6
$\begingroup$

This was a question in a chemistry exam:

How many moles of hydrogen is required to convert 1 mole of this compound into a saturated compound?
$\ce{CH3COCH3}$ (acetone)

Some teachers said that it's saturated so the answer is zero. Others said that one mole is required to break the bond between $\ce{C}$ and $\ce{O}$ and convert it into an alcohol.

So which answer is correct and please mention a reference if available.

$\endgroup$
  • 1
    $\begingroup$ I do not know any official definition of the term. If you define a saturated compound as having only single bonds (whatever that means) there would not be any saturated fatty acids, what contradicts the normal use of the word. So I guess the answer to your question depends on the definition that was given in your class. $\endgroup$ – aventurin Jun 19 '16 at 23:00
  • 1
    $\begingroup$ Willfully "defining" your own chemical nomenclature does not help. The question in this exam is badly posed, and your teachers are afraid to admit that. $\endgroup$ – Karl Jun 19 '16 at 23:22
9
$\begingroup$

Typcially, the terms saturated and unsaturated only apply to the carbon chain in itself. A saturated compound would be one whose systematic name ends in -ane while unsaturated ones would end in -ene or -yne — grossly simplifying. Going by this rather simplistically strict definition, at first sight acetone (propanone) is saturated. But.

You may have heard of the keto-enol tautomery. That is, a ketone is always in equilibrium with its corresponding enol form, for acetone that would be propen-2-ol — an unsaturated compound. It is shown in the scheme below. The first indication of the distinction being difficult.

Keto-enol tautomery of acetone

Furthermore, acetone can be reduced by molecular hydrogen to isopropanol. That means that given the correct conditions, it can basically react much like an unsaturated compound would. It also reacts with elemental bromine similarly to how an unsaturated hydrocarbon would, although only the first mechanistic step is identical; only one carbon is brominated in the product.

There is also the measure of double bond equivalents, that is often used in the process of structure elucitaion. Taking the molecular formula of a compound (which can be deduced well from high-resolution mass spectrometry) and performing a simple calculation allows one to arrive at a number of double bond equivalents for a compound. The formula is:

$$\tag{1} \text{DBE} = \frac{2 \ce{C} - \ce{H} + \ce{N} + 1}{2}$$

Where $\ce{C}$ is the number of carbon atoms (including other tetravalent atoms such as silicon), $\ce{H}$ is the number of hydrogen atoms (including other monovalent atoms such as chlorine) and $\ce{N}$ is the number of nitrogen atoms (including other trivalent atoms such as phosphorus).

However, having 1 double bond equivalent or more does not automatically mean that a molecule is unsaturated, no matter by which definition. Take cyclohexane, $\ce{C6H12}$: It has one double bond equivalent which is caused by the fact that it is cyclic. It will not react with hydrogen or with bromine and no sane chemist would call it unsaturated. So this measure is not of much help.


Concluding, I can only say that both sides of the discussion are wrong. It is wrong that acetone will not react with hydrogen to give isopropanol. It is equally wrong that that makes acetone an unsaturated compound. The concept of unsaturation simply does not extend well to $\ce{C=O}$ double bonds.

The exam question should not be graded.

$\endgroup$
  • $\begingroup$ It seems pretty clear to me that acetone is unsaturated. The question for me is whether isopropanol is unsaturated. $\endgroup$ – jerepierre Jun 20 '16 at 22:54
  • $\begingroup$ I like the example of cyclohexane. Think about the reverse process. Starting from hexane, how do you make cyclohexane? By removing H2, i.e. oxidation. So by that definition, cyclohexane is unsaturated. This is reflected in the DBE calculation, often called 'degrees of unsaturation'. $\endgroup$ – jerepierre Jun 20 '16 at 22:58
  • $\begingroup$ I definitely agree that it's a poorly worded exam question and shouldn't be graded. $\endgroup$ – jerepierre Jun 20 '16 at 22:59
  • 1
    $\begingroup$ @jerepierre You're free to invent your own chemistry, but the established textbook OC nomenclature thinks otherwise. $\endgroup$ – Karl Jun 20 '16 at 23:45
2
$\begingroup$

The degrees of unsaturation for a hydrocarbon of formula $\mathrm{C_xH_y}$ is equal to $\dfrac{(2x+2-y)}{2}$. The addition of an oxygen atom does not change the number of degrees of unsaturation, so acetone has 1 degree of unsaturation. This is confirmed by looking at acetone's structure, which contains a double bond between the carbon and the oxygen. Reacting acetone with $\ce{H2}$ will result in 2-propanol according to the reduction reaction found here, and thus, 1 mole of hydrogen should be required to convert acetone into a saturated compound.

While acetone is not a saturated compound according to the aforementioned criteria found from this source, the degree of unsaturation comes from the carbon-oxygen bond, meaning that within some contexts, it is considered a saturated compound. According to the wikipedia article on saturated vs. unsaturated compounds, unsaturation only counts when it is the result of double or triple carbon-carbon bonds. All of the carbon-carbon bonds on the compound in acetone are single, implying that the compound is saturated according to these criteria.

Either way, this is a poorly written question, and either answer should be considered acceptable.

$\endgroup$
  • 1
    $\begingroup$ Acetone is not a hydrocarbon. Saying that it is "unsaturated" is pointless, you are, as are the OPs teachers, misapplying the term. $\endgroup$ – Karl Jun 19 '16 at 23:20
  • 1
    $\begingroup$ I think it depends on the context. Check out this link. Given the way the problem is worded, the original question seems to be asking about how many moles of H2 will a mole of acetone react with. Regardless, the original question is ill defined, so either answer should be right. $\endgroup$ – Niels Kornerup Jun 19 '16 at 23:26
  • 1
    $\begingroup$ Also, Acetone can tautomerize to make an unsaturated compound. $\endgroup$ – Niels Kornerup Jun 20 '16 at 19:35
  • 1
    $\begingroup$ But it will react with the H2. Even though it is a minor contributor, it is a source of unsaturation. $\endgroup$ – Niels Kornerup Jun 20 '16 at 23:48
  • 1
    $\begingroup$ I am just saying that a solution of acetone contains some unsaturation on a carbon-carbon bond. A large number of carbonyl reactions occur due to the enolate tautomer. One example can be found here $\endgroup$ – Niels Kornerup Jun 21 '16 at 0:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.