CD abhm: Origins of colour in organic compounds; diazonium and azo compounds; dyes Flashcards
Explain in simple terms what causes colouration.
- When light hits a substance, it may be transmitted, reflected or absorbed
- If frequency absorbed is in visible region, substance is coloured
State what is meant by “complementary colours”.
Opposite frequencies of visible light on the colour wheel which, when combined, produce white light.
Explain why some molecules are coloured and others are colourless.
- Electronic energy transitions correspond to visible + ultraviolet light
- When a molecule absorbs light, electrons are excited from ground state to a higher energy level
- Frequency of light is related to ΔE by ΔE = hν
- If light absorbed is in visible region, molecule is coloured. If in ultraviolet region, molecule is colourless
What is a conjugated system?
A sequence of alternating single and double bonds, which allows the overlapping of p-orbitals.
Can also be called “delocalised systems”. All aromatic compounds are conjugated systems (but not vice versa)
Conjugated systems can include alternating C-C and C=C bonds. What else can be part of a conjugated system?
- π electrons from C=O and C=N bonds
- Lone pairs on oxygen and nitrogen, if aligned in correct direction to allow overlap with system (i.e. need to be in py-orbital, not px orbital, where y-direction is perpendicular to ring)
Explain why propene is not an example of a conjugated system.
- For electrons in a double bond to delocalise, electrons in adjacent carbon need to have 5 bonds
- This is energetically feasible in larger conjugated systems, since they have a smaller energy gap between ground + excited electronic states
- Propene is very small, so energy gap is very large + therefore delocalisation is not viable
Think of it like this: there is no energetic incentive for propene to delocalise
Explain what causes some organic molecules containing conjugated systems to be coloured.
- Conjugated system allows p-orbitals to overlap, so π-electrons delocalise across system
- This decreases gap between electronic ground state + excited state
- Allows electrons in molecule to absorb visible light + move up energy level
- Energy absorbed is related to frequency by ∆E = hν
- Light transmitted is complementary colour (due to missing frequency)
- In conjugated systems, double bonds can shift like dominos - single bonds can’t, since they would just break*
- A conjugated system with 5 π-bonds will generally absorb visible light*
Two organic molecules contain conjugated systems. One is coloured and the other is colourless.
Make a prediction about the difference in their structures.
- Conjugated system allows p-orbitals to overlap, so π-electrons delocalise across system
- This decreases gap between electronic ground state + excited state
- Both molecules absorb light + electrons are excited to higher energy level
- Energy absorbed is related to frequency by ∆E = hν
- Coloured molecule absorbs visible light, whereas colourless molecule absorbs ultraviolet light
- Coloured molecule absorbs lower frequency of light
- So coloured molecule has larger conjugated system with smaller ∆E
Will a molecule with 20 π-bonds in a conjugate system absorb radiation of a higher or lower wavelength than a molecule with 10-π bonds in a conjugate system?
Higher
More π-bonds → larger conjugated system → smaller energy gap between ground + excited electronic states → lower frequency absorption → higher wavelength
Diazonium compounds are those which contain the diazonium, or diazo, group. Draw this group.
- The diazo group is unstable. State what is produced when it decomposes.
- Explain why attaching an aromatic compound to a diazonium compound stabilises the latter.
- Nitrogen gas
- π-electrons in diazonium group delocalise across the conjugated system in the aromatic compound, which stabilises the group
Diazonium salts are typically made in situ to form intermediates in organic synthesis.
Why must they be used immediately?
They are unstable & explosive (due to rapid evolution of gaseous nitrogen).
What is diazotisation?
A reaction in which an amine is converted into a diazonium salt.
Aromatic diazonium compounds are prepared from aromatic amines.
- Give the name for this reaction.
- Give the reagents and conditions.
Diazotisation
- Reagents: aromatic amine, conc. HCl, sodium nitrate(III) (NaNO2) solution
- Conditions: Below 5oC / ice-cold
The actual reactant for diazotisation is nitrous acid, HNO2. It is formed by adding dilute hydrochloric acid and sodium nitrate (III) to an amine.
Give the symbol equation for the diazotisation of phenylamine. Represent all organic molecules with skeletal formulae.