Propane bond angle increase?

Question

The observed bond angle in the C-C-C bond of propane is 111.66 degrees. Can someone explain why the bond angle in propane is not 109.5 degrees? I am having difficulty understanding the reason for this increase. Thank you for your help!

Anonymous · Accepted Answer

I got the bond angle as 111.16 degrees. All of my bond angles are higher than 109.5 degrees. I think this is because of the strain. The hydrogens push against each other, forcing the carbons farther apart and creating a larger bond angle.

Look at a picture of propane. There are four hydrogens pointing towards each other on the molecule.

Marianne Towne · Answer

The C-C-C bond angle is higher than 109.5 degrees because the molecule wants to minimize the Van der Waals repulsions between neighboring groups. 
Picture a propane molecule; 3 carbons in a wide v shape. The center carbon (C2) has 2 ethyl groups and 2 hydrogen atoms attached to it. 
In theory, these 4 attached things would try to be as far apart from each other as possible, AKA at 109.5 degree bond angles. 
However, in reality, the methyl groups repel each other more than the hydrogen atoms. To make the methyl groups slightly more "happy", they both move away from each other and closer to the hydrogen atoms, giving you a C-C-C bond angle of 111.66 degrees. You will notice while the C-C-C bond angle is more than 109.5 degrees, the H-C2-H (center carbon atom and directly attached hydrogen atoms) actually decreases from 109.5 degrees- because this makes the overall molecule more "happy".

Anonymous · Answer

I think you are doing O'Chem Lab if I am not mistaken over Molecular Modeling. Anyways I think you may have highlighted or wrote the number down wrong because the bond angle is actually supposed to be lower than 109.5. I actually got that the bond angle was 95.31. I hope this helps a little bit

Anonymous · Answer

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Greetings, XeF4 < H2O < CH4 < BeH2 Xe has 2 unbonded electron pairs which reduce the angle between F - Xe - F to 90 degrees H2O has 2 unbonded e- pairs (104.5 degrees) CH4 does not have any unbonded e- (109.5 degrees) BeH2 has none (180 degrees) Regards

Anonymous · Answer

in ethylene why we want to put big side group R to the backbone?

Dr. Francisco Von II

Propane bond angle increase?

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