Acid Derivatives: Preparation & Uses

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

This lesson will focus on the synthesis and uses of various compounds derived from carboxylic acids. Primary points of discussion will include how a carboxylic acid must be first modified before it can derivatized and the various functional groups that can then be accessed from a common intermediate.

Different Strokes, Same Molecule

Have you ever met someone who just amazes you with how well they do several different things? For instance, maybe they participate in athletics, academics, and music. The astounding thing is that they perform in these fields at an exceptional level, while it takes you maximum concentration and effort just to be able to do one of them moderately well. Those types of people truly are extraordinary some would say.

Did you know that some organic molecules have that capability? That's right, some molecules have the ability to do several things well simultaneously! The class of compound we are going to be learning about are called carboxylic acid derivatives and as the name implies, they come from (or are derived) from carboxylic acids themselves. We will be discussing what we must always do to modify a carboxylic acid first and then we will be exploring how versatile and exceptional the resulting intermediate can be.

First things First

Carboxylic acid derivatives, as their name implies, are derivatives or 'cousins' of carboxylic acids. Carboxylic acids have an organic side chain (designated by 'R') attached to a COOH group. Like we mentioned already, there are almost a plethora of derivatives one can make from a single carboxylic acid. What's important to realize though is that they don't come from the carboxylic acid directly. There's something we have to transform the acid into before it can be derivatized. That something is what's called an acid chloride, also known as acyl chloride.

General form of a carboxylic acid

Why do we need to do this? Well, because of the extremely poor leaving ability of an -OH (formula for a hydroxyl group) group a carboxylic acid is actually pretty un-reactive. A leaving group is something that departs or 'leaves' from our organic substrate. An -OH group isn't a good leaving group because it isn't stable on its own, a quality that a good leaving group must have. How we get around that problem is we exchange the -OH group for a chlorine atom, which is a VERY good leaving group. This can be done by reacting our carboxylic acid with a reagent called thionyl chloride (SOCl2).

Transformation of a carboxylic acid to an acid chloride

Carboxylic Acid Derivatives

Let's take a detailed look at some derivatives that can be made from an acid chloride, using acetyl chloride as our model substrate. (Note: all of the reactions we will mention are general and are not limited to using acetyl chloride as the substrate.)

Synthesis of Amides

Amides are commonly made from acid chlorides. For example, acetamide (a common amide) can be synthesized by reacting acetyl chloride with ammonia (NH3). Acetamide finds application as an industrial solvent and also a plasticizing agent. Acetamide (NH2) has a very high boiling point which is why it is particularly useful as a solvent.

Synthesis of acetamide from acetyl chloride and ammonia

Synthesis of Esters

In a similar fashion as above, esters can be made from acid chlorides, the only change we need to make is substitute an alcohol for the amine. Ethyl acetate, part of the carboxylic acid ester family, can be made from acetyl chloride by reacting it with ethanol (an alcohol). Ethyl acetate is a very useful organic solvent and is also used in things like nail polish remover and some glues. It can also be used to destroy insects, due to its poisonous vapors.

Synthesis of ethyl acetate from acetyl chloride and ethanol

Acetic Anhydrides

Although acetic anhydride is not commonly prepared from acetyl chloride, it is a carboxylic acid derivative nonetheless. Acetic anhydride has two acetyl groups bonded to a central oxygen atom. In general, it finds use primary as an acylating agent (source of an acetyl group) for alcohols and amines. This liquid is also used to make pharmaceuticals such as aspirin and salicylic acid, as well as a preservative for wood.

Structure of acetic anhydride

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