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Methanol and ethanol

The simplest of all alcohols, methanol, has a boiling point of 148.5° F. (64.7° C). It was for­merly known as wood alcohol. For centuries, it was obtained by heating wood to about 900° F. (about 500° C) in the absence of air and distill­ing off (purifying) the liquid formed. Today, the chief source of methanol is synthesis gas, a mixture of carbon monoxide and hydrogen. Nearly half the methanol manufactured is con­verted into methanal (formaldehyde), a starting material for phenolic resins, which are syn­thetic resins that harden and become perma­nently shaped under the continual application of heat. Other derivatives include ethanoic acid (acetic acid) and chloromethane (methyl chloride).

With a boiling point of 172° F. (78.3° 0, etha­nol is a colorless liquid at room temperature. Its preparation by fermentation is an ancient process used over many centuries for produc­ing alcoholic beverages. Until the arrival of the petrochemical industry, ethanol was prepared by fermenting plant-derived carbohydrates such as starch, sugar, or cellulose. Today, most industrial ethanol is prepared syntheti­cally. The major industrial use for ethanol (about 45 per cent of all uses) is in the produc­tion of ethanal (acetaldehyde). Acetaldehyde is



Organic chemistry: Alcohols 81


 

 

 

 

 

 

 

 

A Esterification #   | + •
r + AnAAA
'■r^w'^r^r^^  
Alcohol (ethanol) Carboxylic acid (butanoic acid) Ester (ethyl butanoate) Water
B Saponification      
  HV -^••» *  
a. I IP f L  
Hydroxyl ion (from base) Ester (ethyl butanoate) Alcohol (ethanol) Carboxylate ion (butanoate)
+   ---- ► #OMft • ~CH3 • ~CH2 O =co ^ -OH
Hydrogen ion (from acid) Carboxylate ion (butanoate) Carboxylic acid (butanoic acid)

Two key reactionsof alco­hols are esterification and saponification. Esterification (A) is the reaction between an alcohol and a carboxylic acid to form an ester and water. It is analogous to the neutralization of an acid by a base. Saponification (B) can be considered the re­verse reaction. The action of a base on an ester followed by the addition of excess acid regenerates the origi­nal alcohol and acid.


 



used as a source of acetic acid, as a solid fuel (canned heat), and in the synthesis of many other compounds. About 30 per cent of the ethanol produced annually is used as a sol­vent. The rest is used in various chemical proc­esses.


Date: 2015-12-11; view: 245


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