Common Sense Organic Chemistry

Understanding Acidity in Organic Chemistry

Acids and Bases in Organic Chemistry

Understanding Acidity in Organic Chemistry

Is Understanding Acidity Really Such a Big Deal?

To the student, acidity is rarely given much thought. There are so many other things to worry about. Besides, from earlier courses, you’ve had plenty of exposure to acids and bases from general chemistry. Having calculated and measured pH, memorized examples of strong vs. weak acids, and even performed titrations or neutralizations enough times, there has been plenty of that to assume there isn’t much more to it. Acidity is important in a very different way however in the subject of organic chemistry. How it’s applied in Organic Chemistry is a different animal with several parameters and an understanding of it well is among the most fundamental and useful ideas in the subject. It shows up repeatedly in different forms yet knowing the reasoning to apply it is sometimes never understood. It is little wonder why the subject perplexes so many. Organic chemistry with few exceptions is acid-base chemistry. A solid understanding of it is truly a powerful tool to have and without an unnecessary even crippling burden. While it’s mentioned in lecture with sufficient frequency to understand it is relevant, little context is presented to sufficiently explain how and why so many solutions depend on it. Students rarely understand how it’s relevant and so it adds to the misconception about having to memorize everything. This is not an exaggeration nor it is difficult to see why.

Having a working understanding of acidity makes things much easier. So much easier, in fact, that a great deal of confidence and all the benefits that accompany those feelings depend on it.  It’s an oddity of the human mind that even a little uncertainty about something, even really minor, often does disproportionate damage to one’s feelings of self-assurance. After all, when you sense you lack awareness of how much of something there is that you don’t know, that thing could be a lot, it can be assumed to be extremely serious, and commonly accepted as something that everyone understands, except for you. I know I felt that way.

What students receive at the start of organic chemistry are the two major types of definitions of acids and bases and then an index known as pKa. None of this should be new. A pKa value is a number that can be calculated as easily as calculating a pH value. Unlike, pH values, pKa values are often tabled for many compounds of interest. Unlike in general chemistry, you’re not, for example, given some concentration of acid (or base), make the necessary manipulations, and then take a negative log. Every organic acid has an experimentally determined pKa value that doesn’t change and differs only on the specific solvent used. In introductory organic chemistry the solvent is almost always water and so the values can go from 50-60 in some tables to minus eleven (-11) and just about every number in between sometimes to a couple decimal places, has a compound at that value.

Many classes are provided with what is no small list of structures with their respective values. Most of these structures have functional groups that haven’t been seen or touched upon yet and many that won’t be. Some professors give out several pages of table along with a proviso that you don’t have to memorize them. It truly would not be a productive use of your time. As always, remind yourself that rote memorization is not only unnecessary in this course, if you want to understand and remember anything when it’s over you should not and can’t. If you’re doing it right, there is no more memorization in biology than in organic chemistry. So, not only is it unnecessary to know all the pKa values, but such a thing is also hardly possible even if you tried. That’s the point of having tables. So this collection of numbers you’re told you don’t need to remember is put aside kind of like tossing out the baby with the bath water.

Instead students get by with a different piece of information and that is: in understanding acidity, the lower its pKa, the stronger the acid it is and vice versa. Students leave well enough alone with just that. When needed on an exam, unless instructed otherwise, relevant pKa values will usually be provided. As the semester accelerates, you will also pick up a few important ones with little or no effort. But that little bit of knowledge is not enough, and only later when more is revealed does it become clear why that is.

Like many things at the beginning, acidity (and therefore basicity) doesn’t ever get the treatment it deserves but is something you need to know and will you be repeatedly asked to apply. Since there’s a growing list of higher priority information and skills to learn students leave it at that. Later, however certain questions seem totally foreign. The earlier you understand it in the proper context, the easier and more fully developed your understanding will be. So, pKa values, the numbers themselves are useful, but are just a scratch on the surface. Instead the other factors that determine pKa is what understanding acidity is about. Since it’s a cumulative subject, understanding acidity occupies a large and increasing role in how well and quickly you learn the rest.

Then What is Important to Know?

You should have some sense of the extremes and certain things that fit in between on the scale. There are trends in the periodic table that have a lot to do with it; but that’s a starting point. Don’t worry, a good understanding of acidity isn’t terribly difficult to obtain. But it consists of several parts that you may not get until later nor is the means to have that understanding provided in the context of acids. When the other relevant information is presented, you won’t be told what ties these properties together. It’s assumed you learned that at the beginning. The other seemingly unrelated ideas and concepts that are hard enough on their own, and that influence acidity are provided in different contexts. There are about 4 other concepts, some simple, some more complex, required for you have to fully grasp acidity. Because putting them together isn’t done for you, not in any textbook I’ve seen, you have to do it yourself.

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