CD abhm: Origins of colour in organic compounds; diazonium and azo compounds; dyes

Question 1

Explain in simple terms what causes colouration.

Answer 1

• When light hits a substance, it may be transmitted, reflected or absorbed
• If frequency absorbed is in visible region, substance is coloured

Question 2

State what is meant by “complementary colours”.

Answer 2

Opposite frequencies of visible light on the colour wheel which, when combined, produce white light.

Question 3

Explain why some molecules are coloured and others are colourless.

Answer 3

• 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

Question 4

What is a conjugated system?

Answer 4

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)

Question 5

Conjugated systems can include alternating C-C and C=C bonds. What else can be part of a conjugated system?

Answer 5

• 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 p_y-orbital, not p_x orbital, where y-direction is perpendicular to ring)

Question 6

Explain why propene is not an example of a conjugated system.

Answer 6

• 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

Question 7

Explain what causes some organic molecules containing conjugated systems to be coloured.

Answer 7

• 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*

Question 8

Two organic molecules contain conjugated systems. One is coloured and the other is colourless.

Make a prediction about the difference in their structures.

Answer 8

• 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

Question 9

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?

Answer 9

Higher

More π-bonds → larger conjugated system → smaller energy gap between ground + excited electronic states → lower frequency absorption → higher wavelength

Question 10

Diazonium compounds are those which contain the diazonium, or diazo, group. Draw this group.

Answer 10

Question 11

• 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.

Answer 11

• Nitrogen gas
• π-electrons in diazonium group delocalise across the conjugated system in the aromatic compound, which stabilises the group

Question 12

Diazonium salts are typically made in situ to form intermediates in organic synthesis.

Why must they be used immediately?

Answer 12

They are unstable & explosive (due to rapid evolution of gaseous nitrogen).

Question 13

What is diazotisation?

Answer 13

A reaction in which an amine is converted into a diazonium salt.

Question 14

Aromatic diazonium compounds are prepared from aromatic amines.

• Give the name for this reaction.
• Give the reagents and conditions.

Answer 14

Diazotisation

• Reagents: aromatic amine, conc. HCl, sodium nitrate(III) (NaNO₂) solution
• Conditions: Below 5^oC / ice-cold

Question 15

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.

Answer 15

Question 16

• Name and draw the functional group in azo compounds.
• State the name of the reaction which produces an azo compound, and state the reagents required.

Answer 16

• Azo group, R-N=N-R’
• A coupling reaction between a diazonium compound and a coupling agent

Question 17

• Name the structural feature of aromatic azo compounds that makes them coloured.
• Explain why they are stable enough to be used as dyes.

Answer 17

• Conjugated / delocalised system
• Azo group is stabilised through its π-electrons joining conjugated systems in aromatic residues of both diazonium compound + coupling agent

Question 18

• Describe how a coupling reaction producing an azo compound is carried out.
• State what is observed during the reaction.

Answer 18

• Ice-cold solution of diazonium salt is added to solution of coupling agent
• Coloured precipitate of azo compound forms immediately

Question 19

Name the molecules which typically act as coupling agents for the synthesis of azo compounds.

Answer 19

• Phenols
• Aromatic amines

​I.e. OH or NH₂ groups attached to an arene

Question 20

Using skeletal formulae for organic molecules, draw a general equation for a coupling reaction which produces an azo compound.

Answer 20

• Don’t need to be able to predict where on the ring the diazo group joins.*
• Don’t need to be able to name azo compounds.*
• Lone pair on amine/phenol group of coupling agent does not act as nucleophile directly. Instead, it increases benzene’s electron density, making it a better nucleophile - i.e. it is actually a carbon in the ring which attacks the diazonium group​*

Question 21

In a coupling reaction which produces an azo compound, a new bond forms between the diazo group and the ring of the coupling agent.

Name the mechanism by which the bond forms. Justify your answer.

Answer 21

• Electrophilic substitution
• Diazo(nium) group acts as an electrophile and is attacked by coupling agent
• Proton on benzene ring of coupling agent is substituted for (what is now an) azo group

Question 22

• Suggest why, in the coupling reaction between the benzenediazonium cation and phenol, the coupling agent (i.e. phenol) is dissolved in alkali.
• Using skeletal formulae for the organic molecules, give the equation for this reaction.

Answer 22

• Phenol (OH) group is deprotonated
• Increases electron density in aromatic ring
• Makes coupling agent more susceptible to electrophilic attack by benzenediazonium cation

Question 23

When looking at an azo compound, how do you distinguish the diazonium compound residue from the coupling agent residue?

Answer 23

• Coupling agent residue has phenol or amine group
• Diazonium compound residue is the other one

Question 24

Draw and label the diazonium compound and the coupling agent used to form this azo compound.

Answer 24

Question 25

Define “chromophore”.

Answer 25

The region of a coloured molecule which contributes to the delocalisation of electrons, and is therefore responsible for the absorption of visible light.

