Section 11 Colour By Design Flashcards
(10 cards)
Arene electrophilic substitution reaction with nitronium ions?
Warm benzene with concentrated nitric and sulfuric acids, you get nitrobenzene.
Sulfuric acid as helps to make nitronium ion, NO2+
Arene electrophilic substitution with sulfur trioxide molecules?
To make benzensulfonic acid you could either:
- Boil benzene under reflux with concentrated sulfuric acid
- Warm benzene to 40 Celsius with fuming sulfuric acid for 30 mins
Electrophilic in reaction is sulfur trioxide, SO3
Using halogen carriers to put an alkyl group on benzene?
- Alkyl groups have one H atom fewer than alkane molecules.
- They can be substituted for H atoms on benzene rings by refluxing a chloroalkane and benzene with a halogen carrier such as AlCl3.
- This can be used to put any alkyl group onto a benzene ring and is called Friedel-Crafts alkylation.
General reaction
C6H6 + RCl > C6H5R + HCl
Catalyst AlCl3 and reflux
Kekulé structure vs delocalised structure of benzene?
If kekule was right, there would always be 3 bonds with C-C bond and 3 bonds with C=C. But x-ray diffraction shows all carbon-carbon bonds are same length.
Cyclohexene has one double bond and enthalpy if hydration of -120 kJ mol-¹. If benzene had 3 double bonds it would have enthalpy of hydrogenation of -360 kJ mol-¹.
Benzene is only -208 kJ mol-¹ so more stable than Kekule’s structure.
How are aromatic amines used to make azo dyes?
Azo dyes are man-made dyes that contain the azo group, -N=N-
In most azo dyes, the azo group links two aromatic group, any group with a benzene ring.
Due to delocalised electron system, very stable molecule.
Colours are result of light absorption by the delocalsied electron system.
How are azo dyes made in a coupling reaction?
- React phenylamine with nitrous acid to make a Diazonium Salt.
A. Nitrous acid is unstable, so it had to be made in situ from sodium nitrite and hydrochloric acid.
B. Nitrous acid reacts with phenylamine and HCl to form benzenediazonium chloride. The temperature must be below 5 degrees to prevent phenol forming.
- Make azo dyes by coupling the diazonium salt with a phenol.
a. First the phenol has to be dissolved in NaOH solution to make sodium phenoxide solution.
b. It’s then stood in ice, and chilled benzendiazonium chlorine is added.
c. Azo dyes precipitates out of the solution immediately.
d. Phenol is a coupling agent. Lone pairs on its oxygen increase electron density of the benzene ring. This gives the diazonium ion something to attack.
Chromophores?
Structures in molecules that give them their colour are called chromophores.
When light hits a chromophore, certain wavelengths are absorbed by electrons in the chromophore. Visible wavelength not absorbed will be seen as a particular colour.
Chromophores tend to contain double or triple bonds, lone pairs, or benzene rings.
Modifying the chromophore will change the frequency of light that it absorbs, and so change the colour of the molecule.
Fehling’s solution to test for aldehyde and ketone?
- Add 2 cm³ of Fehling’s or Benedict’s solution to a test tube.
- Add 5 drops of aldehyde or ketone to test tube and place test tube in a bot water bath to warm.
- Aldehyde- the blue solution will give a brick red ppt.
Ketone- nothing happens
Tollens’ reagent can be used to test for aldehyde and ketone?
- Put 2cm³ of 0.1 mol dm-³ silver nitrate solution in a test tube.
- Add new drops of dilute NaOH solution. A light brown ppt should form.
- Add drops of dilute ammonia solution until brown ppt dissolves. This is tollen’s reagent.
- Place test tube in hot water bath and add 10 drops of aldehyde or ketone and wait for few minutes
Aldehyde- silver mirror forms on walls of test tube.
Ketone: nothing happens
Describe gas-liquid chromatography.
- Gas liquid chromatography the stationary phase is a solid or a viscous high boiling point liquid, which coats a porous support inside a long tube. The tubes coiled to save space, and built into an oven.
- The mobile phase is an inert carrier gas such as nitrogen or helium.
- The sample to he analysed is injected into the stream of carrier gas, which carries it through the tube and over the stationary phase.
- The components of the mixture constantly dissolve in the stationary phase, evaporate into the mobile phase and then redissolve as they travel through the tube.
- The solubility of each component of the mixture determines how long it spends dissolved in the stationary phase and how long it spends moving along the tube in the mobile phase. A substance with a high solubility will spend more time dissolved, so will take longer to travel through the tube to the detector then one with a lower solubility.
