4.2.4- Analytical Techniques Flashcards
What happens when a molecule absorbs infrared radiation?
It makes the covalent bond vibrate more in a stretching or bending motion
What factors affect the amount of vibration of a bond?
- Bond strength
- Bond length
- Mass of each atom in the bond
How does infrared spectroscopy work?
- Every bond has a unique vibration frequency in the infrared region of the EM spectrum
- Bonds absorb radiation that has the same frequency as their frequency of vibration
- Infrared radiation emerged from a sample is missing the frequencies that have been absorbed > this information can be used to identify the compound’s functional group
What do the peaks on an infrared spectrum represent?
Absorbance of energy from the infrared radiation
What are 2 peaks that must be present to identify a carboxylic acid?
- Very broad peak at 2500- 3300 cm^-1 -> OH group
- Sharp peak at 1680- 1750 cm^-1-> C=O group
What happens inside a mass spectrometer?
- Organic compound is vaporised and passed through the spectrometer
- Some molecules lose an electron and forms molecular ions
- Excess energy from ionisation makes bonds vibrate more
- Vibration causes bonds to weaken
- Molecular ion splits by fragmentation
What is symbol of molecular ion?
M+
Is fragmentation predictable? Why?
No, because it can happen anywhere in the molecule
In a mass spectrometer, how is a molecular ion represented?
It is the peak with the highest mass/ charge ratio
The molecular mass of the molecular ion is equal to what?
Relative molecular mass of the compound
Will molecular ion peaks of two isomers of the same compound be the same or different?
Same
What is m/z value of CH3+?
15
What is the m/z value of OH- from alcohol?
17
What is the m/z value of C2H5+?
29
What is m/z value of C3H7+?
43
What is the value of m/z value of C4H9+?
57
What are advantages of using mass spectrometry?
- cheap
- small quantities of samples required
What is main disadvantage of using mass spectrometry?
The sample is completely destroyed
