Formation
Home Up Structure Properties Nomenclature Clasification Formation Reactions Lab Work

 

Formation

Now let's consider reactions in which alcohols are formed. Alcohols are the products of several different kinds of reactions. Those include fermentation, addition of water to alkenes, and hydrolysis of alkyl halides.

 

Fermentation

Alcohols are products of naturally occuring fermentation reactions. This involves biological processes in which alcohols are produced from the fermentation of starches and sugars. By far, the most common alcohol formed in this way is ethanol. We will touch upon this process again in the lesson on carbohydrates.

 

Addition of Water to Alkenes

Another method of preparing alcohols is by the addition of water to alkenes. This reaction occurs in the presence of sulfuric acid and various other conditions. The overall reaction is indicated here (and in Example 7 of your workbook).
H H
| |
H-C=C-H

 

 + H2O H2SO4
¾¾¾® 		H H
| |
H-C-C-H
| |
  H OH
The net result of that reaction is that one of the hydrogens from the water gets attached to one of the carbon atoms that was in the double bond and the -OH group from the water gets attached to the other carbon atom that was in the double bond. This is the method by which most of the industrially prepared ethanol is manufactured.

Equation for addition of water to alkene. [63rxn01.JPG]
As shown here (and in Example 7b in your workbook), the -OH does have a preference for attaching to the carbon atom that has the most other carbon atoms bonded to it. Another view of that is that the hydrogen goes to the carbon that has the most hydrogen. Although that preference is significant in other contexts, it is not something you need to remember for this course.
H H H
| | |
H-C=C-C-H
    |
     H 		+ H2O H2SO4
¾¾¾®		 H  H  H
|  |  |
H-C--C--C-H
|  |  |
H  OH H

 

 

Hydrolysis of Alkyl Halides

Alcohols can also be formed by the reaction between alkyl halides and water, as shown here (and in example 8 in your workbook). Because the alkyl halide and the water molecule are split during the reaction, it is called a hydrolysis reaction. (It is similar, in that sense, to the hydrolysis of salts that we dealt with when studying acids and bases.)
H H 
| | 
H-C-C-Cl
| | 
H H  		+ H-O-H ¾¾® H H   
| |  
H-C-C-O-H
| |  
H H   		 + H-Cl
Note that the -OH ends up on the same carbon atom that the halogen was originally on.
H  H  H
|  |  |
H-C--C--C-H
|  |  |
H  Cl H 		+ H2O ¾¾® H  H  H
|  |  |
H-C--C--C-H
|  |  |
H  OH H		 + HCl
You can see that, over all, the chlorine atom comes off of the alkyl halide and bonds to a hydrogen atom from the water molecule and the -OH from the water molecule attaches to the alkyl group, making an alcohol.

Equation showing hydrolysis of alkyl halide. [63rxn02.JPG]

This kind of reaction is not a particularly important way of making alcohols, but it does show a relationship between two of the classes of compounds that we are studying. In addition, it is a relatively simple example of hydrolysis reactions (many more examples of hydrolysis reactions will follow) and it serves as a reference point for the reverse reaction, the formation of alkyl halides from alcohols.

 

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E-mail instructor: Eden Francis

Clackamas Community College
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