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A - Chemistry > Unit 3: Section 4 - Organic analysis MDY * > Flashcards

Unit 3: Section 4 - Organic analysis MDY * Flashcards

tests for functional groups analytical techniques

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1
Q

How can you test whether you’ve got a primary, secondary or tertiary alcohol?

A

add 10 drops of the alcohol to 2cm^3 of acidified potassium dichromate solution in a test tube
warm mixture gently in a hot water bath
watch for colour change

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2
Q

what do the results of the test for primary, secondary, tertiary alcohols show?

A

primary - orange solution slowly turns green as aldehyde forms
secondary - orange solution slowly turns green as ketone forms
tertiary - nothing happens

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3
Q

What must be done to determine whether an alcohol is primary or secondary?

A

add excess alcohol to 2cm^3 of acidified potassium dichromate solution in a round bottomed flask
set up flask in distillation apparatus
gently heat the flask. the alcohol will be oxidised and the product will be distilled off immediately and collected
then they can be tested to see if they are aldehydes or ketones

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4
Q

what are the 3 main reagents that can be used to distinguish between aldehydes or ketones?

A

Fehling’s solution
Benedict’s solution
Tollens’ reagent

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5
Q

how can you use Fehling’s/ Benedict’s solution to test for aldehydes or ketones?

A

add 2cm^3 of solution to test tube
add 5 drops of aldehyde or ketone to test tube
put test tube in hot water bath for a few minutes

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6
Q

what do the results of Fehling’s/ Benedict’s solution show?

A

aldehyde - the blue solution will give a brick red precipitate
ketone - nothing happens

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7
Q

how can you use Tollens’ reafgent to test for aldehydes or ketones?

A

place test tube of Tollens’ reagent in hot water bath
add 10 drops of aldehyde or ketone
wait for a few minutes

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8
Q

how to make Tollens’ reagent?

A

put 2cm^3 of 0.1 mol/dm^3 silver nitrate solution in test tube
add few drops of dilute sodium hydroxide solution - brown precipitate forms
add few drops of dilute ammonia solution until brown precipitate dissolves

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9
Q

what do the results of Tollens’ reagent show?

A

aldehyde - a silver mirror (thin coating of silver) forms on the walls of the test tube
ketone - nothing happens

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10
Q

how can you test for alkenes using bromine water?

A

add 2cm^3 of solution you want to test to test tube
add 2cm^3 of bromine water to test tube
shake test tube

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11
Q

what do results of bromine water test show?

A

alkene - the solution will decolourise (from orange to colourless)
not alkene - nothing happens

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12
Q

what happens when carboxylic acids react with carbonates?

A

salt, carbon dioxide and water are formed

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13
Q

how can you test whether a solution is a carboxylic acid?

A

add 2cm^3 of solution you want to test to test tube
add 1 small spatula of solid sodium carbonate / 2cm^3 of sodium carbonate solution
if solution begins to fizz, bubble gas through limewater in 2nd test tube

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14
Q

what do the results of the test for carboxylic acids with sodium carbonate show?

A

carboxylic acid - the solution will fizz. carbon dioxide gas produced will turn limewater cloudy
not carboxylic acid - nothing happens

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15
Q

what can mass spectrometry be used for?

A

to find the relative atomic/molecular mass of a compound.
a (molecular) ion is formed when a(n) atom/molecule loses an electron, this produced a molecular ion peak on the mass spectrum, this mass/charge value is equal to the molecular mass
abundance of each peak is also found to calculate Ar / Mr

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16
Q

what are high resolution mass spectrometers?

A

mass spectrometers that can measure atomic and molecular masses extremely accurately

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17
Q

why are high resolution mass spectrometers useful?

A

for identifying compounds that appear to have the same Mr when rounded to the nearest whole number
e.g.
C3H8 m/z = 44.0624
CH3CHO m/z = 44. 0302

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18
Q

what happens in infrared spectroscopy?

A

a beam of IR radiation is passed through a sample of a chemical
its absorbed by the covalent bonds, increasing their vibrational energy
bonds absorb IR energy at the same frequency as its bonds vibrate
bonds between different atoms and in different places absorb different frequencies of IR radiation
the radiation that leaves is missing the same frequencies as the bonds in the sample vibrate to

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19
Q

what frequencies do N-H bonds absorb?

A

amines = 3300-3500

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20
Q

what frequencies do O-H bonds absorb?

A

alcohols = 3230-3550

carboxylic acids = 2500-3000

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21
Q

what frequencies do C-H bonds absorb?

A

most organic molecules = 2850-3300

22
Q

what frequencies do C≡N bonds absorb?

A

nitriles = 2220-2260

23
Q

what frequencies do C=O bonds absorb?

