Spectroscopy

Question 1

Describe a stretch

Answer 1

-It is a movement along the line between the atoms so that the distance between the two atomic centres increases and decreases

Question 2

What is a bend?

Answer 2

It result in a change in bond angle

Question 3

What affects the amount that a bond stretched or bends?

Answer 3

1. The mass of the atoms in the bond: the heavier atoms vibrate more slowly than the lighter atoms
2. The strength of the bond: stronger bonds vibrate faster than weaker bonds

Question 4

What affects the frequency of bonds?

Answer 4

1. Any particular bond can only absorb radiation that has the same frequency as the natural frequency of the bond
2. The frequency value are very large, so chemists use a more convenient to scale called wave number, which is proportional to frequency
3. The vibrations of most bonds are observed in the IR wavenumber range of 200cm to 4000cm

Question 5

How is infrared radiation and atmospheric gases related?

Answer 5

1. Much of the Sun’s visible and IR radiation is relatively unaffected by atmospheric gases
2. This radiation passes through the atmosphere to the Earth’s surface, where most of it is absorbed
3. However, some is re-emitted from the Earth’s surface in the form of longer-wavelength IR radiation
4. Water vapour, carbon dioxide, and methane absorb this longer wavelength IR radiation, because it has the same frequency as the natural frequency of their bonds
5. Eventually, the vibrating bonds in these molecules re-emit this energy as radiation that increases the temperature of the atmosphere close to the Earth’s surface, leading to global warming

Question 6

Describe the process of infrared spectroscopy and organic molecules

Answer 6

• Organic chemists use infrared spectroscopy as a means of identifying the functional groups present in organic molecules
  1. The sample under investigation is placed inside an IR spectrometer
  2. A beam of IR radiation in range 200-4000 per cm is passed through the sample
  3. The molecule absorbs some of the IR frequencies, and the emerging beam of radiation so analysed to identify the frequencies that have been absorbed by the sample
  4. The IR spectrometer is usually connected to a computer that plots a graph of transmittance against wavenumber
• In a typical IR spectrum, the dips in the graph are still called ‘peaks; and each peak is observed at a wavenumber that can be related to a particular bond in the molecule

Question 7

What is the fingerprint region?

Answer 7

1. Below 1500 per cm; there are a number of peaks in what is known as the fingerprint region of the spectrum
2. The fingerprint contains unique peaks which can be used to identify the particular molecule under investigation, either using computer software or by physically comparing the spectrum to booklets of published spectra

Question 8

What is the infrared spectra of common functional groups?

Answer 8

1. It is difficult to predict with certainty the identity of functional groups from a peak in the fingerprint region, but outside that region, peaks are clearer
2. You should be able to identify the following function groups in compounds from an IR spectrum
   - O-H group in alcohols
   - C=O group in aldehydes, ketones and carboxylic acid
   - COOH group in carboxylic acid
   - You should also be aware that all organic compound produce a characteristic peak between 2850 and 3100 cm-1 from the presence of C-H bonds and this is often confused with the O-H peak in alcohols, so you will need to take care

Question 9

What does mass spectra show?

Answer 9

Mass spectra is used to identify the molecular mass of an organic compound and to gain further information about its structure

Question 10

What happens in a mass spectrometer?

Answer 10

1. When an organic compound is placed in the mass spectrometer, it LOSES an electron and forms a POSITIVE ION, the MOLECULAR ION
2. The Mass spectrometer detects the mass-to-charge ratio (m/z) of the molecular ion which gives the molecular mass of the compound

Question 11

What is the molecular ion M+?

Answer 11

The molecular ion M+ is the positive ion formed when a molecule loses an electron

Question 12

How do you the find the molecular mass from a mass spectrum?

Answer 12

1. To find the molecular mass, the molecular ion peak (M+ peak) has to be located
2. The molecular ion peak is the clear peak at the highest m/z value in the RHS of the mass spectrum
3. You will usually see a very small peak one unit after the M+ peak and this is refereed to as the M+1 peak and the M+1 peak exists because 1.1% of carbon is present in the carbon-13 isotope

Question 13

What is fragmentation?

Answer 13

1. In the mass spectrometer some molecular ions break down into smaller pieces known as fragments in a process called fragmentation
2. The other peaks in a mass spectrum are caused by fragment ions, formed from the break down of the molecular ion

Question 14

What are the types of fragmentation?

