[Frank Ose]

Hello everyone,

Why is the bond angle between Cl and Cl in trichloromethane more than 109.5degree ?

Is it because the repulsion bonding pair between C-H and C-Cl weaker and hence increasing the angle between C-Cl and C-Cl( As they are stronger and the bonding electrons are closer to Cl atom , making them repel more as Cl is a big atom)?

It is a polar molecule, so the bonding pairs repulsion are stronger between C-Cl and C-Cl and weaker between C-H and C-Cl ( As the distance between the lone pair is more), bear in mind that in tetrahedron, when the C-H and C-Cl angle decreases, the C-Cl and C-Cl's angle will increase.

I don't know if my thinking is right but I would like you to clarify me, thanks in advanced.

[cliverlong]

Hello everyone,

Why is the bond angle between Cl and Cl in trichloromethane more than 109.5degree ?

Is it because the repulsion bonding pair between C-H and C-Cl weaker and hence increasing the angle between C-Cl and C-Cl( As they are stronger and the bonding electrons are closer to Cl atom , making them repel more as Cl is a big atom)?

It is a polar molecule, so the bonding pairs repulsion are stronger between C-Cl and C-Cl and weaker between C-H and C-Cl ( As the distance between the lone pair is more), bear in mind that in tetrahedron, when the C-H and C-Cl angle decreases, the C-Cl and C-Cl's angle will increase.

I don't know if my thinking is right but I would like you to clarify me, thanks in advanced.

Hmmm ...

I don't have an answer but the factors I would consider important are

Start from VSEPR for methane or tetrachloromethane. These will be tetrahedra as all C-other bonds are to identical atoms.

As you wrote, in "mixed" molecules the inter-bond angles are influenced by

• size of atoms bonded to carbon
• the polarity of the C-other atom bond

My feeling is that the size of the Cl atoms is the dominant factor and will reduce the H-C-Cl bond angle to less than 109.5 degrees. Since the H-C-Cl angle has decreased the Cl-C-Cl bond angle must have increased. Does that seem plausible?

You wrote about lone pair in your question. I don't think there are any lone pairs in this situation.


Clive

[Frank Ose]

Sorry it was typing error. There is no any lone pairs there.

Thanks for your helpful reply,

It seems plausible, but I think we have to take hydrogen atom into account. Comparing the size of H and Cl , we can see the that repulsion between H and Cl is not as high as the repulsion between Cl and Cl, resulting in H-C-Cl angle <109.45degree and Cl-C-Cl > 109.45 degree.

If merely considering the size of Cl, that wouldn't seem right because tetrachloromethane has four chlorine atoms and their angles are 109.45degree between each other (Their repulsions are equal, hence distributed equally).Afterall, it is the degree of repulsion that matters, I assume.When there is uneven degree of repulsion, the angle between the respective atoms will be different.

What do you think?

[Borek]

What do you think?

That repulsion between larger atoms is stronger, just because they occupy more volume. Atoms will behave like balloons here.