What Determines C-F Bond Length Order?

[utkarshakash]

Homework Statement

Choose correct order of C-F bond length
A)C^-HF₂<C^-H₂F<C^-F₃
B)C^-F₃<C^-HF₂<C^-H₂F
C)C^-F₃<C^-H₂F<C^-HF₃
D)C^-HF₂<C^-F₃<C^-H₂F

The Attempt at a Solution

What factors should I look for while measuring bond lengths?

 

[AGNuke]

You can take Lone Pair-Bond pair repulsions, Interaction of H and F (due to their size difference), F F interactions, and probably C-H C-F Bond interaction.

 

[utkarshakash]

You can take Lone Pair-Bond pair repulsions, Interaction of H and F (due to their size difference), F F interactions, and probably C-H C-F Bond interaction.

Can you please show through an example?

 

[AGNuke]

Two C-F bonds will repel each other, thus they will try to maximize their angle, but their repulsion with lone pair is even greater, hence they will be forced to elongate themselves in order to face minimal repulsion from each other and the lone pair.

Try to work on this.

 

[utkarshakash]

Two C-F bonds will repel each other, thus they will try to maximize their angle, but their repulsion with lone pair is even greater, hence they will be forced to elongate themselves in order to face minimal repulsion from each other and the lone pair.

Try to work on this.

If I follow through this reasoning, then CF₃ will have maximum C-F bond length because it suffers more repulsion as compared to others.

 

[AGNuke]

Might be. But you should check around the internet for factual data before putting your vote in my "theory".

See, I am assuming that you are preparing for college entrance exam (JEE).

 

[utkarshakash]

Might be. But you should check around the internet for factual data before putting your vote in my "theory".

See, I am assuming that you are preparing for college entrance exam (JEE).

Yes. I'm preparing for JEE and they won't ask questions where factual data is required. But what I wanted to say is that the correct answer is B) which is just the reverse of what I expected.

 

[AGNuke]

Well, what can I say, I'd need to dig up some lost info on this one.