Alkyl halides

The compounds in which one hydrogen atom of alkane is replaced by a halogen atom is called alkyl halide. Alternatively, The halogen derivatives of alkanes with general formula R-X are Halide called alkyl halide.

For example

CH4 (Alkane) CH3-X (Alkyl halide) X = F, Cl, Br and I

Types

Mono, di, tri or poly haloalkane depending upon the number of halogen atom.

For example

Classification of Alkyl Halides

Alkyl halides can be classified as primary, secondary and Tertiary alkyl halides.

1. Primary Alkyl Halides

The alkyl halides in which a halogen atom is bonded with primary carbon is called primary alkyl halide. Primary carbon is the carbon that is attached one or no carbon atom.

The Secondary Alkyl Halides

The alkyl halides in which a halogen atom is bonded with secondary carbon are called secondary alkyl halide. Secondary carbon is the carbon that is attached with two carbon atoms.

For example:

iii. TERTIARY ALKYL HALIDE

The alkyl halogen in which atom is a bonded with tertiary. halogen are called tertiary Alkyl bolide.

·Tertiary carbon is Head Carbon that is attached with Three other carbon atom.

For example:

Nomenclature

2- Common LUPAC system of Naming system of Naming

COMMON SYSTEM

IUPAC System of Naming

Physical Properties

Alkyl halides have very high m.p. This is because of bond polarity of C-X bond (as X halogen is more electronegative than C)

Structure of R-X

Alkyl halide consists of alkyl group which has sp³ hybridized carbon atom bonded with halogen atom.

This C-X band is polar due to move- electronegativity of x.

Preparation of Alkyl halides

(i) From Alcohol

(ii) From Alkane.

Alkyl halide can be obtained from reaction alkane and of sunlight. halogen in the presence

Reactivity of Alkyl Halides

There are two main factor which control the reactivity of alkyl halide.

i. Bond -polarity of C-x bond

ii. Bond energy of C-x bond

i.Bond Polarity

Bond Polarity of R-X is due to move electronegativity of halogen as compared to carbon

As a result carbon acquires partial positive and halogen acquires partial negative charge.

So X becomes nucleophile which can be replaced by an another nucleophile.

On the basis of bond polarity, reactivity of alkyl halide decreases in following order.

R-F>R-a > R-Br > R-I

ii. Bond energy

Energy required to in Lone energy. break bond of same kind ide of bond is called bond

Greater the bond energy, lesser will be reactivity of the beth bond.

Or the basis of bond energy the reactivity of alkyl halide decreases in following order:

R-I > R-Br > R-Cl > R-E

Overall The reactivity order of alky) halide is due to bond energy.

R-I > R-Br > R-Cl > R-F

Definitions

1. Substrate molecule

The alkyl halide molecule on which a nucleophile attacks during Nucleophilic substitution reaction.

Example:

2. Nucleophile

A specie having lone pair of electrons which attacks on an electrophilic carbon is called nucleophile.

Example: OH^-, C₂H₅O^-, HS^-, SCN^-, H₂O, NH₂^- Cl^-, Br^-, NH₂^-

3. Leaving Groups

A leaving group also nucleophile that is Leaves the Substrate. Halogen of an alkyl halide leaving group is called leaving group. Example

Nucleophilic Substitution Reaction (SN)

Definition:

Those chemical reactions where halogen of alkyl halide is substituted or replaced by an attacking nucleophile is called nucleophilic substitution reaction.

During SN two events take place.

i. Formation σ-bond between carbon and

attacking Nucleophile.

ii. Breaking of O-bond between Carbon and

halogen

Mechanism

There are two types of mechanism in SN reaction.

i. Unimolecular nucleophillc substitution

reaction.

ii.

iiBim olecular nucleophillc substitution reaction.

i. Unimolecular nucleophillc substitution reaction (SN1)

The SN reaction in which first bond breaking (between carbon and halogen) and then bond formation (between carbon and nucleophile) takes place simultaneously is called unimolecular nucleophilic substitution reaction.

