18. Organic III (AMINES)
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Give the functional group for amines
-NH2
Give the 3 different types of amines
Primary amines - the nitrogen is bonded to 1 other carbon Secondary amines - the nitrogen is bonded to 2 other carbons Tertiary amine - the nitrogen is bonded to 3 other carbons
Describe how to name amines
Add “amine” to stem of alkyl group - longest chain goes next to the amine eg: N-ethylpentylamine N shows that the alkyl groups are attached to the same nitrogen Conversely , you could say something like 1-aminobutane
Do amines have a high boiling point, and if so which ones
Only primary and secondary amines have N-H bonds so only they can hydrogen bond with neighbouring molecules so boiling point is higher, but as N-H bond is less polar than O-H bond in alcohols, boiling point is lower compared to them Tertiary amines - no N-H bond present so no hydrogen bonds
Which amines are soluble in water
Only primary and secondary amines as they have N-H and so can hydrogen bond with water molecules. But if alkyl groups are too big, this can interfere with hydrogen bonds
Give the 2 ways to synthesise aliphatic amines (ie, amines with NO benzene ring)
1)Halogenoalkane and ammonia 2) Reduction of a nitrile or amide
Describe the type and conditions/ reagents of the reaction between a halogenalkane and ammonia
Nucleophilic substitution Need excess concentrated ammonia in ethanol and HEATED in a sealed tube Forms aliphatic amine and ammonium chloride Eg: CH3CH2Cl + 2NH3 —> CH3CH2NH2 + NH4Cl
Describe the reduction of a nitrile/ amine to form an aliphatic amide
Reduction reaction Need to warm nitrile or amide with LiAIH4 (represented by [H] in dry ether Then follow with dilute acid Eg with nitrile: CH3CN + 4[H] —> CH3CH2NH2 Eg with amide: CH3CONH2 + 4[H] —> CH3CH2NH2 —> H2O
Describe how to synthesis aromatic amines (amines with a benzene ring)
Reflux of nitrobenzene with tin and concentrated HCl Then add excess NaOH and carry out steam distillation to isolate the phenylamine Eg: C6H5NO2 + 6[H] —> C6H5NH2 + 2H2O
Give the three ways amines can act as
A base A ligand A nucleophile —> this is all due to the lone pair of electrons on the nitrogen
Explain why ammonia is more basic (ie, a better base, better proton acceptors) than phenylamine but less basic than primary aliphatic amines
Ammonia better than phenylamine: lone pair of electrons on nitrogen are donated into the delocalised pi system of the benzene ring, so they are less available to accept H+, making it a weaker base Aliphatic amines better than ammonia - alkyl groups donate electron density, making the lone pair of electrons more available, so they are more likely to accept H+
Why do aliphatic amines with longer alkyl groups have a higher pKa for its conjugate acid
Aliphatic alkanes with longer alkyl groups are stronger bases as the alkyl chain has an inductive effect towards the Nitrogen which increases the electron density available to accept a proton Its conjugate acid has a higher pKa, meaning it’s weaker
Describe the reaction between amines (bases) and acid
Amine + acid —> salt Eg: C2H5NH2 + HNO3 —> C2H5NH3+NO3- (ethyl ammonium nitrate) Note: use of + and - in an otherwise dative covalent bond - (lone pair of electrons from nitrogen shared with hydrogen). Also, no water is produced
Describe the reaction of amines (bases) with water
An alkaline solution is formed of alkylammonium ions and hydroxide ions Eg: CH3CH2NH2 + H2O 🔄 CH3CH2NH3+ +OH- Note: reaction is reversible
How can ammonia act as a ligand
Lone pair of electrons on N form a dative covalent bond with empty orbital of a transition metal
Describe the reaction between amines (nucleophile) and acyl chlorides
Lone pair of electrons on nitrogen attacks the delta positive C in C=O. This is known as an addition- elimination reaction Produces a secondary amide Eg: CH3COCl + NH3 → CH3CONH2 + HCl If ammonia is in excess then equation would be: CH3COCl + 2NH3 → CH3CONH2 + NH4+Cl- —> The R group of the acyl chloride (ie everything apart from the Cl) replaces a hydrogen on the original amine —> 1 amine acts as a nucleophile. The other amine acts as a base to react with the HCl formed to make the salt
Describe the reaction between primary amines (nucleophile) and halogenoalkanes
With excess halogenoalkane eg: chloroethane, you get a tertiary or quaternary amine salts Ie: Butylamine + chloroethane –> Ethyl-N-butylamime CH3CH2CH2CH2NH2 + CH3CH2Cl —> CH3CH2CH2CH2NHCH2CH3 with lone pairs of electrons still on nitrogen.
What happens if excess haloalkane is used in the reaction between nucleophilic substitution reaction between the haloalkane and ammonia
The amine produced can act as a nucleophile and attack the delta positive C in the C-X bond. This means the R group from the haloalkane will replace each of the H’s in the amine until eventually a quaternary ammonium salt is produced
Butylamine, C4H9NH2 , reacts with ethanoyl chloride. 2C4H9NH2 + CH3COCl → C4H9NHCOCH3 + C4H9NH3+Cl– Explain how this equation illustrates that butylamine acts as a nucleophile and as a base.
The amine in butylamine acts as the nucleophile. Nucleophiles donate lone pairs of electrons (and attack areas of low electron density). The nucleophile attacks the delta positive carbon in the acyl chloride which produces hydrogen chloride. The second molecule of butylamine will act as a base as the amine group reacts with the acid to gain a proton and form the salt.