3.3.6 Organic Analysis

Question 1

Describe how you would test for alcohols

Answer 1

Adding sodium metal to pure alcohol = fizzing as it gives off H₂ gas

Question 2

Describe how you would test for primary, secondary, tertiary alcohols

Answer 2

1. Add 10 drops of alcohol to 2 cm³ of acidified potassium dichromate solution in test tube
2. Warm mixture gently in hot water bath
3. Watch for colour change
   
   1. Primary: Orange solution turns green as aldehyde forms
   2. Secondary: Orange solution turns green as ketone forms
   3. Tertiary: Nothing happens

Question 3

Describe how you could test for primary alcohol (differentiate it from a secondary alcohol)

Answer 3

1. Add excess alcohol to 2 cm³ of acidified potassium dichromate solution in round bottomed flask
2. Set up flask as part of distillation apparatus
3. Gently heat flask
   
   • Alcohol will be oxidised & product will be distilled off immediately = so you can collect it (if product produced aldehyde, then solution was a primary alcohol)

Question 4

Name 3 reagents you can use to test for an aldehyde or ketone

Answer 4

1. Fehling’s solution
2. Benedict’s solution
3. Tollens’ reagent

Question 5

Describe a test for an aldehyde or ketone using Fehling’s (or Benedict’s) solution

Answer 5

1. Add 2 cm³ of Fehling’s (or Benedict’s) solution to test tube
2. Add 5 drops of aldehyde or ketone to test tube
3. Put test tube in hot water bath to warm it for few minutes
4. Aldehyde: Blue solution will give brick red precipitate
5. Ketone: Nothing

Question 6

Describe a test for an aldehyde or ketone using Tollens’ reagent

Answer 6

1. Put 2 cm³ of 0.1 mol dm^-3 silver nitrate solution in test tube
2. Add few drops of dilute sodium hydroxide to solution
   
   • Light brown precipitate should form
3. Add drops of dilute ammonia solution until brown precipitate dissolves completely
4. Place test tube in a hot water bath and add 10 drops of aldehyde or ketone
5. Aldehyde: silver mirror forms on walls of test tubes
6. Ketone: Nothing

Question 7

Why is a silver mirror produced when Tollens’ reagent is added to an aldehyde?

Answer 7

Aldehyde reduced Ag⁺ ions to silver atoms

Question 8

Describe the test for alkenes

Answer 8

1. Add 2 cm³ of solution you want to test to test tube
2. Add 2 cm³ of bromine water to test tube
3. Shake test tube
4. Alkene: Solution will go from orange to colourless

Question 9

Describe the test for carboxylic acids

Answer 9

1. Add 2 cm³ of solution you want to test to test tube
2. Add 1 small spatula of solid sodium carbonate
3. If solution begins to fizz, bubble gas that it produces through some limewater in 2^nd test tube
4. Carboxylic Acid: Solution will fizz & CO₂ gas produced will turn limewater cloudy

Question 10

Describe the test for halogenoalkanes

Answer 10

1. Add halogenoalkane (R—X) and sodium hydroxide to test tube
2. Warm it by placing it in hot water bath
3. Add dilute nitric acid and silver nitrate solution to test tube to acidify its contents
4. Halogenoalkane: Precipitate of AgX

Question 11

What is mass spectrometry used for?

Answer 11

To find M_r of compound

Question 12

Molecular ion (M⁺) is formed when…

Answer 12

a molecule loses an electron

Question 13

M⁺ produces ____ _____ ____ on mass spectrum of compound

Answer 13

molecular ion peak

Question 14

Mass/charge (m/z) value of molecular ion peak = …

Answer 14

molecular mass of compound

Question 15

Answer 15

Question 16

What does high resolution mass spectrometry measure?

Answer 16

Measure atomic and molecular masses extremely accurately

Question 17

What is high resolution mass spectrometry used for?

Answer 17

Identifying compounds that have same M_r when rounded to nearest whole number

Question 18

Describe how infrared spectroscopy works

Answer 18

1. Beam of IR radiation passed through sample of chemical
2. IR radiation absorbed by covalent bonds in molecules = ↑ vibrational energy
   
   1. Bonds between different atoms absorb different frequencies of IR radiation
   2. Bonds in different places in molecules absorb different frequencies too (e.g. O-H group in alcohol and O-H in carboxylic acid absorb different frequencies)

Question 19

What does an infrared spectrometer produce & what does this show?

Answer 19

Infrared spectrometer produces graph that shows what frequencies of radiation molecules are absorbing

Question 20

What can a graph produced by an infrared spectrometer be used for?

Answer 20

To identify functional groups in molecule

Question 21

The fingerprint region is between ______ and ______

Answer 21

500 cm^-1 and 1500cm^-1

Question 22

Fingerprint region is _____ to a particular compound

Answer 22

Unique

Question 23

Describe how you can use the fingerprint region to identify a molecule

Answer 23

• Can use computer database to check this region of unknown compound’s IR spectrum against known compounds
• If it matches one of them = know what molecule it is

Question 24

Infrared spectroscopy can also be used to find out….

Answer 24

how pure compound is & identify any impurities

Question 25

Impurities produce …. in infrared spectroscopy

Answer 25

extra peaks in fingerprint region

Question 26

Describe how infrared radiation absorption is linked to global warming

Answer 26

1. Some electromagnetic radiation emitted by Sun reaches Earth and is absorbed
   
   • Earth re-emits some of its infrared radiation (heat)
2. Molecules of greenhouse gases (CO₂, CH₄, water vapour) in atmosphere absorb IR radiation (it’s bonds of these molcules that absorb it)
   
   • & re-emit some of it back towards Earth = keeps us warm
   • Called greenhouse effect
3. Human activities, burning fossil fuels & leaving rubbish to rot in landfill sites, caused rise in greenhouse gas concentrations
4. Means more heat being trapped and Earth getting warmer = global warming

Question 27

Infra-red spectrum for ethanoic acid

Describe the 2 main peaks

Answer 27

• Board peak at 2500 – 3000 cm^-1 shows O-H bond (in carboxylic acid)
• Sharp peak at 1680 – 1750 cm^-1 shows C=O

(carboxylic acid = 2 peaks)

Question 28

State the homologous series shown by this infra-red spectrum

Answer 28

Alcohol

(Acid’s peak is boarder than alcohol)

Question 29

State the homologous series shown by this infra-red spectrum

Answer 29

Aldehyde