Groups attached to benzene rings which don’t have electrons in delocalised system are not part of chromophore. However, they do affect colour

Question 26

Lignin contains chromophores which absorb in the UV, whereas its decomposition products are yellow.

Explain what affects the frequency of radiation absorbed, and why the decomposition products are yellow.

Answer 26

What affects the frequency of radiation absorbed:

• Electrons absorb light
• They move to higher energy levels
• Energy + frequency of light absorbed are related by ΔE = hν
• Greater degree of delocalisation causes lower frequency / energy absorbed

​Why the decomposition products are yellow:

• Decomposition products have larger chromophore / larger conjugated system / more delocalisation than lignin
• So former absorb visible light whereas latter absorbs UV
• Former are yellow because they absorb complementary colour, violet

Question 27

List three purposes which functional groups can serve within dye molecules.

Answer 27

Functional groups may:

• Modify the chromophore
• Alter the solubility of the dye
• Allow the dye to bond to fibres

All functional groups affect colour to some extent. Some functional groups serve more than one purpose.

Question 28

Suggest the features of a dye molecule which could serve each of the purposes below.

• Modifying the chromophore
• Altering the solubility of the dye
• Allowing the dye to bond to fibres

Answer 28

Chromophore: groups / bonds which can join conjugated system: alternating C-C and C=C / C=O / C=N / O: / N:, where colons represent lone pairs (aligned in correct direction to allow overlap with system)

Solubility: polar groups

Bonding

• Ionic: acid groups
• Hydrogen: polar groups
• Id-id: any group
• Covalent: groups with lone pairs

Question 29

Functional groups can be attached to dye molecules. These can modify the properties of the dye.

• Give one property of a dye that might be affected if nitro, NO₂, groups are attached.
• Give a different property affected by the attachment of sulfonate, SO₃^-, groups.

Answer 29

• Colour / chromophore
• Solubility (also ionic bonding to fibres?)

Nitro groups don’t affect solubility since N and O are of similar electronegativity, so bonds are not polar

Question 30

• State the source of polyamide (protein) fibres.
• Name the functional group(s) present in these fibres which would be involved in dyeing them.
• Describe how a dye molecule would be bonded to a polyamide fibre.

Answer 30

• Animals (e.g. silk, wool, fur)
• Amine group, NH₂
• NH₂ group is ionised to NH₃⁺ in acidic conditions. Sulfonate group in dye, SO₃H, is ionised to SO₃^- by dissolving in water. Ionic bonding occurs between NH₃⁺ and SO₃^-

Question 31

• State the source of cellulose fibres.
• Name the functional group(s) present in these fibres which would be involved in dyeing them.
• Describe how a dye molecule would be bonded to a cellulose fibre.

Answer 31

• Plants (e.g. cotton, hemp, bamboo)
• Hydroxyl (OH) groups
• OH and NH₂ groups in dye molecules form hydrogen bonds with OH groups in fibre

Question 32

• Dyed cloth is often washed with water. Why might this be a problem for dyes that attach to fibres using hydrogen bonds?
• Dye molecules that attach to fibres through hydrogen bonds are usually long and linear. Explain why this could help to them remain on the fibre when washed.

Answer 32

• Hydrogen bonds between groups in dyes and fibres break. New ones form between groups in dye and water molecules. Dye fades
• Linearity means a dye can line up close to a fibre, and length means it can bond at multiple sites. Decreases the chance of detachment.

Question 33

• State the source of polyester fibres.
• Name the functional group(s) present in these fibres which would be involved in dyeing them.
• Describe how a dye molecule would be bonded to a polyester fibre.

Answer 33

• Synthetic manufacture
• All
• Instantaneous dipole-induced dipole interactions

Question 34

• Explain why the dye molecules which are used to bond to polyesters must contain few polar groups.
• State how they are attached.

Answer 34

• So that they don’t dissolve in water, by forming hydrogen bonds with it, rather than attach to the fibre
• Dye is dispersed through water, then fibre is submerged

Question 35

Name the two types of covalent bonding through which dyes can be attached to fibres.

Answer 35

• Mordanting
• Adding fibre-reactive groups

Question 36

Describe what mordanting involves.

Answer 36

• Groups on fibre + dye form dative covalent bonds to a central metal ion
• Forms a chelate complex

Question 37

What is a chelate?

Answer 37

A complex ion in which a metal ion is covalently bonded to 2 or more atoms from the same molecule.

Question 38

• Describe how fibre-reactive dyes work.
• State the types of fibre to which they attach.

Answer 38

• Reactive groups on dye molecule form covalent bonds with OH or NH₂ groups on fibre, forming a bridge
• Cellulose, some polyamides, some synthetic fibres

These groups include triazine derivatives, which are a class of nitrogen-containing heterocycles. The molecular formula of the parent molecule, triazine, is C₃H₃N₃.

Question 39

Fill in the table.

Answer 39