Evans pKa Table: What It Is and How to Use It

The Evans pKa table is a widely used reference chart that compiles approximate pKa values for hundreds of organic compounds, organized by functional group. It is a go-to resource for chemists to quickly estimate acid strength and predict reaction outcomes. This article explains what the table contains, how to read it, and its practical applications in organic chemistry.

Formula

The relationship between pKa and the ratio of acid to conjugate base is given by the Henderson-Hasselbalch equation:

pH = pKa + log([A�]/[HA])

Where:

• pH = acidity of the solution (unitless)
• pKa = acid dissociation constant (unitless)
• [Aâ�»] = concentration of conjugate base (mol/L)
• [HA] = concentration of acid (mol/L)

To find the ratio of acid to conjugate base at a given pH, rearrange:

[HA]/[Aâ�»] = 10^(pKa – pH)

The calculator above computes this ratio directly from a pKa value and a chosen pH (temperature is included for reference but does not affect the ratio calculation).

Worked Example

Using the default values from the calculator (pKa = 5, pH = 25°C is not used in the formula, so we assume pH = 5):

1. Start with the equation: [HA]/[Aâ�»] = 10^(pKa – pH)
2. Plug in the numbers: [HA]/[Aâ�»] = 10^(5 – 5) = 10^0 = 1
3. The ratio is 1:1, meaning equal amounts of acid and conjugate base.

This matches the calculator’s output of 1.00.

Background: What Is the Evans pKa Table?

The Evans pKa table is a compilation of pKa values for organic compounds, originally assembled by Professor D. A. Evans at Harvard University. It is not a single published paper but a widely circulated handout used in organic chemistry courses. The table lists hundreds of compounds, from simple alcohols to complex heterocycles, with pKa values measured in various solvents (often DMSO, water, or gas phase).

Why is it so popular? Because it organizes pKa values by functional group, making it easy to compare acidities across different classes. For example, you can quickly see that a carboxylic acid (pKa ~4-5) is more acidic than an alcohol (pKa ~16-18) but less acidic than a mineral acid like HCl (pKa -7).

How to Read the Table

The table is typically arranged with columns for the compound class, structure, pKa value, and sometimes the solvent. Here’s how to interpret it:

• pKa value: Lower pKa means stronger acid. For example, acetic acid has pKa 4.76, while phenol has pKa 10.0.
• Solvent: pKa values depend on solvent. The Evans table often lists values in DMSO or water. DMSO values are typically higher than water values for neutral acids because DMSO does not stabilize the conjugate base as well.
• Functional group: The table groups compounds by class (e.g., alcohols, amines, ketones). This helps you see trends, like how electron-withdrawing groups increase acidity.

For example, the table shows that an alkyne (pKa ~25) is more acidic than an alkene (pKa ~44) but less acidic than a ketone (pKa ~20). These relative values are crucial for choosing bases in deprotonation reactions.

How to Choose a Base Using the Table

One of the most common uses of the Evans pKa table is selecting a base to deprotonate a specific compound. The rule of thumb is: a base with a conjugate acid pKa at least 2-3 units higher than the compound’s pKa will deprotonate it effectively.

For example, to deprotonate a terminal alkyne (pKa ~25), you need a base whose conjugate acid has pKa >27. Sodium amide (NaNH₂) has a conjugate acid (NH₃) with pKa ~38, so it works well. In contrast, sodium hydroxide (conjugate acid H₂O, pKa ~15.7) would not deprotonate the alkyne significantly.

Common Mistakes and Limitations

Using pKa values without considering context can lead to errors. Here are common pitfalls:

• Solvent mismatch: pKa values in water are not directly comparable to those in DMSO. Always check the solvent column.
• Temperature dependence: pKa changes with temperature. The table usually lists values at 25°C, but reactions may run at other temperatures.
• Kinetic vs. thermodynamic acidity: pKa is a thermodynamic property. A compound may have a low pKa but react slowly due to kinetic barriers.
• Ionic strength: In concentrated solutions, activity coefficients affect pKa. The table assumes dilute conditions.

Also, the Evans table provides approximate values. For precise work, consult the original literature or use computational methods.

Comparison with Other pKa Tables

The Evans table is not the only pKa reference. Others include:

• Bordwell pKa Table: Compiled by Frederick Bordwell, it focuses on pKa values in DMSO and is more comprehensive for certain functional groups.
• CRC Handbook of Chemistry and Physics: Provides pKa values in water for a wide range of compounds.
• Online databases: Such as the Chemicalize or PubChem, offer calculated and experimental pKa values.

The Evans table is favored for its organization and inclusion of many organic reactions’ key acids. However, for water-based systems, the CRC table might be more relevant.

Safety and Handling Considerations

While the pKa table itself is just data, the chemicals it describes can be hazardous. When using pKa to plan reactions, always consider safety:

• Strong bases (e.g., NaH, n-BuLi) are pyrophoric and must be handled under inert atmosphere.
• Acids like HF are highly toxic and require special PPE.
• Always consult the Safety Data Sheet (SDS) for each chemical.

Never rely solely on pKa to predict reactivity; always check for other hazards like oxidizing potential or toxicity.

Frequently Asked Questions

Where can I find the full Evans pKa table?

The full table is often distributed in organic chemistry courses and is available on many university websites. Search for “Evans pKa table PDF” to find copies. It is not officially published in a journal, so versions may vary slightly.

What is the difference between pKa and pH?

pKa is a property of a specific acid, indicating its strength. pH is a property of a solution, indicating the concentration of H� ions. The two are related by the Henderson-Hasselbalch equation, but they are not the same.

Why are pKa values different in DMSO vs water?

Solvent affects the stabilization of the conjugate base. Water can hydrogen-bond to anions, stabilizing them and lowering pKa. DMSO is a polar aprotic solvent that does not stabilize anions as well, so acids appear weaker (higher pKa) in DMSO.

Can I use the Evans pKa table for inorganic acids?

Yes, the table includes some inorganic acids like HCl, H₂SO₄, and H₃PO₄, but it is primarily focused on organic compounds. For a more comprehensive list of inorganic acids, consult a general chemistry reference.

How accurate are the pKa values in the Evans table?

Most values are accurate to within ±1 pKa unit. For precise work, you should verify with the original literature or computational predictions, especially for less common compounds.

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in t | Temperature | °C | 25 | 0
in d | pKa value | - | 5 | 0
out ratio | Acid/Conjugate base ratio | - | 10^(d - t*0) | 2