Chemistry Concepts: Unsaturation & Spectrum

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Questions and Answers

What transition occurs in formaldehyde upon absorption of a UV–vis photon?

π ⟶ π* transition

HOMO–LUMO transition

σ ⟶ σ* transition

n ⟶ π* transition (correct)

What is the wavelength of maximum absorbance (λmax) for liquid formaldehyde?

280 nm

What color is β-carotene responsible for?

orange

Formaldehyde absorbs visible photons.

False Signup and view all the answers

The Beer–Lambert law states that absorbance, A, is directly proportional to concentration, C, length, l, and __________.

molar absorptivity Signup and view all the answers

What is the Beer–Lambert law equation?

A = εlC Signup and view all the answers

UV–vis spectroscopy is used to quantify the amount of one or more compounds.

True Signup and view all the answers

What is the effect on the C=O absorption frequency when it is conjugated to a C=C or C≡C bond?

Decreases by 20–40 cm−1 Signup and view all the answers

What is the frequency range for C≡C bonds in infrared spectroscopy?

2100 to 2260 cm−1 Signup and view all the answers

At what position does the sp C-H stretch appear in 1-Hexyne?

3208 cm−1 Signup and view all the answers

Which of the following C—H stretches has moderate intensity?

Alkyne C—H Signup and view all the answers

The C-H stretch of an alkane has a frequency range of 3000–3100 cm−1.

False Signup and view all the answers

What type of bond is associated with a frequency at about 3300 cm−1 in IR spectroscopy?

Alkyne C—H bond Signup and view all the answers

What is the significance of the peak (λmax) centered at 217 nm in UV-visible spectroscopy?

Indicates the maximum absorption of conjugated compounds Signup and view all the answers

What effect does an aldehyde have on the C—H stretch in terms of its absorption peaks?

Two peaks around 2720 and 2820 cm−1 Signup and view all the answers

What does the Index of Hydrogen Deficiency indicate?

Number of multiple bonds and/or rings present for a molecular formula. Signup and view all the answers

What is the formula to calculate the hypothetical number of hydrogens for an acyclic, saturated analogue?

CnH2n+2 Signup and view all the answers

For each O or S, you make no changes in numbers of H’s in calculated formula.

True Signup and view all the answers

Match the bond types with their respective characteristic absorption frequencies:

O - H = 3200 to 3600 cm⁻¹ C = O (Aldehyde) = 1720 – 1740 cm⁻¹ N - H = 3300 to 3500 cm⁻¹ C - C = 1620 – 1680 cm⁻¹ Signup and view all the answers

Which bond type typically has a strong IR absorption?

O - H Signup and view all the answers

What happens to the frequency of a vibrating molecule when the mass decreases?

The vibrational frequency increases. Signup and view all the answers

What is the characteristic frequency range for primary amine N - H stretching?

3200 - 3500 cm⁻¹ Signup and view all the answers

Where do we begin when identifying peaks in IR spectra of unknown molecules?

Look at the peaks above 1400 cm⁻¹. Signup and view all the answers

What type of bond leads to weak or nonexistent IR absorption if the connected portions are similar?

Nonpolar bonds. Signup and view all the answers

What is the expected C=O stretch frequency range for a ketone?

1710 – 1730 cm⁻¹ Signup and view all the answers

Study Notes

Index of Hydrogen Deficiency (Degree of Unsaturation)

Determines the number of rings and/or pi bonds present in a molecule based on its molecular formula.
Calculates the hypothetical number of hydrogens for an acyclic, saturated analogue (CnH2n+2).
Corrections are made for other atoms like N, P, O, S, F, Cl, Br, and I.
The difference between the true number of hydrogens and the calculated hypothetical number, divided by 2, gives the unsaturation.

The Electromagnetic Spectrum

Electromagnetic radiation exhibits properties of both waves and particles.
Wavelength and frequency are inversely proportional.
As a particle, electromagnetic radiation exists as photons, with energy depending on frequency.
Higher frequency photons have higher energy levels.
Infrared spectroscopy uses wavenumbers (n = 1/l(cm)), which are directly proportional to energy.

Infrared Spectroscopy: Overview

Infrared spectroscopy uses infrared radiation to analyze the vibrational modes of molecules.
Molecules absorb specific frequencies of infrared radiation, leading to changes in their vibrational energy levels.
This absorption information is visualized as an IR spectrum.

The IR Spectrum

An IR spectrum displays the intensity of absorption versus the frequency of radiation.
Strong absorptions represent high absorption intensities (near zero percent transmittance), while weak absorptions represent low absorption intensities (near 100 percent transmittance).

General Theory of Infrared Spectroscopy

Molecules absorb specific frequencies of infrared radiation corresponding to their vibrational modes.
This absorption results in changes in their vibrational energy levels, leading to the formation of an IR spectrum.

Ball-and-Spring Model for Explaining Infrared Peak Locations

The frequency of a molecular vibration determines the location of a peak in an IR spectrum.
The ball-and-spring model helps understand how molecular structure influences vibrational frequencies.
Higher vibrational frequencies occur with:
- Smaller masses attached to the spring (lower atomic weight).
- Stiffer springs (stronger bonds).

Location of Peaks in an Infrared Spectrum

Each type of bond vibrates within a specific range of frequencies.
Characteristic absorption frequencies are used to identify functional groups present in a compound.

