Organic Analysis

Question 1

Test for Primary, secondary or tertiary alcohol

Answer 1

• 10 drops alcohol to 2cm3 acidified potassium dichromate in test tube
• warm mixture gently in hot water bath
• watch for colour change

Primary- orange solution slowly turn green (aldehyde formed)
Secondary- orange solution slowly turn green (ketone formed)
Tertiary- nothing happens

Question 2

Fehlings and Benedicts test

Answer 2

-add 2cm3 F/B to test tube (clear blue solution)
-add 5 drops aldehyde/ketone to test tube
-put test tube in hot water bath warm for few mins
Aldehyde= brick red precipitate
Ketone= nothing

Question 3

Tollens Reagent

Answer 3

-2cm3 silver nitrate in test tube
-few drops dilute sodium hydroxide solution (light brown precipitate form)
-few drops ammonia solution until brown precipitate dissolve
-test tube in hot water bath, 10 drops aldehyde/ketone
Aldehyde= silver mirror on walls test tube
Ketone= nothing

Question 4

Test for alkenes

Answer 4

-2cm3 of solution in test tube
-add 2cm3 bromine water to test tube
-shake
Alkene- decolourise (orange to colourless)
Not alkene- nothing

Question 5

Test for carboxylic acid

Answer 5

-2cm3 solution you want to test in test tube
-add 1 small spatula solid sodium carbonate
-if solution fizz, collect gas and into test tube containing limewater
Carboxylic acid- solution fizz, CO2 turn limewater cloudy

Question 6

High resolution mass spectrometry

Answer 6

• can measure atomic + molecular masses extremely accurately (to a few decimal places)
• useful for identifying compounds that appear to have to have same Mr when rounded to nearest whole number

Question 7

What is infrared spectroscopy

Answer 7

• a beam of IR radiation is passed through a sample of a chemical
• IR radiation absorbed by covalent bonds in molecule- increase vibrational energy
• bonds between different atoms absorb different frequencies of IR radiation
• infrared spectrometer produces graph- shows what frequencies of radiation molecules are absorbing

Question 8

Equation for wave number

Answer 8

Wave number = 1 / wavelength (cm)

Question 9

Range for IR light absorption

Answer 9

IR light absorbed is in the range 4000 – 400 cm-1

Question 10

What is the fingerprint region

Answer 10

Below 1500 cm-1

• unique to a particular compound
• use computer database to check region of unknown compounds IR spectrum against known, if matches, you know molecule

Question 11

What is infrared spectroscopy used for

Answer 11

• identify functional groups in a molecule

- identify impurity: produce extra peaks in fingerprint region

Question 12

How is IR absorption linked to global warming

Answer 12

• some electromagnetic radiation emitted from sun absorbed and re emitted as infrared by earth
• molecules of greenhouse gases in atmosphere absorb IR then re emit to earth (greenhouse effect)
• burning fossil fuels cause rise in greenhouse gas conc
• more heat trapped, earth gets warmer- global warming