Colour By Design Flashcards
How are azo compounds formed?
Coupling reaction.
Diazonium salt + coupling agent
Azo compounds highly coloured used as dyes
How are the r groups stabilised in an aromatic azo compound
Becoming part of an extended delocalised system involving the arene groups
Why are only aromatic diazonium salts relatively more stable?
Presence of the electron rich benzene ring stabilises the -N-=N (triple bond, +-v charge on first N) group
Whereas the -N///N on non aromatic ones tends to lose the group as N2 (g)
Why are diazonium salts prepared in ice cold solutions
Benzenediazonium chloride decomposes above about 5’c in aqueous solution and the solid compound is explosive
How are diazonium salts prepared in a diazotisation
Cold solution of Sodium nitrate to a solution of an arylamine in dilute acid (below 5 degrees Celsius)
Classify the reaction mechanism for a coupling reaction
Electrophilic substitution
What is a chromophore
An extended delocalised electron system responsible for the colour of a compound
Do electrons in double need more less energy than single bonds to excite them
Electrons in double bonds are spread out and require less energy to excite than those in single bonds. The lower activation energy means these compounds absorb in the visible region
What can be added to enhance or modify colour
Functional groups such as -OH, -NH2 and -NR2 often attached to chromophores
These groups contain lone pairs which become involved in the delocalised system can change the energy of light absorbed by the molecule and so change the colour of the compound
Explain why dyes are coloured and why there can be variation in colour
Electrons are excited
Move up to higher energy level
Absorption of light
^E = HF
Complementary colour transmitted
Different no. Of functional groups, different chromophore
Affects the energy required to excite electrons
Explain in terms of structure and bonding why benzene undergoes mainly substitution rather than addition reactions
Delocalised electron system stability is retained in substitution
Whereas addition loses delocalisation so more energy is required to break delocalisation
Suggest an equation for the reaction of sulfur tetrafluoride with oxygen to for sulfur hexafluoride
3SF4 + O2 –> 2SF8 + SO2
At 298k name the type of bonding present in:
Lithium
Sulfur hexafluoride
Lithium fluoride
Metallic
Covalent
Ionic
In terms of intermolecular bonds explain why SF6 is a gas at room temp whereas hexanediodic acid is a solid (both have approx same Mr)
Sf6 has instantaneous dipole -induced dipole bonds
Hexanediodic acid has hydrogen bongs
Stronger than sf6
More energy is required to overcome bonds to boil/melt acid
What group does the so compound contain?
-N=N- group
