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Chemistry > Topic 7 Modern Analytical Techniques > Flashcards

Topic 7 Modern Analytical Techniques Flashcards

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1
Q

Molecular ion peak

A

Indicates the species formed from the molecule by the loss of one electron.

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2
Q

Fragmentation

A

Occurs when the molecular ion breaks into smaller pieces.

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3
Q

Base peak

A

Indicates the peak with the greatest abundance. The tallest peak. Relative intensity of 100%. It represents the most stable fragment.

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4
Q

In a mass spectrum of an element, what does a peak represent?

A

An isotope of an element.

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5
Q

The peak furthest right on an m/z spectrum

A
  • Has the greatest m/z value.
  • Is the molecular ion peak.
  • Indicates the relative molecular mass of the organic compound.
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6
Q

Equation for the formation of a molecular ion peak (example: butane):

A

C4H10 + e- –> C4H10 + 2e-

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7
Q

How does the molecular ion peak form?

A

An electron collides with the molecule, and knocks out an electron to form a positive ion.

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8
Q

M + 1 peak

A

Small peak just right of the molecular ion peak due to the presence of the 13C isotope of carbon.

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9
Q

13C

A

1% of all naturally-occurring carbon atoms.

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10
Q

14C

A

Used in radiocarbon dating. The proportion of 14C atoms in a sample of an organic compound is negligible.

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11
Q

What causes peaks with smaller m/z values?

A

Fragmentation, rearrangement reactions and the loss of more than one electron.

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12
Q

Rearrangement

A

Beyond the spec. Often unpredictable. Explains the high number of peaks in some spectra.

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13
Q

When a C-C bond breaks by fragmentation, what is produced?

A
  • Another positive ion.
  • A neutral species, usually a radical.
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14
Q

y-axis on a mass spectrum

A

Relative intensity (%) 0%-100%

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15
Q

x-axis on a mass spectrum

A

m/z (no units) Usually 0 to just past the molecular ion peak.

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16
Q

What causes all the peaks formed by fragmentation?

A

Positive ions as radicals are not detected in a mass spectrometer.

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17
Q

The breaking of a carbon-hydrogen bond

A

Can occur, but is not considered fragmentation.

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18
Q

Fragmentation of the molecular ion of ethane (example)

A

(CH3–CH3)+dot –> CH3 dot + CH3+
Fragments to form a methyl cation + a methyl radical, as there is only one C-C bond. Both methyls are equally likely to form the cation, so the products are identical. Thus, the spectrum does not depend on how the bond breaks. There is a peak at m/z= 15.

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19
Q

Fragmentation of the molecular ion of propane (example)

A

There are two equivalent C-C bonds, so there are two possible peaks that form: the methyl m/z= 15 cation & the ethyl m/z= 29 cation.

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20
Q

What is the possible ion for an m/z value of 15?

A

CH3+

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21
Q

What is the possible ion for an m/z value of 17?

A

OH+

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22
Q

What is the possible ion for an m/z value of 28?

A

CO+

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23
Q

What are the possible ions for an m/z value of 29?

A

CH3CH2+ & CHO+

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24
Q

What is the possible ion for an m/z value of 31?

A

CH2OH+

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25
Q

What are the possible ions for an m/z value of 43?

A

CH3CH2CH2+
CH3CHCH3+
CH3CO+

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26
Q

What are the possible ions for an m/z value of 45?

A

COOH+
CH3CHOH+

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27
Q

What are the possible ions for an m/z value of 57?

A

C4H9+ (This represents 4 possible structures.)

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28
Q

Compare the m/z values of the fragment ion & the molecular ion. What does the difference between the two indicate?

A

What has been lost during fragmentation.

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29
Q

What happens when some molecules absorb infrared radiation?

A

Stretching vibrations: where the bond length increases & decreases.
Bending vibrations: where the bond angle increases & decreases.

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30
Q

When a molecule absorbs infrared radiation, upon what factors does the amount absorbed depend?

A
  • The length of the bond.
  • The strength of the bond.
  • The mass of each atom involved in the bond.
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31
Q

Why do simple, non-polar molecules not absorb infrared radiation?

A

The absorption of infrared radiation is linked to changes in the polarity of the molecule.

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32
Q

Infrared radiation

A

The part of the electromagnetic spectrum with frequencies below that of red light.

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33
Q

Stretching

A

Occurs when absorbs infrared radiation and uses it to alter the length of the bond.

34
Q

Transmittance value

A

Value in an infrared spectrum that represents the amount of radiation absorbed at a particular wavenumber.

35
Q

Wavenumber of an infrared absorption

A

Represents the frequency of infrared radiation absorbed by a particular bond in a molecule. Reciprocal of the wavelength.

36
Q

Intensity of an infrared absorption

A

The amount of infrared radiation absorbed.

37
Q

y-axis on an IR spectrum.

A

Transmittance 0% to 100%

38
Q

x-axis on an IR spectrum.

