Question

The compound indene, \(\mathrm{C}_{9} \mathrm{H}_{8}\), found in coal tar, rapidly decolorizes \(\mathrm{Br}_{2} / \mathrm{CCl}_{4}\) and dilute \(\mathrm{KMnO}_{4}\). Only one mole of hydrogen is absorbed readily to form indane, \(\mathrm{C}_{9} \mathrm{H}_{10}\). More vigorous hydrogenation yields a compound of formula \(\mathrm{C}_{9} \mathrm{H}_{16} .\) Vigorous oxidation of indene yields phthalic acid. What is the structure of indene? Of indane?

Short answer

The structure of indene consists of a benzene ring fused with a five-membered ring containing one double bond, as shown here: \( \chemfig{*6(=-=(-*5(-=-))=-=)} \). On the other hand, the structure of indane is formed by hydrogenating the double bond outside the benzene ring in indene, resulting in this structure: \( \chemfig{*6(=-=(-*5(-(-)=))=-=)} \).

Step-by-step solution

Determine the degree of unsaturation for indene

First, we need to figure out the degree of unsaturation (DU) in indene (C9H8) using the following formula: DU = [(2 × Number of Carbons) + 2 - Number of hydrogens]/2 DU = [(2 × 9) + 2 - 8]/2 = 10/2 = 5 Thus, there are 5 degrees of unsaturation in indene.

Analyze the reactions involving indene

Indene decolorizes Br₂/CCl₄ and dilute KMnO₄, which suggests that it contains a double bond or a ring structure. The formation of phthalic acid upon vigorous oxidation also implies the presence of a benzene ring. Additionally, it absorbs one mole of hydrogen to form indane, meaning that there is at least one double bond outside the benzene ring. Considering this information, we can deduce that indene must have a benzene ring and one additional double bond outside the ring.

Determine the structure of indene

Since there are 9 carbon atoms in indene and we have concluded that a benzene ring is present, there must be 3 remaining carbon atoms to accommodate. To connect these carbon atoms to the benzene ring and include the extra double bond, an adequate structure would be the fusion of a five-membered ring with a benzene ring. Thus, the structure of indene is: 

Determine the structure of indane

Since indane has the molecular formula C9H10 and is formed by the addition of one mole of hydrogen to indene, the hydrogenation must occur at the double bond outside the benzene ring. Once the double bond is hydrogenated, the structure of indane would be:  In conclusion, we have determined the structures of indene and indane by analyzing the given reactions and information. The structure of indene consists of a benzene ring fused with a five-membered ring containing one double bond, while the structure of indane is formed by hydrogenating the double bond outside the benzene ring in indene.

Key concepts

Degree of Unsaturation

The concept of the degree of unsaturation is a fundamental aspect of organic chemistry that helps us understand the complexity of a molecule's structure. To calculate the degree of unsaturation (DU), we use a simple formula:
du \text{DU} = \frac{ (2 \times n_{\text{C}}) + 2 - n_{\text{H}}}{2}.
Where \(n_{\text{C}}\) is the number of carbons and \(n_{\text{H}}\) is the number of hydrogens in the molecular formula. Each degree of unsaturation represents a potential ring or double bond in the molecule. For Indene, with a formula of \(\text{C}_9\text{H}_8\), we have a DU of 5 suggesting the presence of a combination of five rings and/or double bonds.

Benzene Ring

A benzene ring is one of the key features in aromatic hydrocarbons; it consists of six carbon atoms linked together in a planar hexagonal ring, with alternating single and double bonds. This unique structure confers upon benzene, and compounds containing benzene rings, a high degree of chemical stability and significant resistance to hydrogenation reactions. In our example, the rapid decolorization of bromine and potassium permanganate by indene hints at the presence of a benzene ring because such reactions often occur with compounds containing areas of high electron density, like those in benzene rings.

Hydrogenation Reactions

Hydrogenation reactions involve the addition of hydrogen (H2) to unsaturated organic compounds—typically alkenes and alkynes—and are widely used in chemistry to reduce or saturate a molecule. In indene \(\text{C}_9\text{H}_8\), hydrogenation occurs readily with the absorption of one mole of hydrogen to form indane \(\text{C}_9\text{H}_{10}\). This signals that indene contains reactive sites that are accessible to hydrogen, specifically the non-aromatic double bond, which saturates to form the indane structure upon hydrogenation.

Aromatic Hydrocarbons

Aromatic hydrocarbons, like indene, have a unique chemical behavior due to the stability provided by their aromatic rings. These compounds contain one or more benzene rings and exhibit distinct reactions, including reluctance to undergo hydrogenation and a tendency to preserve their aromaticity. Aromatic hydrocarbons are also prone to electrophilic substitution reactions rather than the addition reactions typical of alkenes. The formation of phthalic acid upon vigorous oxidation of indene supports the presence of a benzene ring in its structure, which is characteristic of many aromatic hydrocarbons.