22. Analytical Techniques Flashcards
(13 cards)
How does IR spcectroscopy work to elucidate structures?
- A spectrophotometer irradiates a sample (usually an organic compound) with infrared radiation
- A detector measures the absorbance (intensity) of infrared radiation after it has passed through the sample
- As radiation passes through a compound, bonds will vibrate by stretching and twisting at different frequencies depending on the type of bond and their position
- The frequency of vibration dictates the wavelength of radiation that each vibrating bond absorbs
- This is measured as the reciprocal of the wavelength in a unit called wavenumber (cm⁻¹)
How can IR spectra be used to identify functional groups?
- Certain functional groups (containing certain bonds) will vibrate in certain ways, absorbing a specific wavelength and intensity of infrared radiation
- This reduces the absorbance at the detector by a particular amount at a particular wavelength
- Thus, certain functional groups are shown on spectra by characteristically shaped peaks at specific wavenumber values
- If a low amount of radiation is absorbed over a wide range of wavelengths, the peak will be broad and shallow, while if a large amount is absorbed over a small range, the peak will be narrow and deep
IR spectroscopy is limited as there is often overlap between functional groups and it cannot elucidate the entire structure of a molecule, only the presence of certain bonds
What are the key features of this spectrum?
- This spectrum is for propan-1-ol
- The fingerprint region is the group of peaks between 0 and 1500 cm⁻¹ and does not show any specific functional groups
- The broad peak at 3300 cm⁻¹ corresponds to an OH group
- The 3000 cm⁻¹ peak(s) most likely correspond to C-H stretches
How is a spectrum created using mass spectroscopy?
- Electrons are fired at a compound, knocking other electrons off and forming many cationic radicals
- These ions split into fragments, which may have a positive charge
- The fragments are accelerated into an electric field, though only the positive ions are detected
- The ions are deflected into a detector, which measures their mass to charge (m/z) ratio
How are mass spectra interpreted?
- The tallest peaks correspond to the ions with the highest abundances
- These are almost always ions with a charge of 1+, so the m/z ratio is the same as the mass
- There will be small peaks corresponding to ions with other charges, but there will be larger peaks corresponding to the same fragment but with a 1+ charge
- There will be a peak for the entire molecular ion and several peaks for different fragments of it
What is the molecular ion (M⁺) peak?
- The peak with the highest m/z value (with the exception of isotopic molecular ions) on the spectrum, which corresponds to the entire molecular ion (without fragmentation)
- It corresponds to the molecular ion with the most common isotope(s), so it will have the highest abundance of all the peaks in the region
How can mass spectra be used to find the relative atomic mass of an element with isotopes?
- Analyse the spectrum and find the peaks corresponding to the molecular ions featuring each isotope
- Their heights indicate the abundances of the isotopes, which can be used to calculate the relative atomic mass of the element
What is the [M+1]⁺ peak?
- In spectra of organic compounds, it is the molecular ion peak that has an m/z ratio one higher than the main molecular ion peak
- This is due to the presence of one carbon-13 isotope
- Its height will always be smaller than the molecular ion peak
- Organic compounds with multiple carbon-13 isotopes are rare and not considered at AS-Level
- Isotopes of hydrogen are extremely rare so also aren’t considered
How can the number of carbons in an organic compound be deduced from its mass spectrum?
- Using the formula 100 x abundance of [M+1]⁺ ion divided by 1.1 x abundance of M⁺ ion
- This works because carbon-13 has an abundance of 1.1% and the probability of there being a carbon-13 atom in an organic compound increases as the number of carbon atoms increases
What are some examples of fragments that commonly split from organic compounds that show up on mass spectra?
- Alkyl groups like CH₃⁺ or C₂H₅⁺
- Functional groups like COOH⁺
- H₂O⁺, which is often lost from alcohols
Look for their peaks on spectra that correspond to these fragments and what is left of the molecular ion after these fragments detatch to deduce the structure of the compound being analysed
What are the masses and relative abundances of the isotopes of chlorine and bromine?
- Chlorine: ³⁵Cl and ³⁷Cl at a 3:1 ratio
- Bromine: ⁷⁹Br and ⁸¹Br at a 1:1 ratio
What are the [M+2]⁺ and [M+4]⁺ peaks in compounds containing bromine and chlorine?
- The [M+2]⁺ peaks correspond to a molecular ion with one heavy chlorine or bromine isotope
- In compounds with multiple halogen atoms, there will be an [M+4]⁺ peak, which appears when a molecular ion has two heavy isotopes of chlorine or bromine
[M+2]⁺ and [M+4]⁺ peaks generally indicate the presence of chlorine or bromine in a compound
What is the ratio of the peak heights of the molecular ion peaks for a bromine compound with two bromine atoms?
- M⁺ (⁷⁹Br and ⁷⁹Br): 1
- [M+2]⁺ (⁷⁹Br and ⁸¹Br): 2
- [M+4]⁺ (⁸¹Br and ⁸¹Br): 1
- This is because the ratio is 1:1 and there are two ways to achieve the middle peak but only one way to achieve the other two
- One can apply this to the peaks of molecular ions with chlorine molecules, remembering the ratio is 3:1
