The table below (Table 9.5) shows the rate constants foe the reaction of >>elect

Question

The table below (Table 9.5) shows the rate constants foe the reaction of >>elect nucleophiles from Group 6A and 7A with methyl iodide (CH_3I). What is the trend in nucleophile strength for each of the groups of elements. 6A and 7A? Explain answer. What type of solvent is this reaction run in. polar protic or polar aprotic? What does the pKa of the conjugate base tell you about the nucleophile strength m this case? Explain your answer. What effect would switching the solvent of the reaction to DMSO have on the rate of reaction of the nucleophiles in Group 7A with methyl iodide? Dependence of 5_N 2 Reaction Rate on the Basicity of Nucleophiles from Different Periods of the Periodic Table in Methanol

Explanation / Answer

10 (a) The nucleophilic trend in same for both 6A and 7A group, i.e. as we go down the group the nucleophilicity increases. So in group 7A, I-, is most nucleophilic and F-, is least nucleophilic and similarly in group 6A, PhS-, is more nucleophilic than PhO-.

The reason is as we go down the group the size of the nucleophiles increases and electronegativity decreases and thus size of the orbitals of the nucleophile increases and electrons are more available for attack. Due to their enhances size the orbitals are more effective in attacking the electrophile.

(b) The reaction is run in polar aprotic solvent, so that the nucleophile doesn't pick up a proton and becomes a neutral conjugate acid, thus reducing it's nucleophilicity

(c) pKa of conjugate acid tells about the ease of formation of conjugate base, whic acts as a nucleophile. For example in group 6A, pKa of PhS-, is lower than that of PhO-, and thus Ka of  PhS- is higher than that of  PhO-. So higher the Ka means PhSH is more acidic and PhS-, is more stable and more readily formed.

(d) The reaction rate will be enhanced tremendously if the reaction is carried out in DMSO as DMSO is a polar aprotic solvent and thus stabilizes the transition state during the SN2 reaction much more effectively, thus lowering the activation energy of reaction.

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