Organics 3 Flashcards
How to work out of an amine is primary, secondary or tertiary?
Look at how many carbons are attached to the N
Primary amines can
Can hydrogen bond between molecules of itself AND with water
Lone pair on delta negative N with delta positive H from another amine molecule
Secondary amines can
Can hydrogen bond between molecules of itself AND with water
Lone pair on delta negative N with delta positive H from another amine molecule
Tertiary amine can
Cannot hydrogen bond between molecules because they have no delta H to bond to a :N
- lower melting points and boiling points than primary and secondary amines
- less soluble in water than primary and secondary amines
Two ways amines can be prepared
1) Halogenoalkane + concentrated ethanolic ammonia —> primary amine
2) Reduction of nitriles
3) Reduction of nitriles with LiAlH4
Reduction of nitriles to produce amines
What you add
(Conditions and catalyst)
(What you are breaking)
-CN + 2H2 —-> -CH2NH2
Reduce nitriles by adding hydrogen
Break two pi bonds in the triple bond (a triple bond has two pi and one sigma)
Nickel catalyst + high temp&pressure
Reduction of nitriles with LiAlH4
Nitrile reacts with LiAlH4 in ethoxyethane and then dilute acid is added
CN triple bond reduced to primary amine
Eg. Ethanenitrile —> ethylamine
CH3CN + 4[H] —> CH3CH2NH2
Preparation of amines from halogenoalkanes
Equation
(Conditions)
Halogenoalkane + concentrated ethanolic ammonia —> primary amine
Sealed tube (can’t heat under reflux because gas would escape up condenser)
What is an aromatic amine?
An amine with a benzene ring
Preparation of phenylamine
Benzene —> nitrobenzene —-(reduction)—> phenylamine
Concentrated HCl and Sn
nitrobenzene —> phenylamine
Heat under reflux
Reduction because gains e- from tin
What are the 5 reactions of primary aliphatic amines to know for Organic 3?
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- Amine + water forms alkaline solution
- Amine + acids form salts
- Amine + ethanol chloride
- Amine + halogenoalkanes
- Amine + copper(II) ions form complex ions
Tertiary amines less basic than secondary out of trend huh XXX
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What type of chemical property do amine possess?
They are BASES.
The lone pair of electrons on the N means they can accept H+ and form alkaline solutions in water.
Strongest amine base to weakest amine base?
(More methyl groups so larger positive inductive effect so +ve charge on ion decreases so ion more stable)
Tertiary amine > secondary amine > primary amine > ammonia > aromatic amine (eg. Phenylamine)
Weak bases have a
Large positive charge ?
The more CH3 groups…
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The larger the positive inductive effect
Electron density pushed onto positive charge
Positive charge decreases
Stronger attraction to H+ so stronger base
Once it’s an ion: Larger positive inductive effect Electron density pushed onto positive charge Positive charge decreases Ion becomes more stable
Positive charge on N increases
Less stable
Positive charge on N decreases
More stable
Stronger bases form more
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Stable ions
Three reactions where amine acts as a nucleophile
Primary amine + halogenoalkane —> secondary amine
Amine + acyl chloride —> secondary amide
Amine + Cu (II) ions
Formation of amines from aromatic nitro compounds?
Reduction reaction
Tin and concentrated HCl
Half equations for what happens to tin when it is oxidised in the reduction reaction of nitrobenzene —> phenylamine
Sn —> Sn2+ + 2e-
Sn2+ —> Sn4+ + 2e-
Amide functional group
O
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R—C—NH2
What is an N-substituted amide?
Where there is an alkyl group attached to the N rather than a H. (Usually it is NH2 but it might be N with a H and a CH3 attached)
Two ways to form amides?
Acyl chloride + ammonia —> primary amide
Eg. Methanoyl chloride + NH3 —-> methanamide + HCl
Steamy white fumes
Acyl chloride + amine —> N-substituted amide
Eg. Methanoyl chloride + methylamine —> N-methyl methanamide
Steamy white fumes
How to name a primary amide?
Carbon chain
Amide group
Eg. Propanamide
How to name an N - substituted amide?
Thing after N is which alkyl group is attached to the N
Then do what the normal amide would have been called if it didn’t have an alkyl group instead of an H
Eg. N-ethylmethanamide
Why are amides not bases?
Although there is a lone pair on the N they cannot accept another H.
Accepting a hydrogen places a positive charge on the N (because it loses electrons to share with the H). Electronegative oxygen would further pull electrons on the N away and make the +ve charge even larger to the point the ion becomes unstable.
