46 ORGANIC AGRICULTURAL CHEMISTRY 



hyde and rapid evolution as a gas. This is the best method of using 

 formaldehyde as a disinfectant. 



(2) Acetaldehyde. Place 5.0 c.c. of ethyl alcohol in a test tube. 

 Add a few crystals (3.0 g.) potassium dichromate (K2Cr207). Mix 

 15.0 c.c. water + 5.0 c.c. sulphuric acid (concentrated). Add the 

 mixed acid slowly to the alcohol and dichromate. Warm if necessary. 

 Note the action and the sweet odor. The odor is due to acetalde- 

 hyde. The green color is due to chromium sulphate. Explain. 



ACIDS 



When the oxidation of primary alcohols proceeds to its limit, 

 the second stage of the reaction occurs, oxygen is added to the 

 aldehyde, and an acid results. From formaldehyde^ formic acid 

 is obtained, and from acetaldehyde, acetic acid. The composi- 

 tion and constitution of these two acids is shown by the follow- 

 ing reactions : 



/H /H /OH 



H-C^H + 0-^H-C = + (H20)-l-0-^H-C = 

 \0H 



Methyl alcohol Formaldehyde Formic acid 



/H /H /OH 



CH3— Cf-H +0-^CH3-C = 0+(H20)+0-> CHs— C = 

 \0H 



Ethyl alcohol Acetaldehyde Acetic acid 



* 



The formulas just given represent the constitution of the alde- 

 hydes and acids as proven by experimental evidence. The 

 acids, like the alcohols, contain a hydroxyl group (OH), while 

 the aldehydes do not, the hydrogen and oxygen being each 

 united directly to carbon. The common way of writing the 

 formulas is : 



Formaldehyde, H - CHO Formic acid, H - COOH 



Acetaldehyde, CH3-CHO Acetic acid, CH3-COOH 



Any aldehyde, R — CHO Any organic acid, R — COOH 



It is this COOH group, known as the carhoxyl group, which is 

 characteristic of acids, and it is the hydroxyl group within this 



