Chapter 17 - Infrared spectroscopy and mass spectrometry (17.1-17.2) Flashcards

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1
Q

How do you work out the MR from a mass spectrum?

A

It is the last peak on a mass spectrum formed when a molecule loses an electron.

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2
Q

Why will there sometimes be a very small peak after the M+ peak?

A

This is because 1.1% of carbon is present as a carbon-13 isotope and so the molecular ion peak would be one more.

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3
Q

What is fragmentation?

A

It’s when some molecular ions break down into smaller pieces by the breaking of one bond forming a fragment ion

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4
Q

What fragment ion would have the m/z value of 15?

A

CH3+

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5
Q

What fragment ion would have an m/z value of 29?

A

C2H5+

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6
Q

What fragment ion would have an m/z value of 31?

A

CH3O+

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7
Q

What fragment ion would have an m/z value of 43?

A

C3H7+

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8
Q

What fragment ion would have an m/z value of 45?

A

C2H5O+

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9
Q

What fragment ion would have an m/z value of 57?

A

C4H9+

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10
Q

What else can be useful in identifying a compound in mass spectroscopy?

A

Look at the differences between the m/z value of the molecular ion peak and some fragment ion peaks.

(For example a peak at 15 for CH3+ could be small but there could be a larger peak at 15 less than the molecular ion peak, so you know CH3 has been removed)

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11
Q

What happens when a bond absorbs infrared radiation?

A

It makes them bend or stretch more

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12
Q

What does the amount that a bond stretched or bend depend on?

A

The mass of the atoms in the bond (heavier atoms vibrate more slowly) and the strength of the bond (stronger bonds vibrate faster)

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13
Q

What are the three most abundant greenhouse gases?

A

Water vapour, carbon dioxide and methane.

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14
Q

Describe the greenhouse effect

A

1) IR radiation passes through the atmosphere
2) It reaches the earths surface where most of it is absorbed
3) However some is re-emitted as longer-wavelength IR radiation
4) The greenhouse gases absorb this IR radiation as it has the same frequency as the frequency of their bonds
5) They re-emit this energy which increases the temperature of the earth

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15
Q

What is the area on an infrared spectra below 1500?

A

It is the fingerprint region and contains unique peaks which can be used to identify a particular molecule using computer software

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16
Q

How do you recognise the OH- group in an alcohol?

A

There is a very wide “U” shaped peak between 3200 and 3600 which is only given by an OH- group.

17
Q

How do you recognise an aldehyde or ketone form an infrared spectroscopy?

A

There is a very sharp peak showing the C=O around 1700cm but there should not be any other significant peaks apart from C-H

18
Q

How do you recognise the infrared spectrum of a carboxylic acid?

A

Like an alcohol it will have a very broad “U” shaped dip around 3000cm (not just a small peak) but also it will have a very sharp peak at 1700cm like aldehydes and ketones to show the C=O.

19
Q

Give 2 examples of uses of applications of infrared spectroscopy

A

Analysing vechile emissions - measure CO, CO2 and hydrocarbon levels in towns

Breathalysers - passes a beam of IR through breath and checks absorbance to see if it has characteristics of ethanol are detected.

20
Q

What is a typical sequence of how you would identify a compound?

A

Elemental analysis - empirical formula
Mass spectrometry - molecular ion and fragment ion peaks
Infrared spectroscopy - identify bonds and functional groups