Amines
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- Created by: Hindleyc
- Created on: 28-02-19 14:52
What do Primary aliphatic amines act as?
Bronsted-lowry bases
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Why
Because the lone pair of electrons on the nitrogen is readily available for forming a dative covalent bond with a H+ and so accepting a proton
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What type of strength of base?why?
Weak as only a low conc of hydroxide ions produced
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EQN
CH3NH2+H2O=CH3NH3+ +OH-
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Why are primary aliphatic amines stronger bases than ammonia?
The alkyl groups are electron releasing and push electrons towards the nitrogen atom so make it a stronger base
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EQN
NH3(aq)+H2O(l)=NH4^+(aq) + OH-(aq)
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Why are secondary amines stronger bases than primary amines
Because they have more alkyl groups that are substituted onto the N atom in place of H atoms
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Therefore what happens
Electron density is pushed onto the N atom (as inductive effect of alkyl groups is greater than that of H ATOMS)
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When doesn't trend hold
In tertiary amines and corresponding ammonium salts
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Why
They are less soluble in water and this makes them less strong bases than secondary amines
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What don't primary aromatic amines such as phenyl amine form
Basic solutions because the lone pair of electrons on the nitrogen delocalise with the ring of electrons in the benzene ring
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What does this mean
N less able to accept protons
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Overall order of strength
Weaker= aromatic amines, ammonia, primary amines, tertiary amines, secondary amines
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What do all amines react with acids to become
Ammonium salts
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EQ with Primary
RNH2(aq) + H+(aq) = RNH3^+(aq)
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EQ with secondary
RNH(aq)+ H^+ (aq) = RNH2^+ (aq)
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EQ with Tertiary
RN(aq) + H^+ (aq) = RNH^+ (aq)
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What do amines, as bases, react with acids to form
Ammonium salts
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EG
RNH2(aq)+HCl(aq)=RNH3^+Cl^-(aq)
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What will the addition of NaOH to an ammonium salt do
Convert it back to the amine
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What does the ionic salt formed in this reaction mean
Compounds are soluble in the acid eg Phenylamine not v soluble but Phenylammonium chloride is soluble
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What will the ionic salts be?
Solid crystals if the water is evaporated because of the strong ionic interactions
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What can basic buffers be made from
Combining a weak base with a salt of that weak base
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EG
Ammonia and Ammonium chloride, Methyl amine and Methyl ammonium chloride, Ethyl amine and Ethylammonium chloride
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How do you form a primary amine?
One step reaction of halogenoalkanes with ammonia
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What type of rxn
Nucleophilic substitution between halogenoalkane and ammonia in a one step reaction
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Problem?
As the lone pair of electrons is still available on the N in the amine formed
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What does this mean?
The primary amine can react in the same nucleophilic way in a successive series of reactions forming secondary, tertiary and quaternary ammonium salts
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Why is this therefore not a good method for making a primary amine
Because of the further reactions-would mean the desired product would have to be separated from the other produtcts
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What is the initial nucleophile
Ammonia dissolved in ethanol
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EQN
RBr+2NH3=RNH2+NH4Br
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What happens in the first step
Nucleophile attacks the halogenoalkane to form an intermediate
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What happens in the second step
Second ammonia removes a proton from the intermediate (acts as a base) to form the amine
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What can limit the further subsequent reactions and maximise the amount of primary amine formed?
Using an excess of ammonia
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How do you form secondary amine
Primary amine formed in rxn below has a lone pair of electrons on the nitrogen and will react further with the halogenoalkane
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Steps
Lone pair on N in Primary Amine attacks C then arrow from C-X bond to X
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Then
:NH3 attacks H then electrons from N-H bond to N
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In 2nd step, what can remove the proton from the intermediate
Either ammonia or the amine(acts as a base) to from the amine
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WB tertiary amine
Same rxn mechanism occurs with the 2ndary amine reacting to form tertiary amine
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What is a quaternary ammonium salt?
Not amine
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How many H swapped for primary
1
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How many Hydrogen swapped for secondary
2
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3?
3
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Quaternary ammonium salt?
3+extra bon so N has +
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How to promote formation of quaternary ammonium salt?
Using excess of the halogenoalkane
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What step of mechanism occurs when forming the quaternary salt?
1st
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What will maximise the amount of primary amine formed?
Using a large excess of ammonia
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What will promote the formation of the quaternary salt?
Using a large excess of the halogenoalkane
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What can quaternary salts used as
Cationic surfactants
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What do surfactants do
Reduce the surface tension of liquids
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How
Positive N is attracted towards -vely charged surfaces such as glass, hair, fibres and plastics
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What does this help
In their uses as fabric softeners, hair conditioners and sewage flocculants
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What is the method above for reacting halogenoalkanes and ammonia ?
Inefficient to prepare high yield of primary amine
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Why
Because of the further substitution reactions that occur
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What is better method
Convert halogenoalkane to nitrile by using KCN in ethanol Heat under reflux. Reduce nitrile to amine by using LiAlH4 in ether or by reducing with H2 using a Ni catalyst
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EQNS
RBr+CN^- =RCN + Br^-
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2ND EQN
RCN+4[H]= RCH2NH2
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Disadvantage
2 step rxn that may have a low yield, KCN toxic
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What can the nitro group on an arene be reduced to
An amine group
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How
Reagent=Sn/Fe and HCl, Cond=heating, Mechanism=reduction
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EQN
Nitrobenzene+6[H]=Phenylamine+ 2H2O
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As the reaction is carried out in HCl what will be formed?
the ionic salt C6H5NH3+Cl-
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What will reacting this salt with NaOH give
Phenylamine
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How else can this reduction be done
Catalytic hydrogenation (H2 using a Ni catalyst)
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What can aliphatic amines and phenyl amines react with? to form?
Acyl chlorides and acid anhydrides. Amides
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What reaction
Nucleophilic addition elimination
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Change in FG, reagent, cond for Acyl chloride
Acyl chloride to secondary amide, primary amine, room temp
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EQN
RCOCl+2CH3NH2=RCONHCH3+CH3NH3^+Cl^-
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Change in FG, reagent, condition for acid anhydride
Acid anhydride to secondary amide, primary amine, room temperature
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EQN
(RCO)2O+2CH3NH2=RCONHCH3+RCO2^-CH3NH3^+
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Other cards in this set
Card 2
Front
Why
Back
Because the lone pair of electrons on the nitrogen is readily available for forming a dative covalent bond with a H+ and so accepting a proton
Card 3
Front
What type of strength of base?why?
Back
Card 4
Front
EQN
Back
Card 5
Front
Why are primary aliphatic amines stronger bases than ammonia?
Back
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