4.2.4 Analytical Techniques

Question 1

what is mass spectrometry used for

Answer 1

to identify the molecular mass of an organic molecule

Question 2

how does mass spectrometry work

Answer 2

• organic compound is placed in a mass spectrometer
• loses an electron, and forms a positive ion called molecular ion (M+ ion)
• the spectrometer detects the mass-to-charge ratio of the molecular ion (m/z value)
• gives the molecular mass

Question 3

what is the equation for the formation of a propanol molecular ion

Answer 3

CH3CH2CH2OH → CH3CH2CH2OH + e-

Question 4

what is a molecular ion/M+

Answer 4

the positive ion formed when a molecule loses an electron

Question 5

how do you find the molecular mass of a compound from its mass spectrum

Answer 5

• locate the molecular ion M+ peak
• has the highest m/z value and is the furthest to the right

Question 6

explain the m+1 peak

Answer 6

• often see a small peak one unit after the m+ peak
• is the m+1 peak
• there due to the presence of the C-13 isotope

Question 7

what is fragmentation

Answer 7

• when in a spectrometer, some molecular ions break down into smaller pieces of fragments
• called fragment ions
• form the other peaks in the spectrum

Question 8

how can fragmentation occur on a molecule

Answer 8

• breaks the molecular ion into 2 species
• a positively charged fragment ion
• and a radical

Question 9

which fragments are detected in a mass spectrum

Answer 9

• only the positively charged fragment ion
• the radical is uncharged, so goes undetected

Question 10

give an example equation of how fragmentation can occur with propanol

Answer 10

CH3CH2CH2OH+ → CH2OH+ + CH3CH2·

• the fragment ion CH2OH+, shows up with its own m/z value
• the CH3CH2· radical goes undetected

Question 11

why does fragmentation help identify a unique organic compound

Answer 11

• the mass spectrum of each compound will be unique
• as each molecule will fragment differently
• depending on its structure
• even if it has the same molecular mass as another isomer

Question 12

what are the examples of some common fragments

Answer 12

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

• useful to see which fragments may have formed compounds
• can also compare the m/z value of the molecule and of the fragment, to see the other component (the radical that broke off)

Question 13

what is infrared spectroscopy used to identify

Answer 13

functional groups in organic compounds

Question 14

explain the bonds inside organic compounds

Answer 14

• atoms in molecules are joined by covalent bonds
• bonds possess energy
• and vibrate naturally about a central point
• the amount they vibrate increases with increasing temperature
• and atoms in molecules are in constant motion

Question 15

what happens to a bond in an organic molecule when it absorbs infrared radiation

Answer 15

• it vibrates more
• and absorbs energy

Question 16

what are the 2 types of vibrations a bond can undertake

Answer 16

• can stretch (rhythmic movement along the line between atoms so that the distances between their atomic centres increases and decreases)
• can bend (which changes their bond angle)

Question 17

what does the amount of vibration (bend or stretch) of a bond depend on

Answer 17

• the mass of the atoms (heavier vibrate more)
• the strength of the bond (stronger bonds vibrate faster)

Question 18

what is true about the type of frequency a bond can absorb

Answer 18

any bond can only absorb radiation with the same frequency as the natural frequency of the bond

Question 19

how is bond frequency measured

Answer 19

• actually measuring it gives a very large value
• so use wavenumber instead (still proportional to frequency, but more convenient)
• most bonds are between 200 to 4000 cm-1

Question 20

what happens to most of the sun’s visible and infrared radiation

Answer 20

• it is unaffected by atmospheric gases
• it just passes through the earth’s atmosphere and is mostly absorbed

Question 21

what happens to the radiation from the sun that isn’t absorbed

Answer 21

• it is re-emitted
• in a form of longer wavelength IR radiation

Question 22

what are the most abundant greenhouse gases in the atmosphere

Answer 22

water vapour H2O(g)
carbon dioxide CO2
methane CH4

Question 23

what are the impacts of greenhouse gases on global warming

Answer 23

• they absorb the longer wavelength IR radiation re-emitted from the earth’s surface
• as it has the same frequency as the natural frequency of their C=O/O-H/C-H bonds
• the vibrating bonds in these molecules will then reemit this energy eventually as radiation
• which increases the atmosphere close to earth’s surface

Question 24

what are some ways to reduce global warming

Answer 24

• reduce CO2 emissions
• reduce pollution
• and convert to renewable energy sources

Question 25

what is the process of carrying out infrared spectroscopy

Answer 25

- place sample inside IR spectrum - beam of IR radiation is passed through (of wavenumber 200-4000) - the molecules absorb certain IR frequencies (same as their own natural frequencies) - the emerging IR beam is analysed to see the frequencies absorbed

Question 26

what are the peaks on an IR spectrum

Answer 26

- the dips - has its own wavenumber/ range of to show related bond

Question 27

where is the fingerprint region on an IR spectrum, and what is it

Answer 27

- below 1500 cm-1 - contains unique peaks that could identify a molecules using computer software/comparison to published spectra - BUT it is difficult to predict with certainty the functional groups here - as so many peaks and overlap - so to find functional group, use peaks OUTSIDE THE REGION as clearer

Question 28

what peak will all organic compounds have

Answer 28

a peak at 2850-3100 cm-1 due to the presence of the C-H bond

Question 29

how would you find alcohols using a IR spectrum

Answer 29

- have the O-H bond peak - could also use the C-O bond peak - but inside the fingerprint region, so difficult to assign as so many peaks here

Question 30

how would you find aldehydes/ketones using an IR spectrum

Answer 30

find the C=O bond peak

Question 31

how would you find a carboxylic acid using an IR spectrum

Answer 31

- find the C=O bond peak - find the O-H broad bond peak - could also use the C-O bond, but unreliable as inside fingerprint region

Question 32

what are the 2 applications of IR spectroscopy

Answer 32

- identifying the levels of pollutants in a given area - breathalysers

Question 33

how is IR spectroscopy used to identify pollutants

Answer 33

- can identify pollutants and monitor localised pollution - remote sensors can analyse the IR spectra of vehicle emissions - can measure the CO/CO2/hydrocarbons present

Question 34

how is IR spectroscopy used in breathalysers

Answer 34

- a beam of IR radiation is passed through the captured breath in the sample chamber - detects the IR absorbance of the sample - detects the bonds in ethanol - the more IR absorbed, the higher the reading and higher the amount of ethanol in the breath

Question 35

what is the 3 step process of being able to identify a compound analytically

Answer 35

1) use the % composition to identify the empirical formula 2) use a mass spectrum to identify the molecular mass and any fragments present 3) use infrared spectrum to identify functional groups and bonds present