colour by design (CD) Flashcards
What are aromatic compounds (arenes)?
Aromatic compounds contain benzene rings, which are made up of six carbon atoms in a planar, cyclic structure with delocalised π-electrons.
What was Kekulé’s structure for benzene?
Kekulé proposed that benzene has alternating single and double bonds, with the bonds flipping continuously. However, this model is incorrect because all the bonds in benzene are the same length.
What is the delocalised model of benzene?
The delocalised model suggests that each carbon in the benzene ring donates an electron from its p-orbital, forming a ring of delocalised electrons above and below the benzene ring, stabilising the molecule.
How does the delocalised structure of benzene affect its reactivity?
The delocalised electron system in benzene makes it stable, and this stability means benzene typically undergoes substitution reactions instead of addition reactions. The electrons in the delocalised ring are more spread out and less reactive.
What evidence supports the delocalisation of electrons in benzene?
The enthalpy change for the hydrogenation of benzene is less exothermic than expected by Kekulé’s model. Benzene requires more energy to break bonds, indicating that the molecule is more stable than the structure with alternating single and double bonds.
What is electrophilic substitution with nitronium ions?
When warm benzene reacts with concentrated nitric acid (HNO3) and sulfuric acid (H2SO4), sulfuric acid acts as a catalyst to form the nitronium ion (NO2+), which is the electrophile. This reaction introduces a nitro group (NO2) into the benzene ring.
What is the reaction mechanism for the electrophilic substitution with nitronium ions?
HNO3 + H2SO4 → H2NO3+ + HSO4−
H2NO3+ → NO2+ + H2O
The nitronium ion (NO2+) reacts with benzene, substituting a hydrogen atom with a nitro group.
What is electrophilic substitution with sulfur trioxide?
In this reaction, concentrated sulfuric acid (H2SO4) is boiled with benzene to produce benzenesulfonic acid. The sulfur trioxide (SO3) acts as the electrophile in the reaction, replacing a hydrogen atom.
What are halogen carriers, and how do they work?
Halogen carriers, such as AlCl3, are catalysts that help to polarize halogen molecules, making them more electrophilic and capable of reacting with benzene in electrophilic substitution reactions.
What is Friedel-Crafts alkylation?
Friedel-Crafts alkylation is the substitution of a hydrogen atom in benzene with an alkyl group (R). This is done by reacting a chloroalkane (e.g., CH3Cl) with benzene in the presence of a halogen carrier, like AlCl3.
What is Friedel-Crafts acylation?
Friedel-Crafts acylation is the reaction between benzene and an acyl chloride (RCOCl) in the presence of a halogen carrier (e.g., AlCl3) to form an aryl ketone (phenyl ketone).
How do halogen carriers influence halogenation of benzene?
Halogen carriers polarize halogen molecules, allowing them to act as electrophiles in electrophilic substitution reactions with the benzene ring.
What are azo dyes, and how do they work?
Azo dyes contain the azo group (-N=N-) linking two aromatic rings. The azo group becomes part of the delocalised electron system, and the colours of azo dyes result from the absorption of light by this delocalised electron system.
What is a diazonium salt, and how is it formed?
A diazonium salt, such as benzenediazonium chloride, is formed by reacting nitrous acid (HNO2) with an aromatic amine like phenylamine in an acidic environment. The diazonium ion can then react with other compounds to form azo dyes.
What is the mechanism for making a yellow-orange azo dye from a diazonium salt?
To form benzenediazonium chloride, nitrous acid is generated in situ from HCl and sodium nitrite. It reacts with phenylamine at low temperatures to form the diazonium salt, which can then react with phenol in the presence of sodium hydroxide to form an azo dye.
How does a diazonium salt react with phenol to form an azo dye?
The lone pairs on the oxygen of phenol increase the electron density of the benzene ring, allowing it to react with the diazonium ion, leading to the formation of an azo compound.
How do dyes attach to fibers?
Dyes can attach to fibers via ionic interactions or hydrogen bonds, depending on the functional groups in the dye and fiber. For example, amine groups can form hydrogen bonds with cellulose fibers, while acidic groups can bind to -NH- groups in proteins.
What are fiber-reactive dyes?
Fiber-reactive dyes are permanent dyes that form strong covalent bonds with -OH or -NH- groups in fibers, resulting in a more colorfast (long-lasting) bond.
What are chromophores?
Chromophores are structures in molecules that give them their color by absorbing certain wavelengths of light. They typically contain conjugated double bonds or delocalised electron systems.
How does electron excitation lead to color?
When molecules absorb energy, electrons are excited to higher energy levels. If the energy gap corresponds to visible light, the molecule appears colored. The structure of the chromophore and the number of electrons influence the specific color observed.
What is conjugation in terms of color?
Conjugation occurs when alternating single and double bonds (C=C) form a delocalised electron system. More conjugated bonds and functional groups lead to lower-frequency light absorption and different colors.
How can you test for aldehydes and ketones?
Fehling’s solution: Aldehydes turn the solution brick red, while ketones do not react.
Tollen’s reagent: Aldehydes form a silver mirror, while ketones show no reaction.
What happens when Fehling’s solution reacts with an aldehyde?
Fehling’s solution, which contains copper (II) ions, turns brick red when heated with an aldehyde because copper (I) is reduced to form a brick-red precipitate.
What is the result of a reaction with Tollen’s reagent?
When an aldehyde is heated with Tollen’s reagent, a silver mirror forms, indicating the reduction of silver ions. Ketones do not react with Tollen’s reagent.
