Topic 3.4 - Organic Analysis

Question 1

(Tollen’s)
What is the test for aldehydes and ketones?
Give the results

Answer 1

• Tollen’s reagent + gently heat
• Ag+ (aq) + e- > Ag (s)
• Ag+ ions oxidises aldehydes to carboxylic acids
• Aldehyde = colourless > silver mirror
• Ketone = no visible change (colourless)

Question 2

(Fehling’s)
What is the test for aldehydes and ketones?
Give the results

Answer 2

• Fehling’s solution + heat
• 2Cu 2+ (aq) + 2OH- (aq) + 2e- > Cu2O (s) + H2O (l)
• Cu 2+ ions oxidises aldehyde to carboxylic acid
• Aldehyde = Blue > red - orange precipitate
• Ketone = No visible change (blue precipitate)

Question 3

What is the test for alkenes?

Answer 3

• Bromine water
• Orange > colourless

Question 4

What is the test for carboxylic acids?

Answer 4

• Sodium carbonate
  Carboxylic acid + metal carbonate > salt + water + carbon dioxide (gas)
• Formed by oxidising aldehydes or primary alcohols
• Effervescence bubbles through limewater = turns cloudy

Question 5

What is the test for alcohols?

Answer 5

• Potassium dichromate (VI)
  Cr2O7 2- + 6e- + 14H+ > 2Cr 3+ + 7H2O
• K2Cr2O7 oxidises primary and secondary alcohols to aldehydes and ketones
• Primary / secondary = orange > green
• Tertiary = no visible change (green)

Question 6

How do you distinguish between primary and secondary alcohols after they are oxidised?

Answer 6

• Primary alcohols
  > Oxidise alcohol under reflux
  = positive for carboxylic acid
  > Oxidise alcohol under distillation
  = positive for aldehyde
• Secondary alcohols
  > Oxidise alcohol under reflux / distillation
  = positive for ketone

Question 7

Define mass spectrometry

Answer 7

An analytical technique used to identify compounds and determine their molecular formula

Question 8

What three things can mass spectrometry find?

Answer 8

• Relative isotopic masses
• Abundance of different isotopes of an element
• Relative molecular mass (Mr) of a compound
  = Given by molecular ion peak on spectrum

Question 9

Why do different molecules show up as different peaks?

Answer 9

Molecules absorb different amounts of energy

Question 10

What is the standard molecule to compare molecular ion peaks to?

Answer 10

• TMS (Si(CH3)4)
• Standard molecule as four identical carbon and hydrogen environments

Question 11

How is Mr determined by molecular ion peak?

Answer 11

Mass / charge (m/z) value of the molecular ion peak is equal to Mr of molecule

Question 12

Why do Chlorine and Bromine have two molecular ion peaks?

Answer 12

They both have two isotopes

Question 13

Define high resolution mass spectroscopy

Answer 13

• A sensitive way of measuring atomic and molecular masses to 5dp
• Used to differentiate between compounds that appear to have similar Mr

Question 14

Define infrared (IR) spectroscopy

Answer 14

An analytical technique used to identify the functional groups present in a molecule by measuring the vibrational frequencies

Question 15

Why do bonds vibrate?
What does the frequency of vibration in a bond depend on?

Answer 15

• Bonds in different functional groups absorb different frequencies of infrared light
• All bonds vibrate at a characteristic frequency
• Frequency depends on mass of the atoms in the bond, the bond strength and the type of vibration
• Frequencies at which they vibrate are in the infrared region of the electromagnetic spectrum

Question 16

What are the three types of vibration?

Answer 16

• Symmetric stretch
• Asymmetric stretch
• Bonding

Question 17

Describe the process of how IR spectrometry used to identify functional groups in a molecule

Answer 17

1) A beam of IR radiation is passed through a sample of a chemical
2) IR radiation is absorbed by covalent bonds on the molecules, increasing their vibrational energy
3) Bonds between different atoms, absorb different frequencies of IR radiation
4) Wave numbers are used as a measure of the wavelength / frequency of absorption

Question 18

What is the equation to calculate wave number?

Answer 18

Wave number (cm-1) = 1 / wavelength (cm)

Question 19

Define fingerprint region

Answer 19

The region between 1000 cm-1 and 1500 cm-1 on an infrared spectrum which is unique to a particular compound

Question 20

What are some features of the fingerprint region?

Answer 20

• Complicated
• Contains many tiny differences from species to species
• Unique to every compound, and so can be used as a fingerprint
• This region can check if compound is pure (extra peaks in fingerprint region will be produced if impure)

Question 21

How does global warming occur?

Answer 21

• Sun emits UV / visible radiation which is absorbed by the earth’s surface
• Re-emitted as IR radiation
• Carbon dioxide, methane and water vapour have bonds which absorb infrared energy
• Causes heat to be trapped in earth’s atmosphere

Question 22

What are the three greenhouse gases?

Answer 22

• Carbon dioxide
• Methane
• Water vapour

The more IR radiation a molecule absorbs, the more effective they are as a greenhouse gas