Answer 14

1. The simplest fragmentation breaks a molecular ion into two species- a positively charged fragment ion and a radical
2. Any positive ions formed will be detected by the mass spectrometer, but the uncharged radicals are not detected

Question 15

How do you use fragmentation peaks to identify an organic molecule?

Answer 15

1. The mass spectrum of each compound is unique, as molecules will all fragment in slightly different ways depending on their structures
2. Mass spectra can therefore be used to help identify molecules, so even through two molecules may have the same molecular mass and the same molecular ion peak, the fragment ions found in the spectrum may be different

Question 16

What are the m/z values for common fragment ions?

Answer 16

1. CH3 + = 15
2. C2H5+ = 29
3. C3H7+ = 43
4. C4H9+ = 57

Question 17

Give an example of using mass spectrum for ethanol

Answer 17

1. The main features of the spectrum are:
   - Molecular ion peak, M+ at m/z =46
   - Small M+1 peak at m/z =47
   - A number of fragment ion peaks
2. You should be able to link fragments together to obtain an ethanol molecule
   - Peak at m/z = 15 for CH3+
   - Peak at m/z= 29 for CH3CH2 + (C2H5+)
   - Peak at m/z =31 for CH2OH+
   - Peak at m/z = 45 for CH3CH2O+
3. It can also be useful to look at the differences between m/z for the molecular ion and fragments ion although the peak for CH3+ ay m/z=15 is very small, there is a peak at m/z =31, which is 15 less than the value for M+

Question 18

why in infrared radiation used for covalent bonds?

Answer 18

• Atoms in molecules are joined by covalent bonds. These bonds possess energy and vibrate naturally about a central point, the amount of vibration increasing with increasing
• The atoms in molecule are therefore in constant motion
• The bonds can absorb infrared (IR ration, which makes them bend or stretch more

Question 19

Describe the infrared spectrum of an alcohol

Answer 19

• The IR spectrum of an alcohol has an absorbance peak within the range 3200-3600 cm-1 caused by the O-H bond in an alcohol
• There is also a peak between 1000-1300 cm-1 caused by the C-O bond, though this peak is often difficult to assign because of the many other peaks in the fingerprint region
• Look at notes

Question 20

Describe the infrared spectrum of an aldehyde or ketone

Answer 20

• The IR spectrum of an aldehyde or ketone has a key absorbance peak within the range 1630-1820 cm-1 caused by the C=O bond
• This peak typically absorbs close to 1700 cm-1
• Look at notes

Question 21

Describe the infrared spectrum of a carboxylic acid

Answer 21

1. The IR spectrum of carboxylic cid has a key absorb acme peak within the range 1630-1820 cm-1 caused by the C=O bond, and a broad peak at 2500-3330cm-1 caused by the O-H group in the carboxylic acid
2. As with aldehydes and ketone, the C=O peak typically absorbs close to 1700cm-1
3. There is also a peak at 1000-1300 cm-1 that represents the C-O bond
4. The C-O peak is not always reliable, as it is in the fingerprint region
   - Look at notes
   - A C=O peak at round 1700 cm-1 together with a very broad O-H absorption ittint he range 2500-3000 cm-1 indicates for the presence of a carboxylic acid in an organic molecule

Question 22

What are the applications of infrared spectroscopy?

Answer 22

1. Many pollutants can be identified by their IR spectral fingerprints
2. Remote sensors analyse the IR spectra of vehicle emissions to detect and measure carbon monoxide, carbon dioxide and hydrocarbons in busy town centres or by motorways to monitor localised pollution
3. IR based breathalysers pass a beam of IR radiation through the captured break in the sample chamber and detect the IR absorbance of the compounds in the breath
4. The characteristic bonds present in ethanol are detected and the more IR radiation absorbed, the higher the reading, and the more ethanol in the breath

Question 23

What would be a typical sequence for identification? Define each step

Answer 23

1. Elemental analysis: use of percentage composition data to determine the empirical formula
2. Mass spectrometry: use of the molecular ion peak from a mass spectrum to determine the molecular mass; use of fragment ions to identify sections of a molecule
3. Infrared spectroscopy: use of absorption breaks from an infrared spectrum to identify bond and functional groups present in the molecule
   - Once you have both the empirical formula and the molecular mass of a compound, you can determine the molecular formula of your unknown compound. By then using evidence from the infrared spectrum, it may be possible to identify the unknown compound