# Question 32905

Sep 24, 2016

I can think of at least nine possible structures for tropic acid.

#### Explanation:

First Impressions

$\text{Tropic acid + HBr" → "C"_9"H"_9"O"_2"Br" + "H"_2"O}$

∴ The formula of tropic acid must be ${\text{C"_9"H"_10"O}}_{3}$.

The index of hydrogen deficiency is $\frac{\text{IHD" ="20-10}}{2} = \frac{10}{2} = 5$.

Anytime I see "IHD" ≥ 4#, I think of an aromatic ring. Let's try ${\text{C"_6"H}}_{5}$.

There is probably an $\text{OH}$ group (that was replaced by $\text{Br}$).

From the name tropic acid and a remaining $\text{IHD}$ of 1, I would bet there is a $\text{COOH}$ group.

${\text{C"_6"H"_5 + "OH" + "COOH" = "C"_7"H"_7"O}}_{3}$.

That leaves ${\text{C"_2"H}}_{3}$ ($\text{CH"_2"CH}$ or $\text{CH"_3"C}$) for the rest of the molecule.

Possible Structures

(a) If there is a monosubstituted phenyl

The possible structures are $\text{PhCH"_2"CHOHCOOH}$ or $\text{HOCH"_2"CHPhCOOH}$ or $\text{CH"_3"C(OH)PhCOOH}$.

(b) If there is a disubstituted phenyl

If there is a disubstituted phenyl, the possible structures are the ortho, meta, and para isomers of (1-hydroxyethyl)benzoic acid and (2-hydroxyethyl)benzoic acid.

We don’t have enough information to specify the structure of tropic acid.

The reactions

Assume the structure is $\text{HOCH"_2"CHPhCOOH}$.

(a) Conversion of alcohol to alkyl bromide

(b) Dehydrobromination to atropic acid

(c) Hydrogenation of alkene to form hydratropic acid