Explain the basicity of amines in terms of proton acceptance by the nitrogen lone pair;
Amines are weak bases. A base is a proton acceptor. When an amine accepts a proton, a dative covalent bond forms between the lone pair of the nitrogen atom and the proton.
Describe the reactions of amines with acids to form salts;
Amines are neutralised by acids to make an ammonium salt. For example, ethylamine, reacts with hydrochloric acid to form ethylammonium chloride (CH3CH2NH3+Cl-).
Describe the preperation of: alipahtic amines by substitution of halogenoalkanes with excess ethanolic ammonia and aromatic amines by reduction of nitroarenes using tin and concentrated hydrochloric acid;
Amines can be made by heating a halogenoalkane with an excess ethanolic ammonia. You’ll get a mixture of primary, secondary and tertiary amines, and quaternary ammonium salts, as more than one hydrogen is likely to be substituted. You can seperate the products using fractional distillation. Example:
2NH3 + CH3CH2Br → CH3CH2NH2 + NH4Br
Nitro compounds, such as nitrobenzene, are reduced in two steps: 1) heat a mixture of a nitro compound, tin metal and conc. hydrochloric acid under reflux– this makes a salt. 2) then to get the aromatic amine, you have to add sodium hydroxide. Example:
C6H5NO2 +6[H] → C6H5NH2 + 2H2O
Describe the synthesis of an azo dye by reaction of an aromatic amine with nitrous acid (<10 degrees celsius), with formation of a diazonium ion, followed by coupling with a phenol under alkaline conditions; (Nitrous acid is generated in situ from NaNO2/HCl)
The first step in creating an azo dye is to make a diazonium salt. The azo dye is then made by coupling the salt with an aromatic compound that is susceptible to electrophilic attack, e.g. a phenol.
Below is the method for creating a yellow-orange azo dye:
Part 1- making the diazonium salt
1) Nitrous acid (HNO2) is unstable, so it has to be made in situ from sodium nitrite and hydrchloric acid:
NaNO2 + HCl → HNO2 + NaCl
2) Nitrous acid reacts with phenylamine and hydrochloric acid to form benzenediazonium chloride. The temperature must be below 10 degress celsius to prevent a phenol forming instead.
Part 2- coupling the diazonium salt with a phenol
1) First, the phenol has to be dissolved in sodium hydroxide solution to make sodium phenoxide solution.
2) It’s then stood in ice, and chilled in the benzenediazonium chloride is added
3) The azo dye will precipitate out of solution immediately
4) Phenol is a coupling agent. The lone pairs on its oxygen release the electron density of the benzene ring, especially around carbons 2, 4 and 6. This gives the diazonium ion (a weak electrophile) something to attack.
State the uses of reactions, such as (d) in the formation of dye stuffs;
n.b– structures are in the CGP book