Organic chem 1 question

[Gold3nLily]

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My textbook says C-H bonds have a negligible dipole moment b/c their electronegativities are essentially same. However, C-Cl bonds have a large electronegative difference and therefore accounts for most of dipole moment in CH3Cl. Okay, I looked at the values from the electronegativity table: C= 2.5, Cl=3.0, and H=2.1. Why is there so much discrepancy between the electronegativity of the C-H and the C-Cl bonds if their electronegativities only differ by 0.1? The difference between electronegativity in C-H is 0.4 and the difference between C-Cl is 0.5, but they make a point of telling you that C-Cl is much more polar .. why?

Here is something that I actually learned in my Biology class about electronegativity(EN):
If two atoms have a difference in EN that is less than 0.5 it is non- polar.
If two atoms have a difference in EN that is from 0.5 - 2 they are polar.
If two atoms have a difference in EN that is more than 2 then they are ionic.

So i see how a C-H bond would be considered non-polar and a C-Cl bond will be
considered polar even where there is just a 0.1 difference. But I want to understand this better. Can someone explain it better than this rule? (I also know this rule has exceptions like SiF4 and CaS...)

 

[Baylor2011]

I think you're looking too far into electronegativity numbers...

Carbon is NOT a very electronegative atom, therefore when bonded to a hydrogen, the electronegativity difference is negligible. Note whenever you're talking about EAS, alkyl groups are actually electron donating, whereas halogens are electron withdrawing because they're so electronegative

Chlorine is a very electronegative element, therefore creating a net dipole

 

[Paseo Del Norte]

While electronegative plays a big role, I think you also have to consider the molecular geometry (VSEPR theory) and identifying separation of net charge. A classic example would be something like CO2, where you have electronegative oxygen and could be mislead into thinking polar. However, the molecule is linear and there is in fact no separation of charge, the result being a non polar molecule.

I hope that helped?

 

[orgohacks]

It might help if you also think about the electron density on the rest of the atom.

Chlorine has six electrons in lone pairs all to itself, in addition to its high electronegativity, so the electron density around chlorine is high to begin with.

Carbon is usually going to be involved with 3 other atoms where the electrons are shared more or less equally (e.g. with H or C) so the electron density around C will be more spread out and shared.

Hydrogen has no electrons around it except for the two bonding electrons.

Considering the whole picture, the fact that chlorine is bound to only one other atom means that the bonds will often have a dipole moment, whereas with carbon, its tetrahedral nature means that dipole charges usually cancel out (think of the vector sum in a molecule like CH4 or CH3CH3 - pretty small). However the low electronegativity of H does mean that the C is fairly electron rich, which is part of the reason why alkyl groups are considered donor groups.

I have no idea if this is helpful but I hope it was! - James

 

[Gold3nLily]

🙂 Thank you all for you help.

I understand now that I was missing the forest for the trees.

I really appreicate the explanations.

Gold3nLily