Mass Spectrometry, Infrared Spectroscopy, and Ultraviolet/Visible Spectroscopy: Structural Elucidation in Organic Chemistry

Guida di studio - Note intelligenti

Appunti personalizzati basati sui tuoi materiali, ampliati con definizioni chiave, esempi e contesto.

Mass Spectrometry

Principles and Process

Mass spectrometry is a powerful analytical technique used to determine the molecular mass and structural features of organic compounds. The process involves ionizing a molecule with an electron beam, resulting in the formation of a molecular ion (a radical cation) and an electron.

Molecular Ion (M+): The ion formed by the loss of an electron from the molecule; its mass-to-charge ratio (m/z) gives the molecular mass.
Fragmentation: The molecular ion can break into smaller fragments, each with a characteristic m/z value.
Base Peak: The most abundant fragment in the spectrum, often corresponding to the most stable ion.

Formation of molecular ion by electron beam Mass spectrum of pentane showing base peak and molecular ion

Fragmentation Patterns

Fragmentation occurs at specific bonds, often those that produce the most stable cations. The pattern of fragmentation helps deduce the structure of the compound.

C-2—C-3 Fragmentation: Produces a base peak at m/z 43 for pentane, indicating a stable propyl cation.
C-1—C-2 Fragmentation: Produces smaller fragments such as methyl cation (m/z 15).

Fragmentation pathways for pentane Loss of H2 from a fragment Formation of molecular ion for pentane

Mass Spectra of Alkanes and Isomers

Isomers can be distinguished by their fragmentation patterns and the relative abundance of peaks. For example, isopentane shows a more abundant peak at m/z 57 due to the stability of the secondary carbocation.

Fragmentation of isopentane Mass spectrum of pentane Mass spectrum of isopentane Comparison of mass spectra for pentane and isopentane

Mass Spectra of Alkyl Halides

Alkyl halides show characteristic isotopic patterns due to the presence of bromine or chlorine. Bromine has two isotopes (79Br and 81Br) with nearly equal abundance, resulting in two M+ peaks two mass units apart. Chlorine (35Cl and 37Cl) shows a 3:1 ratio in peak intensities.

Heterolytic Cleavage: The carbon-halogen bond breaks, producing characteristic fragments.
α-Cleavage: Homolytic cleavage of the carbon-carbon bond adjacent to the halogen.

Fragmentation of alkyl bromide Mass spectrum of alkyl bromide Fragmentation of alkyl chloride Alpha cleavage in alkyl chloride

Mass Spectra of Ethers, Alcohols, and Ketones

Ethers, alcohols, and ketones exhibit characteristic fragmentation patterns, often involving α-cleavage and rearrangements. The mass spectra provide information about the functional groups present.

α-Cleavage in Ethers: Produces fragments at m/z 57 and 43.
McLafferty Rearrangement in Ketones: Involves migration of a hydrogen atom, producing resonance-stabilized ions.
Alcohols: Show loss of water and α-cleavage fragments.

Mass spectrum of ether Alpha cleavage in ether Mass spectrum of ketone McLafferty rearrangement in ketone Alpha cleavage in alcohol Mass spectrum of alcohol

Fragmentation Behavior Summary

Bonds between carbon and atoms of similar electronegativity break homolytically (e.g., alkanes).
Bonds between carbon and more electronegative atoms break heterolytically (e.g., alkyl halides, alcohols).
Weakest bonds and those forming the most stable cations are most likely to break.

Infrared (IR) Spectroscopy

Principles and the Electromagnetic Spectrum

IR spectroscopy measures the absorption of infrared radiation by molecules, causing vibrational transitions. Only bonds whose dipole moment changes during vibration are IR active.

Energy and Frequency Relationship:
IR region lies between visible and microwave regions of the electromagnetic spectrum.

Electromagnetic spectrum Energy-frequency-wavelength relationship

Types of Vibrations

Molecular vibrations include stretching and bending modes. Stretching occurs along the bond axis, while bending involves changes in bond angles.

Stretching Vibrations: Symmetric and asymmetric.
Bending Vibrations: Scissoring, rocking, twisting, wagging.

Stretching vibration Stretching and bending vibrations

Characteristic IR Absorptions

Each functional group absorbs IR radiation at a characteristic wavenumber. The IR spectrum is divided into the functional group region (4000–1400 cm–1) and the fingerprint region (1400–600 cm–1).

C=O: ~1700 cm–1
O–H: ~3400 cm–1

Characteristic IR absorptions Functional group and fingerprint regions IR spectra of two alcohols

Factors Affecting IR Absorption

Bond Polarity: More polar bonds absorb more intensely (O–H > N–H > C–H).
Bond Order: Higher bond order leads to higher wavenumber absorption.
Resonance and Electron Delocalization: Resonance lowers the frequency of absorption; inductive withdrawal increases it.
Hydrogen Bonding: Hydrogen-bonded O–H stretches appear at lower frequencies.

Relative bond polarities and IR intensities Bond order and wavenumber Resonance effect on carbonyl group IR spectrum of cyclohexanone Resonance and inductive effects on carbonyl group IR spectrum of ester IR spectrum of amide IR absorptions for alcohol, ether, carboxylic acid, ester Hydrogen-bonded dimer Hydrogen bonding in concentrated and dilute solutions

IR Absorption Tables

Tables summarize the characteristic wavenumbers and intensities for various functional groups and bond types.

Table of IR stretching frequencies Table of IR absorptions for C-H bonds

IR Spectra Examples

IR spectra of specific compounds illustrate the assignment of peaks to functional groups.

IR spectrum of 2-propyn-1-ol IR spectrum of N-methylethanamide IR spectrum of ethyl benzyl ketone

Ultraviolet/Visible (UV/Vis) Spectroscopy

Principles and Applications

UV/Vis spectroscopy is used to study compounds with conjugated double bonds. It measures the absorption of ultraviolet (100–400 nm) and visible (400–700 nm) light, causing electronic transitions.

Chromophore: The part of the molecule responsible for UV/Vis absorption.
Auxochrome: A substituent that alters the position and intensity of absorption.
Beer–Lambert Law: (A = absorbance, c = concentration, l = path length, \varepsilon = molar absorptivity).

UV/Vis spectrum Electronic transition in UV/Vis spectroscopy

Conjugation and Wavelength

Greater conjugation in a molecule leads to absorption at longer wavelengths. Colored compounds absorb visible light (>400 nm).

UV/Vis absorption bands are broad due to vibrational sublevels.
Conjugation makes electronic transitions easier and shifts absorption to longer wavelengths.

Effect of conjugation on UV/Vis absorption Beer-Lambert Law Conjugation and wavelength Colored compounds absorb visible light Auxochrome effect

Summary Table: Spectroscopic Techniques

Technique	Information Provided	Key Features
Mass Spectrometry	Molecular mass, fragmentation pattern	m/z values, base peak, isotopic patterns
IR Spectroscopy	Functional groups, bond types	Characteristic wavenumbers, intensity, hydrogen bonding
UV/Vis Spectroscopy	Conjugation, electronic transitions	Absorption wavelength, chromophore, auxochrome

Example: Mass spectrometry distinguishes pentane from isopentane by the abundance of the m/z 57 peak. IR spectroscopy identifies functional groups by their characteristic absorption bands. UV/Vis spectroscopy reveals the extent of conjugation in organic molecules.

Additional info: These notes cover the essential principles and applications of mass spectrometry, IR, and UV/Vis spectroscopy as outlined in Chapter 13 of a typical Organic Chemistry curriculum.