Phosphate Esters
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Phosphate Esters

Esters can also be derived from acids other than carboxylic acids. The structural formulas of some phosphate esters are shown here (and in Example 18 in your workbook). They are derived from phosphoric acid. Compounds of this type are the organic phosphates that you may have heard about in recent years. Two versions of each formula are shown. Both are valid, depending on what you are trying to emphasize. Let me explain why, starting with phosphoric acid.

O
|
HO-P-OH
|
  OH 		O
||
HO-P-OH
|
 OH
phosphoric acid
O 
|  
HO-P-OCH3
|  
OH 		O 
||  
HO-P-OCH3
|  
OH
methyl phosphate
O  
|  
HO-P-OC2H5
|  
  OC2H5 		O  
||  
HO-P-OC2H5
|  
  OC2H5
diethyl phosphate

The first structural formula is the structural formula of phosphoric acid. In the past we simply used the molecular formula H3PO4. The structural formula shows how the oxygen atoms and the hydrogen atoms are arranged.

If you look at the formula on the left with a critical eye, you may notice that the oxygen on the top has only one bond to the phosphorus below it. That doesn't seem right for oxygen. That might create a little confusion for you, which I hope to explain. Sometimes you will see the structural formula drawn using a double bond, as shown in the right side of this example. The reason for doing this is that it gives oxygen the two bonds that it is "supposed" to have. Unfortunately, that puts ten valence electrons around phosphorus. However, it is not really a double bond. Instead, it is what is called coordinate covalent bond.
O
|
HO-P-OH
|
  OH 		O
||
HO-P-OH
|
 OH
phosphoric acid
This diagram (which is also shown in Example 19) will help clarify that. We say that oxygen can form two bonds because it needs to gain two electrons. In this particular case, the top oxygen atom gains both of those electrons from the phosphorus atom (red dots here, xs in workbook). The oxygen atom is not contributing any electrons to that bond. So it is simply two electrons from the phosphorus being shared with the oxygen. Since both electrons come from the phosphorus, it is called a coordinate covalent bond. Since there are only two electrons in the bond (one pair of electrons), it is a single bond.

Lewis diagram for phosphoric acid. [64str12.JPG]

 

The formation of a phosphate ester from phosphoric acid also involves the splitting out of water in an intermolecular dehydration reaction. This results in the compounds such as those shown below (and in Example 18). In these examples, alkyl groups are attached to one or two of the oxygens which are bonded to phosphorus. We will be dealing with such examples later on when we deal with nucleic acids such as RNA and DNA because the phosphate esters are a very important part of those compounds.

O  
|  
HO-P-OCH3
|  
OH 		O  
||  
HO-P-OCH3
|  
OH
methyl phosphate
O   
|  
HO-P-OC2H5
|  
  OC2H5 		O  
||  
HO-P-OC2H5
|  
  OC2H5
diethyl phosphate

Two examples of phosphate esters are shown above. These are named for you, and I am sure you can figure out how we came up with those names just by looking at the names and the structural formulas. Although you should be able to describe and recognize the structural formulas of phosphate esters, you will not be expected to name them.

 

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