3.3.6 Organic analysis

Question 1

What is the test for alcohols?

Answer 1

Add a small piece of solid sodium, if alcohol is present it will fizz and give off H₂ gas

Question 2

How do you test whether an alcohol is primary, secondary, or tertiary using potassium dichromate?

Answer 2

1. Add 10 drops of the alcohol to the 2cm³ of acidified potassium dichromate solution in a test tube
2. Warm the mixture in a water bath
3. Watch for a colour change
   ~ Primary - orange solution turns green
   ~ Secondary - orange solution turns green
   ~ Tertiary - no visible change

The orange dichromate ion turns green when reduced to the chromium (III)

Question 3

How do you distinguish between a aldehyde (from a primary alcohol) or a ketone (from a secondary alcohol) using Fehling’s or Benedict’s solution?

Answer 3

1. Add 2cm³ of Fehling’s/ Benedict’s to a test tube
2. Add 5 drops of the aldehyde or ketone to the test tube
3. Put the test tube in a water bath and warm
   ​
   Aldehyde - blue solution goes to a brick red precipitate
   ​
   Ketone - no visible change

Question 4

How do you distinguish between a aldehyde (from a primary alcohol) or a ketone (from a secondary alcohol) using Tollen’s reagent?

Answer 4

Put 2cm³ of tollens reagent in a test tube in a hot water bath and add 10 drops of aldehyde or ketone and leave for a few minutes
​
Aldehyde - a silver mirror forms on the walls of the test tube
​
Ketone - no visible change

Question 5

How do you make Tollen’s reagent?

Answer 5

1. Put 2cm³ of 0.1 mol dm^-3 silver nitrate solution in a test tube
2. Add a few drops of dilute sodium hydroxide solution, a light brown precipitate should form
3. Add drops of dilute ammonia solution until the brown precipitate redisolves completely

Question 6

How do you test for alkenes (double bonds) using bromine water?

Answer 6

1. Add 2cm³ of the solution that you want to test to a test tube
2. Add 2cm³ of bromine water to the test tube
3. Shake the test tube

Alkene - the solution will decolourise (go from orange to colourless)

Not alkene - no visible change

Question 7

How do you use sodium carbonate to test for carboxylic acids?

Answer 7

Carboxylic acids react with carbonates to form salt, CO₂ and water.

1. Add 2cm³ of the solution being tested to a test tube
2. Add 1 small spatula of solid sodium carbonate
3. If the solution starts to fizz, bubble the gas that it produces through some limewater in a second test tube

Carboxylic acid - The solution will fizz, the CO₂ gas produced will turn limewater cloudy
Not carboxylic acid - nothing happens

Question 8

How can mass spectrometry be used to find identity of a compound?

Answer 8

1. In the mass spectrometer, a molecular ion (M⁺) is formed when a molecule loses an electron
2. The molecular ion produces a molecular ion peak on the mass spectrum of the compound
3. For any compund, the mass/charge (m/z) value of the molecular ion peak will be the same as the molecular mass of the compound

Question 9

Explain how high resolution mass spectrometry can be used to find the identity of a compound?

Answer 9

High resolution mass spectrometers can measure atomic and molecular masses extremely accurately, to several decimal places

This is usefull when compound have the same M_r when rounded to the nearest whole number

E.g. propane and ethanal both have an M_r of 44 to the nearest whole number, on a high resolution mass spectrum, proane has a molecular ion peak with m/z = 44.0624 and ethanal has a molecular ion peak with m/z = 44.0302

Question 10

How can infrared spectroscopy be used to identify molecules?

Answer 10

1. In infrared spectroscopy, a beam of IR radiation is passed through a sample of a chemical
2. The IR radiation is absorbed by the covalent bonds in the molecules, increasing their vibrational energy
3. Bonds between different atoms absorb different frequencies of IR radiation. Bonds in different palces in a molecule absorb different frequencies too - so the O-H group in an alcohol and the O-H group in a carboxylic acid absorb different frequencies
4. An infrared spectrometer produces a graph that shows you what frequencies of radiation the molecules are absorbing, therefore you can identify the functional group

Question 11

Explain how the fingerprint region on an infrared spectrum can be used to identify a molecule

Answer 11

• The fingerprint region is between 500cm^-1 and 1500cm^-1
• It’s unique and particular to a compound
• An unknown compound fingerprint region can be compared to a computer data base of known compounds to discover its identity with one it matches

Question 12

How can infrared spectroscopy be used to find out how pure a compound is

Answer 12

Any impurities will show up as extra peaks on the IR spectrum

Question 13

How is infrared absorbtion linked to global warming?

Answer 13

1. Some of the electromagnetic radiation emited by the sun reaches the earth and is absorbed. The earth then re-emits some of its infrared radiation
2. Green house gasses in the atmosphere like CO₂, methane and water vapour absorb IR and re-emit back towards earth, this is called the green house effect
3. Human activites, such as burning fossil fuels and using landfills have caused a rise in green house gas concentrations
4. More heat is now being trapped by the atmosphere, so earth is getting warmer - this is called global warming