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Chapter 34: Problem 781

Tropinic acid, \(\mathrm{C}_{8} \mathrm{H}_{13} \mathrm{O}_{4} \mathrm{~N}\), is a degradation product of atropine, an alkaloid of the deadly nightshade, Atropa belladonna. It has a neutralization equivalent of \(94 \pm 1 .\) It does not react with benzenesulfonyl chloride, cold dilute \(\mathrm{KMnO}_{4}\), or \(\mathrm{Br}_{2} / \mathrm{CCl}_{4} .\) Exhaustive methylation gives the following results: tropinic acid \(+\mathrm{CH}_{3} \mathrm{I} \rightarrow \mathrm{EEE}\left(\mathrm{C}_{9} \mathrm{H}_{16} \mathrm{O}_{4} \mathrm{NI}\right)\) \(\mathrm{EEE}+\mathrm{Ag}_{2} \mathrm{O}\), then strong heat \(\rightarrow \mathrm{FFF}\left(\mathrm{C}_{9} \mathrm{H}_{15} \mathrm{O}_{4} \mathrm{~N}\right)+\mathrm{H}_{2} \mathrm{O}\) \(+\mathrm{AgIFFF}+\mathrm{CH}_{3} \mathrm{I} \rightarrow \mathrm{GGG}\left(\mathrm{C}_{10} \mathrm{H}_{18} \mathrm{O}_{4} \mathrm{NI}\right)\) \(\mathrm{GGG}+\operatorname{Ag} \mathrm{O}\), then strong heat \(\rightarrow \mathrm{HHH}\left(\mathrm{C}_{7} \mathrm{H}_{8} \mathrm{O}_{4}\right)+\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}\) \(+\mathrm{H}_{2} \mathrm{O}+\mathrm{AgI}\) \(\mathrm{HHH}+\mathrm{H}_{2}, \mathrm{Ni} \rightarrow\) heptanedioic acid (pimelic acid) (a) What structures are likely for tropinic acid? (b) Tropinic acid is formed by oxidation with \(\mathrm{CrO}_{3}\) of tropinone, whose structure has been shown by synthesis to be Now what is the most likely structure for tropinic acid?

Short Answer

Expert verified
Based on the given reactions and information, the most likely structure for tropinic acid is a carboxylic acid with a six-membered ring containing a nitrogen atom and one ether group. The molecular weight of tropinic acid is approximately 185, and it doesn't have alkene or multiple aromatic rings. The plausible structure for tropinic acid is: \[\text{(b)} \hspace{1cm}\]

Step by step solution

01

Determine the functional groups in tropinic acid

The given reactions will provide some important clues regarding the functional groups present in the tropinic acid. 1. Tropinic acid doesn't react with benzenesulfonyl chloride, which suggests that it doesn't have primary or secondary amines. 2. It doesn't react with cold dilute \(\mathrm{KMnO}_{4}\), which suggests that tropinic acid doesn't have alkene groups, nor any activated benzylic hydrogen. 3. It doesn't react with \(\mathrm{Br}_{2}/\mathrm{CCl}_{4}\), which indicates that tropinic acid doesn't have alkene or activated/multiple aromatic rings. 4. Exhaustive methylation hints that there might be an oxygen-containing functional group in tropinic acid since methyl iodide is used for methylation reactions. Based on neutralization equivalent of \(94 \pm 1,\) we can also calculate the molecular weight of tropinic acid: \[\mathrm{C}_{8} \mathrm{H}_{13} \mathrm{O}_{4} \mathrm{~N} = 8 \times 12.01 + 13 \times 1.01 + 4 \times 16.00 + 14.01 \approx 185\] Thus, tropinic acid likely has a carboxylic acid functional group.
02

Propose a likely structure for tropinic acid

Based on the analysis, tropinic acid should have a carboxylic acid group and an oxygen-containing functional group, such as an ether. There should also be no olefinic or multiple aromatic rings. A possible basic structure for tropinic acid is: (Note that there might be multiple condensed structures, and we will narrow it down in the next step.)
03

Modify the tropinic acid structure after discovering tropinone's structure

Given that tropinic acid is formed by oxidation of tropinone, the structure of tropinone is: This structure has a ketone functional group that can be easily oxidized to a carboxylic acid. We now know that the tropinic acid has to have a six-membered ring with a nitrogen atom, and one or two bridged oxygen atoms (ether group). Considering all the information, the plausible structures for tropinic acid are: (a) (b) However, since tropinic acid can undergo exhaustive methylation, there should only be one ether group present. Thus, the most likely structure for tropinic acid is structure (b).

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Most popular questions from this chapter

The sulfonation of p-cymene (1-methyl-4-isopropylbenzene) gives the 2 -sulfonic acid. Is this the expected orientation? Explain. Use this fact to synthesize carvacrol, 2-methyl-5-isopropylphenol, from p-cymene. Carvacrol is found in the essential oils from thyme, marjoram, and summer savory. It has a pleasant thymol-like odor.

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