Dipole moment carbon florides of isotopes of Hydrogen

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Homework Statement

Compare Dipole moment of CH₃F and and CD₃F where D is deuterium.

Homework Equations

Theory questions

The Attempt at a Solution

Our sir had explained that to do this question we muat take into account the vibration of the atom in the respective molecule.
That is that the smaller the molecule is the more easily it can move away from the central atom.
But i don't know how to apply this concept with respect to dipole moment...

The answer given is Dipole moment of CH₃F is more than that of CD₃F

 

[TeethWhitener]

Your profile says you're 17. This seems like a really advanced question for someone your age. Ergo, I'm not sure at what level to address this.

You don't need to take vibration into account per se, but you do need to understand that the vibrational potential is anharmonic. Since a deuterium nucleus is heavier than a proton, its zero point energy will be lower; therefore, it will (on average) sit deeper in the vibrational potential well and the harmonic approximation will work better for deuterium than a proton. The B-level upshot of all this is that the C-H bond length will be slightly longer than the C-D bond length.

What does this mean for the molecule's dipole moment? Well, take a look at the electronegativities of C, H, and F, and try to predict to what extent, and in which direction, the C-H and C-F bonds will be polarized. What happens to the dipole moment when you stretch or shorten a bond?

 

[Navin]

Your profile says you're 17. This seems like a really advanced question for someone your age. Ergo, I'm not sure at what level to address this

Welcome to the beautiful world of JEE(joint enginearing entrance) Advance and indian Entrance exams !

Okay we have zero idea about the vibrational potential wall, but i won't be surprised if we learn it in our course.(i think our Sir mentioned that we shall go deeper about it later on in the course)

Anyway so yea getting back to the question

So since the C-H bond shall be slightly longer and Dipole moment is directly proptional to distance tween the participating atoms that is Bond lenght

I feel CH should have more dipole moment than CD

Now let's look at CH and CF

In CH the direction of polarisation will towards C as it is more Electro Negative than H

But this Reverses in the Case Of CF
Where the direction will be in the direction of Florine as it is more electronegative .

So far so good ?

 

[Navin]

Now if we draw the two tetrahydral structures(i know its frowed upon to send pictures but i really don't know how to draw structures via a keyboard)

We can tell that Since CD has lssser dipole moment than CH ,and CF will have the same dipole moment in both cases,

The sum of the dipole moment in the case of CD₃F is more than that of CH₃F

Therefore i think
CD₃F should have less Dipolemoment than CH₃F

 

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[TeethWhitener]

The sum of the dipole moment in the case of CD3F is more than that of CH3F

Therefore i think
CD3F should have less Dipolemoment than CH3F

This is confusing, but I think it might just be a typo on your part. Everything else looks fine, and is about the level I'd expect from a high school chemistry class.

 

[Navin]

This is confusing, but I think it might just be a typo on your part.

Sorry ! Typo i missed that ! My bad

is about the level I'd expect from a high school chemistry class.

So much for my hopes of being a genius !

 

[Navin]

Thank you so much ! It was really kind of you to help ! Have an awesome day !