A Which S_N 2 reaction in each pair is faster? b. Draw the products of each S_N

Question

A Which S_N 2 reaction in each pair is faster? b. Draw the products of each S_N 2 reaction and indicate the stereochemistry where appropriate. Consider the following S_N 1 reaction. a. Draw a mechanism for this reaction using curved arrows. b. Draw an energy diagram. Label the axes, starting material, product, E_a, and Assume that the starting material and product are equal in energy. c. Draw the structure of any transition states. d. What is the rate equation for this reaction? e. What happens to the reaction rate in each of the following instances? [1] The leaving group is changed from I^- to Cl^-; [2]The solvent is changed from H_2 O to DMF; [3]The alkyl halide is changed from (CH_3)_2 C(I)CH_2 CH_3 to (CH_3)_2 CHCH(I)CH_3; [4]The concentration of H_2 O is increased by a factor of five; and[5]The concentrations of both the alkyl halide and H_2 O are increased by a factor of five.

Explanation / Answer

Characteristics of SN2 mechanism:

*The SN2 reaction is concerted. That is the SN2 occurs in one step, and both the nucleophile ans substrate are involved in the rate determining step. Therefore the rate is dependent on both the concentration of substrate and that of the nucleophile.

*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 of 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).

* For 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).

* The SN2 tends to proceed with strong nuceophiles by this, generally means negatively charged nucleophiles such as CH3O-, CN-, N3-, HO- and others.

* The SN2 reaction is favored by polar aprotic solvents , these are solvents such as acetone, DNSO, acetonitrile or DMF that are polar enough to dissolve the substrate and nucleophile but do not participate in hydrogen bonding with the nucleophile.

* Since SN2 proceeds through a backside attack, if a stereocenter is present the SN2 reaction will give inversion of stereochemistry.

Based above characteristics,

(8)

(a) CH3CH2Br is faster than CH3CH2Cl

Because, - Br- is better leaving group than Cl-

(b) CH3CH2CH2Br+OH- is faster than CH3CH2CH2Br+H2O

Because, OH- is stronger nucleophile than H2O

(c) CH3CH2CH2Cl + NaOH is faster than CH3CH2CH2Cl + NaOCOCH3

Because NaOH is better nucleophile tha NaOCOCH3

(d) CH3CH2CH2I+OCH3- + DMSO is faster than CH3CH2CH2I+OCH3-+CH3OH

Because, DMSO is polar aprotic, but CH3OH is polar ptrotic

(e) CH3CH2CH2CH2Br + -OCH2CH3 is faster than CH3CH2CH(CH3)CH2Br + -OCH2CH3

Because CH3CH2CH(CH3)CH2Br is more sterically hindered than CH3CH2CH2CH2Br

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