Erythrogenic acid, C18 H26 O2, is an acetylenic fatty acid that turns a vivid red on exposure to

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Erythrogenic acid, C18 H26 O2, is an acetylenic fatty acid...

Erythrogenic acid, $\mathrm{C}_{18} \mathrm{H}_{26} \mathrm{O}_{2}$, is an acetylenic fatty acid that turns a vivid red on exposure to light. On catalytic hydrogenation over a palladium catalyst, 5 equivalents of $\mathrm{H}_{2}$ are absorbed, and stearic acid, $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{16} \mathrm{CO}_{2} \mathrm{H},$ is produced. Ozonolysis of erythrogenic acid gives four products: formaldehyde, $\mathrm{CH}_{2} \mathrm{O} ;$ oxalic acid, $\mathrm{HO}_{2} \mathrm{CCO}_{2} \mathrm{H} ;$ azelaic acid, $\mathrm{HO}_{2} \mathrm{C}\left(\mathrm{CH}_{2}\right)_{7} \mathrm{CO}_{2} \mathrm{H} ;$ and the aldehyde acid OHC(CH_) $_{4} \mathrm{CO}_{2} \mathrm{H}$. Draw two possible structures for erythrogenic acid, and suggest a way to tell them apart by carrying out some simple reactions.

Erythrogenic acid, $\mathrm{C}_{18} \mathrm{H}_{26} \mathrm{O}_{2}$, is an acetylenic fatty acid that turns a vivid red on exposure to light. On catalytic hydrogenation over a palladium catalyst,
5 equivalents of $\mathrm{H}_{2}$ are absorbed, and stearic acid, $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{16} \mathrm{CO}_{2} \mathrm{H},$ is produced. Ozonolysis of erythrogenic acid gives four products: formaldehyde, $\mathrm{CH}_{2} \mathrm{O} ;$ oxalic acid, $\mathrm{HO}_{2} \mathrm{CCO}_{2} \mathrm{H} ;$ azelaic acid, $\mathrm{HO}_{2} \mathrm{C}\left(\mathrm{CH}_{2}\right)_{7} \mathrm{CO}_{2} \mathrm{H} ;$ and the aldehyde acid OHC(CH_) $_{4} \mathrm{CO}_{2} \mathrm{H}$. Draw two possible structures for erythrogenic acid, and suggest a way to tell them apart by carrying out some simple reactions.

Key Concepts

Structural Isomerism

Structural isomerism refers to the existence of compounds that have the same molecular formula but different connectivities of atoms. In the context of unsaturated or acetylenic fatty acids, different isomeric forms may arise depending on the positions of double or triple bonds. Differentiating these isomers often requires using reactions like hydrogenation or ozonolysis, which yield distinct products based on the precise arrangement of the atoms in the original molecule.

Ozonolysis

Ozonolysis is a reaction where ozone cleaves carbon–carbon multiple bonds, leading to the formation of smaller molecules such as aldehydes, ketones, or carboxylic acids. Analyzing the breakdown products from ozonolysis allows chemists to deduce the positions and types of unsaturation within the original compound, thereby serving as an important structural diagnostic tool.

Acetylenic Functional Groups

Acetylenic compounds contain carbon–carbon triple bonds, which are reactive sites in organic molecules. These groups can participate in reactions like catalytic hydrogenation, where the triple bonds are reduced to double or single bonds. Their reactivity and presence in specific positions along a fatty acid chain can significantly influence the molecule’s overall chemical behavior, including its photochemical properties.

Catalytic Hydrogenation

Catalytic hydrogenation involves the addition of hydrogen across unsaturated bonds, such as triple or double bonds, using a catalyst like palladium. This reaction not only eliminates unsaturation but also can be used to infer the number and type of unsaturated bonds in a molecule, based on the amount of hydrogen absorbed, making it a key tool in determining molecular structure and saturation levels.

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