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Chemistry A-Level > Mass spectrometry and IR spectroscopy (Chapter 17) > Flashcards

Mass spectrometry and IR spectroscopy (Chapter 17) Flashcards

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1
Q

How are mass spectra used?

A

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

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2
Q

What happens when an organic compound is placed in a mass spectrometer?

A

It loses an electron and forms a positive ion, the molecular ion

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3
Q

What does the mass spectrometer detect?

A

The mass-to-charge ratio (m/z) of the molecular ion which gives the molecular mass of the compound

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4
Q

What is normally (always) the m/z value?

A

1, because z ( the charge) is 1

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5
Q

Give the equation for the molecular ion of octane

A

C8H18(g) => C8H18+(g) + e-(g)

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6
Q

What is the symbol for the molecular ion?

A

M+

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7
Q

How would you find the M+ peak on a mass spectrum?

A

The M+ peak is the clear peak at the highest m/z value on the right-hand side of the mass spectrum

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8
Q

What does the M+ peak indicate?

A

The molecular mass of the compound

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9
Q

What is the M+1 peak?

A

A very small peak one unit after the M+ peak

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10
Q

Why does the M+1 peak exist?

A
  • Due to the natural abundance (1.1%) of carbon-13, a small proportion of the molecules will contain an atom of carbon-13 and therefore have a molecular mass 1 greater than the normal molecular mass
  • The more carbons present, the larger the M+1 peak
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11
Q

What is fragmentation?

A

The process by which in the mass spectrometer some molecular ions break down into smaller pieces known as fragments

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12
Q

What are the other peaks in a mass spectrum caused by?

A

Fragment ions, formed from the breakdown of the molecular ion

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13
Q

Describe what the simplest fragmentation does

A

It breaks a molecular ion into two species - a positively charged fragment ion and a radical
- Any positive ions formed will be detected by the mass spectrometer but the uncharged radicals are not detected

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14
Q

Give the equation for the fragmentation of propan-1-ol into its fragment ion with m/z=31

A

CH3CH2CH2OH = CH2OH+ + CH3CH2•

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15
Q

Why is the mass spectrum of each compound unique?

A
  • Because molecules will all fragment in slightly different ways depending on their structure
  • Two compounds can have the same M+ peak but different fragment ions
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16
Q

What are atoms in molecules joined by?

A

Covalent bonds

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17
Q

Describe what happens with covalent bonds and infrared radiation

A
  • Covalent bonds possess energy and vibrate naturally about a central point, the amount of vibration increasing with increasing temperature
  • The atoms in molecules are therefore in constant motion
  • The bonds can absorb infrared radiation, which then makes them stretch more
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18
Q

What are the two types of vibration in IR spectroscopy?

A

A stretch and a bend

19
Q

What is a stretch?

A

A rhythmic movement along the line between the atoms so that the distance between the two atomic centres increases and decreases

20
Q

What does a bend result in?

A

A change in bond angle

21
Q

What two things does the amount that a bond stretches or bends depends on?

A
  • The mass of the atoms in the bond - heavier atoms vibrate more slowly than lighter atoms
  • The strength of the bond (single, double, polarisation etc) - stronger bonds vibrate faster than weaker bonds
22
Q

What frequency of radiation can any particular bond only absorb?

A

Frequency which is the same as the natural frequency of the bond

23
Q

What scale do chemists use to measure the frequency of the bonds?

A

Wavenumber

24
Q

What is wavenumber proportional to?

A

Frequency

25
Q

Why do chemists use wavenumber to measure frequency?

A

Because the frequency values are very large

26
Q

In what IR wavenumber range are the vibrations of most bonds observed in?

A

200/cm - 4000/cm

27
Q

What happens to the Sun’s visible and IR radiation when it enters the atmosphere?

A
  • It is relatively unaffected by atmospheric gases
  • The radiation passes through the atmosphere to the Earth’s surface, where most of it is absorbed
  • However, some is re-emitted from the Earth’s surface in the form of longer-wavelength IR radiation
28
Q

What absorbs this longer wavelength IR radiation?

A

Water vapour, carbon dioxide and methane (greenhouse gases)

29
Q

Why do greenhouse gases absorb the longer-wavelength IR radiation?

A

Because it has the same frequency as the natural frequency if their bonds

30
Q

What happens when greenhouse gases absorb the longer-wavelength IR radiation?

A
  • Eventually, the vibrating bonds in these molecules re-emit this energy as radiation that increases the temperature of the atmosphere close the the Earth’s surface, leading the global warming
31
Q

What are the three most abundant greenhouse gases?

A

Water vapour, carbon dioxide and methane

32
Q

What is infrared spectroscopy used for?

A

Identifying the functional groups present in organic molecules

33
Q

What happens during IR spectroscopy?

A

1) The sample under investigation is placed inside an IR spectrometer
2) A beam of IR radiation in the range 200-4000/cm is passed through the sample
3) The molecule absorbs some of the IR frequencies, and the emerging beam of radiation is 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

34
Q

What is each dip in an IR spectrum called?

A

A peak

35
Q

What is the fingerprint region of an IR spectrum?

A

A region below 1500/cm where there are a number of peaks

36
Q

How can the fingerprint region be used?

A

It contains unique peaks which can either be used to identify the particular molecule under investigation, either using computer software or by physically comparing the spectrum to booklets of published spectra

37
Q

What functional groups should you be able to identify from an IR spectrum?

A

O-H group in alcohols
C=O group in aldehydes, ketones and carboxylic acids
COOH group in carboxylic acids

38
Q

What peak should you also be aware about in an IR spectrum?

A

A characteristic peak produced by all organic compounds between 2850 and 3100/cm from the presence of C-H bonds (often confused with O-H)

39
Q

What is the characteristic peak of an alcohol in an IR spectrum?

A

A broad peak (due to H-bonding) between 3200-3600/cm

40
Q

What is the characteristic peak of an aldehyde or ketone?

A

A sharp peak at 1700/cm (1630-1820)

41
Q

What are the characteristic peaks of a carboxylic acid?

A

A very broad peak between 2500-3330/cm

A sharp peak at 1700/cm (1630-1820)

42
Q

What are two applications of IR spectroscopy?

A

Identification of pollutants

IR-based breathalysers

43
Q

How can IR spectroscopy be used to identify pollutants?

A
  • Many pollutants can be identified by their IR spectral fingerprints
  • Remote sensors analyse the IR spectra of vehicle emissions to detect and measure CO, CO2 and hydrocarbons in busy town centres or by motorways to monitor localised pollution
44
Q

How can IR spectroscopy be used in breathalysers?

A
  • IR based breathalysers pass a beam of IR radiation through the captured breath in the sample chamber and detect the IR absorbance of the compounds in the breath
  • The characteristic bonds present in ethanol are detected
  • The more IR radiation absorbed, the higher the reading, and the more ethanol in the breath

Chemistry A-Level (27 decks)

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