--%>
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 : Dipole moment direction for the methanol

    Briefly describe the dipole moment direction for the methanol?

    		•
  • Q : Thermodynamics I) Sulphur dioxide (SO2)

    I) Sulphur dioxide (SO2) with a volumetric flow rate 5000cm3/s at 1 bar and 1000C is mixed with a second SO2 stream flowing at 2500cm3/s at 2 bar and 200C. The process occurs at steady state. You may assume ideal gas behaviour. For SO2 take the heat capacity at constant pressure to be CP/R = 3.267

    		•
  • Q : Vapour pressure Vapour pressure of

    Vapour pressure of methanol in water Give me answer of this question. An aqueous solution of methanol in water has vapour pressure: (a) Equal to that of water (b) Equal to that of methanol (c) More than that of water (d) Less than that of water

    		•
  • Q : Rotational energy and entropy due to

    The entropy due to the rotational motion of the molecules of a gas can be calculated. Linear molecules: as was pointed out, any rotating molecule has a set of allowed rotational energies. For a linear molecule the

    		•
  • Q : Preparation of normal solution Give me

    Give me answer of this question. What weight of ferrous ammonium sulphate is requiored to prepare 100 ml of 0.1 normal solution (mol. wt. 392): (a) 39.2 gm (b) 3.92 gm (c)1.96 gm (d)19.6 gm

    		•
  • Q : What is covalent radii? Explain its

    Average covalent radii can be assigned on the basis of molecular structures. The accumulation of structural data by spectroscopic studies and both electron and x-ray diffraction studies allows one to investigate the possibili

    		•
  • Q : Mole fraction of Carbon dioxide Choose

    Choose the right answer from following. If we take 44g of CO2 and 14g of N2 what will be mole fraction of CO2 in the mixture: (a) 1/5 (b) 1/3 (c) 2/3 (d) 1/4

    		•
  • Q : Molarity based question Help me to

    Help me to solve this problem. 4.0 gm of NaOH are contained in one decilitre of solution. Its molarity would be: (a) 4 M (b)2 M (c)1 M (d)1.5 M

    		•
  • Q : Molecular weight of substance The

    The boiling point of a solution of 0.11 gm of a substance in 15 gm of ether was found to be 0.1oC higher than that of the pure ether. The molecular weight of the substance will be (Kb = 2.16)       (a) 148 &nbs

    		•
  • Q : Problem based on molality of glucose

    Select the right answer of the question. If 18 gm of glucose (C6H12O6) is present in 1000 gm of an aqueous solution of glucose, it is said to be: (a)1 molal (b)1.1 molal (c)0.5 molal (d)0.1 molal

    		•