The C―H σ bond length in ethane is 1.09 Å. The C―H

Question

The C―H σ bond length in ethane is 1.09 Å. The C―H σ bond in ethene is 1.07 Å. Explain.

Accepted Answer

All right. Hello everyone. So for this question, let's go ahead and provide an explanation for why the NH sigma bond length is greater in methane amine than in methane. I mean, and here on the screen are the structures for both of these compounds. Beneath this, we have four different answer choices labeled A through D which discuss different possible explanations or the phenomenon described here when it comes to the NH sigma bond length. Now, what's worth noting here is that the explanations provided in these answer choices, either discuss the length of the carbon hydrogen bond or they talk about the s character of the hybridized orbitals in question. So let's go ahead and talk about it specifically about S character. And what that tells us about the properties of certain hybrid orbitals recall that the S character of a given orbitals or a hybrid orbital is the percentage of the S orbital involved in that particular hybrid in organic chemistry. We specifically discuss primarily SP three SP two and SP hybrid orbitals in, in the case of an SP three orbital, for example, it's composed of one S orbital and three P orbitals for a total of four, because the S orbital makes up 1/4 of the SP three hybrid orbital, we say that the SP three orbital has 25% S character. And so the same logic can apply to both the SP two and P hybrid orbitals. Because one third of the SP two orbital is composed of the S orbital, then it has the SP two orbital has 33.33% S character. And lastly because the P orbital is one half S and one half P, then it has 50% S character. So why does this matter? Well, it matters because the greater the ss character, the stronger and shorter the bond happens to be. So if we're discussing bond length, then sp three orbitals are going to contain the, the longest bond length followed by SP two. And finally, sp being the shortest and the strongest. So to determine the S character involved in these different NH bonds, we're going to actually focus on the hybridization of the nitrogen atoms only. And the reason for this is because if you recall, hydrogen only uses an S orbital since hydrogen can only make one covalent bond. So instead we're going to focus on nitrogen. So in order to determine the hybridization of a given atom, it's worth counting the number of quote unquote groups that surround that particular atom, right that we're looking at. So when I refer to groups, I'm referring to two general categories I'm referring to atoms bound directly to the atom of interest as well as any loan payers that that particular atom happens to have. So, in this case, I'm counting the number of atoms bonded to nitrogen as well as any lone pairs to find my total number of groups. And so the total number of groups decides hybridization by taking into account how many orbitals make up each hybrid. For example, an sp three orbital is composed of one S and three P orbitals giving me a total of four, right. So four groups in total would correspond to SP three hybridization. An SP two orbital is composed of one S and two P. So that's a total of three and therefore, three groups would correspond to SP two hybridization. Lastly, an SP orbital is composed of one S and one P for a total of two, meaning two groups would give me sp hybridization. So first, let's go ahead and discuss the hybridization of the nitrogen atom in methane amine. In this case, when counting the total number of groups, I can see that excuse me, three atoms are connecting to this nitrogen, right, I have two hydrogens and one carbon atom. So that's three other atoms and nitrogen has one lone pair of electrons attached to it. So three and one gives me a total of four groups. Therefore, the nitrogen atom of methane amine is sp three hybridized. On the other hand, the nitrogen atom of methane, I mean, is a little bit different in the case of the nitrogen atom of methane, I mean, I only have two other atoms attached to it. In this case, one carbon and one hydrogen. So that's two as well as one lone pair giving me a total of three groups which corresponds to sp two hybridization. And so with this, we can actually understand why the bond length is different because the nitrogen atom of methane amine is sp three hybridized, its sigma bond length is going to be longer because its hybridized orbital has less s character. It is P three where the nitrogen of methane iine is sp two hybridized. So this means that our answer choice is going to be option d in the multiple choice, the methane amine NH bond is longer because its hybridized orbital has less s character. And so with that being said, thank you so very much for watching. And I hope you found this helpful.

The C―H σ bond length in ethane is 1.09 Å. The C―H σ bond in ethene is 1.07 Å. Explain.

Key Concepts

Bond Length

Hybridization

Bond Order

Watch next