--%>
Assignment Help - homework help

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.

1475_nucleophillic.png 

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.

2268_nucleophillic1.png 

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 : What is Spectroscopy? This is a very

    This is a very important aspect of Physical Chemistry in which knowledge of the size, shape, rigidity and electronic structure of molecules deduced from the experimental methods treated here goes hand in hand with the theoretical approaches of chemical reactions. Spec

    		•
  • Q : P block bif3 is ionic while other

    bif3 is ionic while other trihalides are covalent in nature

    		•
  • Q : Molar mass lculwhat is the equation for

    lculwhat is the equation for caating molar mass of non volatile solute

    		•
  • Q : Wavelengths which the human eye can see

    Briefly state the wavelengths which the human eye can see?

    		•
  • Q : Alkaline medium The amount of KMnO 4

    The amount of KMnO4 required to prepare 100 ml of 0.1N solution in alkaline medium is: (a) 1.58 gm (b) 3.16 gm (c) 0.52 gm (d) 0.31 gmAnswer: (a) In alkaline medium KMnO4 act as ox

    		•
  • Q : Electrons present in a benzene How

    How many electrons are present in a benzene?

    		•
  • Q : Diffusion Molecular View When the

    When the diffusion process is treated as the movement of particles through a solvent the diffusion coefficient can be related to the effective size of diffusing particles and the viscosity of the medium.To see how the experimental coefficients can be treat

    		•
  • Q : Explain Phase Rule The relation between

    The relation between the number of phases, components and the degrees of freedom is known as the phase rule. One constituent systems: the identification of an area on a P-versus-T with one phase of a component system illustrates the two degrees of freedom that

    		•
  • Q : Describe physical adsorption and its

    When the forces of attraction existing between adsorbate and adsorbent are van der Waal's forces, the adsorption is called physical adsorption. This type of adsorption is also known as physisorption or van der Waal's adsorption. Since the forces existing between adsorbent and adsorbate are very w

    		•
  • Q : Describe Thermodynamics Properties The

    The free energy property leads to convenient expressions for the volume and pressure dependence of internal energy, enthalpy and the heat capacities.All the properties of a chemical system, a sample of a substance, or a mixture of substances have some fixe

    		•