Question

Deduce the structure of each compound from the information given. All unknowns in this problem have molecular formula C₈H₁₂.

(a) Upon catalytic hydrogenation, unknown W gives cyclooctane. Ozonolysis of W, followed by reduction with dimethyl sulfide, gives octanedioic acid, HOOC — (CH₂)6 — COOH. Draw the structure of W.

(b) Upon catalytic hydrogenation, unknown X gives cyclooctane. Ozonolysis of X, followed by reduction with dimethyl sulfide, gives two equivalents of butanedial, O=CH — CH₂CH₂—CH = O. Draw the structure of X.

(c) Upon catalytic hydrogenation, unknown Y gives cyclooctane. Ozonolysis of Y, followed by reduction with dimethyl sulfide, gives a three-carbon dialdehyde and a five-carbon dialdehyde. Draw the structure of Y.

(d) Upon catalytic hydrogenation, unknown Z gives cis-bicyclo[4.2.0]octane. Ozonolysis of Z, followed by reduction with dimethyl sulfide, gives a cyclobutane with a three-carbon aldehyde (—CH₂—CH₂—CHO) group on Cl and a one-carbon aldehyde (—CHO) group on C2. Draw the structure of Z.

Answer 1

(a)

First, look at the molecular formula. A structure with eight carbons and 12 hydrogens has 3 elements of unsaturation. Since hydrogenation of this unknown molecule gives cyclooctane, cyclooctyne is a possible match for the unknown starting material. The ring would count as one element of saturation, and the alkyne would account for the other two.

For this to be correct it has to match the other data we were given. We were told that ozonolysis / mild reduction with dimethyl sulfide led to an eight carbon dicarboxylic acid. You know that ozonolysis of an alkyne splits the triple bond down the middle, leaving each carbon fragment with a carboxylic acid group. Treatment of cyclooctyne with ozone / dimethyl sulfide would indeed lead to octanedioic acid. This unknown is most likely cyclooctyne.

(b)

First, look at the molecular formula. A structure with eight carbons and 12 hydrogens has 3 elements of unsaturation. Since hydrogenation of this unknown molecule gives cyclooctane, 1,5-cyclooctadiene is a possible match for the unknown starting material. The ring would count as one element of saturation, and the two alkenes would account for the other two.

For this to be correct it has to match the other data we were given. We were told that ozonolysis / mild reduction with dimethyl sulfide led to two equivalents of butanedial. You know that ozonolysis of an alkene splits the double bond down the middle, leaving each carbon fragment with an aldehyde group. Treatment of 1,5-cyclooctadiene with ozone / dimethyl sulfide would indeed lead to two equivalents of butanedial. This unknown is most likely 1,5-cyclooctadiene.

(c)

First, look at the molecular formula. A structure with eight carbons and 12 hydrogens has 3 elements of unsaturation. Since hydrogenation of this unknown molecule gives cyclooctane, 1,4-cyclooctadiene is a possible match for the unknown starting material. The ring would count as one element of saturation, and the two alkenes would account for the other two.

For this to be correct it has to match the other data we were given. We were told that ozonolysis / mild reduction with dimethyl sulfide led to a three-carbon dialdehyde and a five-carbon dialdehyde. You know that ozonolysis of an alkene splits the double bond down the middle, leaving each carbon fragment with an aldehyde group. Treatment of 1,4-cyclooctadiene with ozone / dimethyl sulfide would indeed lead to two equivalents of butanedial. This unknown is most likely 1,4-cyclooctadiene.

(d)

First, look at the molecular formula. A structure with eight carbons and 12 hydrogens has 3 elements of unsaturation. Since hydrogenation of this unknown molecule leads to a bicyclo[4.2.0]octane means that (since the product has two rings, accounting for two elements of unsaturation), the starting material must be an alkene. We can place the alkene by looking at the additional information we were given (ozonolysis).

You know that reaction of an alkene with ozone gas / dimethyl sulfide leads clips the alkene in half, leaving each carbon fragment with an aldehyde. For there to be a cyclobutane ring substituted with a one-carbon aldehyde along with a three-carbon aldehyde, the alkene would have to be in a specific place in the starting material.

Note that the alkene couldn’t have been placed in the four-membered ring. It would be too strained and would react in the ozonolysis step to give the wrong aldehyde products.