Contents
III. Factors Affecting the Acidity of Organic Compounds
I. Introduction
In this post, we shall start by looking at a list of common organic compounds and their respective pKa values, known as the pKa table. We will then have a look at more specific compounds and trends to see what factors affect the acidity of organic compounds.
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II. The pKa Table
Below are the list of common organic compounds with their repsective conjugate bases and pKa values.
IMPORTANT: I’m not sure if your computer shows it properly, but be very careful which ones have the approximate symbol (that squiggly thing called tilde: ∼), and which ones have the negative symbol. The compounds are ordered from the largest pKa (positive large pKa), to the smallest pKa (negative large pKa), so above hydrofluoric acid, there’s no negative pKa. Anyone with an idea to make the squiggly thing clearer, do leave a comment. I’d like to hear from you.
Click here to get a PDF copy of the table.
This list is by no means exhaustive. Furthermore, it only shows the approximate pKa values for series of functional groups. For specific compounds, there are plenty of sources that have information about the pKa values of specific organic compounds, such as your textbooks. For online sources, I often use Evans pKa Table or Bordwell pKa Table.
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III. What Information pKa Table Tells You
The pKa table tells you a lot more information than you realise. They don’t only tell you the pKa values for each set of species with certain functional groups, they also tell you the relative strength of each species. In future posts we will see how these values can be used to predict the course of reactions, how we can determine the acidity and basicity of these species in different solvents, and whether they are strong or weak acids/bases in certain solvents. Basically the pKa table is very important and useful, and you will gain a lot of advantages by understanding how to use it.
You can use the pKa table to gauge the strength of acids with the same functional group
Before I explain the pKa table further, I would like to point out something important. Most of the species listed on the table are not specific. Yes, some are specific (like HCl, HBr, etc), but most aren’t. They are drawn with R groups to indicate that these groups can have many variations of alkyl groups but they will have approximately similar pKa as long as we’re comparing the same functional groups.
For example, carboxylic acids have a pKa of approximately 5. The value will change according to what the R group is, and you can find the specific values for specific carboxylic acids in many resources online and offline. However, if you run into a carboxylic acid with an unknown pKa, 5 is a good estimate to start. You can even make an educated guess whether the pKa of the unknown carboxylic acid would be greater or smaller than 5. We will discuss this in the next post: Acid-Base IV, about factors that affect the acidity of organic compounds.
Strong acids have pKas?
You may recognise some, if not all, of the species listed on that table. I believe you are familiar with HF, HCl, HBr, and HI, so if you haven’t read Acid-Base II yet, you may think, ‘these are strong acids, and so they should have no pKa because they completely dissociates in water’. After all, that may be how some of you were taught when you first learned acid-base theory.
If you did think that way, have another read at Acid-Base II. Yes, these are strong acids and yes they have their own pKa (and therefore Ka) values. These values indicate that the dissociation reaction in water proceeds very favourably to the right side of the equilibrium. For all intents and purposes, these reactions completely form the products; which is why these are considered strong acids. In Acid-Base VI, you will see several examples that require the pKa values of these acids and you will see why we need them.
Alcohols, ammonium salts, and amines can act as acids too, specifically weak acids
You may also recognise some weak acids such as carboxylic acids and phenol, and seeing that they have pKa values should not surprise you.
You may feel weird to see some alcohols, ammonium salts, and amines because you may be at the stage where you’re not used to see the OH in alcohols and NH2 in amines get deprotonated. Don’t worry, we’ll talk about this in a bit.
Alkanes, alkenes, and alkynes have pKas too
Lastly, you may not believe your eyes when you see that alkanes, alkenes, alkynes, and compounds with α-hydrogens have their own conjugate bases.
Have a look at the reactions of butane and benzene below:
Judging by the Ka, we know that these equilibriums will never ever go to the right. For all intents and purposes, these reactions will not take place.
So why do we bother ourselves with reactions that will never take place?
You need to keep in mind that the dissociation of acids is takes place in an equilibrium. If the equilibrium to the right is unfavourable, then the equilibrium to the left (the reverse reaction) is favourable. Because we’re now talking about the reverse reaction, the equilibrium constant is the reciprocal of Ka.
And indeed it is fairly common for these reactions to happen in real life:
The reactions above are deprotonation of acids (represented by H+ only) by butyllithium and phenyllithium. Because the equilibrium constants are the reciprocal of each Kas, the K value of these reactions are very large. This means that the equilibrium to the right is very favourable.
This is why we bother with species with such large pKas because even though their forward reaction practically does not happen, the reverse does and is fairly common. The same reason also applies to other species such as alcohols, amines, and compounds with α-hydrogens.
One last important note…
Before we end the post, I would like to let you know that species such as alkanes, alkenes, alkynes, and compounds with α-hydrogens as acids produce species with carbanion as conjugate bases. Carbanion is an anion of carbon (C–).
Species that produces carbanion as conjugate bases is known as carbon acids.
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IV. Conclusion
We have seen the pKa table and what it can potentially tell you about certain group of species in general.
The acid dissociation reaction of several species with large pKas will not take place, however, the reverse reaction does and it is what we’re concerned about.
In the next post, we will see factors that affect the acidity of organic compounds. These factors can help you make an educated guess when you encounter certain species with unknown pKas. They will also help you see why and how different species with the same functional group can have different pKas.
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Page last updated: 07vi17
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