March 29, 2024, 10:11:22 AM
Forum Rules: Read This Before Posting


Topic: This Order of acidity is soo confusing .  (Read 1698 times)

0 Members and 1 Guest are viewing this topic.

Offline AdiDex

  • Full Member
  • ****
  • Posts: 305
  • Mole Snacks: +16/-12
  • Gender: Male
This Order of acidity is soo confusing .
« on: September 27, 2016, 10:00:46 AM »
I am facing this question from quite long time (Since 2 year ;D ) , Got different Explanations from different teacher . But none of them satisfied me .
I will reply the explanations later which I got from different teachers .
So observation is , The order of acidity is  Methanol>Water>Ethanol .

What the basic knowledge says , the order should be Water>Methanol>Ethanol , Due to inductive effect .
So first Thought came to in my mind -:
there should be some solvent effect , like in their "conjugated base form" the solvation is almost equal since basically you have to solvate O- .
Solvation of Water will be highest , since it has 2 partially positively proton (Small in size , High charge density , better solvation) this make the ΔG reaction more positive , since reactant side is getting stabilized .

Whereas in case of Methanol,Ethanol the solvation will be poorer . So Inductive effect and Solvation have just opposite effect on acidity . So In case of Methanol it most balanced , So It has Highest acidity .

But I got confused again when I saw the data of Proton Affinity (in KJ/mol) .
Hydroxide (1635) > Methoxide (1587) > Ethoxide (1574)
So hydroxide in gaseous phase is best base , so poorest base .  ??? ???
Just opposite of Inductive effect . (Note: I've read that It is assumed that alkyl group has +I effect , The results are not clear when alkyl is connected to a saturated atom )

Also Second thing which made me double confused that
In gaseous phase the the proton affinity is
Water<Methanol<Ethanol< Propanol

As predicted ,bigger alkyl have more powerful +I .

So whats going on ?? Is there any connection with Electron affinity of Oxygen ?? As Electron affinity of O- is negative and O (neutral) is positive (at the end this is coming in my mind) ??

Offline Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Posts: 5592
  • Mole Snacks: +319/-22
Re: This Order of acidity is soo confusing .
« Reply #1 on: September 27, 2016, 11:47:26 AM »
The order of acidity in the gas phase can be explained by picturing the negative charge on oxygen being slightly disbursed onto the carbon atom(s) of the alcohol.  The more carbon atoms, the better the dispersal.  If this explanation is correct, then the conjugate base of isopropanol should be slightly lower in proton affinity than the conjugate base of ethanol.  I am not sure whether or not this is the generally accepted explanation, but I do recall reading it somewhere.  Certainly the effect of solvation must also be considered.

Offline AdiDex

  • Full Member
  • ****
  • Posts: 305
  • Mole Snacks: +16/-12
  • Gender: Male
Re: This Order of acidity is soo confusing .
« Reply #2 on: September 29, 2016, 12:26:10 AM »
So basically it's a type of -I effect ...!! Is there any other similar cases where R- groups shows -I type of effect ?

Carbon has greater Electrons than hydrogen so they can donate more electrons than hydrogen , In case of Carbocations ..!! But in case of MEthoxide , they are providing more surface area  so its kind of -I Effect , Since they are somehow dispersing the negative charge on themselves .

Then why Methyl anion is less basic than ethane ??

Sponsored Links