Question

(a) What is a 'red shift'? (b) Suggest why the methoxy (MeO) substituent in the compound \(\mathrm{MeOCH}=\mathrm{CHCH}=\mathrm{CH}_{2}\) causes a red shift in the UV-VIS spectrum with respect to the spectrum of \(\mathrm{CH}_{2}=\mathrm{CHCH}=\mathrm{CH}_{2}\)

Short answer

A red shift is the movement of spectral lines to longer wavelengths. The methoxy group in the compound increases conjugation, lowering transition energy and causing a red shift.

Step-by-step solution

Define Red Shift

A 'red shift' in spectroscopy refers to the phenomenon where absorption or emission lines move to longer wavelengths or lower energy in a spectrum. This shift towards the red part of the spectrum indicates that the energy required for electronic transitions is lower.

Methoxy Group Introduction

The compound in question, \(\text{MeOCH} = \text{CHCH} = \text{CH}_{2},\) contains a methoxy (MeO) group attached to a butadiene backbone. Evaluating the structure, the presence of the methoxy group introduces lone pair electrons from the oxygen atom into the conjugation system.

Analyze Effect on Conjugation

The lone pairs from the methoxy group's oxygen atom can participate in resonance with the conjugated diene system, thus increasing the overall conjugation. More conjugation often leads to a lower energy gap between the HOMO and LUMO, resulting in a red shift as the energy required for electronic transitions is reduced.

Compare to the Unsubstituted Diene

The unsubstituted compound \(\text{CH}_{2} = \text{CHCH} = \text{CH}_{2},\) lacks the methoxy group and its electronic effects. Hence, it has less conjugation and a higher energy transition compared to the methoxy-substituted compound. The methoxy group's ability to participate in resonance lowers the transition energy.

Key concepts

Spectroscopy

Spectroscopy is an essential analytical technique widely used to study the interaction between matter and electromagnetic radiation. In simple terms, spectroscopy helps us understand how molecules absorb or emit light at different wavelengths. This science is crucial for identifying chemical compositions and structural information about various substances.
The process of spectroscopy can categorize into types based on the regions of the electromagnetic spectrum they analyze, such as infrared, ultraviolet-visible, and nuclear magnetic resonance (NMR) spectroscopy.
Through spectroscopy, scientists can obtain absorption or emission spectra that depict how a molecule absorbs or emits light at specific wavelengths, providing insight into various electronic transitions happening within the molecule.

Ultraviolet-visible spectrum

The ultraviolet-visible (UV-VIS) spectrum is a part of the electromagnetic spectrum that deals with light in the ultraviolet and visible range. It spans wavelengths from approximately 200 nm to 800 nm. Within this range, different molecules absorb light that results in electronic transitions, typically between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).
When we talk about a red shift in the UV-VIS spectrum, it means that the absorption of light happens at longer wavelengths or lower energies. The prospect of detecting a red shift can be crucial in the fields of organic and inorganic chemistry, as it indicates changes in a molecule's electronic structure due to modifications such as substituent effects or increased conjugation.
This spectrum can be used to understand how different chemical structures and functional groups affect the absorption behavior of a molecule, enabling deeper insights into molecular stability and electronic interaction mechanisms.

Conjugation

Conjugation in organic chemistry refers to the overlap of p-orbitals across adjacent single and multiple bonds in a molecule, which leads to a delocalization of electrons. This delocalization makes the molecule more stable and lowers the energy gap between electronic states.
In the context of the butadiene backbone in the exercise's example, the introduction of a methoxy group increases conjugation by allowing resonance with lone pair electrons from the oxygen atom.
Increased conjugation results in a lowering of the energy difference between the HOMO and LUMO, which manifests as a red shift when observed in the ultraviolet-visible spectrum.

• More conjugation = lower energy transitions
• This results in absorption at longer wavelengths

Conjugation is thus a key factor in determining the stability and optical properties of organic compounds.

Methoxy Group Influence

The methoxy group (MeO) has a significant impact on the electronic properties of a molecule due to its ability to donate electrons through resonance. Its presence in conjugated diene systems, such as the example compound MeOCH = CHCH = CH_2, influences both the absorption characteristics and the overall conjugation of the system.
The methoxy group contributes lone pair electrons from the oxygen atom, which can participate in resonance with adjacent double bonds. This increases the net electron delocalization across the system, enhancing the conjugative effect.

• Methoxy groups often result in red shifts in absorption spectra
• This is due to increased electron delocalization and reduced excitation energy requirements

By introducing a methoxy group, the diene's redshifted UV-VIS spectrum reflects how its electronic environment becomes more stabilized and extended compared to its unsubstituted counterpart. This demonstrates the power of substituent groups in modulating the optical and chemical characteristics of molecular systems.