UNIT 4
A2 ORGANIC CHEMISTRY
MECHANISMS AND REAGENTS.
ALDEHYDES & KETONES
Identified by
• Tollens reagent
• Fehling’s Solution
Aldehydes are oxidised to carboxylic acids. Therefore, the Tollens’ and Fehling’s must
be reduced.
Tollens’ Reagent
Sodium hydroxide, aqueous silver nitrate this produces a precipitate. Ammonia is then
added to make aqueous [Ag(NH3)2]+
This is reduced to silver when an aldehyde is added.
Fehling’s Solution
This is a blue solution containing Cu2+ ions. When aldehydes are added and heated,
these ions are reduced to cooper (I) oxide Cu2O, producing a brick-red ppt.
AQA A2 text book,
p64
ALCOHOLS FROM ALDEHYDES &
KETONES
ALDEHYDE
KETONE
Lithium tetrahydridoaluminate
LiAlH4
[H]
Sodium tetrahydridoborate
NaBH4
PRIMARY ALCOHOL
SECONDARY ALCOHOL
Mechanism
• Nucleophilic addition
• The reducing agents produce the H- ion (the nucleophile).
• The carbonyl bond is polar, due to oxygen being more electronegative.
• First Step is the hydride ion being attracted to the ƍ+ carbon.
• Second step, the product of first step gains a proton.
AQA A2 text book,
p66
HYDROXYNITRILES FROM ALDEHYDES
& KETONES
ALDEHYDE
HCN
HYDROXYNITRILE
KETONE
KCN/H2SO4
Mechanism
• Similar to making alcohols!
• Nucleophilic addition
• The reducing agents produce the CN- ion (the nucleophile).
• The carbonyl bond is polar, due to oxygen being more electronegative.
• First Step is the cyanide ion being attracted to the ƍ+ carbon.
• Second step, the product of first step gains a proton from the solvent or
hydrogen cyanide.
AQA A2 text book,
p68
HYDROLYSIS OF THE NITRILE GROUP
The nitrile is hydrolysed in acidic conditions, producing a carboxylic group.
HYDROXYNITRILE
H2O/ H+
CARBOXYLIC ACID
AMMONIUM
AQA A2 text book,
p69
ESTERS FROM CARBOXYLIC ACIDS &
ALCOHOLS
CARBOXYLIC ACID
ALCOHOLS
H2SO4 or HCl cat, reflux
ESTER
WATER
No need for Mechanism
• But you do need to be able to draw and name ester.
• OH from carboxylic acid and H from OH in alcohol bond to produce water.
• First part of name comes from the alcohol, second from the carboxylic acid.
AQA A2 text book,
p71
ESTER HYDROLYSIS
Acid Catalysed Hydrolysis
• Reverse of esterification.
• Dilute sulfuric acid or hydrochloric acid.
• Heat under reflux.
ESTER
CARBOXYLIC ACID
WATER
ALCOHOLS
Base-Catalysed Hydrolysis
• Form a salt
• Dilute strong base, sodium hydroxide.
• Heat under reflux.
• This method is quicker than acid catalysed.
Heat with excess dilute
acid to produce
carboxylic acid
ESTER
SODIUM CARBOXYLATE
SODIUM HYDROXIDE
ALCOHOLS
AQA A2 text book,
p74
VEGETABLE OILS & ANIMAL FATS
Alkyl chain in Vegetable oils are unsaturated.
Alkyl chain in Animal fats are saturated.
TRIACYLGLYCEROLS or TRIGLYCERIDES (three “esters”) TRIESTERS
No need for Mechanism
Hydrolysed by heating with aqueous sodium hydroxide or potassium hydroxide.
Produces glycerol (propane-1,2,3-triol) and sodium/potassium salt.
TRIACYGLYCEROL
PROPANE-1,2,3-TRIOL
SODIUM HYDROXIDE
SODIUM CARBOXYLATE
Three moles
Three moles produced
AQA A2 text book,
p74-75
BIODIESEL
No need for Mechanism
• Base catalysed Transesterification.
• Heating vegetable oil with methanol and sodium hydroxide.
• Produces methyl esters with long chains and propane-1,2,3-triol.
TRIACYGLYCEROL
PROPANE-1,2,3-TRIOL
SODIUM HYDROXIDE
METHYL ESTER
METHANOL
Three moles
Three moles
AQA A2 text book,
p75
ACYL CHLORIDES
Mechanism
• Nucleophilic Substitution reactions. (two steps addition + elimination)
• Greater ƍ+ on carbon due to Cl and O bond.
• 1st Step (addition) nucleophile attacks carbon ƍ+ . Pair of electrons in C=O is
transferred to oxygen.
• 2nd Step (elimination) lone pair of electrons on oxygen forms C=O, chlorine is
eliminated. Hydrogen attached to oxygen atom breaks.
CARBOXYLIC ACID
water
alcohol
ESTER
ACYL CHLORIDE
N-SUBSTITUTED AMIDE
AQA A2 text book,
p76-81
Primary
amine
ammonia
Hydrogen chloride, HCl is the secondary product.
AMIDE
ACID ANHYDRIDE
Synthesis
Dehydrate a carboxylic acid (P4O10 cat)
Mechanism
• Same as acyl chlorides.
• Secondary product is a carboxylic acid.
CARBOXYLIC ACID
water
alcohol
ESTER
ACID ANYDRIDE
N-SUBSTITUTED AMIDE
AQA A2 text book,
p76-81
Primary
amine
ammonia
AMIDE
Preferred in synthesis, cheaper, less violent reactions, no fumes of HCl
NITRO COMPOUNDS FROM BENZENE
Mechanism
• Generate the nitronium ion. Nitric acid and sulfuric acid.
