Rank the following compounds from strongest to weakest acid:

Question

CH₃CH₂OH; CH₃CH₂NH₂; CH₃CH₂SH; CH₃CH₂CH₃

Accepted Answer

Hey everyone. And welcome back in today's video, we are going to rank these compounds in order of increasing acidity, we are given four structures and all of them share that metal group. Now, the main difference is that when we look at our next atom for structure A, that's carbon bonded to hydrogen, for B that's oxygen bonded to hydrogen. For C that's nitrogen bonded to hydrogen and for D that's sulfur bonded to hydrogen. So it makes sense to draw their structures. First of all, for A, we have C three C two bonded to H, right? So essentially that's what we get. We, we are essentially expanding that metal group in a form of siege to H. Now, for B, we have C three bonded to O which is bonded to H or structure C, we have C three NH bonded to H and for D, we have C three bonded to S which is bonded to H OK. So essentially we are comparing the following bond strand ch for the first one, right? This is the bond we are analyzing for the second one. We are comparing a wage bond strength. Then we have NH and for the final one we are considering sh remember that the more acidic the structure is the easier it is for us to break our atom hydrogen bond, right, and release that hydrogen in the form of an acidic proton into a solution. So let's remember what kind of factors we have to keep in mind whenever we are considering acidity. And for this purpose, we are going to add a basic P periodic table so that we can see all of the, the relevant information for us whenever we are solving this problem. So below, you can see the periodic table and we want to identify positions of four atoms, carbon oxygen nitrogen. So we have carbon nitrogen, oxygen and then we have sulfur. Now remember that the atomic radius increases down the group, right. So we can essentially say that when you go down the group, the atomic radius increases and size is one of the major effects that increases acidity with an increase in size, acidity increases as well. For example, as you know, H I is more acidic than HBR because iodine it has a greater radius than bromine, right? And essentially this is explained using the kom attraction force because the radius of iodine is greater, the attraction force between hydrogen and iodine become slower and the lower the attraction, the easier it is for us to release that hydrogen in the form of an acidic proton. So size becomes really important whenever you have elements in different periods. And sulfur is the only one that belongs to three. So it has the greatest size out of all of them. So we can say that Methanol structure D will be number one in acidity because sulfur is the biggest atom out of these four atoms. But when we start comparing CN and O, even though they're smaller, they're in the same period. So their size becomes comparable. And we need to think about electron negativity. You may recall that the more electron negative your atom is the more polarized your bond becomes and the more polarized your bond becomes the weaker. It is right because the electron density shifts towards your electron negative atom, you may also remember that electron negativity increases from left to right. Fluorine is one of the most electron negative atoms, right. So we can say that electron negativity increases from left to right. And therefore oxygen will be the most electron negative atom. If oxygen is the most electron negative atom, that means out of these three structures, methanol will be the most acidic. So we give it a rank of two followed by nitrogen because electron negativity decreases from right to left. So our methylamine will be structure three overall and finally, carbon is least electron. So if carbon is the least electron negative, that means it will correspond to the weakest asset because carbon hydrogen bond is least polarized. And therefore, we can conclude that from least acidic to most acidic. We have the following trend starting with four. We first will have ethane C three C three followed by methylamine ch three NH two, followed by methanol ch 30 followed by methane thol. So that would be CH three sh. So we can conclude that ethane will be the least acidic followed by methylamine, methanol and methane will be the most acidic. Once again, we had to consider two factors size and electron negativity because sulfur has a much greater size than any of the atoms within the previous period. That's why size matters is basically impaired at three while the three other atoms are imper at two. So we chose methane to be the most acidic because sulfur has the biggest size. But out of the three remaining structures, we had to consider their electron negativities since their sizes are comparable. So the most electron negative oxygen would produce the most acidic alcohol followed by a and then finally, we get our ALCA ethane, right, which is the least acidic because carbon is the least electron negative. And based on this analysis, we can conclude that the correct answer to the multiple choice question is A B Thank you for watching.

Rank the following compounds from strongest to weakest acid:
CH₃CH₂OH; CH₃CH₂NH₂; CH₃CH₂SH; CH₃CH₂CH₃

Key Concepts

Acidity and pKa

Functional Groups and Their Effects

Inductive Effect and Hybridization

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