Question

The basicities (as measured by $p K_a$ values) of trimethyl-, triethyl-, and tri-n-propylphosphine are nearly identical. However, in reaction 13-5, if tri-n-propylphosphine is replaced by $\mathrm{Et}_3 \mathrm{P}$ the rate is enhanced; even greater enhancement is gained by replacement with $\mathrm{Me}_3 \mathrm{P}$. Why should this be the case?

Name: Problem 3, Chapter 13: Inorganic Chemistry
Uploaded: 2022-01-28T20:59:53-08:00
Duration: 1 min 26 s
Channel: Aadit Sharma
Description: The basicities (as measured by p Ka values) of trimethyl-, triethyl-, and tri-n-propylphosphine are nearly identical. However, in reaction 13-5, if tri-n-propylphosphine is replaced by Et3 P the rate is enhanced; even greater enhancement is gained by replacement with Me3 P. Why should this be the case?

   The basicities (as measured by $p K_a$ values) of trimethyl-, triethyl-, and tri-n-propylphosphine are nearly identical. However, in reaction 13-5, if tri-n-propylphosphine is replaced by $\mathrm{Et}_3 \mathrm{P}$ the rate is enhanced; even greater enhancement is gained by replacement with $\mathrm{Me}_3 \mathrm{P}$. Why should this be the case?

Inorganic Chemistry

Keith F. Purcell,… 1st Edition

Chapter 13, Problem 3

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Solved by Expert Aadit Sharma

01/28/2022

Step 1

The question discusses the basicities of different trialkylphosphines (trimethyl-, triethyl-, and tri-n-propylphosphine) and their effects on the rate of a specific reaction (reaction 13-5). It notes that while the basicities are nearly identical, the rate of the Show more…

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The basicities (as measured by $p K_a$ values) of trimethyl-, triethyl-, and tri-n-propylphosphine are nearly identical. However, in reaction 13-5, if tri-n-propylphosphine is replaced by $\mathrm{Et}_3 \mathrm{P}$ the rate is enhanced; even greater enhancement is gained by replacement with $\mathrm{Me}_3 \mathrm{P}$. Why should this be the case?

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Key Concepts

Nucleophilicity versus Basicity

While basicity is typically measured by how readily a molecule accepts a proton (indicated by pKa values), nucleophilicity is a kinetic property that describes how easily a molecule donates an electron pair to form a new chemical bond. In many reactions, especially those involving nucleophilic catalysts or reagents, less sterically hindered phosphines can act as more effective nucleophiles, even when their basicities are similar. This explains why phosphines with smaller alkyl substituents show enhanced reaction rates.

Steric Effects

Steric effects refer to the way that the size and spatial arrangement of substituents on a molecule can hinder or facilitate the approach of reactants to a reactive center. In the context of phosphines, even if the intrinsic electronic basicities are similar, bulkier alkyl groups such as ethyl or n-propyl can obstruct the efficient approach to the reaction site, thus impacting the overall reaction rate.

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