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Alcohol Classification & Reactions

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In organic chemistry, alcohol means a lot more than an alcoholic drink, as it commonly known. In this lesson, we'll chemically define an alcohol, look at the different oxidizing reactions that it can undergo, and the resulting products that are formed.

Types of Alcohols

We all know what alcohol is when it is referred to as a drink, and there are many different types of alcoholic drinks. In chemistry, however, all of these alcohols are exactly the same and fall under one name - ethanol. Also present in chemistry are many other types of alcohols.

alcohol types

Alcohols are any carbon chain with an 'OH' group attached to one of the carbon atoms. These alcohols can be primary, secondary or tertiary. A primary alcohol is one in which the 'OH' group is attached to a terminating carbon (the last carbon on a carbon chain). A secondary alcohol is when the carbon that is attached to the 'OH' group is attached to 2 other carbon atoms. Finally, a tertiary alcohol is when the carbon that is attached to the 'OH' group is not attached to any hydrogen atoms. Instead, it is attached to 3 other carbon atoms.

Oxidizing Primary Alcohols

Since the carbon that is attached to the 'OH' group has 2 hydrogen atoms, we have the potential to easily replace each of those carbon-hydrogen bonds with carbon-oxygen bonds. So, we can either form an aldehyde (if only 1 carbon-hydrogen bond is replaced with a carbon-oxygen bond), or we can form a carboxylic acid (if both carbon-hydrogen bonds are replaced with a carbon-oxygen bond).

primary alcohol

There are many different oxygen sources that can be used for the oxidation process. Some of the most common ones are acidified dichromate salts. In this equation, the O (in square brackets) indicates any oxygen source.

If you want to stop at aldehydes (that is if you don't want to fully oxidize the carbon), then we need to use excess of the alcohol. That way it is much more likely that the oxidizer will 'find' the alcohol, instead of one of the aldehydes, to oxidize further into a carboxylic acid. But we also distill the aldehyde off as soon as it is formed (so it is no longer in the solution to be reacted).

If we want to make carboxylic acids then we use equal equivalents of the oxidizer and the alcohol and we let it react together until it has reached completion.

Oxidizing Secondary and Tertiary Alcohols

Now remember that that the carbon on secondary alcohols are only attached to 1 hydrogen atom. So we only have the option to create 1 more carbon-oxygen bond. Since there are no more hydrogen atoms on that carbon, it will be a ketone that is formed (instead of an aldehyde).

alcohol to ketone

They are oxidized in the same manner as primary alcohols, using oxidizers such as potassium dichromate. In this equation we see that the oxygen source, O, reacts with a secondary alcohol and forms a ketone. There are no more hydrogen atoms on that carbon to further react and make a carboxylic acid.

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