Mechanism of nucleophilic substitution reaction of alkyl halide

A.K.GuptaPGT Chemistry

Kendriya Vidyalaya BallygungeKolkata 700019

ashokkgupta62@rediffmail.com

Learning ObjectivesAt the end of this class, pupil should be able to-•Understand the meaning of Nucleophilic substitution(SN) reaction.•Explain the factor affecting SN reaction•Differentiate between SN1 and SN2 reaction

•Predict whether the given halide will undergo SN1

and SN2 reaction

(a) Substitution nucleophilic bimolecular (SN2)

SN1 - Racemization - Inversion

SN1 - Racemization - Retention

SN2 concerted reaction inversion

SN2 Inversion

Steric effects in SN2 reaction-The relative rate of SN2 reaction is given in parenthesis

(b) Substitution nucleophilic unimolecular (SN1)

We can sum up the order of reactivity of alkyl halides towards SN1 and SN2 reactions as follows:

Compare the reactivity of allylic and benzylic halides towards the SN1

reaction-

Allylic Halide Benzylic Halide

The carbocation formed gets stabilised through resonance asshown below-

What order do you expect regarding the reactivity of alkyl halide for a given Alkyl group ?

• R–I> R–Br>R–Cl>>R–F

1 . Which statement best describes a carbon-chlorine bond?   A.The bond is covalent and polar, with chlorine

carrying a partial positive charge.  B.The bond is ionic and polar, with chlorine

carrying a full positive charge.  C.The bond is covalent and nonpolar.  D.The bond is covalent and polar, with chlorine

carrying a partial negative charge.  E.The bond is ionic and polar, with chlorine

carrying a full negative charge.

2 .     Which of the following does not affect the mechanism of a nucleophilic substitution reaction?

A. the intensity of the light present  B. the structure of the alkyl halide  C. the polarity of the solvent  the

concentration of the nucleophile  D. the reactivity of the nucleophile

4 . Which type of reactant shows the greatest reactivity in an SN2 reaction?  A. quaternary alkyl halides  B. methyl halides  C. tertiary alkyl halides  D. secondary alkyl halides  E. primary alkyl halides

6 .     Which of the following represents the relative leaving group abilities of the halide ions?

A. iodide < bromide < chloride > fluoride  B. iodide > chloride > bromide > fluoride  C. iodide < bromide < chloride < fluoride  D. iodide > bromide > chloride > fluoride  E. iodide < bromide > chloride > fluoride

7 .     Which statement best describes the mechanism of an SN2 reaction?A. front side and back side attack with

racemization  B. front side attack with inversion of configuration  C. back side attack with retention of configuration  D. back side attack with inversion of

configuration  E. front side attack with retention of configuration

8 .     Which of the following is not a correct statement?

A. Alkyl halides react with -NH2 to form amines. 

B. Alkyl halides react with RS- to form thiols.  C. Alkyl halides react with RCΞC- to form alkynes.  D. Alkyl halides react with HO- to form alcohols.  E. Alky halides react with RO- to form ethers.

     Which of the following is the rate law for an SN1 reaction?

A. Rate = k[nucleophile]2  B. Rate = k[alkyl halide][nucleophile]  C. Rate = k[alkyl halide]2  D. Rate = k[alkyl halide]  E. Rate = k[nucleophile]

Which of the following is the rate-limiting step for the hydrolysis of tert-butyl bromide?

A. loss of water from the carbocation  B. addition of water to the carbocation  C. deprotonation of the addition product formed

by addition of water to the carbocation  D. dissociation of the alkyl halide into a

carbocation and a bromide ion  E. reaction of the carbocation with bromide ion

Which of the following alkyl bromides is the most reactive in an SN1 reaction?

A. 1-bromopropane  B. bromomethane  C. 1-bromo-2-methylpropane  D. 2-bromopropane  E. 2-bromo-2-methylpropane

Which of the following is the most nucleophilic towards CH3I?

A. NH3 

B. -CN  C. F-  D. CH3S- 

E. H2O

Home work Assignment:TBQ 10.9 Which compound in each of the following

pairs will react faster in SN2 reaction with –OH?(i) CH3Br or CH3I (ii) (CH3)3CCl or CH3ClTBQ 10.16 Arrange the compounds of each set in order

of reactivity towards SN2 displacement:(i) 2-Bromo-2-methylbutane, 1-Bromopentane,

2-Bromopentane(ii) 1-Bromo-3-methylbutane,

2-Bromo-2-methylbutane, 3-Bromo-2-methylbutane