Question

Compounds A, B and C have the formula \(\mathrm{C}_{5} \mathrm{H}_{8}\) and on hydrogenation all yield n-pentane. Their ultraviolet spectra show the following values of \(\lambda_{\max }: A, 176 \mathrm{~nm} ; \mathrm{B}, 211 \mathrm{~nm}\) C, \(215 \mathrm{~nm}\). (1-Pentene has \(\lambda_{\max } 178 \mathrm{~nm}\).) (a) What is a likely structure for \(\mathrm{A} ?\) For \(\mathrm{B}\) and \(\mathrm{C}\) ? (b) What kind of information might enable you to assign specific structures to \(\mathrm{B}\) and \(\mathrm{C}\) ?

Short answer

The likely structure for compound A is an alkene similar to 1-pentene: \[ \textnormal{CH}_{3}\textnormal{CH}_{2}\textnormal{CH}\textnormal{CH}\textnormal{CH}_{3} \] Compounds B and C may be cycloalkenes with 5-membered rings and different arrangements of the double bond or substituents within the ring. To assign definite structures to B and C, additional information such as NMR spectroscopy, IR spectroscopy, mass spectrometry data, or comparison with known structures in the literature would be needed.

Step-by-step solution

Understand the given information

Compounds A, B, and C have the same molecular formula \(\mathrm{C}_{5} \mathrm{H}_{8}\) and on hydrogenation, all convert to n-pentane. Their ultraviolet spectra have λmax values given - A: 176 nm, B: 211 nm, and C: 215 nm. 1-Pentene has λmax 178 nm.

Identify the type of compounds

Since the compounds have the general formula \(\mathrm{C}_{5} \mathrm{H}_{8}\), they are likely to be alkenes (carbon-carbon double bonds) or cycloalkenes. As all compounds yield n-pentane upon hydrogenation, they must have one double bond (addition of hydrogen to form single bonds).

Predict the structure of compound A

Given that compound A has a λmax of 176 nm, which is very close to that of 1-pentene (178 nm), it is likely that compound A is also an alkene with a similar structure to 1-pentene. Hence, the structure of compound A can be: \[ \textnormal{CH}_{3}\textnormal{CH}_{2}\textnormal{CH}\textnormal{CH}\textnormal{CH}_{3} \]

Predict the structures of compounds B and C

On the other hand, the λmax values of compounds B (211 nm) and C (215 nm) are quite different from 1-pentene. This suggests they might be cycloalkenes, which would still yield n-pentane upon hydrogenation. Possible structures for compounds B and C can be: \[ \textnormal{B: }\textnormal{C}_{5} \mathrm{H}_{8} \textnormal{with a five-membered ring and one double bond}\\ \textnormal{C: }\textnormal{C}_{5} \mathrm{H}_{8} \textnormal{with a different arrangement of the double bond or substituents within the ring} \]

Determine information needed to assign specific structures to B and C

In order to assign definite structures to compounds B and C, we would need additional information that could differentiate them. Possible sources of information include: - NMR spectroscopy data (that could reveal information about the connectivity and the environment of protons and carbons in the molecule) - IR spectroscopy data (that could provide information about the presence of specific functional groups and/or substituents) - Mass spectrometry data (that could provide information about the molecular fragments when the compounds are subjected to ionization) - Comparison with known structures of cycloalkenes with similar λmax values in the literature (to identify common structural features).

Key concepts

Hydrogenation of Alkenes

Hydrogenation is a chemical reaction where hydrogen molecules are added across the double bonds of alkenes, converting them into alkanes. This process is key in determining the structure of such organic compounds, because it can reveal how many carbon-carbon double bonds are present. In the case of compounds A, B, and C, which all have the formula \(\mathrm{C}_{5} \mathrm{H}_{8}\), we can infer that they each possess one double bond. During hydrogenation, n-pentane is produced, indicating that the initial alkenes or cycloalkenes are straight-chain or cyclic hydrocarbons with a similar carbon skeleton to n-pentane.

For a clearer understanding, consider the visual of adding two hydrogen atoms across the double bond of an alkene: \[\textnormal{R-CH=CH-R} \rightarrow \textnormal{R-CH}_{2}\textnormal{-CH}_{2}\textnormal{-R}\] This transformation is fundamental in organic chemistry because it not only saturates the molecule but also provides insights into molecular geometry and possible isomers.

Cycloalkene Structure

Cycloalkenes are cyclic hydrocarbons with one or more carbon-carbon double bonds. These structures are significant in organic chemistry due to their unique physical and chemical properties. For instance, the stability of a cycloalkene is influenced by the size of the ring and the location of the double bond. In our exercise, compounds B and C were suspected to be cycloalkenes because their ultraviolet (UV) spectra differed from that of a typical straight-chain alkene. The UV spectra can provide clues about the electronic environment in a molecule, which is influenced by the ring strain and substitution pattern on cycloalkenes.

It's helpful to visualize a five-membered ring, like compound B, which could have a structure similar to: \[\textnormal{Cyclic-C}_{5}\mathrm{H}_{8}\] with variations in the double bond's position for compound C. The specific placement of this bond affects the physical and spectral properties of the cycloalkene.

NMR Spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful analytical technique used to determine the structure of organic compounds. It works by exposing the nuclei of atoms within a molecule to a magnetic field and measuring their interactions with radio waves. There are two main types of NMR: \( ^1H-NMR \) for hydrogen atoms and \( ^{13}C-NMR \) for carbon atoms. These methods offer detailed information about the connectivity and environment of the individual atoms in a molecule.

For instance, \( ^1H-NMR \) can reveal the number of hydrogen environments and their relationship to each other, differentiating between hydrogen atoms on a straight chain and those in a cyclic structure. \( ^{13}C-NMR \) provides similar information for carbon atoms. In the exercise, the NMR data could allow us to distinguish between compound A, which is likely an alkene, and compounds B and C, which we suspect to be cycloalkenes.

IR Spectroscopy

Infrared (IR) spectroscopy is an instrumental analysis technique used to identify functional groups within a molecule based on the absorption of infrared light. Molecules absorb IR at specific frequencies that cause bond vibrations, and these vibrations are characteristic of certain functional groups.

For alkenes and cycloalkenes, IR spectroscopy can be particularly revealing. The presence of a carbon-carbon double bond will generally show an absorption band due to the stretching vibration in the region of 1640-1680 cm^-1. This band can provide evidence supporting the existence of such a bond in the compounds we're studying. Different types of alkenes, cis or trans, and their corresponding substituents, can also result in variations in the IR spectra, offering clues to the molecular structure.

Mass Spectrometry

Mass spectrometry is an analytical tool that sorts ions based on their mass-to-charge ratio. When a molecule is subjected to ionization in a mass spectrometer, it breaks into various fragments. These fragments form a pattern or 'fingerprint' that can be used to deduce structural information about the original molecule.

In the context of the exercise, mass spectrometry would allow us to see the different fragmentations for compounds A, B, and C. It might reveal that compound A, as a straight-chain alkene, breaks down in a manner different from the cyclic structures of B and C. By comparing the fragmentation patterns, chemists can infer the positions of double bonds and other structural features that distinguish one compound from another.

This technique complements the other spectroscopic methods by providing a different perspective on the molecule's structure, especially when considering isotopic patterns or the presence of molecular rearrangements.