Structural Formulas

sugar,

In organic chemistry it is a common thing for several compounds to contain the same elements in the same proportions. Thus we have three different sugars with the formula C12H22O11 cane sugar, milk and malt sugar. Such compounds are said to be isomers, or isomeric compounds. In many instances we are able to distinguish isomers from one another by representing their atoms as differently joined in the molecule. Thus the formula, C2H6O, represents two compounds, viz. ethyl alcohol and dimethyl ether. These are distinguished as follows:

Ethyl alcohol is the hydroxide of the ethyl radicle, while dimethyl ether is the oxide of the methyl radicle. In the former the two carbon atoms are represented as joined directly together and the oxygen atom as connecting one of the hydrogen atoms to a carbon atom. In the latter all six hydrogen atoms are joined directly to carbon, and the oxygen atom unites the two carbon atoms.

Formulas of this kind are called structural or graphic formulas.

The structural formulas of all alcohols have the hydroxyl group, OH, joined to a carbon atom to which no other oxygen atom is attached.

It will be noted that in all these formulas the carbon atom is represented as having four valence bonds, the oxygen atom two, and the hydrogen atom one; thus:

CHAPTER XXV

ESTERS. FATS

In their behavior towards acids, alcohols resemble, but differ from, bases. It will be remembered that a base reacts with an acid to give water and a product called a salt. Similarly, an alcohol reacts with an acid to give water and a product analogous to a salt. For instance

+ Acid =

Ethyl alcohol + Acetic acid

=

+ Water

Ester
Ethyl acetate + Water

C2H5OH + CH3COOH = CH3COOC2H5 + H2O

Thus the esters bear the same relation to acids and alcohols that salts bear to acids and bases. The ester is obtained from the acid by the replacement of the hydrogen of the acid by the radicle of the alcohol. In their physical properties, however, esters are no more like salts than alcohols are like bases. Ethyl acetate and other esters of the simpler alcohols with the simpler organic acids are neutral, volatile liquors, with pleasant fruity odors. They are nearly insoluble in water and do not ionize when dissolved. The flavors and odors of fruits and wines are in part due to the esters they contain. Some esters are manufactured and sold as flavoring matters or perfumes. Amyl acetate, for example, is sold as pear oil, methyl butyrate as pineapple oil, etc. Esters of more complex alcohols and acids are important constituents of the waxes - beeswax, Carnauba wax, etc.

Experiment 78.

Examine specimens of methyl acetate, ethyl acetate, propyl acetate, amyl acetate (pear oil), methyl butyrate (pineapple oil), ethyl citrate, methyl salicylate (oil of wintergreen).

The reactions between alcohols and acids are much slower than those between bases and acids. When ethyl alcohol and acetic acid are mixed and kept at ordinary room temperature, the formation of ethyl acetate goes on slowly for several months. If the mixture is kept hot, the reaction goes on more rapidly. There are also certain substances, such as sulphuric acid, which will accelerate the action. A common method of preparing esters is to mix the alcohol with sulphuric acid, add the acid whose ester is wanted, and distill out the ester.

Experiment 79.

Mix equal volumes of alcohol and concentrated sulphuric acid. Add acetic acid and boil. Note the odor and compare with those of alcohol, acetic acid, and ethyl acetate. Has ethyl acetate been formed? Write equations for the reaction (omitting the sulphuric acid from the equation).

Fats

The esters of the alcohol, glycerol, with certain organic acids, constitute the fats. Natural fats and animal and vegetable oils - lard, tallow, butter, lard oil, olive oil, cottonseed oil, linseed oil, castor oil, etc. are mixtures of the glycerol esters of a number of different acids.

The most common of these acids are palmitic, HC16H3102, a soft solid; stearic, HC18H35O2, a soft solid; and oleic, HC18H3302, a liquid.

The most common simple fats are therefore:

Glyceryl palmitate, C3H5(C16H31O2)3 known as palmitin or tripalmitin. This, like palmitic acid, is a solid.

Glyceryl stearate, C3H5(C18H35O2)3, stearin or tristearin also a solid.

And glyceryl oleate, C3H5 (C18H33O2)3, olein or triolein a liquid.

Experiment 80.

Examine specimens of palmitic, stearic, and oleic acids, and of the three simple fats, tripalmitin, triolein, and tristearin.

Palmitic and stearic acids belong to the same series of acids as acetic acid. In structure they differ from acetic acid in having a long chain of carbon atoms (each carrying two hydrogen atoms) between the carbon atom of the CH,— radicle and that of the -COOH radicle.1 Palmitic acid has 14, stearic acid 16, of these intermediate carbon atoms in its chain.

Tripalmitin may, therefore, be represented by the following structural formula:

1 The -COOH radicle, called carboxyl, is the characteristic radicle of organic

acids.