AS - Organic Analysis

Question 1

Outline the method of the test used to distinguish between primary, secondary and tertiary alcohols. What happens with each type of alcohol?

Answer 1

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

Question 2

What causes the colour change with primary and secondary alcohols when mixed with acidified potassium dichromate?

Answer 2

The orange dichromate(VI) ion (Cr2O7^2-) is reduced to the green chromium (III) ion (Cr^3+)

Question 3

What is the problem with the test used to distinguish between the classifications of alcohols?

How is this overcome?

Answer 3

The test shows the same result for primary and secondary alcohols. To find out which is present, you need to collect the product using distillation apparatus and test to see whether it’s an aldehyde or ketone.

Question 4

How can you test for alcohols using sodium metal?

Answer 4

If you add a small piece of sodium metal to a pure alcohol, it will fizz as it gives of hydrogen gas.

Question 5

What are the three main reagents used to distinguish between aldehydes and ketones?

Answer 5

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

Question 6

Outline the method used to distinguish between aldehydes and ketones using Fehling’s and Benedict’s solutions.

Answer 6

These tests both work in the same way.

1. Add 2cm^3 of Fehling’s or Benedict’s solution to a test tube (this has a clear blue solution).
2. Add 5 drops of the aldehyde or ketone to the test tube.
3. Put the test tube in a hot water bath to warm it for a few minutes.

If an aldehyde is present, a brick red precipitate will form, if a ketone is present, nothing will happen.

Question 7

Outline the method used to make Tollen’s reagent.

Answer 7

1. Put 2cm3 of 0.10 mol dm-3 of silver nitrate solution into a test tube (this is a colourless solution).
2. Add a few drops of dilute sodium hydroxide solution. A light brown precipitate will form.
3. Add drops of dilute ammonia solution until the brown precipitate dissolves completely. This is Tollen’s reagent.

Question 8

Outline the method used to distinguish between aldehydes and ketones using Tollen’s reagent.

Answer 8

1. Place test tube in a hot water bath and add 10 drops of aldehyde or ketone using a pipette.
2. Wait a few minutes.

If an aldehyde is present, a silver mirror will form (a thin coating of silver) on the walls of the test tube.

If a ketone is present, nothing will happen.

Question 9

In the test using Tollen’s reagent to distinguish between aldehydes and ketones, why must a hot water bath be used?

And how is the silver mirror formed when an aldehyde is present?

Answer 9

Aldehydes and ketones are flammable so cannot be heated over a flame.

A silver mirror forms as the aldehyde reduces the Ag+ ions to silver atoms.

Question 10

Outline the method used to test for alkenes.

Answer 10

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

If an alkene is present, the solution will decolourise (go from orange to colourless).

If an alkene is not present, nothing happens.

Question 11

Carboxylic acids react with carbonates to form what?

Answer 11

A salt, carbon dioxide and water.

Question 12

Outline the test used to detect the presence of a carboxylic acid.

Answer 12

1. Add 2cm3 of the solution you want to test to a test tube.
2. Add 1 small spatula of solid sodium carbonate or 2cm3 of sodium carbonate solution.
3. If the solution begins to fizz, collect the gas and bubble it through another test tube containing limewater.

If a carboxylic acid is present, the solution will fizz and the carbon dioxide gas produced will turn the limewater cloudy.

If a carboxylic acid is not present, nothing will happen.

Question 13

In mass spectrometry, how is a molecular ion formed?

Answer 13

When a molecule loses an electron.

Question 14

What does the molecular ion produce on a mass spectrum? How can this be used to determine the molecular mass of a compound?

Answer 14

A molecular ion peak. The mass/charge value of the molecular ion peak will be the same as the molecular mass of the compound, assuming the ion has a 1+ charge which it usually will have.

Question 15

What is high resolution mass spectrometry?

Answer 15

Mass spectrometers which measure atomic and molecular masses extremely accurately, to several decimal places.

Question 16

In any spectrum of an organic compound there will be a tiny peak one mass unit to the right of the molecular ion. What is this caused by?

Answer 16

Ions containing the 13C isotope.

Question 17

Infrared spectroscopy helps you identify organic molecules, explain how this works.

Answer 17

1. 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. Bond between different atoms absorb different frequencies of IR radiation. Bond s in different places in a molecule also absorb different frequencies.
4. An infrared spectrometer produces a graph that shows you what frequencies of radiation the molecules are absorbing so you can use it to identify the functional groups in a molecule.

Question 18

On an infrared spectrum, what do the peaks pointing downwards show you?

Answer 18

Where radiation is being absorbed.

Question 19

What region of an infrared spectrum identifies a molecule?

Answer 19

The fingerprint region - between 500cm-1 and 1500cm-1.

Question 20

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

Answer 20

Impurities will produce extra peaks in the fingerprint region on an infrared spectrum.

Question 21

Explain how infrared radiation absorption is linked to global warming.

Answer 21

Some of the electromagnetic radiation emitted from the sun reaches the earth and is absorbed. The earth then re-emits some of this as IR radiation (heat).

Bonds of molecules of greenhouse gases, like carbon dioxide, methane and water vapour in the atmosphere absorb this IR radiation. Greenhouse gases then re-emit some of it back towards the earth, causing a warming effect (this is known as the greenhouse effect).

Human activities has led to a rise in greenhouse gas concentrations which means the earth is getting warmer due to increased greenhouse effect resulting in global warming.

Question 22

A pair of atoms joined by a chemical bond is always vibrating. How do stronger bonds vibrate? What effect do heavier atoms have on the vibration of a bond?

Answer 22

Stronger bonds vibrate faster, at a higher frequency, and heavier atoms make the bond vibrate more slowly, at a lower frequency.

Question 23

What is the wavenumber?

Answer 23

The measure used for the frequency (1/wavelength(cm))