What is mass spectrometry ?

Mass spectrometry is an analytical technique used to identify unknown compounds by bombarding molecules with high-energy electrons to form ions, which are separated by their mass-to-charge (m/z) ratio.

What is infrared (IR) spectroscopy ?

Infrared (IR) spectroscopy is an analytical technique used to identify compounds by measuring which frequencies of infrared radiation are absorbed, causing bonds between atoms to vibrate by stretching, bending, and twisting.

True or False? The [M+1] peak in a mass spectrum is caused by the natural abundance of carbon-13.

True. Approximately 1% of carbon atoms in nature are C-13. Molecules containing C-13 produce a molecular ion one mass unit heavier than the main M+ peak. The more carbon atoms in the molecule, the larger the [M+1] peak.

True or False? Hydrogen bonding causes O–H absorptions in IR spectra to appear as broad peaks.

True. Hydrogen bonding in alcohols and carboxylic acids causes a range of O–H bond lengths and vibrational energies, broadening the absorption peak. In contrast, C=O bonds produce a strong, sharp peak because they are not involved in hydrogen bonding.

Why do halogenoalkanes show multiple peaks close to the molecular ion peak in their mass spectra?

Halogenoalkanes show multiple peaks close to the molecular ion peak because halogens exist as more than one naturally occurring isotope . Each isotope combination produces a separate molecular ion at a slightly different m/z value, giving a characteristic cluster of peaks.

Wavenumber is the unit used in IR spectroscopy, measured in cm -1 . It is the reciprocal of the wavelength of the absorbed infrared radiation, and indicates the position of absorption peaks in an IR spectrum.

Define molecular ion .

A molecular ion is a positively charged species formed when a molecule loses one electron during mass spectrometry, giving it one unpaired electron. Its m/z value equals the molecular mass of the compound.

In IR spectroscopy, the O–H group in an alcohol produces a .......... absorption around 3200–3750 cm -1 , while the C=O group in a ketone produces a .......... , sharp absorption around 1700–1720 cm -1 .

In IR spectroscopy, the O–H group in an alcohol produces a broad absorption around 3200–3750 cm -1 , while the C=O group in a ketone produces a strong , sharp absorption around 1700–1720 cm -1 .

When an alcohol undergoes fragmentation in a mass spectrometer, it commonly loses a .......... molecule, producing a peak .......... mass units below the molecular ion peak.

When an alcohol undergoes fragmentation in a mass spectrometer, it commonly loses a water molecule, producing a peak 18 mass units below the molecular ion peak.

How is IR spectroscopy used to identify an unknown organic compound?

The IR spectrum of the unknown compound is compared to the spectra of known compounds. Characteristic absorption peaks at specific wavenumber ranges identify functional groups present, and the fingerprint region (below ~1500 cm -1 ) can be matched to a reference spectrum to confirm the compound's identity.

The base peak is the peak in a mass spectrum corresponding to the most abundant ion fragment.

An IR spectrum shows a strong, sharp absorption at 1710 cm -1 and no broad absorption between 2500–3300 cm -1 . What functional group is present, and what class of compound does this suggest?

The strong, sharp absorption at 1710 cm -1 indicates a C=O carbonyl group. The absence of a broad O–H absorption rules out a carboxylic acid, suggesting the compound is a ketone or aldehyde.

True or False? In a mass spectrum, fragment ions with a smaller m/z value are deflected less by the magnetic field.

False. Fragment ions with a smaller m/z value — either because they are lighter or carry a greater positive charge — are deflected more by the magnetic field and are detected first.

True or False? The O–H absorption in carboxylic acids appears at a higher wavenumber than the O–H absorption in alcohols.

False. The O–H stretch in carboxylic acids (3300–2500 cm -1 ) is shifted to lower wavenumbers compared to alcohols (3750–3200 cm -1 ). Strong hydrogen bonding in carboxylic acid dimers broadens and lowers the absorption.

A mass spectrum shows a peak at m/z = 29. What fragment ion does this correspond to, and in which type of compound is it commonly seen?

A peak at m/z = 29 corresponds to the C 2 H 5 + fragment ion. It is commonly seen in the mass spectra of alkanes and other organic compounds containing an ethyl group, arising from cleavage of a C–C bond.

Define fingerprint region .

The fingerprint region is the area of an IR spectrum below approximately 1500 cm -1 . It contains a complex pattern of absorptions unique to each molecule and is used to confirm the identity of a compound by comparison with a reference spectrum.

Define fragment ions .

Fragment ions are positively charged species formed when the molecular ion breaks apart inside the mass spectrometer. Each fragment produces a peak at a characteristic m/z value that helps identify the compound's structure.

What is the approximate wavenumber range for the N–H stretching absorption in amines, and how does it differ from the O–H absorption in alcohols?

N–H stretching in amines absorbs in the range 3300–3500 cm -1 , similar in position to the O–H absorption in alcohols (3200–3750 cm -1 ). The O–H peak is typically broader due to stronger hydrogen bonding, while the N–H peak is narrower and may appear as a doublet in primary amines.

A LevelChemistryEdexcelFlashcards3. Organic ChemistryModern Analytical Techniques I

Modern Analytical Techniques I (Edexcel A Level Chemistry): Flashcards

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What is mass spectrometry?

