[Borek]

I am a little bit late today, so this is not my original question, but a borrowed one (I will give source next week). Besides, we will venture into a simple organic chemistry for a change.

Achiral organic compound A has a molar mass of 100 g/mol. From other trials we know that the compound is free of geometric isomerism and doesn't contain quaternary carbon. Complete combustion of 100 mg of the compound gives 372 mg of mixture of H₂O and CO₂. Gentle oxidation of the compound A with potassium dichromate gives compound B with molar mass of 98. Compound A doesn't react with bromine dissolved in carbon tetrachloride.

Give structural formulas of both compounds A and B.

[Dan]

Nice little problem. I got it, but will leave it open.

[XGen]

Here is what I have; please tell me if any steps have any error in reasoning.

The molar mass of CO₂ is 44 g/mol and the molar mass of H₂O is 18 g/mol.

We can then say that the sum of the mass of the products can be represented with the expression of 18x + 44y = 372. Solving for integer solutions, the only integer solutions for the pair (x, y) is (6, 6). Therefore, the products side of the equation has the form 6CO₂ + 6 H₂O, meaning that Compound A has 6 carbons and 12 hydrogens at the very least.

Two pieces of information verify my next step; in order for Compound A to be oxidized and lose 2 grams from its molar mass to form Compound B, two hydrogens must be lost. This can be done in the oxidation of an alcohol to either a ketone or an aldehyde. Therefore, there must be at least one oxygen.

One oxygen, with a molar mass of 16g, sums with 6 carbons (6 x 12 = 72) and 12 hydrogens (12 x 1 = 12) to 100g.

After determining that the formula of Compound A is C₆H₁₂O, the next step is to determine the structure. Assuming that there will be a hydroxide group to create the alcohol, effectively the formula is C₆H₁₂ in terms of structure. This means that the structure must be an alkene or a cycloalkane. If it is an achiral compound and does not react to Br₂, then it can not be an alkene. Therefore, it must be a cycloalkane. If it is a cycloalkane, then there are a few options for its structure. However, only cyclohexanol fits the bill.

The structural formula for Compound A is (CH₂)₅CHOH, and the structural formula for Compound B is (CH₂)₅CO.

[Borek]

The structural formula for Compound A is (CH₂)₅CHOH, and the structural formula for Compound B is (CH₂)₅CO.

These are not structural formulas, and it would be better if you would give names of both compounds, but you are right. A is cyclohexanol and B is cyclohexanon.

Question taken from the first stage of the Polish 45^th Chemistry Olympiad.

[XGen]

I apologize, I seem to have misread D:

[Yggdrasil]

Couldn't cyclopentylmethanol/cyclopentylmethanal also work?  Since you specify gentle permanganate oxidation, that could mean that the primary alcohol cyclopentylmethanol is oxidized only to the aldehyde and not to the acid.

[Borek]

For the record: dichromate, not permanganate.

Original answer to the question states that gentle oxidation of the primary alcohol would end with an acid, so it has to be a secondary alcohol. Could be it depends on the meaning of "gentle".

[XGen]

Original answer to the question states that gentle oxidation of the primary alcohol would end with an acid, so it has to be a secondary alcohol. Could be it depends on the meaning of "gentle".

With which piece of information can you conclude that gentle oxidation would end up with an acid?

Also, what is the use of bromine dissolved in carbon tetrachloride?

[Borek]

With which piece of information can you conclude that gentle oxidation would end up with an acid?

As I mentioned earlier - it may depend on the meaning of "gentle". My organic chemistry is pretty rusty, so I would prefer someone else to chime in, but as far as I remember oxidation of aldehyde to acid is not harder than the oxidation of the alcohol to aldehyde, so if the first step is observed, second would most likely follow in the same conditions. Secondary alcohol would not get oxidized further than to ketone - unless we are talking about brute force oxidation that ends just with water and carbon dioxide.

Also, what is the use of bromine dissolved in carbon tetrachloride?

Double bonds.