unit 4 a2 organic chemistry mechanisms and reagents
TRANSCRIPT
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UNIT 4
A2 ORGANIC CHEMISTRY
MECHANISMS AND REAGENTS.
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ALDEHYDES & KETONESIdentified 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
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ALCOHOLS FROM ALDEHYDES & KETONES
ALDEHYDE
KETONE
PRIMARY ALCOHOL
SECONDARY ALCOHOL
Lithium tetrahydridoaluminate LiAlH4
Sodium tetrahydridoborate NaBH4
[H]
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
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HYDROXYNITRILES FROM ALDEHYDES & KETONES
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.
ALDEHYDE
KETONEHYDROXYNITRILE
HCN
KCN/H2SO4
AQA A2 text book, p68
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HYDROLYSIS OF THE NITRILE GROUP
AQA A2 text book, p69
The nitrile is hydrolysed in acidic conditions, producing a carboxylic group.
HYDROXYNITRILEH2O/ H+
CARBOXYLIC ACID
AMMONIUM
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ESTERS FROM CARBOXYLIC ACIDS & ALCOHOLS
CARBOXYLIC ACID
ALCOHOLS
ESTER
WATER
H2SO4 or HCl cat, reflux
AQA A2 text book, p71
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.
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ESTER HYDROLYSIS
AQA A2 text book, p74
ESTER
WATER
CARBOXYLIC ACID
ALCOHOLS
Acid Catalysed Hydrolysis• Reverse of esterification.• Dilute sulfuric acid or hydrochloric acid.• Heat under reflux.
Base-Catalysed Hydrolysis• Form a salt• Dilute strong base, sodium hydroxide.• Heat under reflux.• This method is quicker than acid catalysed.
ESTER
SODIUM HYDROXIDE ALCOHOLS
SODIUM CARBOXYLATE
Heat with excess dilute acid to produce carboxylic acid
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VEGETABLE OILS & ANIMAL FATSAlkyl chain in Vegetable oils are unsaturated. Alkyl chain in Animal fats are saturated.
TRIACYLGLYCEROLS or TRIGLYCERIDES (three “esters”) TRIESTERS
AQA A2 text book, p74-75
TRIACYGLYCEROL
No need for MechanismHydrolysed by heating with aqueous sodium hydroxide or potassium hydroxide.Produces glycerol (propane-1,2,3-triol) and sodium/potassium salt.
SODIUM HYDROXIDE
PROPANE-1,2,3-TRIOL
SODIUM CARBOXYLATE
Three moles Three moles produced
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BIODIESELNo 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
SODIUM HYDROXIDE
PROPANE-1,2,3-TRIOL
METHYL ESTER
AQA A2 text book, p75
METHANOL
Three moles
Three moles
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ACYL CHLORIDESMechanism• 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.
AQA A2 text book, p76-81
ACYL CHLORIDE
ESTER
N-SUBSTITUTED AMIDE
CARBOXYLIC ACID
AMIDE
water alcohol
Primary amine
ammonia
Hydrogen chloride, HCl is the secondary product.
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ACID ANHYDRIDE
Mechanism• Same as acyl chlorides.• Secondary product is a carboxylic acid.
Preferred in synthesis, cheaper, less violent reactions, no fumes of HCl
ACID ANYDRIDE
ESTER
N-SUBSTITUTED AMIDE
CARBOXYLIC ACID
AMIDE
water alcohol
Primary amine
ammonia
SynthesisDehydrate a carboxylic acid (P4O10 cat)
AQA A2 text book, p76-81
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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
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ICE DYES & DIAZONIUM SALTS
PHENYLAMINE DIAZONIUM SALT
Sodium nitrate(III), HClIce bath (chilled)
Ice DyesNo need for mechanism but need to be able to draw compounds.Sodium chloride and water is also formed.
AQA A2 text book, p92-93
Coupling Reaction (azo dyes)
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.
WATER
SODIUM CHLORIDE
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TRINITROTOLUENE, TNTNo need for mechanism but need to be able to draw compounds.
METHYLBENZENE 2-METHYL-1,3,5-TRINITROBENZENE
conc. HNO3
conc. H2SO4
AQA A2 text book, p93
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FIEDEL-CRAFTS ACYLATIONMechanism• 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
UNSTABLE POSITIVELY CHARGED
INTERMEDIATE
PHENYLETHANONE
HYDROGEN CHLORIDE
AQA A2 text book, p94
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MAKING ALIPHATIC AMINES
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 HaloalkanesExcess ammonia reacting with primary haloalkanes.
AMMONIA
HALOALKANE
AMMONIUM SALT
HALIDE ION
AMINE
AMMONIUM HALIDE
nucleophile NH3
base NH3
From NitrileAmine can be made by reducing nitrile
NITRILE AMINE LiAlH4
H2/NiAQA A2 text book, p100-107
4[H]
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MAKING AROMATIC AMINES
Reduction of aromatic nitro compounds
NITROBENZENE AMINE
conc.HCl/Snor
conc. HCl/Fe
6[H] WATER
AQA A2 text book, p101
NH3+
INTERMEDIATE
NaOH
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ZWITTERIONSActing as a BaseAccepting protons.
Acting as an AcidDonating protons.
AMINO ACID H+
AMINO ACID OH-
AQA A2 text book, p108-109
HCl SALT
WATERNaOH SALT
NITROGEN IS PROTONATED
NEGATIVE CHARGE ON OXYGEN
With Acid
With Base
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PROTEINS FROM AMINO ACIDS
AQA A2 text book, p110
Amino group reacting with carboxylic group of another amino acid. PEPTIDE LINKEach link produces one water molecule. Condensation reaction.Can form ploymers (amino acids)
AMINO ACID
AMINO ACIDDIPEPTIDE WATER
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AMINO ACIDS FROM PROTEINS
Hydrolysis of peptide link.
DIPEPTIDE
AMINO ACID
AMINO ACID Heat Water
6 moldm-3 HCl for 24 hours
AQA A2 text book, p112
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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.
AQA A2 text book, p114-115
AQA AS text book, p218-219
ALKENE POLY(ALKENE)
Monomer Polymer
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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.
AQA A2 text book, p116-117
DICARBOXYLIC ACID
DIOL
POLYESTER
WATER
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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
DIAMINE
POLYAMIDE
WATER
AQA A2 text book, p118-119
Questions on Nylon often appear on exams.
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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!
AQA A2 text book, p120-121
POLYAMIDE
DICARBOXYLIC ACID DIAMINE
DICARBOXYLIC ACID DIAMINE
SODIUM SALT OF DICARBOXYLIC ACID DIAMINE
Water
NaOH
ACID
With polyesters the secondary product is a diol.
You need to be able to identify repeating units.