Simple question but I don't get it

[mariposas905]

This is a pretty simple question, but I don't really get it. Why is the C-H bond shorter than C-O? I figured that C-O would be shorter because the electronegativity of O will pull the electrons to itself and make for a tighter bond between C and O....I guess this reasoning is wrong?

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[pretty_positron]

I think I know what passage you're talking about and I answered that based on the values they gave for bond energies. Also the C-O bond is different from a C=O bond. I believe the answer is because H is so tiny that its bond will be very short.

 

[mariposas905]

I think I know what passage you're talking about and I answered that based on the values they gave for bond energies. Also the C-O bond is different from a C=O bond. I believe the answer is because H is so tiny that its bond will be very short.

This is passage 9 from the C/P section of AAMC FL 2. I didn't see them give any bond energy values? How did you answer it based on those values if they did?

Yes, that was the rationale behind the correct answer, but if H is so tiny, wouldn't that mean it has a longer bond? I'm trying to draw a picture in my head with a large C atom and a small H atom with one valence shell and I feel like it'd be longer esp since O would pull the other atom's electrons towards itself...

Oh well, maybe I'm overthinking it.

 

[mariposas905]

Also, from the same passage, there is another question that says the C-C bond is less polar than the C-H bond. Does this mean polarities can be less than zero, since the C-H bond polarity is equal to zero? I didn't know they could be negative...

 

[DBC03]

My guess is that the electrons around the H are closer to the nucleus than the valence electrons around the O (those are in the next shell out)


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[pretty_positron]

Here:

You can see that C-H has the highest bond energy, thus requiring the highest amount of energy to break the bond which indicates that it is a strong (short) bond.

Also, bond length is defined as the length between the radii of the two atoms. Considering H has such a tiny atomic radius, this results in a tiny bond length.

Also, the C-H bond is slightly polar. I don't know where you are getting that they're non-polar from. C is more EN than H so it pulls electron density from the H resulting in a polarity.

C-C is clearly non-polar considering that they are the same exact atoms with equal EN's and thus equal pulls on one another.

 

[mariposas905]

Here:

You can see that C-H has the highest bond energy, thus requiring the highest amount of energy to break the bond which indicates that it is a strong (short) bond.

Also, bond length is defined as the length between the radii of the two atoms. Considering H has such a tiny atomic radius, this results in a tiny bond length.

Also, the C-H bond is slightly polar. I don't know where you are getting that they're non-polar from. C is more EN than H so it pulls electron density from the H resulting in a polarity.

C-C is clearly non-polar considering that they are the same exact atoms with equal EN's and thus equal pulls on one another.

Thank you! This helps explain things a lot. I got that C-H polarity was zero from TPR textbook. I guess that was kind of inaccurate...

But do you know if negative polarities are even possible or is the least one can have zero?

 

[pretty_positron]

Thank you! This helps explain things a lot. I got that C-H polarity was zero from TPR textbook. I guess that was kind of inaccurate...

But do you know if negative polarities are even possible or is the least one can have zero?

I don't know what negative polarity even means. A polarity means that there is more of a pull of electrons by one atom within a bond. This happens whenever there is a difference in electronegativities. This polarity creates partial charges as the more EN element is pulling electron density towards itself from the less EN atom, thus creating a partial negative charge for itself and a partial positive charge for the other.

Take carbonyl for instance, C=O, the O is the more EN atom so it is pulling electron density from the C. This is why there's a partial positive charge on the C and partial negative on the O.

 

[pretty_positron]

I got that C-H polarity was zero from TPR textbook. I guess that was kind of inaccurate...

Are you sure you're not thinking of CH4? In that case the polarity is zero, due to symmetry.

 

[mariposas905]

Are you sure you're not thinking of CH4? In that case the polarity is zero, due to symmetry.

Im pretty sure they said CH, but it was from an old MCAT textbook...maybe I'm remembering it wrong idk. But yes, it makes sense that CH4 is zero polarity. And the C-H polarity is like 0.3 according to Google so yeah, since it's so small, it is considered non-polar.

 

[CommyO]

Im pretty sure they said CH, but it was from an old MCAT textbook...maybe I'm remembering it wrong idk. But yes, it makes sense that CH4 is zero polarity. And the C-H polarity is like 0.3 according to Google so yeah, since it's so small, it is considered non-polar.

C-H is indeed considered a nonpolar bond
Recall a polar covalent bond being described with a net dEN (delta electronegativity b/w atoms) dEN = .5-1.7

for a nonpolar bond: dEN = <.5
for an ionic bond/polar covalent: 1.7+ (with exceptions such as metal/nonmetal which are obviously ionic)

C-H has a dEN of 2.55-2.2=.35 ; this means it is nonpolar

For this question; I think you can compare atomic radii such that H has a smaller radius than O

 

[PlsLetMeIn21]

Maybe it's me, but it seems like everyone is overanalyzing this one. C and C are the same atom, while C and H are different atoms, so a bond between two identical atoms has to be less polar than a bond between two different atoms, with different electronegativities.

Pretty positron, you mentioned a passage on energies where this came up. Are you talking about chapter 1 in TBR organic? That passage beat me up, but I know my stuff so much better now.