Welcome Guest Search | Active Topics |

Tag as favorite
Alcohol acidity
Sukham_5277
#1 Posted : Saturday, July 11, 2020 5:06:38 PM
Rank: Member

Groups: Registered
Joined: 4/10/2020
Posts: 28

Thanks: 0 times
Was thanked: 0 time(s) in 0 post(s)
Why is methanol more acidic than diols?

Thanks!
INSTR_Molly_59
#2 Posted : Saturday, July 11, 2020 7:08:09 PM
Rank: Member

Groups:
Joined: 7/12/2019
Posts: 28

Thanks: 0 times
Was thanked: 0 time(s) in 0 post(s)
Hi Sukham, did you have a specific question/problem you were attempting?

It depends on what kind of diol; for instance, a methanediol (CH2(OH2)) will be more acidic than methanol.

It really depends on the nature of the molecule. Please let me know what you were referring to so I can better assist you.
Sukham_5277
#3 Posted : Sunday, July 12, 2020 5:03:14 PM
Rank: Member

Groups: Registered
Joined: 4/10/2020
Posts: 28

Thanks: 0 times
Was thanked: 0 time(s) in 0 post(s)
Hi,
Sorry I was referring to a 1,3-propanediol.
Thanks!
INSTR_Molly_59
#4 Posted : Sunday, July 12, 2020 9:29:35 PM
Rank: Member

Groups:
Joined: 7/12/2019
Posts: 28

Thanks: 0 times
Was thanked: 0 time(s) in 0 post(s)
Yeah totally! Lets take a look.

When looking at alcohols, we get an interesting situation, since we know that oxygen can act as both an electron donator while it also inductively withdraws electrons due to EN. When looking at acids, the strength of the acid is dependent on the stability of the conjugate base. In short alkyl chains like 1,3-propanediol, the electron donating effect of the second alcohol's oxygen atom takes over to add more electron density to the negative charge of the alcohol conjugate base, making 1,3-propanediol a more acidic molecule.

Thats the short of it. It can get a lot more complicated if you are considering alcohol trends. Realistically, this is not likely to come up on the MCAT, unless they provide this information in a passage some how.

In looking at trends of diols, because oxygen acts as both an electron donator and also inductively withdraws electrons, the effects are a bit all over the place when it comes to short alkyl chains with, ie. methanediol's dual EN inductive effect taking over to make the molecule more acidic than methanol. The electron donating effect of diols start tappering off when you get to longer chains, when the inductive effect finally takes over. There's also more crazy stuff involved too like intermolecular bonding, hydrogen bonding forces, etc. so its not really worth getting into the extreme details of this.

You can see the weird trend by looking at the pKa values if you are curious (remembering that the lower pKa means more acidic). Remember that this is NOT really important to know for the MCAT, its just for pure interests sake:
Methanediol - 13.47
1,2-ethanediol - 15.1
1,3-propanediol - 15.6
1,4-butanediol - 14.5 <-- This is when inductive withdrawing effect of oxygen takes over to make it a much better acid!
1,5-pentanediol - 14.83
1,6-hexanediol - 14.87
1,7-heptanediol - 14.89 <-- You can see the induction wearing off as the distance grows greater!

Hope this helps/has been interesting! :) Cheers, and happy studying!
Sukham_5277
#5 Posted : Monday, July 13, 2020 1:03:29 AM
Rank: Member

Groups: Registered
Joined: 4/10/2020
Posts: 28

Thanks: 0 times
Was thanked: 0 time(s) in 0 post(s)
Thank you so much! That was very helpful.
Users browsing this topic
Guest (3)
Tag as favorite
You cannot post new topics in this forum.
You cannot reply to topics in this forum.
You cannot delete your posts in this forum.
You cannot edit your posts in this forum.
You cannot create polls in this forum.
You cannot vote in polls in this forum.

Clean Slate theme by Jaben Cargman (Tiny Gecko)
Powered by YAF | YAF © 2003-2009, Yet Another Forum.NET
This page was generated in 0.091 seconds.