Chemical Analysis - Organic Tests (10.1)

Question 1

What does qualitative analysis help with?

Answer 1

• what molecules/functional groups = present in a sample

- is the sample pure?

Question 2

What does quantitative analysis help with?

Answer 2

• how much of a substance is present in a sample

Question 3

What does structure determination help with?

Answer 3

• what the molecule looks like

Question 4

How do we test for alkenes?

Answer 4

• add bromine water to the suspected sample
• if the water turns colourless - alkene
• if it stays red-brown - no alkene present
• bromine adds across the double bond

Question 5

How do we test for the presence of alcohols?

Answer 5

• add phosphorus pentachloride to a suspected sample
• white misty fumes of HCl will be released (HCl = formed)
• fumes = acidic + turn universal indicator paper, red

Question 6

How else do we test for the presence of alcohols? (primary, secondary, tertiary)

Answer 6

• can be done by oxidation
• use a strong oxidising agent e.g. acidified potassium dichromate (K2Cr2O7)
• reflux it with alcohol
• primary alcohol: orange to green
• secondary alcohol: orange to green
• tertiary alcohol: orange to orange
• reduction equation: (Cr2O72- + 14H+ + 6e- —–> 2Cr3+ + 7H2O) - Alcohol = oxidised, chromium = reduced

Question 7

Aldehydes can be oxidised to carboxylic acids, whereas ketones cannot be oxidised further. True or False?

Answer 7

True

Question 8

What happens in aldehydes in an acidified potassium dichromate test?

Answer 8

• goes from orange to green

Question 9

What happens in ketones in an acidified potassium dichromate test?

Answer 9

• no change (stays orange)

Question 10

What happens in aldehydes in Fehling’s solution (containing Cu2+ ions) test?

Answer 10

• goes from blue to form red/orange precipitate
• production of CuO
• aldehyde = oxidised
• Cu = reduced

Question 11

What happens in ketones in Fehling’s solution (containing Cu2+ ions) test?

Answer 11

• no change (remains blue)

Question 12

What happens in aldehydes in Tollen’s reagent (ammoniacal silver nitrate) test?

Answer 12

• a silver mirror = formed
• solid silver = formed
• aldehyde = oxidised
• Ag = reduced

Question 13

What happens in ketones in Tollen’s reagent (ammoniacal silver nitrate) test?

Answer 13

• no change (stays colourless)

Question 14

What happens in aldehydes when they are warmed with iodine and sodium hydroxide solution?

Answer 14

• no change

Question 15

What happens in ketones when they are warmed with iodine and sodium hydroxide solution?

Answer 15

• a yellow precipitate is formed (iodoform)

- with an antiseptic smell

Question 16

What happens when acyl chlorides are reacted with water?

Answer 16

• acyl chlorides = extremely reactive
• react with moisture in the air
• form carboxylic acids
• releases misty fumes of acidic HCl gas

Question 17

Where are carboxylic acids in the pH scale?

Answer 17

• weak acids (between 3 - 7)

Question 18

Where are amines in the pH scale?

Answer 18

• weak bases (8 - 11)

Question 19

How do we test for haloalkenes?

Answer 19

• warm the suspected haloalkenes gently in water
• add silver nitrate
• free halide ions react with the silver ions
• give a coloured precipitate:
  > AgCl, white
  > AgBr, Cream
  > AgI, Yellow
  Equation: X-(aq) + Ag+(aq) ——-> AgX(s)