infra-red spectroscopy

Question 1

why do we use infra-red spectroscopy?

Answer 1

infra-red spectroscopy is used to identify the functional groups present in organic compounds

Question 2

how can we identify certain bonds on the infra-red spectrum?

Answer 2

chemical bonds absorb infra-red radiation of specific frequencies so a particular bond can be matched to a particular absorption range on the infra-red spectrum

Question 3

explain what infra-red does to bonds?

Answer 3

• covalent bonds between atoms vibrate naturally.
• infra-red radiation is absorbed by the bonds and the absorbed energy makes the bonds vibrate even more
• bonds can stretch and bend

Question 4

draw arrows to show stretching and bending in water molecules

Question 5

Different bonds absorb infra-red radiation at different frequencies. the frequency absorbed depends on?

Answer 5

1. the bond strength
2. the masses of the atoms

Question 6

what is a high energy bond?

Answer 6

strong bonds are formed between light atoms
e.g O-H, C-H, N-H

Question 7

What are low energy bonds?

Answer 7

weak bonds are formed between heavy atoms
e.g C-O, C-C, C-halogen

Question 8

what corresponds to a particular wavelength of electromagnetic radiation?

Answer 8

the energy corresponding to a bond stretch or bend

Question 9

two equation which can be used to frequency, wavelength and wavenumber

Answer 9

• E = planks constant (h) x frequency (v)
• v = velocity of light (c) / wavelength

Question 10

what are wavenumbers?
why are wavenumbers used?

Answer 10

• wavenumbers are reciprocal wavelengths they are numerically more convenient than frequency or wavelength

Question 11

the sample will absorb infra-red frequencies corresponding to..?

Answer 11

energies of the bond stretches and bends in tis molecules

Question 12

how can a spectrum be plotted?

Answer 12

by recording the intensity of the remaining (transmitted) infra-red radiation a spectrum can be plotted

Question 13

what does the infra-red spectrum allow us to do?

Answer 13

identify the functional groups and the compound

Question 14

when infra-red radiation is absorbed by the bonds, what is reduced?

Answer 14

the transmission (the light passing through the sample)

Question 15

how can we identify a C-H bond?

Answer 15

has a typically sharp/jagged trough in all organic compounds

Question 16

what is the fingerprint region?

Answer 16

complex region of the spectrum that contains many peaks (1500cm-1 - 400cm-1)

Question 17

how can we find a compound based on the fingerprint region?

Answer 17

the fingerprint region is unique to each compound

Question 18

what would a no match be..?

Answer 18

possibly a new compound

Question 19

how could we possibly not the difference of a carbonyl compound?

Answer 19

• very sharp peak at C=O

Question 20

how could we find a carboxylic acid?

Answer 20

show a broad absorbtion at O-H and strong broad absorbtion at C=O

Question 21

how could we identify an alcohol?

Answer 21

broad absorbtion at O-H

Question 22

how could we identify an ester?

Answer 22

strong broad absorbtion at C=O

Question 23

What are examples of greenhouse gases?

Answer 23

Carbon dioxide, methane and water vapour

Question 24

explain a disadvantage of infra-red radiation?

Answer 24

may be absorbed by greenhouse gases which may contribute to global warming

Question 25

why does methane, water vapour and carbon dioxide get absorbed by infra-red radiation?

Answer 25

• the vibration of their bonds ( O-H in water , C-H in methane, C=O in carbon dioxide corresponds to the energies in the infra-red spectrum

Question 26

how does infra-red radiation lead to global warming?

Answer 26

• green house gases absorb infra-red radiation
• the energy absorbed by these molecules can be transferred to other molecules when they collide (e.g as kinetic energy)
• there is therefore an increase in average energy of molecules in the atmosphere –> global warming

Question 27

which greenhouse gases absorb the most amount of radiation and how does it effect global warming?

Answer 27

• co2 is a great absorber of infra-red but it is not the most significant greenhouse gas because it only makes up a tiny percentage of the atmosphere
• water vapour is fairly present and is responsible for most infra-red absorbed

Question 28

why don’t gases like oxygen and nitrogen absorb infra red?

Answer 28

because they are not polar bonds, molecules need polar bonds in order to absorb infra-red

Question 29

interpreting a mass spectrum

Question 30

what happens in the mass spectrum when ethanol is injected into a mass spectrometer.

how is a molecular ion peak formed/ what is the molecular ion peak?

Answer 30

• a sample of ethanol vapour is injected into a mass spectrometer
• electrons from an electron gun knock electrons off the molecules of ethanol (ionisation)
• molecules usually contain pairs of electrons, so when an electron is removed an unpaired electron remains and the ion has a positive charge - this is called a radical ion
• ethanol molecules lose an electron to become [C2H5OH]+*
  -the positively charged ethanol ions are accelerated by an electric field, pass through a drift area and then they are detected
• the [C2H5OH]+* ions are the heaviest that can be made by a sample of ethanol and the peak they produce on a mass spectrum is called the molecular ion peak

Question 31

what symbol does the molecular ion have?

Answer 31

M+*

Question 32

what is the molecular ion peak?

Answer 32

the m/z value for the molecular ion peak is the molecular mass
e.g the mass of the ethanol molecular ion = 46 so peak will be at m/z value = 46

Question 33

so what is the overall rule?

Answer 33

• when an organic compound M is put through a mass spectrometer, the peak at it’s maximum value of m/z corresponds to the molecular ion M+*. the value of the m/z for this ion is equal to the relative molecular mass.

Question 34

why would this not be accurate at assuming the compund?

Answer 34

e.g C3H8 - 44 C2H40 - 44 both have the same molecular mass

Question 35

what could be used to get a more accurate mass?

Answer 35

a high resolution mass spectrometer

Question 36

what are the following atoms mass to 4 decimal places:
Hydrogen, nitrogen and Oxygen

Answer 36

H = 1.0078
N = 14.0031
O = 15.9949

Question 37

what are the two most important isotopes in organic chemistry?

Answer 37

chlorine and bromine

Question 38

what are the two naturally occurring isotopes and what are their abundances?

Answer 38

cl35 and cl37 in the abundance ratio 3:1

Question 39

so the mass spectra of monochloroalkanes exhibit …?
due to what?

Answer 39

2 molecular ions 2 mass units apart in a 3:1 intensity ratio

due to the presence of R35cl and R37CL

Question 40

for dichloroalkanes how many molecular peaks are there?

Answer 40

3 molecular ion peaks 2 mass units apart from each other in a 9:6:1 ratio

Question 41

what is the abundance of bromine isotopes?

Answer 41

they are approximately equally abundant

Question 42

so what is the mass spectra of monobromoalkanes exhibit?

Answer 42

exhibit 2 molecular ions 2 mass units apart in an intensity ration 1:1

Question 43

explain how these features are useful?

Answer 43

in distinguishing the presence of these two isotopes in compounds

Question 44

FINISH