Organic 6: Organic Analysis

Question 1

How do you test for alkenes?

Answer 1

Mix with bromine water
Positive result: Turns colourless from orange-brown

Question 2

How do you test for halogenoalkanes?

Answer 2

Test for halogenoalkanes:

1) Add aqueous warm NaOH to neutralise any acids present or remove interfering substances

2) Acidify with nitric acid to remove carbonate ions and acidify the solution

3) Add silver nitrate:

If chlorides are present: White precipitate

If bromides are present: Cream precipitate

If iodides are present: Pale yellow precipitate



To further test to identify specific halides:

Chlorides: dissolves in dilute ammonia

Bromides: dissolves in concentrated ammonia

Iodide: insoluble in concentrated ammonia



Question 3

How do you test for primary or secondary alcohols?

Answer 3

Add acidified potassium dichromate (K2Cr2O7)
Dichromate ions (orange) - Cr2O72
Reduced to chromium(III) ions (green) - Cr3+
Positive result = orange –> green

Question 4

How do you test for aldehydes?

Answer 4

Warm with Fehling’s or Tollen’s solution
Positive result for Fehling’s: Blue –> red precipitate
Positive result for Tollen’s: Forms silver mirror

Question 5

How do you test for carboxylic acids?

Answer 5

Add sodium carbonate (Na2CO3)
Gives off bubbles of CO2
Also produces water and sodium carboxylate

Question 6

If a compound is acidic, what does this suggest?

Answer 6

It could be a carboxylic acid

Question 7

If a compound is solid, what does this suggest?

Answer 7

Long carbon chain
Ionic bonding

Question 8

If a compound is liquid, what does this suggest?

Answer 8

Medium carbon chain
Polar/hydrogen bonding

Question 9

If a compound is gaseous, what does this suggest?

Answer 9

Short carbon chain
Little to no polarity

Question 10

What does solubility in water suggest?

Answer 10

Polar groups are present

Question 11

What is mass spectrometry used for?

Answer 11

Finding the relative molecular mass of organic compounds

Question 12

Describe how infrared spectroscopy works.

Answer 12

1) A beam of infrared radiation containing a spread of frequencies is passed through a sample
2) The radiation that emerges is missing the frequencies that correspond to the types of bonds found in the sample
3) The instrument plots a graph of the intensity of the radiation emerging from the sample (transmittance) against frequency of radiation
4) Frequency is expressed as a wavenumber, measured in cm-1

Question 13

Describe the IR peak of an OH bond in an alcohol.

Answer 13

Smoother, rounded curve

Question 14

Describe the IR peak of an OH bond in a carboxylic acid.

Answer 14

Broader trough
More jagged than alcohol

Question 15

Describe the IR peak of a C=O bond.

Answer 15

Long narrow peak
Around the middle of the spectrum

Question 16

Describe the IR peak of a C-O bond in alcohols and esters.

Answer 16

Long jagged peak
To the right of the middle of the spectrum

Question 17

What is the fingerprint region of an IR spectrum?

Answer 17

Many peaks caused by complex vibrations
Unique for every substance
Used to identify the chemical
Chemists use a computer to match this fingerprint region to those on a database

Question 18

Predict what functional group(s) this spectrum might show.

Answer 18

Alcohol

Question 19

Predict what functional group(s) this spectrum might show.

Answer 19

Alcohol

Question 20

Predict what functional group(s) this compound might show

Answer 20

C=O
O-H in an acid
Suggests carboxylic acid