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.
