Chapter 16: Organic Analysis

Question 1

What is Potassium Dichromate and what does it do

Answer 1

• Is an oxidising agent and can oxidise primary alcohols to form aldehydes and secondary alcohols to form ketones.
• Tertiary alcohols can’t be oxidised

Question 2

How do you test for the presence of primary, secondary and tertiary alcohol

Answer 2

1. Add 10 drops of alcohol to 2cm³ to acidified potassium dichromate solution and warm the mixture
   
   1. Watch for a colour change: orange - green for an aldehyde/ketone. No colour change = tertiary alcohol.
2. Can’t test for primary or secondary in this test

Question 3

How do you test for aldehydes and ketones using Fehiling’s or Benedict’s

Answer 3

Aldehydes are easily oxidised to carboxylic acids and ketones can’t be oxidised further.

• Fehling’s or Benedict’s solution - blue solution of copper ions dissolved in sodium hydroxide

1. Add 2cm³ of Fehling’s to a test tube and and aldehyde or ketone to the test tube
2. Put the test tube in a hot water bath. Solution turns brick red when warmed with aldehyde. No reaction happens with Ketone

Question 4

How do you test for aldehydes and ketones using Tollen’s reagent

Answer 4

• Tolen’s reagent contains Silver nitrate.

1. Put 2cm³ of silver nitrate in a test tube
2. Add few drops of diulte sodium hydroxide and dilute ammoni solution = Tollen’s reagent
3. Silver ions in Tollen’s reagent are reduced to silver metal when warmed with aldehyde not ketone and will produce silver mirror effect

Question 5

How to test for Carboxylic Acid

Answer 5

1. Add aolution to test tube and add solid sodium carbonate to solution
2. The solution will fizz bubble the gas through limewater.
3. If the solution contains carboxylic acids then carbon dioxide is produced. Limewater turns cloudy

Question 6

How to test for alkenes

Answer 6

1. Add 2cm³ of solution to a test tube
2. Add Bromine water
3. Shake the test tube - If alkene is present the bromine water will turn from orange to colourless. Decolourises bromine water.

Question 7

What can mass spectrometry be used to determine

Answer 7

• The molecular formula of a compound
• It measures the relative atomic mass of atoms and the molecular mass of organic compounds

Question 8

Why is a high resolution mass spectrometry used

Answer 8

• Can measure atomic and molecular masses extremely accurately (to several decimal places)
• This can be used for identifying compounds that appear to have the same Mr when they are rounded to the nearest whole number.

Question 9

What is Infrared Spectroscopy

Answer 9

• Is another analystical technique and uses the fact that the bonds in different functional groups absorb different frequencies of infrared light.
• Is a beam of IR radiation is passed through a sample of a chemical.
• The IR is absorbed by the covalent bonds increasing their vibrational energy. Bonds between different atoms absorb different frequencies.

Question 10

What are the different bonds that an IR spectrometer can detect

Answer 10

• N-H - Amines
• O-H (alcohols)
• C-H
• O-H (acids)
• C≡N - Nitriles
• C=O - Carboxylic acids, aldehydes, ketones, acids
• C-O - alcohols, carboxylic acid
• C-C

Question 11

What is the fingerprint region

Answer 11

• The region between 1000cm and 1550cm is called the fingerprint region
• It is unique to a particular compound
• You can check this region of an unknown compounds IR spectrum against known compounds.
• Infrared Spectroscopy can be used to identify impurities (they produce extra peaks in the fingerprint region)

Question 12

How does the absorbtion of infrared lead to global warming

Answer 12

• The sum emits UV which is absorbed by the Earth’s surface and re-emitted as IR
• Greenhouses gases (Co2, methane) bonds absorb IR - so if amount of these gases increase more IR is absorbed which leads to global warming