NItrogen Compounds Flashcards
Why do amines have higher boiling points than hydrocarbons with similar Mr?
This is because amines have stronger hydrogen bonding (Except for tertiary amines).
However amines have lower boiling points than alcohols of similar Mr due to the hydrogen bonds in amines being weake than those in alcohols since the N-H bond is less polar than the O-H bond.
Why is hydrogen bonding in secondary amines less extensive than that of primary amines?
With one more R group bonded directly to the N atom in secondary amines, there is greater steric hindrance which makes the lone pair of electrons on the N atom is less accessible for hydrogen bonding as compared to primary amines. Hence, the hydrogen bonding in secondary amines is less extensive.
Why are amines with small molecular size soluble in water?
Amines can form hydrogen bonds with water molecules. The hydrogen bonds formed between amines and water molecules releases sufficeint energy to overcome the strong hydrogen bonds between amines and water molecules.
Why does the solubility in water of bulkier/heavier amines decreases?
The pd-pd interactions between the larger non-polar alkyl group and water molecules do not release sufficient energy to overcome the strong hydrogen bonds between amines and water molecules. furthermore, the larger alkl group also hinders the formation of hydrogen bonds between the amine functional group and water molecules.
Why are ammonium salts soluble in water?
Ammonium salts are soluble as ionic quaternary ammonium salts can form ion-dipole interactions with water molecules which releases sufficient energy to overcome the ionic bonds in the crystal lattice structure.
How are amines prepared from halogenoalkanes?
Nucleophilic substitution.
Reagent and conditions: excess NH3 in ethanol, heat in sealed tube
Excess ammonia is used to ensure high yield of the primary amine as a mixture of product will be formed without it.
Why can’t helogenoarenes be used in preparing amines?
The p-orbital of the halogen can have a side-on overlap with the pi electron cloud of the benzene ring. As a result, the lone pair of electrons on the halogen atom can delocalise into the benzene ring. Therefore there is a partial double bond characte in the C-X bond and is thus stronger and not easily broken.
How can primary amines be prepared from nitriles?
Redcution of intriles
Reagent and conditions:
(1) LiAlH4 in dry ether, r.t
OR
(2) H2 with Ni/Pd/Pt catalyst. r.t
How can amines be prepared from amides?
Reduction of amides
Reagent and conditions: LiAlH4 in dry eher. r.t
How can aromatic amines be prepared from nitrobenzenes?
Reduction of nitrobenzenes
Reagent and conditions: Sn, concentrated HCl, heat
Due to the highly acidic concentraed HCl used, the basic phenylamine is obtained in the form of its ammonium salt, C6H5NH3+Cl-. To free phenylamine, NaOH(aq) can be added to neutralise the acid
C6H5NH3+(aq) + NaOH(aq) -> C6H5NH2(l) + H2O(l)
Describe the basicity of amines in the gaseous state.
In the gaseous phase, amines behave like a Lewis base (electron pair donor) This is due to the availability of the lone pair of electrons on the nitrogen atoms for donation to an electron pair acceptor. Their basic strength increases with the availability of the lone pair of electons on the nitrogen atom for donation to an electron pair acceptor.
Why does basicity decrease from trimethylamine to ammonia?
From methylamine to trimethylamine, the number of electron-donating alkyl groups attached to the N atom increases. This increases the electron density on N atom of the amine. Hence the lone pair of electrons on N atom is more avilable for donation
Describe the basicity of amines in the aqueous state.
Amines are weak bases in aqueous medium as they behave like a Bronsted-Lowry base (proton acceptor). This is due to the ability of the lone pair of electrons on Natom to accept a proton from H2O. The basic strenght increases with the avilability of the lone pair of electrons on the nitrogen atom for donation.
Why is phenylamine a weaker base than ammonia?
In phenylamine, the lone pair of electrons on the nitrogen atom is delocalised into the benzene ring due to the p-orbital of the nitrogen atom having a side-on overlap with the pi electron cloud of the benzene ring. The delocalisation of the lone pair on N atom makes it less available for donation to a proton.
How do substituents effect the basicity of amines?
Electron-donating substituents cause an increase in basicity as they increase the electron density at the N atom of the amine group, making the lone pair of electrons on the N atom more available for donation to a proton.
Electron-withdrawing substituents cause a decrease in basicity as they decrease electron density at the N atom of the amine group, making the lone pair of electrons less available for donation to a proton.
How is salt formed from amines?
Acid-base reaction/neutralisation
Reagents and conditions: HCl(aq) or H2SO4(aq) r.t
How are amides formed from amines?
Nucleophilic substitution/condensation reaction with acyl chlorides.
Reagents and conditions: anhydrous acyl chloride, RCOCl, r.t
Observations: white fumes of HC;
This is the only method to synthesise amides + tertiary amides cannot be used.
What reaction can phenylamines undergo?
Electrophilic substitution of phenylamines.
Reagents and conditions: aqueous bromine
Observations: orange bromine solution decolourised with the formation of a white ppt of 2,4,6-tribromophenylamine and white fumes of HBr evoloved
What is a distinguishing test for phenol and phenylamine?
Add aqueous neutral FeCl3 solution seperately to each unknown compound.
Phenol - forms a violet complex
Phenylamin - does not form a violet complex
Why don’t tertiary amides form hydrogen bonds with each other?
They have no hydrogen atoms directly bonded to the electronegative N atom.
Describe the solubility of amides in water.
Aliphatic amides with a small number of carbon atoms are soluble in water because the amide molecules can interact with water molecules via hydrogen bonding. Solubility decreases with increasing size of the non-polar hydrocarbon chain.
Why are amides neutral?
This is due to the lone pair of electrons on the nitrogen atom delocalising into the pi-electon cloud of the adjacent C-O double bonds. The lone pair of electrons on the nitrogen atom is therefore not available for donation to a proton.
State the reagent and conditions of the hydrolysis of amides.
Amides can only undergo hydrolysis when heated in a strong acid or alkali.
Reagent and conditions:
(Strong acid): HCl(aq) or H2SO4(aq), heat
(Strong base): NaOH(aq) or KOH(aq), heat
Heating is required to hydrolyse amides
The alkaline/base hydrolysis reaction can be used as a distinguishing test for amides (Observation would be a pungent gas of CH3CH2NH2 evolved which turns moist red litmus paper blue) + ONLY owrks if amines produced contain 5 carbons or less as they are volatile enough to be vaporised by the heat
State the reagent and conditions of the reduction of amides.
LiAlH4 in dry ether, r.t
Amides are reduced to amines
