Ketone Reactions

Instructor: Laura Foist

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

In this lesson we will learn about ketones and the reactions they undergo, including addition reactions, reduction reactions, and substitution reactions. We will also learn the relationship between ketones and diabetes.

A 'Key-Tone'

A ketone is a common structure in chemical compounds. It's a carbon that is double bonded to an oxygen on one side, then on the other two sides it is bonded to two more carbons. We can remember the name 'ketone' by thinking about it as a 'key-tone'. So the general structure for a ketone is this:

The general formula for a ketone
Ketone

Now let's picture a key super-imposed over the ketone.

We can remember the name ketone by thinking of it as a key
key-tone

The oxygen is the bow (the round part at the top of the key), the double bond is the stem of the shaft, and the R-groups on either side are the bits on both sides of the shaft. This way you can remember that a carbon double bonded to an oxygen with R-groups on either side is a 'key-tone'.

Ketone Addition Reactions

A ketone can add another oxygen onto the carbon. This reaction needs to add an oxygen on because currently the oxygen 'has' the electrons, and it likes those electrons because it is very electronegative. Electronegativity refers to the strength with which an element will attract, or hold onto, electrons, and something that is highly electronegative holds on to electrons really strongly.

So since oxygen is very electronegative, it won't easily give up its electrons to something less electronegative than itself (carbon is less electronegative). But it is willing to give up its electrons to something that is as electronegative as itself (another oxygen).

The oxygen from water can be added to the ketone
Ketone addition

Here's an example of an addition reaction between ketone and water. The orange line to the orange oxygen shows you where the original bond is. Then we have the blue line going to the purple oxygen. This oxygen came from the water. The hydrogen atoms on the water are also added onto the oxygen atoms in order to make sure the oxygen atoms don't have a negative charge.

The oxygen added to a ketone can have another R-group attached to it
Ketone addition

In this example, we have a ketone reaction with an oxygen (in red) that is already connected to its own 'R' group. In this case, it's an ethyl (2-carbon) group. The oxygen connects to the carbon, bringing the ethyl group with it. The oxygen (shown in black) that was a part of the ketone before leaves (with the hydrogen ions) as water.

Ketone Reduction Reactions

Remember that oxygen is very electronegative, so it is not going to easily be reduced - it doesn't want to give up those electrons. Thus ketone reductions only occur when there is a strong acid present, such as hydrochloric acid (HCl), or a strong reducing agent, such as lithium aluminum hydride. A strong acid will easily give up its hydrogen, and then the oxygen can take the hydrogen. A strong reducing agent is more electronegative than the oxygen, so it can take the electrons from the oxygen.

Take a look at this example of a reduction reaction between ketone and lithium aluminum hydride. You can see that the lithium aluminum hydride gives up two of its hydrogens, one to the carbon and one to the oxygen. In this way, the carbon has been reduced (it is not connected to as many oxygen atoms).

A ketone can be reduced using a strong reducing agent
Ketone reduction

In order for a ketone to be completely reduced (which means there will be no more oxygen in it), then we need to use a strong acid like hydrochloric acid. The hydrochloric acid wants to give up its hydrogens; those hydrogens can then attach to the carbon and kick off the oxygen.

A ketone can be completely reduced using a strong acid
Ketone reduction

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