The PKa Table Is Your Friend - Master Organic Chemistry

The importance of pKa values in organic chemistry can’t be overestimated, in my opinion.  Not knowing pKa  values in organic chemistry is like not knowing the value of the hands in poker. In this scheme, alkyl anions are the equivalent of the royal flush – they win the proton from everything underneath them in the table.

Why is pKa so important? Because every nucleophile is potentially a base, and vice versa. If you have a reaction where it looks like you might get SN2 or E2, look closely first – is there any chance of a simple acid-base reaction? For instance, take NaOH plus an alkyl thiol, R–SH. Is it an SN2? Or possibly an E2? Both are incorrect. The reaction that happens is the simplest one – deprotonation of SH, to provide water and the deprotonated thiol.

Also, the pKatable tells you about leaving group ability. Good leaving groups are weak bases!

If you don’t know the relative values of the pKa‘s of the major functional groups, you’ll be flying blind in the course. Expect to hit a tree.

PDF VERSION NOW AVAILABLE (click here)

For more complete lists, be sure to check out Evans and Reich. (check out the resources on Reich’s page by the way – fantastic!) Blessed are the OCD, for they produce the most beautiful and complete web resources.

Notes

Related Articles

• What makes a good leaving group?
• Acid-Base Reactions: Introducing Ka and pKa
• How to Use a pKa Table
• Walkthrough of Acid-Base Reactions (3) – Acidity Trends
• Five Key Factors That Influence Acidity
• Acid Base Practice Problems (MOC Membership)

Quiz Yourself!

Become a  MOC member to see the clickable quiz with answers on the back.

Become a  MOC member to see the clickable quiz with answers on the back.

Become a  MOC member to see the clickable quiz with answers on the back.

Become a  MOC member to see the clickable quiz with answers on the back.

Become a  MOC member to see the clickable quiz with answers on the back.

Become a  MOC member to see the clickable quiz with answers on the back.

Become a  MOC member to see the clickable quiz with answers on the back.

Become a  MOC member to see the clickable quiz with answers on the back.

Appendix: Tables of pKa Values By Functional Group

Some pKa tables broken down by functional group (Source: ACS Division of Organic Chemistry and references within)

Rather than memorize these values, just try to spot the key trends:

• What is the effect of additional electron-withdrawing groups?
• How does addition of different halogen atoms compare?
• What is the effect of being able to delocalize the conjugate base through resonance
• What is the effect of moving electron-withdrawing groups further away?

Alcohols

Carboxylic Acids

Protonated Nitrogen