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Reaction of Acetone with Water

Acetone in water can react in three ways:

posssible reactions of acetone in water

The different reactions change the concentrations of $\ce{H+}$ and, consequently the pH, differently. My question is what is the pH of acetone in water? Is it greater than (basic), less than (acidic), or equal to (neutral) seven.

Context

Labeled Oxygen Exchange

In case of labeled water, exchange does occur. Since I couldn't find a source for this, I have outlined three pathways, that I could think of, for the same.

Neutral Pathways for Exchange

neutral exchange pathways for acetone in labelled water

Since the pathways contains either both acidic and basic or only neutral intermediates, acetone in water should be, more or less, neutral ($\mathrm{pH} \approx 7$).

Basic Pathway for Exchange

Exchange can also occur via another pathway.

basic exchange pathway for acetone in labelled water

Only basic intermediates are involved in this pathway suggesting acetone in water should be slightly basic ($\mathrm{pH} > 7$).

Acidic, Basic, or Neutral?

Since, in the three pathways, I could come up with were either neutral or basic solutions, I am thinking acetone in water should be either neutral ($\mathrm{pH} = 7$) or basic ($\mathrm{pH} > 7$). However, going back to the first figure, I cannot discard the possibility of an acidic ($\mathrm{pH} < 7$) solution.

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    $\begingroup$ Is water in water "Acidic, Basic, or Neutral?" Same thing. You should reword it to ask about specific equilibrium constants - hydration, pKa of diol and of protonated aceton, instead of something as vague as "neutral". $\endgroup$
    – Mithoron
    Jun 6 at 0:32
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    $\begingroup$ @Mithoron water is known to have equal concentrations of $\ce{H+}$ and $\ce{HO-}$ which is why we call it neutral and, having a pH of 7 (at Room Temp.), we have decided to make that the standard for neutrality. Since water can react with both acidic and basic moieties, we call it amphoteric. So, water is an amphoteric neutral moiety. It is very common for ketones to show nucleophilic addition at $\alpha$-carbon, and electrophilic addition at $\ce{O}$, I am asking which reaction is favored, and if we measure the pH of a solution of acetone and water, would it be greater or less than seven. $\endgroup$
    – ananta
    Jun 6 at 0:42
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    $\begingroup$ "we have decided to make that the standard for neutrality", "we call it amphoteric" who is "we"? Water is just both acidic and basic, like virtually everything. If you want to edit according to my pointers than rethink and edit it. If not than my downvote stays. That would be all. $\endgroup$
    – Mithoron
    Jun 6 at 1:13
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    $\begingroup$ Adding a cosolvent changes equilibrium constants so a substance that is neither acid nor base still can change the pH of a solution: pubs.rsc.org/en/content/articlepdf/1994/p2/… $\endgroup$
    – Karsten ♦
    Jun 6 at 1:15

2 Answers 2

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The problem stated in the question can be reduced to a problem of labeling something as acidic, basic or neutral. All human devised labels break down under one condition or the other. This is why there are plenty of definitions of acids and bases.

Concurrently, then people assume that acid means the pH is < 7 and neutral means pH=7, and base means pH>7 in water. This label also breaks down. As mentioned in a recent question, atoms, molecules, solutions do not care what humans call them or how people label them hydrogen isotopes and Bronsted-Lowry , they will behave the way they have been designed to. Recall the ambiguous issue of labelling chemical vs. physical changes.

  1. Now, if we were to dissect your thesis, conventional pH is strictly defined for aqueous solutions of substances which partially or completely ionize in water and maintain this equilibrium. Acetone, certainly does not ionize in water in a traditional electrolyte sense (no molecular rearrangement). So the question of acetone being acidic, neutral or basic in water does not arise. Alternatively, one better refine the thought process to asking if I add a small amount of acetone or any other water miscible ketone (without another functional group) to water, will the aqueous pH change if we measure it with a pH probe (glass electrode)? Even then, electrochemical purists will raise their eyebrows because acetone is not a conventional electrolyte.

  2. Next issue with this thought process is what percentage of acetone are we talking about 1% mole ratio, 60% mole ratio acetone:water? In that case, the former solution is aqueous, and the latter the solution is more organic. The experimental measurement of pH and its interpretation becomes murky in organic solvents.

If you are solely thinking about isotope exchange, in that case, the total mass balance of free protons will not change, and the bulk solution pH of acetone (again measured with a glass electrode) should remain the same. In short, if you add a small amount of acetone in water, the pH will not change measurably.

If you are more interested, search how the pKa of acetone or any other ketone is measured in water. The experiment would certainly be interesting and worth knowing.

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    $\begingroup$ One can take acetone with pKa about 19 as any other very weak acid. The fact it is solvent may not be relevant, we consider diluted solutions as for any other substances where properties of the solvent are assumed not to be altered. $\endgroup$
    – Poutnik
    Jun 6 at 5:45
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    $\begingroup$ pKa of protonated acetone is reportedly -7.5 and I remember ketone protonization in context of superacids. $\endgroup$
    – Poutnik
    Jun 6 at 6:18
  • $\begingroup$ I am actually curious how do people experimentally determine pKa of extremely weak acids or bases let us say acetone with a pKa of 19 in water! This must be either thermodynamic measurements or spectroscopic calculations. Yes, indeed superacids can protonate almost everything! $\endgroup$
    – AChem
    Jun 6 at 6:24
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The exchange with H2O[18] is either acid or base catalyzed because of the amphoteric nature of the carbonyl group. You will have to research the rates in pure water-acetone mixes. Your diagrams seem to be confusing you as much as they are confusing me.

Acetone in water is a less polar solvent. Two simple reasons [to me at least] First: the molecule exists in the keto not the enol form, no very polar and acidic OH bond; and Second: the addition of acetone lowers the water activity in the solution. This means any mechanism involving ionic intermediates such as ketal or enol formation should be slower in pure water-acetone solution.

In water: H2O = H+ + OH-. Keq[H2O] = [H+][OH-]. If Keq remains constant [this requires research], as the water concentration is lowered the H3O+ concentration will be lower. The response of the glass electrode could be tested by addition of measured amounts of strong acid or base in the solvent mixture.

My prediction: pH will increase with additions of acetone. Mettlar-Toledo has a white paper that tries to explain the use of glass pH electrodes in homogeneous organic-water solutions. It is simplistic and empirical. They assign the effects to changes in the ion activities not the change in water activity and discuss many technical caveats. It is difficult to make accurate absolute measurements in such solutions.

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