Nucleophilic Substitution Reactions - Haloalkanes and Haloarenes, Class 12, Chemistry PDF Download

Chemical reactions of alkyl halide:

Nucleophilic substitution reaction:

Those organic compounds in which an sp³ hybridized carbon is bonded to an electronegative atom or group can undergo two type of reaction e.g. substitution reactions in which the electronegative atom or group is replaced by another atom or group. Sec

ond is elimination reaction in which the electronegative atom or group is eliminated along with hydrogen from an adjacent carbon. The electronegative atom or group which is substituted or eliminated is known as leaving group. 

Because of more electronegativity of halogen atom it has partial negative charge and partial positive cha develops on carbon atom.

X = F, Cl, Br, I

Due to this polar carbon - halogen bond alkyl halides shows nucleophilic substitution and elimination reaction.

There are two important mechanisms for the substitution reaction

(1) A nucleophile is attracted to the partially positively charged carbon. As the nucleophile approaches the carbon,

it causes the carbon - halogen bond to break heterolytically (the halogen keeps both of the bonding electrons.)

(2) The carbon-halogen bond breaks heterolytically without any assistance from the nucleophile, by the help of polar protic solvent and carbocation is formed (solvolysis). Formed carbocation then reacts with the nucleophile to form the substitution product.

(A) Bimolecular nucleophilic substitution reaction (S_N²)

The mechanism of S_N² reaction

                                        transition state

Characteristic of S_N²

(1) It is bimolecular, unistep process

(2) It is second order reaction because in the Rds two species are involved

(3) Kinetics of the reaction → rate ∝ [alkyl halide] [nucleophile]

                                               rate ∝  k[alkyl halide] [nucleophile]

If the concentration of alkyl halide in the reaction mixture is doubled, the rate of the nucleophilic substitution reaction is double. If the concentration of nucleophile is doubled the rate of reaction is also double. If the concentration of both are doubled then the rate of the reaction quadriples.

(4) Energetics of the reaction →

Figure : A free energy diagrams for a hypothetical S_N² reaction that takes place with a negative DGº

(5) No intermediates are formed in the S_N² reaction, the reaction proceeds through the formation of an unstable arrangement of atoms or group called transition state.

(6) The stereochemistry of S_N² reaction → As we seen earlier, in an S_N² mechanism the nucleophile attacks from the back side, that is from the side directly opposite to the leaving group. this mode of attack causes a inversion of configuration at the carbon atom that is the target of nucleophilic attack. This inversion is also known as walden inversion.

(7) Factor's affecting the rate of S_N² reaction → Number of factors affect the relative rate of S_N² reaction, the most important factors are

(i) Structure of the substrate

(ii) Concentration and reactivity of the nucleophile

(iii) Effect of the solvent

(iv) Nature of the leaving group

(i) Effect of the structure of the substrate →

Order of reactivity in S_N² reaction : - CH₃ > 1º > 2º >> 3º (unreactive)

the important factor behind this order of reactivity is a steric effect. Very large and bulky groups can often hinder the formation of the required transition state and crowding raises the energy of the transition state and slows down reaction.

Table : 6 Relative rates of reactions of alkyl halide in S_N² reaction.

(ii) According to kinetics of S_N² increasing the concentration of the nucleophile increases the rate of an S_N² reaction. The nature of nucleophile strongly affect the rate of S_N² reaction. A stronger nucleophile is much more effective than a weaker. For example we know that a negatively charged nucleophile is more reactive than its conjugate acid e.g. HO^! > H₂O, RO^! > ROH.

Table : 7

Steric effects on nucleophilicity

t-butoxide ,Stronger base, nucleophile cannot approach the carbon atom so easily.

CH₃ - CH₂ - O^!
 ethoxide,weaker base, yet weaker yet stronger nucleophile

(iii) The effect of the solvent ° In polar protic solvent large nucleophiles are good, and the halide ions show the following order

I^! > Br^! > Cl^! > F^! (in polar protic solvent)

This effect is related to the strength of the interaction between nucleophile and solvent molecules of polar protic solvent forms hydrogen bond to nucleophiles in the following manner.

