Esters

Figure 71.

Figure 71 also shows a cryptand molecule, which is made so that it nearly completely hides the metal ion molecule. Instead of a crown ether, which contains just carbon, hydrogen, and oxygen atoms, the cryptand consists of a nitrogen atom as well, which adds to the electronegativity of the center of the molecule along with the oxygen molecule. Both cryptands and crown ethers will help solvate a metal ion because they allow the metal ion to have a place to dissolve.

ESTERS

Esters are well-known in nature in that they are responsible for the smells of different organic molecules. The structure of the ester molecule is RCOOR’. These are formed through the mixture of an acid and an alcohol with water as an elimination product. In an earlier chapter, we discussed the oxidation and dehydration of alcohols; this reaction refers to the esterification of alcohols.

The naming of esters involves the alkyl chain being the one that contains the carboxylic acid component with the opposite alkane attached as a side chain. The main chain has the “-e” removed and “-oate” added as an ending. Methyl ethanoate is made by the carboxylic acid CH3COOH plus the methyl group attached instead of the hydrogen atom. Instead of ethanoic acid (a carboxylic acid), the methyl group is attached. When an ester group is attached to a ring, the ester is named as a side chain on the ring.

The most common ester talked about in organic chemistry circles is ethyl ethanoate. This is not a symmetrical molecule in that one side is ethanoic acid and the other side is the ethyl or “ethane” side chain. As others are named besides this, they are named so that the side chain is first in the molecule and the parent chain (with the carboxylic acid) is always second.

Esters are important because animal and vegetable oils and fats are made from complex, long-chain esters. Some are in liquid form and others are solid at room temperature. Consider the complex number of molecules that can be made from glycerol or propane1,2,3-triol. This can be “esterified” to make three ethanoate groupings. When the carboxylic acid is a very long chain and when all three hydroxyl groups on the glycerol molecule are esterified, this is called a triglyceride. This is looked at in the following example structure seen in figure 72:

Figure 72.

The triglyceride in figure 72 combines three molecules of octadecanoic acid with glycerol. The chemical name is propane-1, 2, 3-triyl trioctadecanoate or glyceryl tristearate. This is considered a saturated triglyceride or saturated fatty acid. Any time that a carbon-carbon bond is unsaturated one time, it is called a monounsaturated fatty acid and if there is more than one carbon double bond, it is called polyunsaturated.

You should know that omega-3 and omega-6 fatty acids can be esterified to make a triglyceride; however, they are not named chemically correctly. The omega fatty acids are unsaturated with the name omega-6 referring to a carbon-carbon double bond at the

sixth carbon from the methyl end and not from the carboxylic acid end. The same is true of omega-3 fatty acids.

Esters are like aldehydes and ketones in that they are polar molecules that have both dipole-dipole interactions and van der Waals dispersion forces. There are no ester-ester hydrogen bonds unlike carboxylic acids so they have lower boiling points than acids of the same number of carbon atoms.

The smaller esters will be soluble in water; however, the larger ones are not as soluble in water. They will not be able to hydrogen bond with each other but will hydrogen bond with water, making them somewhat soluble in water. As the length increases, however, the molecule becomes more hydrocarbon-like and will be less soluble in water. Fats and oils are so long-chained that they aren’t soluble in water.

When it comes to fats and oils, there are factors that determine their solidity or liquidity at room temperature. Fats tend to be called “fats” because they are saturated chains of fatty acids. This saturation allows for greater van der Waals dispersion forces so that there is more energy required to break their solid structure. The greater the degree of unsaturation, the lower the melting point. These unsaturated molecules have more disordered packing, resulting in a lesser number of forces between different molecules.

Cis-fatty acids are highly disordered because of the cis bonding. These are liquid at room temperature. Trans-fatty acids are generally solid at room temperature. This is because the molecules are more tightly packed and behave more like saturated fats. Figure 73 shows a cis and trans fatty acid triglyceride.

Figure 73.