ALDEHYDES. HALOID DERIVATIVES. 315 



42. ALDEHYDES. HALOID DEKIVATIVES. 



Aldehydes. The name aldehyde is derived from alcohol dehydro- 

 genatum, referring to its method of formation, viz., by the removal 

 of hydrogen from alcohols, as, for instance : 



C 2 H 6 O 2H : C 2 H 4 0. 

 Ethyl alcohol. Acetic aldehyde. 



This removal of hydrogen may be accomplished by various methods, 

 as, for instance, by oxidation of alcohols, when one atom of oxygen 

 combines with two atoms of hydrogen, forming water, whilst an alde- 

 hyde is formed at the same time. Aldehydes, when further oxidized, 

 are converted into acids ; aldehydes are, consequently, the interme- 

 diate products between alcohols and acids, and are frequently looked 

 upon as the hydrides of the acid radicals. The constitution of acetic 

 acid may be represented by the formula CH 3 .CO.OH ; the radical of 

 acetic acid or acetyl is the group CH 3 .CO, and the hydride of acetyl 

 is acetic aldehyde, CH 3 .COH. It is the group COH which is char- 

 acteristic of, and found in, all aldehydes. Only a few aldehydes are 

 of practical interest, as, for instance, acetic aldehyde, paraldchyde, and 

 benzoic aldehyde, which latter substance will be more fully considered 

 in connection with the aromatic substances. 



Acetic aldehyde, C 2 H 4 O or CH 3 COH. Alcohol may be converted 

 into aldehyde by the action of various oxidizing agents ; the one 

 generally used is potassium dichromate, which oxidizes two hydrogen 

 atoms of the alcohol molecule, converting it into aldehyde : 

 C 2 H 6 O + O = : C 2 H 4 O + H 2 0. 



Experiment 44. Place in a 500 c.c. flask, provided with a funnel-tube and 

 connected with a Liebig's condenser, 6 grammes of potassium dichromate. 

 Pour upon this salt through the funnel-tube, very slowly, a previously pre- 

 pared and cooled mixture of 5 c.c. of sulphuric acid, 24 c c. of water and 6 c.c. 

 of alcohol. Chemical action begins generally without application of heat, and 

 often becomes so violent that the liquid boils up, for which reason a large flask 

 is used. The escaping vapors, which are a mixture of aldehyde, alcohol, and 

 water, are collected in a receiver kept cold by ice. From this mixture pure 



406. By what process is methyl alcohol obtained, under what other names is it 

 known, and what are its properties ? 407. Describe the manufacture of pure 

 alcohol from sugar. 408. Give the alcoholic strength of the alcohol and diluted 

 alcohol of the U. S. P., and also of spirit of wine, proof-spirit, light wines, heavy 

 wines, beers, and spirits. 409. What are the general properties of common 

 alcohol? 410. What is glycerin, how is it found in nature, how is it obtained, 

 and what are its properties? 