• Protonation of nitric acid, then protonated nitric acid breaks down.
• Electrophilic substitution(addition then elimination).
• Electrophile is attracted to the delocalized electron.
• C-H bond breaks and restores the stable delocalized electron structure.
BENZENE
UNSTABLE
POSITIVELY
CHARGED
INTERMEDIATE
NITROBENZENE
Temperature should be kept at 50oC. Further substitution occurs at 100oC
(1,3-dinitrobenzene).
AQA A2 text book,
p90-91
ICE DYES & DIAZONIUM SALTS
Ice Dyes
No need for mechanism but need to be able to draw compounds.
Sodium chloride and water is also formed.
PHENYLAMINE
Sodium nitrate(III), HCl
Ice bath (chilled)
DIAZONIUM SALT
WATER
Coupling Reaction (azo dyes)
SODIUM CHLORIDE
No need for mechanism but you need to be able to draw compounds.
Diazonium salt reacts with phenylamine or other aromatic compounds to produce azo
dyes. HCl is also produced.
AQA A2 text book,
p92-93
TRINITROTOLUENE, TNT
No need for mechanism but need to be able to draw compounds.
METHYLBENZENE
AQA A2 text book,
p93
conc. HNO3
conc. H2SO4
2-METHYL-1,3,5-TRINITROBENZENE
FIEDEL-CRAFTS ACYLATION
Mechanism
• Generate the electrophile. Aluminium chloride and acyl chloride.
• Electrophilic substitution(addition then elimination).
• Electrophile is attracted to the delocalized electron.
• C-H bond breaks and restores the stable delocalized electron structure.
• Aluminium chloride is regenerated whren [AlCl4]- reacts with H+.
BENZENE
AQA A2 text book,
p94
UNSTABLE
POSITIVELY
CHARGED
INTERMEDIATE
PHENYLETHANONE
HYDROGEN CHLORIDE
MAKING ALIPHATIC AMINES
From Haloalkanes
Excess ammonia reacting with primary haloalkanes.
AMMONIA
nucleophile
NH3
base
NH3
AMMONIUM SALT
HALOALKANE
HALIDE ION
AMINE
AMMONIUM HALIDE
Mechanism
• If you can do this you can describe
• Amine acting as a nucleophile
• Amine acting as a base
• How to make secondary, tertiary amines and quarternary ammonium salts
From Nitrile
Amine can be made by reducing nitrile
NITRILE
AQA A2 text book,
p100-107
4[H]
LiAlH4
H2/Ni
AMINE
MAKING AROMATIC AMINES
Reduction of aromatic nitro compounds
NITROBENZENE
WATER
AMINE
6[H]
NaOH
conc.HCl/Sn
or
conc. HCl/Fe
AQA A2 text book,
p101
NH3+
INTERMEDIATE
ZWITTERIONS
Acting as a Base
Accepting protons.
NITROGEN IS PROTONATED
AMINO ACID
H+
With Acid
HCl
SALT
AMINO ACID
OH-
NEGATIVE CHARGE ON OXYGEN
With Base
NaOH
SALT
Acting as an Acid
Donating protons.
AQA A2 text book,
p108-109
WATER
PROTEINS FROM AMINO ACIDS
Amino group reacting with carboxylic group of another amino acid. PEPTIDE LINK
Each link produces one water molecule. Condensation reaction.
Can form ploymers (amino acids)
AMINO ACID
AMINO ACID
AQA A2 text book,
p110
DIPEPTIDE
WATER
AMINO ACIDS FROM PROTEINS
Hydrolysis of peptide link.
Heat
Water
AMINO ACID
6 moldm-3 HCl for
24 hours
AMINO ACID
DIPEPTIDE
AQA A2 text book,
p112
ADDITION POLYMERS
Mechanism
• Addition polymerisation
• Involves one functional groups.
• Carbon, carbon double bond.
• Need to be able to identify repeating units
Taught in AS Chemistry Unit 2.
ALKENE
Monomer
AQA A2 text book,
p114-115
AQA AS text book,
p218-219
POLY(ALKENE)
Polymer
POLYESTERS
Mechanism
• Condensation polymerisation
• Involves two functional groups.
• Polyesters are formed from a dicarboxylic acid and a diol.
• Carboxyl group forms an ester link with the hydroxy group.
• Need to identify ester link.
• Identify the repeating units.
DICARBOXYLIC
ACID
POLYESTER
DIOL
WATER
AQA A2 text book,
p116-117
POLYAMIDES
Mechanism
• Condensation polymerisation
• Involves two functional groups.
• Polyamide are formed from a dicarboxylic acid and a diamines.
• Hydroxyl group from dicarboxylic acid reacts with hydrogen attached to nitrogen of
polyamide.
• Forms an amide link.
• Need to identify amide link.
• Identify the repeating units.
DICARBOXYLIC
ACID
POLYAMIDE
DIAMINE
WATER
AQA A2 text book,
p118-119
Questions on Nylon often appear on exams.
BIODEGRADABLE POLYMERS
Esters and amides can be hydrolysed by water, acids, alkalis and enzymes.
This is the same for polyesters and polyamides.
Break ester link or amide link!
POLYAMIDE
NaOH
DICARBOXYLIC ACID
DIAMINE
SODIUM SALT OF
DICARBOXYLIC ACID
DIAMINE
DICARBOXYLIC ACID
DIAMINE
With polyesters the secondary product is a diol.
You need to be able to identify repeating units.
AQA A2 text book,
p120-121