Understanding and Using the Bordwell pKa Table
The Bordwell pKa table is a valuable tool for chemists to compare acidities of organic molecules, revealing how structural differences profoundly affect pKa values. A notable example is acetonitrile and malononitrile: acetonitrile’s pKa is about 31, while malononitrile’s pKa drops drastically to around 11.
Why Are Acetonitrile and Malononitrile’s pKa So Different?
This large difference initially surprises many. However, it aligns with the trend observed by comparing similar molecules like methane (pKa ~50). Replacing hydrogens with electron-withdrawing nitrile groups lowers pKa dramatically:
- Methane (no nitriles): pKa ~50
- Acetonitrile (one nitrile): pKa ~31
- Malononitrile (two nitriles): pKa ~11
The strong electron-withdrawing effect of nitrile groups stabilizes the conjugate base formed after deprotonation. This stabilizing effect increases as more nitriles are added, thereby making the hydrogen more acidic and lowering the pKa value.
Applying Bordwell’s Table: Consideration of Solvent Effects
It is crucial to consider solvent effects when using Bordwell pKa values. The table provides data for different solvents, primarily water and DMSO (dimethyl sulfoxide).
Water solvates anions very effectively, which influences acidity measurements differently from aprotic solvents like DMSO or DMF. When reactions occur in aprotic solvents, Bordwell’s DMSO pKa values offer more relevant guidance. This is because in such solvents, electron density distribution within the molecule has a stronger impact on acidity compared to environments where solvation controls acidity.
How This Information Helps in Organic Chemistry
- Understanding acid strength aids in selecting appropriate bases or acids in synthesis.
- Recognizing solvent influence allows accurate interpretation of experimental outcomes.
- Comparing pKa values deepens insight into the role of substituents on molecular stability.
Key Takeaways
- The Bordwell pKa table shows how electron-withdrawing nitriles drastically lower acidity, explaining the large pKa gap between acetonitrile (31) and malononitrile (11).
- Adding nitriles systematically reduces pKa by stabilizing the conjugate base.
- Always use pKa values appropriate to the solvent of your reaction; DMSO data suits aprotic solvents better than water data.
- Solvent solvation effects can reorder acidities; consider this when analyzing or predicting reactions.
How often should I consult the Bordwell pKa Table in my work?
Use the table whenever you need to estimate acidity or basicity in organic reactions. It helps predict proton transfer and reaction outcomes. Checking pKa values early can guide your choice of reagents and conditions.
Why is the pKa of acetonitrile (31) so much higher than malononitrile (11) in the Bordwell Table?
Adding nitrile groups lowers pKa significantly. Methane’s pKa is about 50. One nitrile in acetonitrile drops it to around 31. A second nitrile in malononitrile pulls it down further to near 11 due to strong electron-withdrawing effects.
Does the Bordwell pKa Table take solvent into account?
Yes, solvent matters greatly. The table provides pKa values in different solvents, commonly DMSO and water. Use the DMSO values if your reaction is in an aprotic solvent because they better reflect acidity under those conditions.
How does solvent affect the acidity values in the Bordwell pKa Table?
Water stabilizes anions through solvation more than DMSO does. This affects acidity ranking because electron density distribution influences acidity differently. For aprotic solvents, DMSO pKa values show more relevant trends.
Can the Bordwell pKa Table help explain surprising or confusing acidities?
Yes, it reveals how structural changes, like adding electron-withdrawing groups, impact acidity dramatically. It helps rationalize surprising values, such as why malononitrile is far more acidic than acetonitrile despite similar structures.
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