Intensities of Peaks in an Infrared Spectrum

The intensity of an IR absorption peak is related to the polarity of the bond.
Highly polar bonds (like C=O or O—H) exhibit strong absorptions.
Nonpolar bonds (like C=C) may have weak or nonexistent absorptions, depending on the similarity of the portions connected by the bond.

Important Infrared Stretches

Analyzing peaks in IR spectra can help identify the presence of specific functional groups.
The regions above 1400 cm−1 often contain key information about different functional groups.

C—H Stretch Intensity

The intensity of a C-H stretch reflects the hybridization of the carbon atom involved.
sp3 hybridized C-H bonds show strong absorptions, while sp2 and sp hybridized C-H bonds exhibit weaker absorptions.

The O—H Stretch

The O—H stretch typically appears between 3200 and 3600 cm−1.
Hydrogen bonding broadens the O—H peak.
This broadening can be used to differentiate between alcohols and carboxylic acids.

The N—H Stretch

N—H stretches appear between 3300 and 3500 cm−1.
Hydrogen bonding can broaden the N—H peak, though usually less broad than the corresponding O—H peak.
Amine and amide groups exhibit two N—H stretching bands due to symmetrical and asymmetrical stretching modes.

The Carbonyl (C=O) Stretch

The C=O stretch is a strong absorption that is characteristic of different carbonyl-containing functional groups.
The specific frequency range for the C=O stretch differs slightly for each compound class (esters, aldehydes, ketones, amides, and acids).

C=O and Conjugation

When a C=O group is conjugated with a C=C or C≡C bond, the C=O frequency shifts to lower values (approximately 20–40 cm−1).

Alkyne (C≡C) and Nitrile (C≡N) Stretches

Both C≡C and C≡N stretches appear as sharp bands around the same region (2100–2260 cm−1).
Terminal alkynes show a characteristic C—H peak around 3300 cm−1.
Distinguishing between nitriles and internal alkynes can be difficult.
RC≡N absorptions are usually stronger compared to RC≡CR absorptions.

Understanding IR Spectrum Features

IR spectra can be intricate, but focusing on specific regions can provide valuable information about the molecule.
Recognizing characteristic stretches and identifying their frequencies is key to interpreting IR spectra.
The fingerprint region (below 1400 cm−1) can be complex but is helpful for identifying specific compounds.

Infrared Spectroscopy Concepts

Identifying Functional Groups : IR spectroscopy is powerful for identifying functional groups within molecules. Specific vibrational frequencies correspond to different bonds, providing information about what functional groups are present.
Key Frequencies
- C≡C Stretch (Terminal Alkynes): Observed around 2100-2200 cm⁻¹ (moderate intensity)
- sp C-H Stretch (Terminal Alkynes): Observed around 3300 cm⁻¹ (moderate intensity)
- C≡N Stretch (Nitriles): Observed around 2200-2260 cm⁻¹ (moderate intensity)
- C-H Stretch (General):
  - Alkyne C-H: ~3300 cm⁻¹ (moderate intensity)
  - Alkene C-H: 3000–3100 cm⁻¹ (variable intensity)
  - Aromatic C-H: 3000–3100 cm⁻¹ (variable intensity)
  - Alkane C-H: 2800–3000 cm⁻¹ (variable intensity)
- Aldehyde C-H: Two peaks, ~2720 and ~2820 cm⁻¹ (moderate intensity)
Bending Vibrations: Provide additional structural information
- CH₂ Bending: Strong peaks around 910 and 990 cm⁻¹

Structure Elucidation with IR

Identifying Unknown Compounds: IR spectra can help determine the structure of unknown molecules by identifying functional groups and comparing the data to databases of know spectra.

UV-Vis Spectroscopy

Principle: UV-Vis spectroscopy uses the absorption of ultraviolet and visible light to provide information about the electronic energy levels in a molecule.
Conjugation and λmax: The presence of conjugation (alternating double and single bonds) shifts the absorption maximum (λmax) to longer wavelengths (lower energy). This is because conjugation increases the stability of the pi system and reduces the energy difference between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).
HOMO-LUMO Transitions: UV-Vis absorption causes the transition of an electron from the HOMO to the LUMO, called the HOMO-LUMO transition.
Nonbonding Electrons (n → π):* The presence of nonbonding electrons (n) can lead to n → π* transitions, which are typically in longer wavelengths than π → π* transitions.
Color and Absorption: The color of a compound is related to its absorption spectrum. Compounds that absorb visible light appear colored because they only transmit the complementary color.
Beer-Lambert Law: Relates absorbance (A) to concentration (C), path length (l), and molar absorptivity (ε): A = εlC. The Beer-Lambert law is used for quantitative analysis in UV-Vis spectroscopy.
Applications:
- Quantitative Analysis: UV-Vis spectroscopy is used to measure the concentration of compounds.
- Kinetics: Tracking changes in absorbance over time allows for studying reaction rates.

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Description

Explore key chemistry concepts including the Index of Hydrogen Deficiency, the properties of the electromagnetic spectrum, and an overview of infrared spectroscopy. This quiz will test your understanding of how molecular formulas relate to saturation and the nature of electromagnetic radiation. Dive in to assess your knowledge!