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how to know that which reaction will be sn1 or sn2 ?

how to know that which reaction will be sn1 or sn2 ?

Grade:12

1 Answers

Saurabh Kumar
askIITians Faculty 2411 Points
7 years ago
Let’s compare the SN1 and the SN2.
The MechanismThe SN2 reaction is concerted. That is, the SN2 occurs in one step, and both the nucleophile and substrate are involved in the rate determining step. Therefore the rate is dependent on both the concentration of substrate and that of the nucleophile.245-145_compare-sn1sn2.png

The MechanismThe SN2 reaction is concerted. That is, the SN2 occurs in one step, and both the nucleophile and substrate are involved in the rate determining step. Therefore the rate is dependent on both the concentration of substrate and that of the nucleophile.
The SN1 reaction proceeds stepwise. The leaving group first leaves, whereupon a carbocation forms that is attacked by the nucleophile.

The Big Barrier– this is the most important thing to understand about each reaction. What’s the one key factor that can prevent this reaction from occurring?

In the SN2 reaction, the big barrier is steric hindrance. Since the SN2 proceeds through a backside attack, the reaction will only proceed if the empty orbital is accessible. The more groups that are present around the vicinity of the leaving group, the slower the reaction will be. That’s why the rate of reaction proceeds from primary (fastest) > secondary >> tertiary (slowest)
In the SN1 reaction, the big barrier is carbocation stability. Since the first step of the SN1 reaction is loss of a leaving group to give a carbocation, the rate of the reaction will be proportional to the stability of the carbocation. Carbocation stability increases with increasing substitution of the carbon (tertiary > secondary >> primary) as well as with resonance.The dependence of rate upon the substrateFor the SN2, since steric hindrance increases as we go from primary to secondary to tertiary, the rate of reaction proceeds from primary (fastest) > secondary >> tertiary (slowest).
For the SN1, since carbocation stability increases as we go from primary to secondary to tertiary, the rate of reaction for the SN1 goes from primary (slowest) << secondary < tertiary (fastest)Remember that SN1 and SN2 reactions only occur for alkyl halides (and related compounds like tosylates and mesylates). If the leaving group is directly attached to an alkene or alkyne, SN1 or SN2 will not occur!
The NucleophileThe SN2 tends to proceed with strong nucleophiles; by this, generally means negatively charged nucleophiles such as CH3O(-), CN(-), RS(-), N3(-), HO(-), and others.
The SN1 tends to proceed with weak nucleophiles – generally neutralcompounds such as solvents like CH3OH, H2O, CH3CH2OH, and so on.The SolventThe SN2 reaction is favored by polar aprotic solvents – these are solvents such as acetone, DMSO, acetonitrile, or DMF that are polar enough to dissolve the substrate and nucleophile but do not participate in hydrogen bonding with the nucleophile.
The SN1 reaction tends to proceed in polar protic solvents such as water, alcohols, and carboxylic acids. These also tend to be the nucleophiles for these reactions as well.StereochemistrySince the SN2 proceeds through a backside attack, if a stereocenter is present the SN2 reaction will giveinversionof stereochemistry.
By contrast, if the SN1 leads to the formation of a stereocenter, there will be a mixture of retention and inversionsince the nucleophile can attack from either face of the flat carbocation.So does the story about the hobo on the bench make sense now?In the SN2, the nucleophile (you) forms a bond to the substrate (bench) at the same time the leaving group (hobo) leaves.
In the SN1, the leaving group (hobo) leaves the substrate (bench), and then the nucleophile (you) forms a bond.For now, this concludes the series on substitution reactions.

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