acid is required as catalyst This reaction allows the rearrangement at intermediate step if possible and also carbocation intermediate formed in this reaction.

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Elim

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Elimination Reaction- Evidences • Follows first Order Kinetics • Strong Base Not Required • Allows rearrangement • Don’t Shows isotope effect

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MECHANISM FOR REACTION OF ALKENE OZONOLYSIS

Step 2: The cyclic species called the malozonide rearranges to the ozonide

Step3 The ozonide decomposes on Reduction with (usually Zn / acetic acid) gives the two carbonyl groups

Step 1:

The π electrons act as the nucleophile, attacking the ozone at

the electrophilic terminal O. A second C-O is formed by the

nucleophilic O attacking the other end of the C=C.

malozonide

ozonide



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poc – 1 unit – 2 alkenes...

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