Question

Treatment of 1,3 -dichloropropane with potassium cyanide results in the formation of pentanedinitrile. The rate of this reaction is about 1000 times greater in DMSO than in ethanol. Account for this difference in rate.

Short answer

Answer: The rate of this reaction is about 1000 times greater in DMSO than in ethanol because DMSO is a polar aprotic solvent, which does not stabilize the nucleophilic cyanide ion (CN-) through hydrogen bonding. This results in the cyanide ion being more reactive in DMSO, leading to a faster reaction rate. In contrast, ethanol is a polar protic solvent that solvates and stabilizes the cyanide ion through hydrogen bonding, making it less reactive and resulting in a slower reaction rate.

Step-by-step solution

Write down the reaction equation

First, let's write down the reaction equation: 1,3-dichloropropane + 2 KCN → pentanedinitrile + 2 KCl

Analyze the properties of DMSO and ethanol

Compare the properties of the two solvents, dimethyl sulfoxide (DMSO) and ethanol (EtOH). DMSO is a highly polar aprotic solvent, meaning it has a large dipole moment but does not donate protons in hydrogen bonding. Ethanol, on the other hand, is a polar protic solvent, which means it has a smaller dipole moment and can participate in hydrogen bonding.

Examine the effect of solvents on nucleophilicity of cyanide ion

The cyanide ion (CN-) is a strong nucleophile and is responsible for attacking the electrophilic carbon in 1,3-dichloropropane. In polar protic solvents like ethanol, the nucleophilicity of negatively charged species gets reduced, as the solvent forms strong hydrogen bonds with the negatively charged nucleophile. This results in solvation and stabilization of the nucleophile, making it less reactive. In polar aprotic solvents like DMSO, hydrogen bonding with the nucleophile is not possible, so there is less stabilization of the nucleophile. Consequently, the nucleophile is more reactive and can attack the electrophilic carbon more easily.

Explain the difference in reaction rate

The reaction rate depends on the nucleophilicity of the cyanide ion (CN-) in this reaction. As DMSO is polar aprotic solvent, it does not solvate and stabilize the cyanide ion, making it more reactive and resulting in a faster reaction rate. In ethanol, a polar protic solvent, the cyanide ion gets solvated and stabilized, making it less reactive and resulting in a slower reaction rate. Taking all of these factors into account, we can conclude that the rate of reaction is about 1000 times greater in DMSO than in ethanol because the nucleophilicity of the cyanide ion is significantly higher in DMSO due to the lack of stabilization by hydrogen bonding.