ALDEHYDES. 



Aldehydes are the first products of oxidation of primary alcohols, 

 e.g. : 



CH 3 CH 3 CH. 



I -* I / OH - I 



CHoOH CH( CHO 



\OH 



Ethyl alcohol. Hypothetical. Acetaldehyde. 



The hypothetical intermediate compound does not exist; it at 

 once loses a molecule of water and is converted into aldehyde. Two 

 OH groups cannot exist attached to one carbon atom ; aldehyde is 

 formed by loss of water. There are a few exceptions, such as chloral 

 hydrate, 



CC1 3 



\ /OH 



CH^ 



\OH. 



Formaldehyde is formed from methyl alcohol, propyl aldehyde 

 from primary propyl alcohol, etc. 



/ H 



The group CHO or C<f is characteristic of aldehydes. 



% 



Preparation. 



When the alcohol is available the aldehyde is usually prepared by 

 oxidation ; otherwise it may be prepared by the dry distillation of 

 molecular proportions of calcium formate and the calcium salt of the 

 corresponding acid (compare ketones). 



FORMALDEHYDE. 



Preparation. 



Formaldehyde is prepared by passing the vapour of methyl alcohol 

 mixed with air over heated platinum or copper, or other substances. 

 The formaldehyde formed by oxidation is passed into water. 



Formol, or formalin, is a commercial aqueous solution containing 

 40 per cent, of formaldehyde. 



ACETALDEHYDE. 



Preparation. 



25 gm. of coarsely powdered potassium bichromate and 100 c.c. of 

 water are placed in a distilling flask of 250 c.c. capacity. The flask is 

 connected with a condenser and a strong current of cold water is made 

 to flow through it. Through a tap funnel, secured in the neck of the 



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