Amines Flashcards
What are the two main properties of ammonia and amines? What is responsible for these two properties?
1) Their tendency to act as nucleophiles and their action as Brønsted-Lowry bases
2) The lone pair on the nitrogen atom
How are quaternary ammonium ions produced?
From tertiary amines when the lone pair on the N forms a dative bond to a carbocation to form a fourth hydrocarbon group
Like ammonium salts, quaternary ammonium salts are…
… crystalline, ionic solids
Are amines and ammonia weak or strong bases? Why?
1) Weak
2) When in aqueous solution, an equilibrium is formed between the products and reactants
Why are aromatic primary amines weaker bases than ammonia?
An overlap occurs between the lone pair on the N and the delocalised electron system of the benzene ring, and the nitrogen lone pair is delocalised towards the benzene ring. The electron density on the nitrogen atom decreases which reduces the ability of the N to accept a proton
Why are aliphatic primary amines stronger bases than ammonia?
They push electron density away from the alkyl group towards the amine group. This increase the electron density on the N atom which increase it’s ability to accept a proton
How can primary aliphatic amines be formed?
By the nucleophilic substitution of the halogen atom in a halogenoalkane with ammonia. Excess ammonia is dissolved in ethanol and heated with the halogenoalkane in a sealed vessel. Sealed vessel leads to increased pressure which increases the rate of reaction
In nucleophilic substitution to produce primary amines, what must be ensured to get the desired amine?
- The halogenoalkane must have the same number of carbon atoms as the required amine product
- The halogen must be positioned on the carbon atom(s) to which the amino groups need to be bonded
What are the three stages of nucleophilic substitution when preparing a primary amine?
STAGE 1: The lone pair on the N atom is attracted to the +ve dipole on the carbon atom in the polar C-Halogen bond
STAGE 2: The lone pair on the N forms a covalent bond with the carbon atom and the C-Halogen bond is broken
STAGE 3: The amine is released by the removal of a proton by a base. This base can be added to the reaction mixture (e.g aqueous NaOH) or the excess ammonia can act as the base
Why are chloro- and bromoalkanes preferred sources of the hydrocarbon groups for nucleophilic substitution reactions than iodoalkanes and fluoroalkanes?
Thy are more readily available than iodoalkanes and they are more reactive than fluoroalkanes
When halogenoalkanes undergo nucleophilic substitution with ammonia, why are mixtures or primary, secondary and tertiary amines and quaternary ammonium salts produced?
Alkyl groups push electron density towards the N atom, increasing its ability as a nucleophile. Therefore the product of each successive alkylation is a better nucleophile than the starting material. As a result some of them will go on to react agiain with a halogenoalkane and form the next product
How can we increase the yield of the primary amine when a halogenoalkane does nucleophilic substitution with ammonia?
Use excess ammonia because it is less likely that a second halogenoalkane will react with a primary amine as there are relatively a lot more unreacted ammonia molecules still available
How can we increase the yield of the quaternary ammonium salt when a halogenoalkane does nucleophilic substitution with ammonia?
Use excess amounts of the halogenoalkane as this will ensure that each ammonia molecule reacts with four halogenoalkane molecules
What is the most efficient method for producing the highest yield of primary amines? How else can aliphatic primary amines be made?
1) Reflux an appropriate halogenoalkane with a solution of KCN dissolved in water and ethanol. The lone pair on the cyanide ion allows nucleophilic substitution which produces a nitrile. The reduction of the triple bond between the CN with two H molecules forms the amine. The reduction can be achieved by using lithium tetrahydridoaluminate (LiAlH4), dissolved in dry ether, followed by hydrolysis. Alternatively H gas with a nickel catalyst can cause the reduction.
2) By reducing nitro compounds and amides
Why don’t halogenobenzenes react strongly with ammonia?
- The N atom in ammonia is nucleophilic, benzene molecules usually react with electrophiles. The high electron density of the delocalised ring of electrons repels nucleophiles
- The aromatic ring is an electron-attracting group. The lone pairs on the halogen atom are delocalised towards the benzene ring, this shortens the C-Halogen bond making it less reactive than an aliphatic halogenoalkane
How is phenylamine made?
By nitrating benzene with conc. nitric acid and sulphuric acid at 50°C. The nitro group is then reduced to a primary amine group using an acid together with a metal. On an industrial scale, scrap iron and HCl are used, but in the lab tin and HCl are used. Since excess acid is present, the initial product is the protonated form of phenylamine. A base is added (e.g. NaOH) which removes the proton. This reduction can also be achieved using H gas and a nickel catalyst.
Why is the conversion of aromatic nitro compounds into aromatic amines important?
It is important in the dye industry. It is used to introduce primary amine groups into a wide variety of aromatic molecules used for making dyes or into dye molecules themselves, causing changes in colour and ability to bond to fabrics
Another important reaction of ammonia and primary amines is with acyl chlorides and acid anhydrides to give amides. What are by-products of these reactions? What does this mean for the quantities used in the reaction?
For each mole of acyl chloride or acid anhydride, two miles of ammonia or amine are needed. The first to act as a nucleophile, the second to act as a base and react with the generated acid
What are the names of the three steps in nucleophilic addition-elimination reactions?
Addition -> Elimination -> Deprotonation
What is a cationic surfactant and an anionic surfactant?
The polar head is positively charged on a cationic surfactant
What can be used as cationic surfactants? How do they work? What are they used in?
1) Quaternary ammonium salts
2) The positive charge on the N atom is attracted to negatively charged surfaces (glass, hair, fibre, metals and plastics)
3) Used in fabric conditioners, leather softeners, sewage flocculants (for bringing particles to the surface), corrosion inhibitors, hair conditioners, anti-static agents and emulsifiers. Some with shorter carbon chains are also used in disinfectants, where they combine surfactant action with germicidal properties
