4.6 amines Flashcards
what is a primary amine?
an amine that has 1 carbon bonded to the N
H I R - N I H
what is a secondary amine?
an amine that has 2 carbons bonded to the N
when drawing compounds, need to add H to the skeletal formula if bonded to anything other than carbon
how do you name amines?
- amino-
e.g 1-aminopropane or 2-amino-2-methylpropane
what is the functional group of amines?
-NH2
what is the type of reaction for the formation of aliphatic amines from halogenoalkanes?
nucleophilic substitution
what are the reagents/conditions needed for the formation of aliphatic amines from halogenoalkanes?
(excess, alcoholic) ammonia
- reflux in excess, alcoholic solution under pressure
(the nucleophile is ammonia (NH3)
what is an example of an equation for the formation of aliphatic amines from halogenoalkanes?
e.g
C2H5Br + NH3 (alc) —> C2H5NH2 + HBr
why is excess ammonia used in the formation of aliphatic amines from halogenoalkanes?
- ammonia attacks halogenoalkanes because it has a lone pair and is a nucleophile
- the amine produced also has a lone pair C2H5NH2 so can also attack a halogenoalkane; this leads to the formation of substituted amines
- using excess ammonia ensures that all the halogenoalkane molecules react with the ammonia before having the chance to react with any amines produced
.
- if want to create a monosubstituted amine, use excess
- bc lots of CH3CH2Br left over if NH3 not in excess
what is the product of the formation of aliphatic amines from halogenoalkanes?
amine (or its salt due to a reaction with the acid produced)
how can ammonium salts be converted to the amine?
by adding alkali (e.g NaOH)
if e.g BrCH2CH2Br undergoes nucleophilic substitution to form an amine, what amine is formed?
BrCH2CH2Br + 2NH3 —> NH2CH2CH2NH2 + 2HBr
what is the preferred reducing agent when reducing nitriles to amines?
LiAlH4
(but hydrogen with a nickel catalyst can also be used)
what are some properties of amines?
- the LONE PAIR on the nitrogen atom in 1°, 2° and 3° amines makes them:
• basic - can be proton acceptors
• nucleophiles - provide a lone pair to attack an electron deficient centre - their boiling point increases with molecular mass
- amines have higher boiling points than corresponding alkanes because of their intermolecular hydrogen bonding
- therefore, lower mass compounds are soluble in water due to the H bonding
- solubility decreases as the molecules get heavier
- soluble in organic solvents
- basic
- high b.p
- high solubility
explaining the base strength of amines:
- amines are weak bases
- lone pair on N can accept a proton
- base strength depends on how well N lone pair can accept H+
- the higher the electron density of the N lone pair, the stronger the base
- CH3CH2CH2NH2 vs CCl3CH2CH2NH2
- A is a stronger base as B has Cl- which is electronegative so withdrawing e- density from N
- alkyl groups push electrons towards the N better than N (remember carbocation stability), hence say that the lone pair is more ‘available’ in tertiary amines
- 3° > 2° > 1° > NH3
amines are (strong/weak) (bases/acids)
weak bases
what does the base strengh of amines depend on?
- how well the N lone pair can accept H+
- (the greater the electron density of the N lone pair, the stronger the base - this is affected by the groups attached to the nitrogen)
why are the lone pair in tertiary amines said to be more ‘available’ than secondary or primary amines?
- alkyl groups push electrons towards the N better than H (remember carbonation stability)
- hence say that the lone pair is more available in tertiary amines than 2° or 1°
what is an aromatic amine?
the amino group must be directly bonded to the benzene ring
aromatic amines are (more/less) basic than tertiary/secondary/primary amines?
less basic
why are aromatic amines less basic than tertiary/secondary/primary amines?
as the lone pair on the N atom delocalises into the ring, making it less available to accept a proton
(therefore not very basic)
what is the general rule of base strength of amines?
3° > 2° > 1° > NH3 > aromatic
what are the 2 reactions of primary amines?
- ethanoylation (acylation) of primary amines
- making alcohols
what is the reaction of the ethanoylation (acylation) of primary amines?
- acid chloride + amine —> N-substituted amide + HCl
- primary amines react quickly with ethanoyl chloride to from substituted amides (e.g N-ethylethanamide)