Module 4 - [ch17] (spectroscopy)

Question 1

what is the molecular ion

Answer 1

when the organic compound is placed into the mass spectrometer it loses and electron to form a positive ion

Question 2

where do you find M+ peak on a mass spectrum

Answer 2

at highest m/z value
however, often before the very highest due to the M+1 peak

Question 3

what are the axis on a mass spectrum graph

Answer 3

relative intensity against m/z

Question 4

why does the M+1 peak exist

Answer 4

1.1% of carbon is present as the C13 isotope.

Question 5

what is fragmentation

Answer 5

some molecular ions break down into smaller pieces known as fragments
fragment ions cause the other peaks in a mass spectrum

Question 6

What is the simplest way a molecular ion can fragment

Answer 6

fragment cation + radical

Question 7

4 common fragment ions and their corresponding m/z values

Answer 7

CH3+ = 15
C2H5+ = 29
C3H7+ = 43
C4H9+ = 57

Question 8

what happens when a molecule absorbs infrared radiation

Answer 8

covalent bonds absorb energy and vibrate about a central point more in a stretching and bending motion

Question 9

what are the types of vibration

Answer 9

stretch
increases and decreases the distance between the atomic centres

bend
results in a change of bond angle

Question 10

Which factors affect the amount of vibration of a bond

Answer 10

mass of atoms in the bond ( heavier atoms vibrate slower than lighter ones )
strength of bond ( stronger bonds vibrate faster than weaker ones )

Question 11

what is the condition for the radiation absorbed by a particular bond

Answer 11

has to have the same frequency as the natural frequency of the bond

Question 12

why is wavenumber used?

Answer 12

frequency values are very large
wavenumber is proportional to frequency
vibrations of most bonds are observed between wavenumber 200cm^-1 to 4000cm^-1

Question 13

greenhouse effect

Answer 13

IR radiation emitted by sun
absorbed by earth’s surface, some re-emitted in the form of longer-wavelength IR radiation
greenhouse gases absorb this radiation
vibrating bonds in these molecules re-emit this energy as radiation which increases the temp of the earth’s atmosphere near the surface - global warming

Question 14

how does IR spectroscopy work

Answer 14

1. the sample is placed inside an IR spectrometer
2. A beam of IR radiation is passed through the sample
3. the molecule absorbs some of the IR frequencies, the emerging beam is analysed to identify the frequencies that are missing (that have been absorbed)
4. spectrometer connected to a computer which plots a graph of transmittance against wavenumber

Question 15

where is the fingerprint region

Answer 15

below 1500cm^-1

Question 16

what does the alcohol (O-H) peak look like

Answer 16

smooth peak
(looks like a positive quadratic)
3200 - 3600cm^-1

Question 17

what to look for when searching for an aldehyde or ketone

Answer 17

C = O bond peak
close to 1700cmm^-1

Question 18

what to look for when searching for a carboxylic acid

Answer 18

broad peak at O-H
and C=O peak

Question 19

applications of infrared spectroscopy

Answer 19

pollutants can be identified by the IR spectral fingerprints
IR based breathalysers,
pass a beam of IR radiation through breath

Question 20

what happens in a mass spectrometer

Answer 20

sample vaporised and ionised to form molecular ions
excess energy from ionisation causes bonds to vibrate more
vibration causes bonds to weaken
molecular ion splits by fragmentation

Question 21

how do you determine the Mr of a compound using a mass spectrum

Answer 21

molecular masses of the molecular ions is equal to the Mr of the compound

Question 22

what are the advantages of mass spectrometry (2)

Answer 22

cheap
small quantities of sample needed

Question 23

what is the main advantage of using mass spectrometry

Answer 23

the sample is destroyed