Consider Table 8.4 in Carey\'s Organic Chemistry, 9th Ed. Reproduce an approxima
ID: 580820 • Letter: C
Question
Consider Table 8.4 in Carey's Organic Chemistry, 9th Ed. Reproduce an approximation of Table 8.4 and write the dielectric constant next to each solvent. 1. 1. Please define the term dielectric constant. 2. Can the rate data be explained solely by comparing dielectric constant values? 3. Is acetonitrile a better solvent for these types of reactions than dimethyl sulfoxide? 4. Can vou rationalize the differences in rate based on the structures of the solvent molecules? 5. How do polar protic solvents affect the activation energy of charged transition states? (10 points) Relative Rates of SN2 Displacement of 1-Bromobutane by Azide in Various Solvents. Solvent 3SN2 Solvent Type Dielectric Constant Solvent Line- angle structure of solvent Relative rate Polar protic Methanol -OH Water Dimethyl sulfoxide DMFExplanation / Answer
Answer : Dielectric constant: it is the measure of separation of two oppositely charged species.
2. The above reaction is nucleophilic substitution reaction, and particularly proceeds through SN2 reaction.
In SN2 reaction the rate of the reaction depends on both the concentrations of alkyl halide and incoming nucleophile, in other words it depends on the dielectric constant of C-X bond (X = Cl, Br or I) and C-Nu bond (Nu = incoming nucleophile). If dielectric constant of C-X is more when compared C-Nu, then the rate of the reaction is faster and gives good yields in short time.
3. Acetonitrile is non polar solvent compared with DMSO. The above reaction proceeds through SN2 mechanism, in which without formation of carbocation intermediate takes place. This process is favourable in nonpolar solvents like Acetonitrile.
DMSO is good solvent in SN1 mechanism, in which formation of carbocation intermediate takes place which can be stabilized by polar solvents like DMSO.
4.In DMSO S=O is polar bond, alkyl groups attached to the Sulphur atom, due to positive inductive effect of alkyl groups neutralize the partial +ve charge on sulphur atom and increase the bond polarity.
5. In Acetonitrile C triple bond N is slightly polar compared with DMSO.
6. Polar protic solvent increases the C-X bond polarity by the formation hydrogen bonding with hetero atom meanwhile C-X bond gets weakened and the activation energy decreases so the rate of the reaction increases.
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