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Chemistry Y12 > infra-red spectroscopy > Flashcards

infra-red spectroscopy Flashcards

1
Q

why do we use infra-red spectroscopy?

A

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

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2
Q

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

A

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

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3
Q

explain what infra-red does to bonds?

A
  • 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
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4
Q

draw arrows to show stretching and bending in water molecules

A
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5
Q

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

A
  1. the bond strength
  2. the masses of the atoms
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6
Q

what is a high energy bond?

A

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

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7
Q

What are low energy bonds?

A

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

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8
Q

what corresponds to a particular wavelength of electromagnetic radiation?

A

the energy corresponding to a bond stretch or bend

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9
Q

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

A
  • E = planks constant (h) x frequency (v)
  • v = velocity of light (c) / wavelength
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10
Q

what are wavenumbers?
why are wavenumbers used?

A
  • wavenumbers are reciprocal wavelengths they are numerically more convenient than frequency or wavelength
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11
Q

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

A

energies of the bond stretches and bends in tis molecules

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12
Q

how can a spectrum be plotted?

A

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

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13
Q

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

A

identify the functional groups and the compound

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14
Q

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

A

the transmission (the light passing through the sample)

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15
Q

how can we identify a C-H bond?

A

has a typically sharp/jagged trough in all organic compounds

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16
Q

what is the fingerprint region?

A

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

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17
Q

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

A

the fingerprint region is unique to each compound

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18
Q

what would a no match be..?

A

possibly a new compound

19
Q

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

A
  • very sharp peak at C=O
20
Q

how could we find a carboxylic acid?

A

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

21
Q

how could we identify an alcohol?

A

broad absorbtion at O-H

22
Q

how could we identify an ester?

A

strong broad absorbtion at C=O

23
Q

What are examples of greenhouse gases?

A

Carbon dioxide, methane and water vapour

24
Q

explain a disadvantage of infra-red radiation?

A

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

25
Q

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

A
  • 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
26
Q

how does infra-red radiation lead to global warming?

A
  • 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
27
Q

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

A
  • 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
28
Q

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

A

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

29
Q

interpreting a mass spectrum

A
30
Q

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?

A
  • 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
31
Q

what symbol does the molecular ion have?

A

M+*

32
Q

what is the molecular ion peak?

A

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

33
Q

so what is the overall rule?

A
  • 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.
34
Q

why would this not be accurate at assuming the compund?

A

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

35
Q

what could be used to get a more accurate mass?

A

a high resolution mass spectrometer

36
Q

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

A

H = 1.0078
N = 14.0031
O = 15.9949

37
Q

what are the two most important isotopes in organic chemistry?

A

chlorine and bromine

38
Q

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

A

cl35 and cl37 in the abundance ratio 3:1

39
Q

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

A

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

due to the presence of R35cl and R37CL

40
Q

for dichloroalkanes how many molecular peaks are there?

A

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

41
Q

what is the abundance of bromine isotopes?

A

they are approximately equally abundant

42
Q

so what is the mass spectra of monobromoalkanes exhibit?

A

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

43
Q

explain how these features are useful?

A

in distinguishing the presence of these two isotopes in compounds

44
Q

FINISH

A

Chemistry Y12 (11 decks)

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