Question Video: Predicting the Likely Bond Angle of the C–O–H Bond in a Propanol Molecule Chemistry

What is the most likely bond angle of the C–O–H bond in a molecule of propanol?


Video Transcript

What is the most likely bond angle of the C–O–H bond in a molecule of propanol?

Let’s begin by examining the provided model of the propanol molecule. In the model, white circles represent hydrogen atoms, black circles represent carbon atoms, and red circles represent oxygen atoms. The bond angle we are trying to determine is between a carbon atom, an oxygen atom, and the hydrogen atom that is bonded to the oxygen atom.

To help us, we will make use of the valence shell electron pair repulsion model. This model helps us to predict the shapes of molecules based on the arrangement of electron pairs. The model assumes that there are always electrostatic repulsions between pairs of valence electrons. And the shape the molecule has comes from minimizing these high-energy repulsive interactions.

Now, let’s draw a Lewis structure of propanol so that we can determine how electron pairs are organized in the molecule. We will need to determine the shape of only the portion of the molecule that is pink. To help us, let’s look at the simple Lewis structure of methane. The methane molecule contains four bonded electron pairs. Each pair is shared between the central carbon atom and one hydrogen atom.

Let’s write an AXE formula for methane. Carbon is the central atom of the molecule, so we begin by writing A. Carbon is surrounded by four bonded electron pairs, so we write X4. Methane belongs to the AX4 group and therefore has a tetrahedral shape. Because there are equal repulsive forces between all the bonded electron pairs, all bond angles around the central atom are the same. Each of these angles is 109.5 degrees.

Now let’s take a look at the Lewis structure of water. A water molecule has two bonded pairs of electrons. Water also contains two nonbonded electron pairs, which are valence electrons that are not shared in a covalent bond. Therefore, water belongs to the AX2E2 group and has a bent shape. The nonbonded electron pairs take up more space than the bonded pairs and have greater repulsive forces than the bonded pairs. Therefore, the hydrogen–oxygen–hydrogen bond angle is less and is only 104.5 degrees.

Now, let’s return to the propanol molecule. Like water, the oxygen atom in propanol is surrounded by two bonded pairs and two nonbonded pairs. Therefore, we can infer that the C–O–H bond angle should be closer to 104.5 degrees like the H–O–H bond angle in water.

In conclusion, the most likely bond angle of the C–O–H bond in the propanol molecule is 104.5 degrees.

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