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What is mass spectrometry?
Mass spectrometry is an analytical technique used to identify unknown compounds by bombarding molecules with high-energy electrons to form ions, which are separated by their mass-to-charge (m/z) ratio.
What is infrared (IR) spectroscopy?
Infrared (IR) spectroscopy is an analytical technique used to identify compounds by measuring which frequencies of infrared radiation are absorbed, causing bonds between atoms to vibrate by stretching, bending, and twisting.
True or False?
The [M+1] peak in a mass spectrum is caused by the natural abundance of carbon-13.
True.
Approximately 1% of carbon atoms in nature are C-13. Molecules containing C-13 produce a molecular ion one mass unit heavier than the main M+ peak. The more carbon atoms in the molecule, the larger the [M+1] peak.
True or False?
Hydrogen bonding causes O–H absorptions in IR spectra to appear as broad peaks.
True.
Hydrogen bonding in alcohols and carboxylic acids causes a range of O–H bond lengths and vibrational energies, broadening the absorption peak. In contrast, C=O bonds produce a strong, sharp peak because they are not involved in hydrogen bonding.
Why do halogenoalkanes show multiple peaks close to the molecular ion peak in their mass spectra?
Halogenoalkanes show multiple peaks close to the molecular ion peak because halogens exist as more than one naturally occurring isotope. Each isotope combination produces a separate molecular ion at a slightly different m/z value, giving a characteristic cluster of peaks.
Define wavenumber.
Wavenumber is the unit used in IR spectroscopy, measured in cm^-1. It is the reciprocal of the wavelength of the absorbed infrared radiation, and indicates the position of absorption peaks in an IR spectrum.
Define molecular ion.
A molecular ion is a positively charged species formed when a molecule loses one electron during mass spectrometry, giving it one unpaired electron. Its m/z value equals the molecular mass of the compound.
In IR spectroscopy, the O–H group in an alcohol produces a .......... absorption around 3200–3750 cm^-1, while the C=O group in a ketone produces a .......... , sharp absorption around 1700–1720 cm^-1.
In IR spectroscopy, the O–H group in an alcohol produces a broad absorption around 3200–3750 cm^-1, while the C=O group in a ketone produces a strong, sharp absorption around 1700–1720 cm^-1.
When an alcohol undergoes fragmentation in a mass spectrometer, it commonly loses a .......... molecule, producing a peak .......... mass units below the molecular ion peak.
When an alcohol undergoes fragmentation in a mass spectrometer, it commonly loses a water molecule, producing a peak 18 mass units below the molecular ion peak.
How is IR spectroscopy used to identify an unknown organic compound?
The IR spectrum of the unknown compound is compared to the spectra of known compounds. Characteristic absorption peaks at specific wavenumber ranges identify functional groups present, and the fingerprint region (below ~1500 cm^-1) can be matched to a reference spectrum to confirm the compound's identity.
Define base peak.
The base peak is the peak in a mass spectrum corresponding to the most abundant ion fragment.
An IR spectrum shows a strong, sharp absorption at 1710 cm^-1 and no broad absorption between 2500–3300 cm^-1. What functional group is present, and what class of compound does this suggest?
The strong, sharp absorption at 1710 cm^-1 indicates a C=O carbonyl group. The absence of a broad O–H absorption rules out a carboxylic acid, suggesting the compound is a ketone or aldehyde.
True or False?
In a mass spectrum, fragment ions with a smaller m/z value are deflected less by the magnetic field.
False.
Fragment ions with a smaller m/z value — either because they are lighter or carry a greater positive charge — are deflected more by the magnetic field and are detected first.
True or False?
The O–H absorption in carboxylic acids appears at a higher wavenumber than the O–H absorption in alcohols.
False.
The O–H stretch in carboxylic acids (3300–2500 cm^-1) is shifted to lower wavenumbers compared to alcohols (3750–3200 cm^-1). Strong hydrogen bonding in carboxylic acid dimers broadens and lowers the absorption.
A mass spectrum shows a peak at m/z = 29. What fragment ion does this correspond to, and in which type of compound is it commonly seen?
A peak at m/z = 29 corresponds to the C₂H₅⁺ fragment ion. It is commonly seen in the mass spectra of alkanes and other organic compounds containing an ethyl group, arising from cleavage of a C–C bond.
Define fingerprint region.
The fingerprint region is the area of an IR spectrum below approximately 1500 cm^-1. It contains a complex pattern of absorptions unique to each molecule and is used to confirm the identity of a compound by comparison with a reference spectrum.
Define fragment ions.
Fragment ions are positively charged species formed when the molecular ion breaks apart inside the mass spectrometer. Each fragment produces a peak at a characteristic m/z value that helps identify the compound's structure.
What is the approximate wavenumber range for the N–H stretching absorption in amines, and how does it differ from the O–H absorption in alcohols?
N–H stretching in amines absorbs in the range 3300–3500 cm^-1, similar in position to the O–H absorption in alcohols (3200–3750 cm^-1). The O–H peak is typically broader due to stronger hydrogen bonding, while the N–H peak is narrower and may appear as a doublet in primary amines.

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