Question

Compound \(\mathrm{A}\left(\mathrm{C}_{9} \mathrm{H}_{12}\right)\) shows prominent peaks in its mass spectrum at \(\mathrm{m} / \mathrm{z} 120\) and 105. Compound \(\mathrm{B}\) (also \(\mathrm{C}_{9} \mathrm{H}_{12}\) ) shows prominent peaks at \(m / z 120\) and 91 . On vigorous oxidation with chromic acid, both compounds give benzoic acid. From this information, deduce the structural formulas of compounds A and B.

Short answer

Answer: Compound A has the structural formula of para-Methyl Toluene or para-Xylene (\(\displaystyle \mathrm{C}_{6} \mathrm{H}_{4}( \mathrm{H}\mathrm{C})\left(\mathrm{CH}_{3}\right)_{2}\)), and compound B has the structural formula of ortho-Ethyl Toluene (\(\displaystyle \mathrm{C}_{6} \mathrm{H}_{4}\left(\mathrm{CH}_{3}\right)\left(\mathrm{CH}_{2} \mathrm{CH}_{3}\right)\)).

Step-by-step solution

Deduce possible formulas based on mass spectral data

Prominent peak at m/z 120 is due to molecular ion or parent peak, so the molecular weight of both compounds A and B is 120. Given that both compounds have molecular formula C9H12, we can proceed further. Compound A displays a prominent peak at m/z 105, which is a difference of 15 units from the parent peak. Most likely, this difference corresponds to the loss of a methyl (CH3) group. Compound B shows a prominent peak at m/z 91, that is a difference of 29 units from the parent peak. This difference likely corresponds to the loss of an ethyl (C2H5) group.

Consider oxidation product - benzoic acid

We know that both compounds A and B, on vigorous oxidation with chromic acid, give benzoic acid. Benzoic acid has the molecular formula C7H6O2. This tells us that our original compounds A and B should have two additional carbons when compared to benzoic acid, with one of them likely derived from the CH3 or C2H5 group losses observed in the mass spectrum.

Deduce the structure of compound A

Combining the information from Steps 1 and 2, we can deduce that compound A is a derivative of benzoic acid with an extra methyl group and one extra carbon. Adding this extra carbon to the benzoic acid structure as a side chain and methyl group at the end gives the structure for toluene which is C7H8. With a methyl group at the para-position (opposite to the methyl group in toluene), the structure will be C9H12. Thus, the structure of compound A is para-Methyl Toluene or para-Xylene: para-Xylene (A): \(\displaystyle \mathrm{C}_{6} \mathrm{H}_{4}( \mathrm{H}\mathrm{C})\left(\mathrm{CH}_{3}\right)_{2}\)

Deduce the structure of compound B

Following a similar approach as compound A, compound B is a derivative of benzoic acid, but with an extra ethyl group instead of a methyl group. Also, we will add an extra carbon to the benzene ring. One possibility is to add ethyl to a methylbenzene (Toluene) at the ortho position (next to the methyl group). This gives us the structure: ortho-Ethyl Toluene (B): \(\displaystyle \mathrm{C}_{6} \mathrm{H}_{4}\left(\mathrm{CH}_{3}\right)\left(\mathrm{CH}_{2} \mathrm{CH}_{3}\right)\) Both para-Xylene (A) and ortho-Ethyl Toluene (B) have a molecular formula of C9H12 and exhibit the correct loss of CH3 and C2H5 groups in their mass spectra. They also produce benzoic acid after vigorous oxidation with chromic acid, as required by the given information.

Key concepts

Structural Analysis

Understanding structural analysis is essential in determining the molecular composition of compounds using mass spectrometry. This technique allows us to identify different fragments of a compound based on their masses.

In the exercise, both compound A and B have a molecular weight reflected by their mass spectrometric peak at \( m/z 120 \). This corresponds to the entire molecule, termed the 'parent ion' or molecular ion.

Fragmentation is the process involved in structural analysis. When molecules break down into smaller pieces during ionization, they create smaller peaks like those of \( m/z 105 \) and \( m/z 91 \) for compounds A and B, respectively.

By understanding these patterns, we can predict possible molecular structures, such as identifying which functional groups or side chains have been lost.

• Peak at \( m/z 105 \) for compound A suggests loss of a methyl group \((\text{CH}_3)\).
• Peak at \( m/z 91 \) for compound B indicates the loss of an ethyl group \((\text{C}_2\text{H}_5)\).

Organic Compounds

Organic compounds are the building blocks of chemistry that primarily consist of carbon atoms. These compounds often include hydrogen, oxygen, and nitrogen atoms, among others.

Contextually, compounds A and B both have the molecular formula \( \text{C}_9\text{H}_{12} \), categorizing them under aromatic hydrocarbons since they produce benzoic acid upon oxidation.

• Aromatic hydrocarbons, like benzene and toluene, have a unique structural cyclic arrangement of carbon bonds, offering stability and distinct chemical properties.
• The molecular formula \( \text{C}_9\text{H}_{12} \) suggests these compounds may have additional alkyl groups like methyl \((\text{CH}_3)\) or ethyl \((\text{C}_2\text{H}_5)\) attached to a benzene ring structure.

Understanding the composition of these organic compounds in their unaltered state aids in predicting their behavior in reactions, such as oxidation with chromic acid.

Chromic Acid Oxidation

Chromic acid oxidation is a robust chemical reaction widely used for converting alcohols and other functional groups into carboxylic acids. This is a well-recognized process in organic chemistry.

In our exercise, both compounds A and B undergo vigorous chromic acid oxidation to produce benzoic acid.

• Benzoic acid has the molecular formula \( \text{C}_7\text{H}_6\text{O}_2 \), which reflects the aromatic nature of the resultant compound.
• The process of vigorous oxidation implies the complete alteration of certain functional groups or side chains, often involving the aromatic ring becoming a carboxylic acid group.

Through oxidation, labile groups such as methyl \((\text{CH}_3)\) and ethyl \((\text{C}_2\text{H}_5)\) are removed, leaving a simplified base structure suitable for forming benzoic acid. Understanding this transformation process is critical for predicting reaction products in organic chemistry.