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AQA A Level Organic Chemistry > 6. Organic Analysis > Flashcards

6. Organic Analysis Flashcards

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1
Q

What is the test for an alkene?

A

Add bromine water

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2
Q

What is the result for when bromine water is added to an alkene?

A

Colour change from orange to colourless

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3
Q

What is the test for an aldehyde?

A

Tollen’s reagent/Fehling’s solution

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4
Q

What is the test for a carboxylic acid?

A

Add sodium hydrogencarbonate

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5
Q

What is the result for when sodium hydrogencarbonate is added to a carboxylic acid?

A

Effervesces

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6
Q

What happens when a compound is analysed in a mass spectrometer?

A
  • gaseous molecules bombarded with high speed electrons from an electrospray
  • these knock out electrons from some molecules, creating +ve ions which travel to detector plates
  • mass spectra then generated
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7
Q

Where do the high speed electrons come from in mass spectrometry?

A

An electrospray

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8
Q

Which peak on a mass spectra represents the molecular ion?

A

The peak with the largest m/z ratio

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9
Q

Why is the molecular ion the peak with the largest m/z ratio?

A

Because the molecule hasn’t been fragmented into smaller pieces

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10
Q

Why do larger molecules tend to have more peaks on a mass spectra?

A

They can be fragmented in many different ways

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11
Q

Why is it not possible to deduce the exact molecular formula of a compound using mass spectrometry?

A

Different compounds may have the same relative formula

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12
Q

What does C₄H₁₀ and C₃H₆O both having a peak of 58 on the mass spectrum mean for the substances’ molecular formula?

A

It is unknown which molecular formula is that of the substance

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13
Q

How can the exact molecular formula of a substance be determined using mass spectrometry?

A

If the mass spectrum is examined under high resolution

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14
Q

What do high resolution mass spectrometers do?

A

Measure the m/z values to enough precision to find the molecular formula

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15
Q

What can high resolution spectrometers not identify? Why is this?

A

They cannot identify compounds - often there is more than one compound with the same molecular formula

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16
Q

What can high resolution spectrometers determine?

A

Molecular formula

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17
Q

What happens when covalent bonds are subjected to infra-red radiation with the same frequency as their characteristic frequency?

A

The bonds will bend or stretch more and the radiation of that frequency will be absorbed

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18
Q

What are covalent bonds constantly rotating, bending and stretching with?

A

Characteristic frequencies

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19
Q

What frequencies does each of covalent bond absorb?

A

Only absorbs one or two distinct frequencies

20
Q

What will happen to the frequencies in infrared spectroscopy that are not absorbed?

A

They pass through unaffected

21
Q

What would the spectra formed from infrared spectroscopy be like if there was no sample?

A

A horizontal line straight across

22
Q

What regions can be analysed on an infrared spectrum?

A
  • region above 1500 cm⁻¹

* region below 1500cm⁻¹

23
Q

On an infrared spectrum, what is the region below 1500cm⁻¹ known as?

A

The fingerprint region

24
Q

Where is the fingerprint region on an infrared spectrum?

A

Below 1500cm⁻¹

25
Q

Why is the area below 1500cm⁻¹ on an infrared spectrum called the fingerprint region?

A

Unique for each molecule

26
Q

What are the peaks on an infrared spectrum?

A

If transmittance is below 50% (i.e. significant quantity at that frequency is absorbed)

27
Q

What are strong, sharp peaks?

A

Peaks that occur at very precise frequencies

28
Q

What are medium, broad peaks?

A

Peaks that occur over a range of frequencies

29
Q

What can the peaks on an infrared spectrum be used to determine?

A

The different bonds present in the molecule (therefore different functional groups)

30
Q

In a certain region on a mass spectrum, what do most functional groups show?

A

Characteristic absorptions

31
Q

Can using the area above 1500cm⁻¹ on an infrared spectrum identify a molecule?

A

No - it cannot identify a specific molecule, only functional groups

32
Q

What can be used on an infrared spectrum to identify a specific molecule?

A

The fingerprint region (below 1500cm⁻¹)

33
Q

On which side of an infrared spectrum is the region above 1500cm⁻¹?

A

Left

34
Q

On which side of an infrared spectrum is the region below 1500cm⁻¹?

A

Right

35
Q

What is fingerprinting?

A

When the fingerprint regions of the unknown sample is compared to databases of pre-recorded infrared spectra until a match is identified

36
Q

What can infrared spectroscopy be used for, as well as determining functional groups?

A

Identify impurities

37
Q

How are impurities determined using an infrared spectrum?

A

The bonds present in the impurity but not in the desired product will absorb radiation, giving unexpected peaks

38
Q

What peaks should a sample of ethanal have on an infrared spectrum?

A

Sharp absorption at around 1630-1815⁻¹ for C=O bond, as well as C-C and C-H bonds

39
Q

How can you tell a sample is pure?

A

On the infrared spectrum, if the spectrum of the sample matches the spectrum of a sample known to be pure

40
Q

What happens to radiation from the sun?

A

It is absorbed by the earth and emitted at a lower frequency

41
Q

What frequency is radiation from the sun usually re-emitted at?

A

In the infrared region

42
Q

Why can visible and ultra-violet light reach the earth’s surface?

A

Molecules like water, CO2 and methane do not absorb visible or UV light

43
Q

When do molecules like water, CO2 and methane absorb radiation?

A

When radiation is remitted from earth, as when remitted it is in the infrared region, which the bonds can absorb

44
Q

Why can molecules like water, CO2 and methane absorb infrared when it is remitted from earth?

A

It matches the natural frequency of vibration or rotation of bonds in these molecules

45
Q

What is the ability of CO2 and water vapour to absorb radiation emitted by the earth called?

A

The greenhouse effect

46
Q

What is causing global warming?

A

Increased levels of CO2 - so that more of the planet’s remitted heat to be trapped

AQA A Level Organic Chemistry (16 decks)

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