So amides don’t act as bases because the ions they would form would break down.
Which two groups do amino acids contain?
Amine (NH2)
Carboxyl (COOH)
What is the Kekulé model of benzene?
Benzene ring with alternating double bond single bond with one hydrogen attached to each carbon
Benzene observations
1) All C-C bonds are the same length and strength (they have a value between C-C and C=C)
2) relatively reactive compared to alkenes
3) only undergoes electrophilic substitution reactions
Explain how the enthalpy of hydrogenation disproves the kekulé structure of benzene.
(Equation for hydrogenation of C6H10)
(Expected vs actual values for hydrogenation of C6H6)
C6H10 + H2 —-> C6H12
Enthalpy of hydrogenation = -120Kj mol^-1 (breaking one double bond)
Expected:
C6H6 with three double bonds so hydrogenation enthalpy would be 3x-120=-360Kj mol^-1
Actual hydrogenation enthalpy is -205Kj mol^-1 (much less, C6H6 structure less exothermic)
Actual structure is more stable
Explain the delocalised ring of benzene
Caused by unintentional overlap of all the p orbitals
Constant overlap means the 6 electrons can move around the entire ring structure in one molecule
6 electrons come from the three pi bonds that are from the three double bonds had there not been a delocalised ring and it was the predicted structure
Why is benzene a stable structure?
The electrons are delocalised so repel each other less
The delocalised system requires a lot of energy to break
Describe the structure and bonding of benzene in the delocalised model
C6H6
Each carbons bonded to two other carbons and a H
The 4th bonding pair of electrons for each C is delocalised
Due to every p orbital overlapping this means there is a delocalised cloud of electrons above and below the plane
Hexagonal & 120degrees between C atoms
All bond lengths are equal
Stable due to delocalised ring
Because of its high stability it does not readily undergo addition reactions but does undergo substitution reactions
What is the main mechanism that benzene rings undergo?
Electrophilic substitution
Why does benzene not undergo addition reactions?
Would involve breaking the delocalised ring and losing stability
Instead substitution reactions occur so the pi bonds are temporarily broken and at the end of the reaction the delocalised ring still has 6 electrons and is preserved
In benzene electrophilic substitution reactions what must the electrophile have?
A full +ve charge because benzene is less reactive than alkenes
Explain the mechanism for benzene electrophilic substitution
1) 2 electrons from weakest pi bond in the delocalised system are attracted to the electrophile
2)Delocalised ring now has 4 electrons because those 2 electrons have been used to create a new bond between the C and electrophile
[drawing: circle should be 2/3 and gap pointing towards C with electrophile on it & +ve charge from E placed in centre of circle]
3) want to reform delocalised ring so 2 electrons from the C-H bonds are used to reform it. C-H bond breaks and left with substituted benzene ring and H+
4)
Draw the general mechanism for benzene electrophilic substitution
1) curly arrow from ring to electrophile
2) skeletal formula only showing electrophile and H attached to same carbon. 2/3 circle and gap pointing to carbon with electrophile on it & +ve charge in middle
3) curly arrow from C-H back into circle
4) substituted benzene ring + H+
What are the 4 main benzene electrophilic substitution reaction mechanisms to know?
1) halogenation of benzene
2) nitration of benzene
3) Friedal Crafts Alkylation
4) Friedal Crafts Acylation
What is alkylation?
Substituting an alkyl group into a benzene ring
Friedal Crafts Alkylation
Benzene treated with chloroalkane
Aluminium chloride catalyst
Equation for formation of electrophile (eg. CH3):
CH3Cl + AlCl3 —> CH3+ + AlCl4-
Equation for regeneration of AlCl3 catalyst:
AlCl4- + H+ —> AlCl3 + HCl
Steamy white fumes
What is an acyl group?
An alkyl group attached to a C=O double bond
RCO-
What is acylation?
Substituting an acyl group into a benzene ring
Friedel Crafts Acylation
Benzene treated with acyl chloride
Aluminium chloride catalyst
Equation for formation of electrophile (eg. CH3CO+):
CH3COCl + AlCl3 —> CH3CO+ + AlCl4-
Equation for regeneration of AlCl3 catalyst:
AlCl4- + H+ —> AlCl3 + HCl
Steamy white fumes
What is the electrophile in friedel crafts acylation?
The C=O bond with the alkyl group attached (aka. The acyl group) and
What is the electrophile in friedel crafts alkylation?
