What are different mechanisms for nucleophilic substitution?

Nucleophilic substitution reactions in halides containing  - X bond may take place through either of the two different mechanisms,SN1 and SN2.

SN1 Mechanism (unimolecular Nucleophilic Substitution)

In this type, the rate of reaction dependent only on the concentration of alkyl halide, i.e.

Rate = k [RX]

The tertiary alkyl halides react by SN1 mechanism via formation of carbocations as intermediates as given below:

Step I: in the first step the alkyl halide slowly dissociates into halide and carbocation.


This step is the slowest and reversible. It involves the cleavage of C-Br bond for which the energy is obtained through salvation of halide ion with the proton of protic solvent. Since the rate of reaction depends upon the slowest step, the rate of reaction depends only on the concentration of alkyl halide and not on the concentration of nucleophile.

Step IInd: in the second step, carbocation at once combines with the nucleophile to form the final substituted product.


The order of reactivity of a variety of alkyl halides from SN1 mechanism is as below:

The 3+ alkyl halides are most reactive because the intermediate carbocation formed in their case is the most stable. The more stable intermediate is formed at faster rate.
SN2 Mechanism (Bimolecular Nucleophilic Substitution)

In this type of reaction is dependent on the concentration of alkyl halide as well as nucleophile, i.e. 

Rate = k [RX] [Z-]

In this mechanism the incoming nucleophile interacts with alkyl halide causing the carbon-halide bond to break while forming a new carbon nucleophile bond. These two processes occurs at the same time in a single step and no intermediate is formed. As the reaction progresses and the bond between the nucleophile and the carbon atom starts forming and the bond between carbon atom and leaving group starts breaking. Finally, the product formed and the leaving group goes away.

In the transition state, the carbon atom is simultaneously bonded to incoming nucleophile and the leaving group. Such structures formed are unstable and cannot be isolated. This is due to the carbon atom in the transition state is at the same time bonded to five atoms and consequently is unstable.
The order of reactivity can be explained in terms of stability of transition state. Bulky alkyl groups attached to the carbon carrying halogen make the transition state unstable due to crowding (steric hindrance and decrease the reactivity of the alkyl halide through SN2mechanism. In 3° alkyl halide three alkyl groups are attached to the carbon carrying halogen. Therefore, transition state in this case has maximum energy and hence the reactivity is least. The 2° alkyl halides with two alkyl groups are most reactive whereas 1° alkyl halide with one alkyl group is most reactive.
Starting with an optically active alkyl halide, the reaction through SN2 mechanism results in complete inversion of configuration as it involves attack of nucleophile from backside. For example, when (-) -2-bromoethane is allowed to react with sodium hydroxide, (+)-2-octanol is formed. In (+)-2-octanol the position of -OH group is opposite to what bromide had occupied in (-)-2-bromooctane

   Related Questions in Chemistry

  • Q : Molarity of acid solution If 20ml of

    If 20ml of 0.4N, NaoH solution completely neutralises 40ml of a dibasic acid. The molarity of the acid solution is: (a) 0.1M (b) 0.2M  (c) 0.3M (d) 0.4M Choose the right answer fron above.

  • Q : How much phosphorus is in superphosphate

    Superphosphate has the formulate: CaH4 (PO4)2 H2O calculate the percentage of Phosphorus in this chemical. Show your calculations

  • Q : Amount of glucose in blood What is the

    What is the normal amount of glucose in 100ml of blood (8–12 hrs after meal) is: (i) 8mg (ii) 80mg (iii) 200mg (iv) 800mg Choose the right answer from above.

  • Q : Mole fraction of water Give me answer

    Give me answer of this question. A solution contains 25%H2O 25%C2H5OH , and 50% CH3 COOH by mass. The mole fraction of H2O would be: (a) 0.25 (b) 2.5 (c) 0.503 (d) 5.03.

  • Q : Describe characteristics of halides and

    Halides characteristics

  • Q : What are homogenous catalyst? Give few

    When a catalyst mixes homogeneously with the reactants and forms a single phase, the catalyst is said to be homogeneous and this type of catalysis is called homogeneous catalysis. Some more examples of homogeneous catalysis are:    SO2

  • Q : Pressure Phase Diagrams The occurrence

    The occurrence of different phases of a one component system can be shown on a pressure temperature. The phases present in a one line system at various temperatures can be conveniently presented on a P- versus-T diagram. An example is pro

  • Q : Effect of addition of mercuric iodide

    Give me answer of this question. When mercuric iodide is added to the aqueous solution of potassium iodide, the:(a) Freezing point is raised (b) Freezing point is lowered (c) Freezing point does not change (d) Boiling point does not change

  • Q : Statement of Henry law Determine the

    Determine the correct regarding Henry’s law: (1) The gas is in contact with the liquid must behave as an ideal gas (2) There must not be any chemical interaction among the gas and liquid (3) The pressure applied must be high (4) All of these.

  • Q : Molecular Structure type The ionic

    The ionic radii of Rb+ and I- respectively are 1.46 Å and 2.16Å. The very most probable type of structure exhibited by it is: (a) CsCl type  (b) ZnS type  (c) Nacl type  (d) CaF2 type

    Discover Q & A

    Leading Solution Library
    Avail More Than 1448912 Solved problems, classrooms assignments, textbook's solutions, for quick Downloads
    No hassle, Instant Access
    Start Discovering




    Active Tutors



    Start Excelling in your courses, Ask an Expert and get answers for your homework and assignments!!

    Submit Assignment

    ©TutorsGlobe All rights reserved 2022-2023.