Because small nucleophile is solvated more by the polar protic solvent thus its nucleophilicity decreases and rate of SN² decreases

Relative nucleophilicity in polar protic solvent

SH^! > CN^! > I^! > OH^! > N₃^! > Br^! > ACO^! > Cl^! > F^! > H₂O

So, polar protic solvents are not useful for rate of S_N², if nucleophile is anionic. But polar aprotic solvent does not have any active hydrogen atom so they can not forms H bond with nucleophiles. Polar aprotic solvent have crowded positive centre, so they do not solvate the anion appreciably therefore the rate of S_N² reactions increased when they are carried out in polar aprotic solvent.

Examples of polar aprotic solvent.

In DMSO, the relative order of reactivity of halide ions is

F^! > Cl^! > Br^! > I^!

(iv) The nature of the leaving group ° The best leaving groups are those that become the most stable ion after they leave, because leaving group generally leave as a negative ion, so those leaving group are good, which stabilise negative charge most effectively and weak base do this best, so weaker bases are good leaving groups. A good leaving group always stabilize the transition state and lowers its free energy of activation and thereby increases the rate of the reaction.

Order of leaving ability of halide ion

I^! > Br^! > Cl^! > F^!

Other leaving groups are

Strongly basic ions rarely act as leaving group →

Table : 8 Examples of SN² reactions of alkyl halide →

Nucleophile		Product	Class of Product
		R -	Alkyl halide
		R -	Alcohol
		R -	Ether
		R -	Thiol(mercaptan)
		R -	Thioether (sulphide)
		R -	Amine
		R -	Azide
		R -	Alkyne
		R -	Nitrile
		R - COO - R	Ester
		[R - PPh3]+	Posphonium salt

Ex. Complete the following reactions with mechanism

(a)  

Sol.

(b) ?

Sol. (p-Nitroanisole)

(c) Ph - CH₂Cl

Sol. CH₃-CH₂-O^! is present in excess and it is stronger nucleophile than Ph - O^! so product is Ph-CH₂ - OEt

(d) CH₃ - C º CH X Y

Sol.

(e) Ph₃ → Salt

Sol.

Ex. When the concentration of alkyl halide is tripled and the concentration of OH^- ion is reduced to half, the rate of S_N² reaction increases by :

(A) 3 times (B) 2 times (C) 1.5 times (D) 6 times

Ans. C

Ex. In the given reaction, CH₃CH₂ - X CH₃SNa °

The fastest reaction occurs when 'X' is -

(A) - OH (B) - F (C) - OCOCF₃ (D) OCOCH₃

Ans. C

Ex. Correct decreasing order of reactivity towards S_N² reaction

(I)
(II)
(III) CH₃CH₂CH₂CH₂ Cl
(IV)

(A) IV > I > II > III (B) III > II > I > IV (C) IV > I > III > II (D) II > I > IV > III

Ans. B

Q.1 ?

Q.2 KF Products.

What is the function of 8 corwn-6?

Q.3 Write mechanisms that account for the product of the following reactions:

(a) HOCH₂CH₂Br

(b) NH₂-CH₂-CH₂-CH ₂-CH₂-Br

Q.4 Draw a flscher projection for the product of the following S_N² reaction

(a) (b)

(B) Unimolecular nucleophillic substitution reaction (S_N¹) :

Characteristics of S_N¹ reactions

1. It is unimolecular, two step process and intermediate is formed, (intermediate is carbocation)

2. It is first order reaction

3. Kinetics of the reaction

Rate ∝ [Alkyl halide]

Rate = k[(CH₃)₃C-X]

Rate of SN¹ reaction is independent of concentration and reactivity of nucleophile.

4. Energetics of the S_N¹

Figure : free energy diagram for the SN¹ reaction.

5 Factor's affecting the rates of S_N¹

5.(i) The structure of the substrate °

The Rds of the S_N¹ reaction is ionization step, in this step form a carbocation. This ionisation is strongly endothermic process, rate of S_N¹ reaction depends strongly on carbocation stability because carbocation is the intermediate of S_N¹ reaction which determines the energy of activation of the reaction.

