Structure

Ketones have as their functional group the carbonyl group, which has a carbon atom double bonded to an oxygen atom. Also, in order to be a ketone, the carbon atom has to be bonded to two other carbon atoms. Another way of putting this is that the carbonyl group cannot be at the end of a carbon chain - it must be somewhere in the middle.

Structure of ketone functional group. [63str01.JPG]

Hybridization

The carbon atom in the carbonyl group has sp² hybridization. It bonds to three other atoms (two carbon atoms and one oxygen atom) in a flat triangular plane using sigma bonds with 120 degree bond angles. It also forms a pi bond to the oxygen atom. This bonding arrangement dictates the shape, polarity, physical properties and chemical reactivity of ketones. The carbonyl group is a polar functional group.

Models

In this ball and stick model of a small ketone notice how the oxygen sticks out quite a bit.

Model of acetone (ball and stick). [63mod07.JPG]

This makes the molecule polar and allows it to bond to and dissolve in water.

Models of acetone and water. [63mod08.JPG]

These model kits were not designed to show sp² hybridization for carbon so I had to use a different color for the central carbon atom in this space-filling model of the same molecule as shown above.

Models of acetone and water (space filling). [63mod09.JPG]

In this model of a larger ketone notice how the carbonyl group is a less significant portion ofthe molecule. It is still there and can still react in the same way. However, this molecule should be considered nonpolar with one small polar section and is not soluble in water.

Model (space filling) of large ketone along with acetone and water. [63mod10.JPG]

This model points out something that is important to remember about organic molecules, particularly the large ones. The structural formulas are generally drawn out in straight lines. The models can be made to look somewhat the same but with zigs and zags.

Model of large ketone stretched out. [63mod11.JPG]

The molecule itself is able to rotate on every single bond and its most likely shape will be quite different than the straight line you might imply from the structural formula.

Model of large ketone bunched up. [63mod12.JPG]

Now, or when you are in the lab, go to the demonstration table and identify the space-filling model of the ketone there. Also make a ball-and-stick model of the same ketone and compare the two models. See for yourself why it is polar and how it would form intermolecular bonds to water molecules and thus this compound is soluble in water.

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E-mail instructor: Sue Eggling

Clackamas Community College
Š2001, 2003 Clackamas Community College, Hal Bender