Esters

Copyright © Karl Dahlke, 2023

As you recall, three fatty acids join with a glycerol molecule to build animal fat (if the chains are long and saturated), or vegetable oil (if the chains are short and unsaturated). Each connection is made through an ester bond. In general, an ester bond connects an acid and an alcohol, liberating a water molecule in the process. Yes, glycerol is an alcohol, thus the letters "ol" at the end of glycerol. It is different from most alcohols however, because it has three OH groups, not just one. Each of these OH groups joins with an organic acid, via the ester bond, to build a triglyceride. Thus three chains hang off the glycerol backbone. Before we dive into the ester bond, let's look at acids in general.

Every acid contains hydrogen, and when that acid is dissolved in water, the hydrogen ion, which is just a single proton, disassociates from the molecule and is free to roam about. The simplest example is HCl, hydrochloric acid. Remember that NaCl is sodium chloride, or salt. When salt is dissolved in water, the sodium and chloride ions separate, and roam free, giving salt water. The oceans are full of it. No big deal. But when HCl dissolves in water, the hydrogen ions roam free, and the result is a strong acid. If you dip your finger in concentrated HCl, it will eat your skin away. Not a good idea!

Fortunately, organic acids are weaker. Vinegar, for example, is an organic acid, and it is safe to eat in moderation. My wife is a big fan of salt and vinegar chips. Citric acid contributes to the delicious taste of oranges and lemons, and malic acid lends its sour flavor to grapes and green apples. In fact, malic comes from the Latin word for apple. So organic acids are an important part of the foods we eat, and essential for biochemistry.

Every organic acid ends in COOH. This is shorthand for carbon, double bond oxygen, OH. Since carbon has 4 bonds, and 2 are consumed by O, and 1 by OH, there is one bond left to connect COOH to the rest of the molecule. The electrons of COOH are such, that H⁺ is released when dissolved in water, thus an organic acid. Below is the layout for acetic acid, also known as vinegar. I put COOH at the left, rather than the right, because I will soon connect this to an alcohol, (alcohol + acid), using an ester bond. Acetic acid has 2 carbons in its chain. Butyric acid has 4 carbons in its chain. Acetic acid is shown on the left, and butyric acid is on the right.

In the following depiction, X and Y are generic pieces of an alcohol and an acid, respectively. The OH from the alcohol joins with H from the acid to make H₂O, also known as water, whereupon the remaining portions, X and Y, join together. This is the ester bond, similar to base + acid = salt + water.

In certain situations this process can be reversed, breaking the ester bond and recreating the original alcohol and acid. This requires water - one water molecule for each ester bond broken. When you digest fats, the ester bond is broken, freeing the chains from the glycerol backbone. This is just the first step; then the chains are broken down further, and eventually burned to produce water and carbon dioxide. Yes, water is an end product of fat metabolism, but water is required to break down the molecule at the outset. The same is true of starch; water is needed to break the bonds and separate starch into its sugar molecules. Water is also required to disassemble proteins. That is why we drink water with our meals.

The ester bond does more than build fat molecules. In fact, certain fragrant compounds were called esters in the 19^th century, then chemists realized that these compounds were build from an alcohol and an acid using a special bond, which they called the ester bond. So - what is an ester? I will present three examples, though there are many more.

Pentane is a chain of 5 carbons, with hydrogens all around. It is so named because pent is Latin for 5. Replace the hydrogen at one end with OH and get pentanol, a straight chain alcohol. Pour some in a beaker, along with some acetic acid, i.e. vinegar. To speed the reaction along, add a third chemical that actively pulls out and absorbs water. Conditions are now favorable for the formation of the ester. OH and H make water, which is pulled away, then the alcohol joins with the acid to make pentyl acetate, as shown below. This is a very nice ester - it smells like banana, and is sometimes used as a flavoring agent.

I ran this experiment in high school, under the guidance of a teacher who was so wonderful, so inspiring, that I was forever hooked on science. I learned more chemistry from him than I did in college. On that day the smell of banana filled the air, but we were sternly warned not to taste the concoction. The third reagent, that pulled the water out and catalyzed the reaction, was rather toxic.

We made another ester that day, ethyl butyrate, having a fruity smell similar to pineapple. This time the alcohol is ethanol, with 2 carbons, and the acid is butyric acid, with 4 carbons. The result looks almost like the ester above, yet the smell is completely different. It's amazing how those little carbons on either side of the ester bond can change the smell completely.

There is a catch however. If you are doing this experiment at home, and there is a little bit of butyric acid left over, that rancid smell, a principal component of vomit, will overpower the pineapple. As a fun exercise in high school chemistry, compute the molar masses of the alcohol and the acid, and mix them in proper proportions, so they consume each other completely, leaving only pineapple behind. (This is called stoichiometry.) If unsure, throw in some extra ethanol, which has just a slight scent, and will not overpower the pineapple.

The last ester was methyl salicylate, which is present in wintergreen lifesavers. It too has a nice smell, but can be toxic in high doses.

Plants produce many different esters, to attract animals to fruit, or insects to nectar. We try to replicate these scents in candy, and cooking, and air fresheners, and perfumes, but the result, while pleasant, is slightly off target. Nothing matches the smell of fresh flowers.