A

Wavenumber (cm-1). 4000-500 cm-1. The scale changes at 2000 cm-1.

39
Q

Weak intensities

A

High transmittance value/weak absorption

40
Q

Strong intensities

A

Low transmittance value/strong absorption

41
Q

Fingerprint region

A

1500-500 cm-1. Unique to each molecule, and can be used to identify a molecule by comparing it with known samples in a database. Results from vibrations produced by the whole molecule.

42
Q

Why are ranges of values given for each bond?

A

A functional group is affected by other functional groups nearby. A functional group will give one value in one compound & another value in another compound.

43
Q

IR spectrum exam technique: what should be stated?

A

The bond responsible for the peak & the wavenumber range.

44
Q

What causes the broad peak for O-H bond in a carboxylic acid?

A

The formation of dimers: two carboxylic acids forming hydrogen bonds with each other.

45
Q

How does mass spectrometry work?

A

The gaseous sample is injected into the mass spectrometer. The molecules are ionised, & then fragmented.

46
Q

Exam technique for mass spectrometry

A

Always identify fragments as + ions.

47
Q

The fragmentation pattern

A

Unique to each compound. The presence or absence of peaks can be used to identify compounds.

48
Q

High-resolution mass spectrometry

A

Uses more precise isotopic masses to measure the mass of a molecular ion.

49
Q

m/z

A

Mass-to-charge ratio. If charge= +1, then m/z value = mass of the ion.

50
Q

Molecules with polar bonds absorb IR. What happens when this occurs?

A

The polarity of the molecule changes. The vibrational energy of the molecule increases, so stretching and bending of the bonds occurs. Each bond vibrates at a particular frequency & absorbs IR at a particular frequency.

51
Q

Method to find Mr from the molecular ion m/z

A

Start with the hydrocarbon with the maximum number of C atoms that has an Mr less than the m/z of the molecular ion. Then add O atoms.

52
Q

Why does the OH bond absorb over a wide range of frequencies?

A

Hydrogen bonding. Produces a broad band.

53
Q

How can IR spectra show whether a reaction has been successful?

A

It can show if a functional group has changed.

54
Q

Greenhouse gases, specifically CO2

A

H2O, CO2, CH4 & NO all absorb IR radiation.
In CO2, the centres of partial + & - charge are in the centre. Symmetric stretching CO2 does not absorb in the infrared region, as the centres of partial + & - are unchanged. Asymmetric stretching in CO2 does absorb in the IR spectrum as the centre of partial - is unaltered.

55
Q

Why don’t molecules such as H2, and atmospheric gases such as O2 & N2, absorb infrared?

A

The atoms are identical, so there is no change in dipole when they vibrate.

56
Q

C-H aldehyde

A

2700-2900 cm-1 medium-strong

57
Q

C-H alkene

A

3095-3010 cm-1 (just above 3000) medium-strong

58
Q

C-H alkane

A

2962-2853 cm-1 (just below 3000) medium-strong

59
Q

C=C alkene

A

1669-1645 cm-1 medium

60
Q

C=O aldehyde

A

1740-1720 cm-1 strong

61
Q

C=O ketone

A

1720-1700 cm-1 strong

62
Q

C=O carboxylic acid

A

1725-1700 cm-1

63
Q

C-O alcohols

A

1000-1300 cm-1 strong

64
Q

C-Cl

A

700-800 cm-1 strong

65
Q

O-H carboxylic acid

A

3300-2500 cm-1 medium/broad

66
Q

N-H amine

A

3500-3300 cm-1 strong

67
Q

O-H alcohols

A

3750-3200 cm-1 strong/broad

68
Q

Pros of chemical tests

A

Rapid result.
Tests are readily available.

69
Q

Cons of chemical tests

A

Not as sensitive.
Uses some of the sample, which cannot be recovered.

70
Q

What must be included when giving the formula of a fragment in mass spectrometry?

A

+ charge.
DO NOT include the bond to the fragment.

71
Q

When deducing the Mr of a compound from a mass spectrum, what must be included?

A

State the m/z of the molecular ion peak. State that the molecular ion peak has the highest m/z.

72
Q

What factor determines the size of the titre in titration?

A

The number of moles of substance being titrated.

73
Q

Typical ion responsible for a peak at 69 m/z.

A

[C5H9]+

74
Q

Greenhouse gases.

A

Absorb IR radiation.

75
Q

81Br79Br & 79Br81Br

A

Appear as the same peak in the mass spectrum.

76
Q

Pros of chemical tests:

A

Rapid result, less expensive; tests are readily available.

77
Q

Cons of chemical tests:

A

Not as sensitive. Use some of the sample, which cannot be recovered.

78
Q

IR OH stretch of a carboxylic acid.

A

Very broad. Less intense.

79
Q

When does an alkene have E/Z isomers?

A

There is restricted rotation around the double bond. Each carbon in the double bond is attached to 2 different groups.

80
Q
A

Chemistry (23 decks)

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