Step 1:

Step 2:

Characteristics of SN1 reaction

1. It is unimolecular reaction.

2. It is two step reaction.

3. First step is slow and it involves ionization

of alkyl halide. It is rate determining step.

4. During first step, carbocation is formed and

carbon bonded with X changes from Sp³ to Sp².

5. Second step is fast and it involves bond

formation between carbon and nucleophile.

6. During second step, carbon of carbocation

changes from sp2 to sp³to sp³.

7. It is favored by Polar Solvent.

8. Tertiary alkyl halide gives this reaction.

9. It gives product with 50% inversion and 50%

retention în configuration.

10. Rate = k[R-x].

Kinetic Evidence of SN1 reaction

Experiments show that the rate of an S_N1 reaction depends upon the concentration of alkyl halide. However change in concentration of nucleophile does not affect the rate of reaction. Rate = k [R-X]¹. It is a first order reaction that proves S_N1mechanism.

Stereo Chemical evidence of SN1 reaction

Since during reaction, carbon atom of carbonation is Sp² hybridized and it has unhybridized p orbital. The nucleophile can attack on P orbital either from right side or from left side.

So it gives a racemic mixture (50% inversion and 50% retention in configuration).

SN2 (Bimolecular nucleophilic substitution reaction).

The SN reaction in which bond breaking (between carbon and halogen) and bond formation (carbon and nucleophile) take place simultaneously is called SN2.

Characteristics of SN2

1. It is bimolecular reaction.

2. It is one step reaction that is rate determining step.

3. In transition state, carbon bonded to halogen atom becomes sp² hybridized. Both attacking nucleophile and leaving are partially bonded with p orbital in opposite directions.

4. It is favored by non-polar Solvent.

5. Usually primary alkyl halide gives this reaction.

6. It gives product with 100% inversion in configuration.

7. Rate = k[R-x][Nu].

Kinetic evidence of SN2reaction

Experiments show that rate of SN₂ reaction depends upon conc. of alkyl halide and conc. of nucleophile.

Rate= K(N8] [Alkyl halide]. It is a second order reaction that proves S_N2 mechanism.

Stereochemical evidence of SN2reaction

In SN₂ reaction, nucleophile attacks from opposite side of leaving group (19). 5. it gives product with 100% invension in configuration.

1,2 Elimination Reaction or beta elimination reaction

The chemical reaction in which two -groups (halogen and B- Hydrogen) are removed from two adjacent carbon atoms of alkyl halide to form carbon-carbon double band is called 1,2- elimination reaction, B-elimination reaction.

These are two types of elimination reaction

E1-mechanism

The elimination reaction in which first leaving group is removed to form carbocation and then base removes beta hydrogen to form carbon –carbon double bond is called E1 mechanism .

First step:

Second step:

Example 2

Characteristics of E1 mechanism

1. It is imolecular elimination reaction.

2. It is two step reaction.

3. First step is slow step and involves ionization of alkyl halide. It is rate determining step.

4. Second step is fast step which involves attack of base at beta hydrogen.

Kinetic Evidence

The experiment show that rate of beta elimination reaction depends upon the concentration of alkyl halide and does not depend upon the concentration of base.

Rate = k[R-X]

E2-Mechanism

It is bimolecular elimination reaction in which attack of base on beta hydrogen and removal of leaving group with formation of carbocation double take place simultaneously.

Characteristics of E2 mechanism

1. It is a single reaction which is rate determining step.

2. Since two molecule are involves in rate determining step. So, it bimolecular elimination reaction.

3. Kinetic Evidence

The experiment show that reaction rate of E2 depend upon the concentration of alkyl and base.

Rate = k [B] [R-X]

Organometallic Compounds (Grignard's Reagents)

Preparation of Grignard's Reagents from Alkyl halides

When alkyl halide is treated with. Mg in the presence of dry ether.

Alky magnecium halide

Reactivity of Grignard's Reagents

Grignard Reagent show nucleophilic reaction due to partial negative charge an carbon of alkyl group carbon is more electronegative than Mg, so it gets partial negative charge.