topic 7 - modern analytical techniques Flashcards
What happens when a molecule is put through a mass spectrometer?
It often breaks up and gives a series of peaks caused by the fragments.
What does the peak with the highest mass/charge ratio usually represent?
The original molecule that hasn’t fragmented.
Why is the mass/charge ratio equal to the Mr in mass spectrometry?
Because the charge of the ion is +1.
What is the molecular ion peak (M+)?
It is due to the molecule losing an electron and becoming both an ion and a free radical: M → [M]+• + e-
What causes the M+1 peak in a mass spectrum?
The presence of the carbon-13 isotope.
What can excess energy from ionisation do in mass spectrometry?
It can make the molecular ion unstable, causing it to vibrate and potentially break covalent bonds, leading to fragmentation.
What is fragmentation in mass spectrometry?
The process where a molecular ion splits into fragments: [M]+ → X+ + Y
What are examples of relatively stable ions in mass spectrometry?
Carbocations R+ such as CH3CH2+ and acylium ions [R-C=O]+.
What affects peak intensity in mass spectrometry?
The stability of the ion – the more stable, the greater the peak intensity.
How can mass spectra help determine the structure of an unknown compound?
Because each organic compound has a unique mass spectra pattern used as a ‘fingerprint’ for identification.
Why can mass spectra of two isomers differ despite having the same M+ peak?
Because their fragmentation patterns will be different.
How is mass spectrometry commonly used today?
It is often linked to a database that helps identify unknown compounds.
What does an infrared spectrometer do?
Passes IR radiation through a sample and measures how much is absorbed by covalent bonds in the sample.
What type of molecules can absorb infrared radiation?
Molecules that change their polarity as they vibrate, e.g., C-H, C=O, O-H.
Why does each bond absorb IR radiation differently?
Because each bond has a different absorption frequency. By passing a range of frequencies through the sample, the transmission of the radiation to the analyser will correspond to the bonds present in the molecule.
What is an IR spectrum?
A graph that plots % transmitted against wavenumber, showing which IR frequencies were absorbed by bonds in the molecule
What is the main purpose of IR spectrometry?
To identify functional groups in molecules.
How is the identification of functional groups done using IR spectra?
By recognizing the position/shape of peaks and matching them to known wavenumber data.
What is the region below 1500 cm⁻¹ in an IR spectrum called?
The fingerprint region.
Why is the fingerprint region important and what are its issues?
- Complicated and contains many signals – picking out functional group signals difficult
- This part of the spectrum is unique for every compound
- This region can also be used to check if a compound is pure as the computer will compare the IR spectra against a database of known pure compounds to identify the compound
How can infrared spectroscopy show changes in functional groups during a reaction?
Certain peaks disappear and new ones appear indicating functional group transformations.
What happens to the IR spectrum when an alcohol is oxidised to an aldehyde?
The O-H peak disappears and a C=O peak appears.
What changes occur in the IR spectrum when an aldehyde is oxidised to a carboxylic acid?
An O-H peak at a slightly lower frequency than before appears alongside the C=O peak.
Why don’t H2, O2, and N2 show up in IR spectra?
Because they cannot change their polarity as they vibrate and thus do not absorb infrared radiation.
