3.4 Organic analysis

Question 1

How you find out if you’ve got primary or secondary alcohol?

Answer 1

Oxidised and separate the mixture by distillation so you only have the aldehyde or ketone. Then add 2cm³ of either Benedict’s or Fehling’s solution (Cu²⁺) to 5 drops of the solution you are testing. Warm them together, if you get a brick red precipitate it’s an aldehyde, if not then it’s a ketone.

Question 2

Give the 4 steps of how to test for an aldehyde or ketone by using Tollen’s reagent

Answer 2

1. Add 2cm³ of 0.1 mol dm^-3 silver nitrate solution into a test tube.
2. Add a few drops of dilute sodium hydroxide solution to form a brown precipitate.
3. Add drops of dilute ammonia solution until the precipitate dissolves, this is Tollen’s reagent.
4. Place the reagent into a water bath and add 10 drops of the aldehyde/ketone. If it’s an aldehyde a silver layer will form on the test tube, if it’s a ketone nothing happens.

Question 3

How do you test between an alkene and an alkane?

Answer 3

Add 2cm³ of the solution to 2cm³ of bromine water in a test tube. Shake the mixture, if the solution contains alkenes the solution will turn colourless, if it contains alkanes there will be no change.

Question 4

How do you test for carboxylic acids?

Answer 4

Add a spatula of sodium carbonate to 2cm³ of the solution. If it contains carboxylic acids, CO₂ will be produced which can be tested with limewater (making it cloudy).

Question 5

How can you calculate the M_r of a sample to several decimal places?

Answer 5

Use high resolution mass spectrometry

Question 6

How does infrared spectroscopy detect bonds?

Answer 6

Certain bonds absorb specific wave numbers of IR radiation meaning it will be missing from a detectors results.

Question 7

What is the area from 1500 to 500 wave numbers called and used for?

Answer 7

The fingerprint region which is individual to every molecule so it used to identify compounds.

Question 8

How do you test if you’ve got a primary/secondary or tertiary alcohol?

Answer 8

Add 10 drops of alcohol to 2cm³ of acidified potassium dichloromate. If it is a primary or secondary alcohol it will change from orange to green. If it is a tertiary alcohol there won’t be any change.