A

aldehydes, ketones, carboxylic acids, esters = 1680-1750

24
Q

what frequencies do C=C bonds absorb?

A

alkenes = 1620-1680

25
Q

what frequencies do C-O bonds absorb?

A

alcohols, carboxylic acids = 1000-1300

26
Q

what frequencies do C-C bonds absorb?

A

most organic molecules = 750-1100

27
Q

what does an infrared spectrometer produce?

A

the frequencies that aren’t absorbed by the sample hit a detector which plots a graph that shows what frequencies of radiation the molecules are absorbing. it can be used to identify the functional groups in a molecule

28
Q

what is the fingerprint region?

A

the region between 500 and 1500 cm-1 on the infrared spectrum.
its unique to a particular compound

29
Q

what can the fingerprint region be used for?

A

a computer database can be used to check the region of an unknown compound’s IR spectrum against those of known compounds. if it matches one, you know what the molecule is.
it can also test purity as impurities produce extra peaks in the fingerprint region

30
Q

how does the ‘greenhouse effect’ work?

A

some electromagnetic radiation emitted by the sun reaches the earth and is absorbed
some of it is re-emitted as infrared radiation
molecules of greenhouse gases in the atmosphere absorb this infrared radiation and re-emit some of if back towards us

31
Q

what causes global warming?

A

human activities like burning fossil fuels and leaving rubbish to rot in landfill sites cause greenhouse gas concentrations to rise
more heat is trapped and the earth gets warmer

32
Q

what is infrared spectrometry?

A

pair of atoms within a covalent bond are always vibrating
stronger bonds vibrate faster and heavier bonds slower
each bond has its own vibration frequency

33
Q

what can IR spectroscopy be used to identify?

A

alcohols
carbonyl components
carboxylic acids
alkenes

34
Q

what are the 4 things that happen in a time of flight mass spectrometer?

A
  1. ionisation
  2. acceleration
  3. ion drift
  4. detection
35
Q

what are the 2 types of ionisation?

A

electrospray ionisation

electron impact ionisation

36
Q

what happens in electrospray ionisation?

A

the sample is dissolved and pushed through a hypodermic needle at high pressure. a high voltage is applied to it, causing each particle to gain an H+ ion. the sample is turned into a gas made of positive ions

37
Q

what happens in electron impact ionisation?

A

sample is vaporised, an electron gun is used to fire high energy electrons at it from a hot wire. this knocks 1 electron off each particle so they become +1 ions

38
Q

when iselectrospray used?

A

for large molecules that electrons can’t pass through to knock electrons off, and will get damaged instead

39
Q

when is electron impact used?

A

on small molecules or elements

40
Q

what happens in acceleration in mass spectrometry?

A

positively charged ions are accelerated by an electric field so they all have the same kinetic energy. (lighter ions move faster than heavier ones)

41
Q

what happens in ion drift?

A

ions enter a vacuum with no electric field so they drift through and allow the different isotopes to separate

42
Q

what happens in detection?

A

lighter ions travel faster in drift region so reach the detector first. the detector is a negative plate and every time a positive ion hits it a current is produced (current proportional to number of ions) and a mass spectrum is created

43
Q

what are the axis labelled on a mass spectrum graph?

A 
y-axis = abundance, height of peak = relative isotopic abundance
x-axis = mass/charge ratio
44
Q

how is a mass spectrum from electrospray different to electron impact?

A

in electrospray each atom gains an H+ ion so the relative mass on the graph will be 1 unit greater than the relative mass of the isotope

45
Q

how to work out relative atomic mass from a mass spectrum?

A

multiply abundance by relative isotopic mass for each peak add all of these values divide by combined abundance

46
Q

how can a mass spectrum be used to identify elements?

A

each element has different abundances of each of its isotopes if the abundances match up with the ones on the mass spectrum its the same element

47
Q

how is mass spectrometry used on molecules?

A

a molecular ion is formed by removing an electron from the molecule. this produces a peak with the same mass/charge ratio as the molecules relative molecular mass. this can be used to identify unknown molecules

48
Q

how can a molecule produce different peaks on a mass spectrum?

A

if the elements that make it up have isotopes there’ll be a peak for every combination of isotopes that produces a different Mr

49
Q

what problem might there be in electron impact ionisation?

A

there may be peaks at lower m/z values due to fragments caused by the break up of a molecular ion - fragmentation pattern

50
Q

what relationship do to different ions have in electrospray based on KE=1/2 m v^2?

A

ion 1 has m1 and takes t1
ion 2 has m2 and takes t2
m1/t1^2 = m2/t2^2

51
Q

what can the fragmentation pattern be used for?

A

to deduce the structural formula

wet tests will give clues to some fragments that you should expect to see

A - Chemistry (26 decks)

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