S_N¹ reactivity : 3º > 2º > 1º > CH₃ - X

5.(ii) Concentration and reactivity of the nucleophile °

The rate of S_N¹ reactions are unaffected by the concentration and nature of the nucleophile

5.(iii) Effect of the solvent ° the ioizing ability of the solvent:

Because to solvate cations and anions so effectively the use of polar protic solvent will greatly increase the rate of ionization of an alkyl halide in any S_N¹ reaction. It does this because solvation stabilizes the transition state leading to the intermediate carbocation and halide ion more than it does the reactant, thus the energy of activation is lower.

Table : 9 Dielectric constants (ä) and ionization rates of t-Butylchloride in common solvents

5.(iV) The nature of the leaving group °

In the S_N¹ reaction the leaving group begins to acquire a negative charge as the transition state is reached stabilisation of this developing negative charge at the leaving group stabilizes the transition state and : this lowers the free energy of activation and thereby increases the rate of reaction.

6. Stereochemistry of S_N¹ reactions → In the S_N¹ mechanism, the carbocation intermediate is sp² hybridized and planar. A nucleophile can attack on the carbocation from either face,if reactant is chiral than after attack of nucleophile from both faces gives both enantiomers of the product, which is called recemization.

Mechanism of recemization (S_N¹) →

Comparison of SN¹ and SN² reactions :

Ex. 6 Predict the compound in each pair that will undergo solvolysis (in aqueous ethanol) more rapidly.

Sol. (a) II > I (b) II > I (c) I > II (d) II > I (e) II > I

Ex. 7 Give the solvolysis products expected when each compound is heated in ethanol

(a)  
(b)  
(c)  
(d) 

Sol. (a)  
(b)  
(c)  
(d) 

Ex.8 The rate of SN¹ reaction is fastest with

Ans. (A)

Reaction of RX with aq. KOH

1.  R - OH + KCl

2. CH₃-CH₂-Cl  CH₃-CH₂-OH

3. CH₃-CH   CH₃-CH   CH₃-CHO

4. CH₃-C CH₃-C  CH₃COOH

5. C - C - C - C - Br  C - C - C - C - OH

6.   

7. ¹⁴C = C - C - I  ¹⁴C = C - C - OH + C = C - C¹⁴ - OH

8. 

Other Nucleophilic reaction of R - X :-

WillionSon's Ether Synthesis: (SN²)

1. 

2. EtONa + Me - Cl  EtOMe

3.   EtOMe

Rate (2) > (3) 2 is better method. (Due to less steric hindrence)

4. MeONa + PhCl  No reaction

5.  + NaCl

6. Me₃CO^!Na^Å + MeCl  Me₃COMe + NaCl

7. MeO^!Na^Å + Me₃C - Cl   + MeOH + NaCl

                                                                          Major

8. PhONa + Me₃C-Cl   + PhOH + NaCl

9. Me₃CONa + Ph-Cl  No. reaction

Me₃CO-Ph can not prepared by willionson's ether synthesis.

1. 

2. 

3.  + HCl

4.  + HBr

5.  (Pyridinium salt)

Some Other reactions

Hydrolysis of Ether

1. 

2. MeOEt  Et -  + Me - OH

3. 

4.

5. 

Reaction of ether with HI :

1. Me - O - Et 

2.  (Due to formation of more stable carbocation)

With moist and dry Ag₂O :

1. 

2. 2Me - Cl  Me - O - Me + 2AgCl

3. Me - Cl + EtCl  Me - O - Et + Me - O - Me + EtOEt

SN_i (Nucleophilic substitution intramolecular )

(Darzon's process)

Mech.

R - Cl + O = S = O ­ (g)

Note : (1) In SNi retention of configuration takes place.

Note : (2) In presence of pyridene above reaction follow the SN² reaction mechanism.

SN^NGP(Neighbouring group participation)

Increase in rate of SN reaction due to attack of internal nucleophie is called as SN_NGP is also known as Anchimeric assistence.

For SN^NGP:-

1. Internal nucleophile must be present

2. Internal nucleophile must be anti to lg.

During NGP :-

1. 

2.  (enantiomers)

3.  + (enantiomer) Rate 3 > 2 > 1

4.