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Which of the following compounds exhibits dipole-dipole forces as its strongest attraction between molecules?

a) $\ce{CO2}$
b) $\ce{CH3NH2}$
c) $\ce{Kr}$
d) $\ce{H2}$
e) $\ce{HBr}$

From what I understand, $\ce{CH3NH2}$ has an H-bond and it's the strongest form of dipole-dipole. So I thought that would be the answer.

I also knew that $\ce{HBr}$ shows the greatest electronegativity difference between $\ce{H}$ and $\ce{Br}$ atom, it would be a polar covalent bond (?!).

In that case, $\ce{HBr}$ may have stronger attraction between molecules than $\ce{CH3NH2},$ but it is not dipole-dipole. Isn't that correct? Then shouldn't the answer be $\ce{CH3NH2}$ instead of $\ce{HBr}?$

I must have some concept issues here.

Molly_K
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    Mistaking bonds for intermolecular interactions is a serious "concept issue". – Mithoron Jul 18 '20 at 15:15
  • Wait, I thought there would be H-bonds between CH3NH2? And each HBr molecule is attracted to other HBr molecules by a mixture of permanent dipole-dipole and dispersion forces. This is intermolecular bonding. So Hbond vs dipole-dipole, why is HBr stronger than CH3NH2? I AM trying to compare intermolecular interactions, not the bonds? Please tell me where I am making a mistake. I am going in circles and very confuse. Thank you. – Molly_K Jul 18 '20 at 15:29
  • Well, that's what this terrible title suggests, so edit it. – Mithoron Jul 18 '20 at 15:32
  • Thank you for the suggestion, I edited and made it specific. – Molly_K Jul 18 '20 at 15:41

1 Answers1

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EDIT (after title edited): $\ce{HBr}$ doesn't have stronger interaction than $\ce{CH2NH2}$, but it has dipole-dipole interaction as the strongest forces between it's molecules, which is obviously weaker than H-bonding.

Hydrogen bonding is the strongest intermolecular attraction. It is a type of dipole-dipole interaction1, but it is specific to Hydrogen.

In general, dipole-dipole interactions are considered weaker than H-bonding. The relative strength of forces are as follows,

Hydrogen bonding > Van der Waals dipole-dipole interactions > Van der Waals dispersion forces

As per your question, (a), (c) and (d) have dispersion forces (as they're non-polar), while (b) has H-bonding as it's strongest interaction (not dipole-dipole).

Finally, as only (e) has dipole-dipole as it's strongest attraction between molecules, hence it's the answer.

Notes:

1: Dipoles are created due to difference in electronegativity. Below image shows that there is sufficient electronegativity difference to create a dipole. H-bonding and dipole-dipole interactions have same origins.

enter image description here

![H-BONDING and dipole-dipole interactions](https://cdn.masterorganicchemistry.com/wp-content/uploads/2019/11/3-van-der-waals-dipole-dipole-interactions-acetone-methyl-acetate-propyl-fluoride.gif)

(Source: Master Organic Chemistry)

Rahul Verma
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  • So H-bond is not a dipole-dipole? But it's said to be the strongest dipole-dipole? – Molly_K Jul 18 '20 at 15:22
  • @Molly_K https://chemistry.stackexchange.com/questions/35488/is-hydrogen-bonding-a-type-of-dipole-dipole-interaction https://chemistry.stackexchange.com/questions/21743/why-is-hydrogen-bonding-more-significant-than-any-other-interaction-between-dipo – Mithoron Jul 18 '20 at 15:29
  • @Molly_K: H-bond is the strongest dipole-dipole interaction. But, dipole-dipole forces are said to be other interactions than H-bond, in general – Rahul Verma Jul 18 '20 at 16:28
  • @RahulVerma Thank you for the clarification. But why is "dipole-dipole forces are said to be other interactions than H-bond, in general" yet the textbook emphasized H-bond it the strongest dipole-dipole intermolecular force? This is so confusing! – Molly_K Jul 18 '20 at 21:06
  • @RahulVerma I looked up the melting point forHbr, which is -124.6°F, and is -135.4°F for methylamine. As we also learned that higher intermolecular force (IMF) will have a higher melting point, we know that HBr is having a stronger IMF.

    The question is why this is making an exception from the rule we learn in the textbook. I would really appreciate how to interpret this and also learn if there's any rule that I missed from reading.

     – Molly_K Jul 19 '20 at 00:51
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    @YiKo: 1) have a look at this image (src: masterorganicchemistry) 2) H-bonding is theoretically separated from other dipole-dipole interactions. But, in essence both of them have same origin, as said in image. 3) intermolecular forces aren't significant in determining melting point, packing fraction and crystal structure are used to compare MP. 4) intermolecular forces are significant in comparison of BP, BP(hbr) = -60, BP(ch3nh2) = -6 – Rahul Verma Jul 19 '20 at 03:01