The alkyl group from the chloroalkane
Nitration of benzene
Benzene treated with mixture of concentrated HNO3 and H2SO4
Concentrated H2SO4 catalyst
Temp <50 (as temperature increases greater chance of substituting more than one nitro group)
Equation for formation of electrophile (NO2+):
HNO3 + 2H2SO4 —> NO2+ + 2HSO4- + H3O+
HNO3 + H2SO4 —> NO2+ + HSO4- + H2O
Forms nitrobenzene
What is the electrophile in nitration of benzene?
NO2+ (nitronium ion)
Halogenation of benzene
(Benzene reacting with bromine or chlorine in the presence of a catalyst)
XXX
Benzene reacts with either chlorine or bromine
Aluminium chloride/bromide catalyst
Equation for formation of electrophile:
Cl2 + AlCl3 —> Cl+ + AlCl4-
Equation for regeneration of catalyst:
H+ + AlCl4- —> HCl + AlCl3
Steamy white fumes
Forms chlorobenzene
In all 4 of the benzene reactions what can you use instead of chlorine? And what would the halogen carrier be?
Bromine
FeBr3 as a halogen carrier rather than AlCl3
Draw methylbenzene
Benzene ring with methyl group sticking out
How does methylbenzene react when there is UV light?
Free radical substitution
Cl free radical reacts with CH3 so CH2Cl is attached to benzene ring
How does methylbenzene react when there is a halogen carrier, AlCl3, present?
Electrophilic substitution
Remove H from benzene ring and sub in a Cl so it’s a benzene ring with a CH3 and Cl sticking out
Explain why benzene is resistant to bromination compared with alkenes?
X
Combustion of benzene
Combustion hardly ever complete due to high proportion of carbon needing lots oxygen
Produces smoky flame
Draw phenol
Benzene ring with an OH attached (doesn’t matter which carbon the OH is on)
Explain the effect of the electronegativity of the oxygen in phenol
Electronegative oxygen is electron withdrawing and pulls electron density out of the benzene ring making it less attractive to electrophile and less reactive (compared to benzene)
Explain the effect of the two lone pairs on the oxygen in phenol?
Two one pairs on the oxygen atom are located in p orbitals. One of the p orbitals overlaps with the p orbital on the carbon attached to the oxygen.
Lone pairs become part of the delocalised ring, ring structure now has 8 delocalised electrons (instead of 6).
There is a higher electron density in the ring.
It is more attractive to electrophiles.
Phenol is more reactive than benzene.
Is phenol more or less reactive than benzene?
MORE REACTIVE
Explain the lack of acidity of normal alcohols
Negative charge on oxygen in OH attracts very strongly to the H+ the alcohol donated (proton donator = acid) so equilibrium remains mainly on LHS as an alcohol.
CH3OH CH3O- + H+
Explain why phenol is classed as weakly acidic (but still less acidic than carboxylic acids)
Oxygen is part of the delocalised ring (due to overlapping p orbitals) so charge is spread across whole ring (ion)
Weaker attraction to H+ it has lost (proton donator = acid) so equilibrium more to RHS than normal alcohols. Phenol slightly more acidic.
C6H6OH C6H6O- + H+
Phenol reacting with bromine water
Bromine water decolourised
White ppt forms = 2,4,6-tribromophenol
Antiseptic smell
Phenol + 3Br2 —> 2,4,6-tribromophenol + 3HBr
Why is phenol easier to brominate compared to benzene?
XXX
Phenol oxygen in OH group is part of the delocalised ring due to overlapping p orbitals. It’s delocalised ring has more electron density so easier to induce a dipole in Br2 compared to benzene.
Solubility of amine a
Amines contain hydrogen atom directly bonded to nitrogen atom so can’t pram H bonds
Amines with short hydrocarbon chains are miscible with water
When dissolved in water they form an alkaline solution
As carbon chain length increases amine solubility decreases and they become weaker alkalis
Phenylamine is slightly soluble in water due to large carbon group
C
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G
G
Amines bases
Amines are bases and react with acids to form salts
This enables phenylamine to dissolve in HCl
Add NaOH to reverse the reaction
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X
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General equation for reaction of primary amine + ethanol chloride
RNH2 + CH3COCl —> RNHCOCH3 + HCl
Example of this reaction is formation of paracetamol
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General equations for amines
RNH2 + R’X ——> RNHR’ (secondary amine)+ HX
RNHR’ + R’X ——> RNR’2 (tertiary amine) + HX
RNR’2 + R’X ——> RNR’3+ X- (quaternary ammonium salt)
