Chapter 17 - Spetroscopy Flashcards
When an organic compound is placed inside the mass spectrometer, what happens?
It loses an electron and forms a positive ion, which is called the molecular ion
How to find the molecular ion?
It will be the largest peak furthest to the right
What is the M+1 peak?
A very small peak after the molecular ion on the spectrometer
Why is the M+1 peak formed?
Isotopes - normally due to the carbon-13 isotope
What is fragmentation?
The process in which the molecular ion is broken down into smaller fragments
What are the smaller peaks on a spectrometer caused by?
Fragments
What two things are produced when a molecular ion is fragmented?
A positively charged ion and a free radical
Out of the two things produced by fragmentation, which will be detected by the spectrometer?
The positively charged ion
What does the unit m/z show?
It is a ration of mass to charge
How is it possible to tell two compounds with the same Mr apart using mass spectrometry?
They have different structures so won’t produce the same fragments, which will be seen on the spectrum
How to combine spectrometry techniques to work out a compound
1) Use the composition to work out the molecular mass of the compound
2) Work out what functional groups are in the compound from its infrared spectrum
3) Use the mass spectrum to work out the structure of the molecule
What is the m/z value of CH3+ ion?
15
What is the m/z value of a C2H5+ ion?
29
What is the m/z value of a C3H7+ ion?
43
What is the m/z value of a C4H9+ ion?
57
How to calculate the number of carbon atoms in a compound using the spectrum?
(Height of M+1 peak/height of M peak) x 100
What are the two ways in which a bond can move?
It can bend or it can stretch
What happens when a bond absorbs infrared radiation?
They bend or stretch more
What happens when a stretch occurs?
The distance between the two atomic centres increase and decrease
What happens when a stretch occurs?
The bond angle changes
What does the amount that a bond stretches or bends depend upon?
The mass of atoms in the bond
The strength of a bond
Will stronger bonds vibrate more or less?
More
When will a bond absorb infrared radiation?
When it has the same frequency as the bonds in the molecule
What can infrared spectroscopy be used for?
Identifying different functional groups because they absorb different frequencies of infrared radiation
The process of infrared spectroscopy
The sample is placed inside an infrared spectrometer
A beam of IR radiation is passed through the sample
The molecule absorbs some of the IR frequencies and the emerging beam is analysed to work out what frequencies these are
A computer then plots transmission against wavenumber
What is the fingerprint region?
Unique peaks that can be used to identify the molecule
From an infrared spectrum, how do you identify the different functional groups?
Compare the frequencies against the ones found in the data table
What will all organic compounds produce on a spectrum?
A peak between 2850 and 3100 from the C-H bond
What are the three functional groups you should be able to identify from a spectrum?
O-H group in alcohols
C=O group in aldehydes, ketones and carboxylic acids
COOH group in carboxylic acids
What will the spectrum of an alcohol show?
A peak between 3200 and 3600 because of the O-H group
What will the spectrum of an aldehyde or ketone show?
A peak between 1630 and 1820 because of the C=O bond
What will the spectrum of a carboxylic acid show?
A peak at between 1630 and 1820 caused by the C=O bond
A broad peak between 2500 - 3300 caused by the O-H group
Main use of infrared spectroscopy
Breathalysers
Why can infrared spectroscopy be used in breathalysers?
The intensity of the peaks of bonds associated with ethanol, e.g. C-H
Why is infrared spectroscopy used to monitor the concentrations of polluting gases?
Peaks associated with the bonds found in the pollutants can be measured for intensity
What is elemental analysis?
Use of percentage composition data to determine the empirical formula
What is mass spectrometry?
Use of a molecular ion from a spectrum to determine the molecular mass
What is infrared spectroscopy?
Use of absorption peaks from a spectrum to identify bonds and functional groups that are present
