C17- Spectroscopy

Question 1

What is mass spectra used for?

Answer 1

To identify the molecular mass of an organic compound and to gain further information about its structure.

Question 2

What is a molecular ion?

Answer 2

The positive ion formed when an organic compound loses an electron in the ionisation area of the mass spectrometer.
M+

Question 3

How do you determine molecular mass from a mass spectrum (graph)?

Answer 3

To find molecular mass the molecular ion (M+) peak has to be located.

• this is the clear peak at the highest m/z value on the right hand side of the mass spectrum.
• the m/z value of the peak is the molecular mass.

Question 4

What is the M+1 peak?

Answer 4

A very small peak, one unit after the M+ peak.
It exists because many compounds like alcohols contain an atom of the carbon-13 isotope (1.1% only).
This gives the small M+1 peak.

Question 5

What is fragmentation and fragment ions?

Answer 5

The process in which some molecular ions break down into smaller pieces called fragments in the mass spectrometer.
This forms fragment ions and these cause the other peaks in the mass spectrometer.
- the simplest fragmentation breaks a molecular ion into two species: a +charged fragment ion and a radical. Only the fragment ions are detected by the mass spectrometer.

Question 6

What do fragment ion peaks provide?

Answer 6

Information about the structure of the organic compound.
Specific fragment ions have a specific m/z values.
By identifying these on the mass spectrum you can work out the structure of the compound.

Question 7

What are the m/z values for the 4 main fragment ions?

Answer 7

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

Question 8

How do you calculate the number of carbon atoms from a mass spectrum?

Answer 8

(Height of M+1 peak / Height of M peak) x 100

Question 9

Explain vibration of atoms in molecules.

Answer 9

• atoms in molecules are joined by covalent bonds.
• these bonds possess energy and vibrate naturally about a point.
• amount of vibration increases with increasing temperature. Due to more kinetic energy.
• the atoms are therefore in constant motion.
• the bonds can absorb IR, which makes the vibrate more.

Question 10

What are the two types of vibration?

Answer 10

1. A stretch- a rhythmic movement along the line between the atoms so that the distance between the the two atomic centres increases and decreases.
2. A bend- results in a change in bond angle.

Question 11

What affects the amount a bond stretches/bends?

Answer 11

1. The mass of the atoms in the bond- heavy atoms vibrate more slowly than lighter atoms.
2. The strength of the bond- stronger bonds vibrate faster than weak ones.

Question 12

How much radiation can a bond absorb?

Answer 12

Any particular bond can only absorb radiation that has the same frequency as the natural frequency of the bond.

Question 13

What is wavenumber?

Answer 13

The frequency values are large so wavenumber is used as the scale. Wavenumber is proportional to frequency.

Question 14

Explain how IR radiation leads to global warming.

Answer 14

• the suns IR radiation passes through the atmosphere to the earth’s surface where most of it is absorbed.
• some is re-emitted from the surface as longer wavelength IR radiation.
• water vapour, carbon dioxide and methane are greenhouse gases which absorb this longer wavelength radiation as it has the same frequency as the natural frequency of their bonds.
• the vibrating bonds in these molecules eventually re-emit this energy as radiation that increases the atmospheres temperature close to Earth, leading to global warming.

Question 15

Why is infrared spectroscopy used?

Answer 15

Organic chemists use infrared spectroscopy in order to identify the functional groups present in organic molecules.

Question 16

How does infrared spectroscopy work?

Answer 16

1. Sample is placed inside the IR spectrometer.
2. Beam of IR radiation in the range 200-4000 cm-1 is passed through the sample.
3. molecule absorbs some of the IR frequencies. The emerging beam of radiation is analysed to identify the frequencies that have been absorbed by the sample.
4. IR spectrometer is usually connected to a computer which plots a graph of transmittance against wavenumber.

Question 17

What is the fingerprint region of an infrared spectrum?

Answer 17

The region below 1500cm-1 which contains a number of unique peaks that can be used to identify the particular molecule.
This can be done using a computer software or physically comparing the spectrum to booklets of published spectra.

Question 18

What peak do all organic compounds produce on an IR spectrum?

Answer 18

A peak between 2850- 3100.

- due to the presence of C-H bonds.

Question 19

infrared spectrum of an alcohol?

Answer 19

• has an O-H peak within 3200-3600.
• C-O peak between 1000-1300. This is not always reliable and hard to assign due to the numerous other peaks in the fingerprint region.

Question 20

infrared spectrum of an aldehyde/ketone?

Answer 20

• C=O peaks within 1630-1820.

- this peak is typically close to 1700.

Question 21

Infrared spectrum of carboxylic acid?

Answer 21

• C=O peak within 1630-1820 (close to 1700 usually)
• Broad O-H peak at 2500- 3330.
• C-O peak at 1000-1300. Not always reliable as its in fingerprint region.

Question 22

What are the applications of infrared spectroscopy?

Answer 22

• pollutants can be identified by their IR spectral fingerprints.
• remote sensors analyse IR spectra of vehicle emissions to detect CO2, CO or hydrocarbons in order to monitor pollution.
• IR based breathalysers pass beam of IR radiation through breath and detect the characteristic bonds. More ethanol in breath= more IR radiation absorbed = higher reading.

Question 23

What is the typical sequence for identifying an organic compound?

Answer 23

1. Elemental analysis- use of % composition data to determine the empirical formula.
2. Mass spectrometry- use of the molecular ion peak from a mass spectrum to determine molecular mass. Use of fragment ions to identify sections.
3. IR spectroscopy- use of absorption peaks to identify bonds and functional groups present.