an introduction to synthetic organic chemistry knockhardy publishing
TRANSCRIPT
AN INTRODUCTION TOAN INTRODUCTION TO
SYNTHETIC ORGANIC SYNTHETIC ORGANIC CHEMISTRYCHEMISTRY
KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING
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INTRODUCTION
This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.
Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available.
Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...
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ORGANIC REACTION SEQUENCES ORGANIC REACTION SEQUENCES
ESTERSESTERS
REACTIONS OF ORGANIC COMPOUNDSREACTIONS OF ORGANIC COMPOUNDS
ALKANESALKANES ALKENESALKENES
HALOGENOALKANESHALOGENOALKANES
ALCOHOLSALCOHOLS
AMINESAMINES
ALDEHYDESALDEHYDES
KETONESKETONES
CARBOXYLIC ACIDSCARBOXYLIC ACIDS
POLYMERSPOLYMERS
NITRILESNITRILES
DIBROMOALKANESDIBROMOALKANES
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ESTERSESTERS
REACTIONS OF ORGANIC COMPOUNDSREACTIONS OF ORGANIC COMPOUNDS
ALKANESALKANES ALKENESALKENES
HALOGENOALKANESHALOGENOALKANES
ALCOHOLSALCOHOLS
AMINESAMINES
ALDEHYDESALDEHYDES
KETONESKETONES
CARBOXYLIC ACIDSCARBOXYLIC ACIDS
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POLYMERSPOLYMERS
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NITRILESNITRILES
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DIBROMOALKANESDIBROMOALKANES
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CHLORINATION OF METHANECHLORINATION OF METHANE
Initiation Cl2 ——> 2Cl• radicals created
Propagation Cl• + CH4 ——> CH3• + HCl radicals used and
Cl2 + CH3• ——> CH3Cl + Cl• then re-generated
Termination Cl• + Cl• ——> Cl2 radicals removed
Cl• + CH3• ——> CH3Cl
CH3• + CH3• ——> C2H6
SummaryDue to the lack of reactivity of alkanes you need a very reactive species to persuade them to reactFree radicals need to be formed by homolytic fission of covalent bondsThis is done by shining UV light on the mixture (heat could be used)Chlorine radicals are produced because the Cl-Cl bond is the weakestYou only need one chlorine radical to start things offWith excess chlorine you will get further substitution and a mixture of chlorinated products
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ELECTROPHILIC ADDITION OF HBrELECTROPHILIC ADDITION OF HBr
Reagent Hydrogen bromide... it is electrophilic as the H is slightly positive
Condition Room temperature.
Equation C2H4(g) + HBr(g) ———> C2H5Br(l) bromoethane
Mechanism
Step 1 As the HBr nears the alkene, one of the carbon-carbon bonds breaksThe pair of electrons attaches to the slightly positive H end of H-Br.The HBr bond breaks to form a bromide ion.A carbocation (positively charged carbon species) is formed.
Step 2 The bromide ion behaves as a nucleophile and attacks the carbocation.Overall there has been addition of HBr across the double bond.
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Reagent Bromine. (Neat liquid or dissolved in tetrachloromethane, CCl4 )
Conditions Room temperature. No catalyst or UV light required!
Equation C2H4(g) + Br2(l) ——> CH2BrCH2Br(l) 1,2 - dibromoethane
Mechanism
It is surprising that bromineshould act as an electrophileas it is non-polar.
CC ELECTROPHILIC ADDITION OF BROMINEELECTROPHILIC ADDITION OF BROMINE
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DIRECT HYDRATION OF ALKENESDIRECT HYDRATION OF ALKENES
Reagent steam
Conditions high pressure
Catalyst phosphoric acid
Product alcohol
Equation C2H4(g) + H2O(g) C2H5OH(g) ethanol
Use ethanol manufacture
Comments It may be surprising that water needs such vigorous conditions to react with ethene. It is a highly polar molecule and you would expect it to be a good electrophile.
However, the O-H bonds are very strong so require a great deal of energy to be broken. This necessitates the need for a catalyst.
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HYDROGENATIONHYDROGENATIONEE
Reagent hydrogen
Conditions nickel catalyst - finely divided
Product alkanes
Equation C2H4(g) + H2(g) ———> C2H6(g) ethane
Use margarine manufacture
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POLYMERISATION OF ALKENESPOLYMERISATION OF ALKENES
ETHENE
EXAMPLES OF ADDITION POLYMERISATION
PROPENE
TETRAFLUOROETHENE
CHLOROETHENE
POLY(ETHENE)
POLY(PROPENE)
POLY(CHLOROETHENE)
POLYVINYLCHLORIDE PVC
POLY(TETRAFLUOROETHENE)
PTFE “Teflon”
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AQUEOUS SODIUM HYDROXIDE
Reagent Aqueous* sodium (or potassium) hydroxideConditions Reflux in aqueous solution (SOLVENT IS IMPORTANT)Product AlcoholNucleophile hydroxide ion (OH¯)
Equation e.g. C2H5Br(l) + NaOH(aq) ———> C2H5OH(l) + NaBr(aq)
Mechanism
* WARNING It is important to quote the solvent when answering questions. Elimination takes place when ethanol is the solvent The reaction (and the one with water) is known as HYDROLYSIS
NUCLEOPHILIC SUBSTITUTIONNUCLEOPHILIC SUBSTITUTIONGG
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NUCLEOPHILIC SUBSTITUTIONNUCLEOPHILIC SUBSTITUTION
AMMONIA
Reagent Aqueous, alcoholic ammonia (in EXCESS)Conditions Reflux in aqueous, alcoholic solution under pressureProduct AmineNucleophile Ammonia (NH3)
Equation e.g. C2H5Br + 2NH3 (aq / alc) ——> C2H5NH2 + NH4Br
(i) C2H5Br + NH3 (aq / alc) ——> C2H5NH2 + HBr
(ii) HBr + NH3 (aq / alc) ——> NH4Br
Mechanism
Notes The equation shows two ammonia molecules.The second one ensures that a salt is not formed.Excess ammonia is used to prevent further substitution (SEE NEXT SLIDE)
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NUCLEOPHILIC SUBSTITUTIONNUCLEOPHILIC SUBSTITUTION
AMMONIA
Why excess ammonia?The second ammonia molecule ensures the removal of HBr which would lead to the formation of a salt. A large excess ammonia ensures that further substitution doesn’t take place - see below
ProblemThe amine produced is also a nucleophile (lone pair on N) and can attack another molecule of halogenoalkane to produce a 2° amine. This in turn is a nucleophile and reacts further producing a 3° amine and, eventually a quarternary ammonium salt.
C2H5NH2 + C2H5Br ——> HBr + (C2H5)2NH diethylamine, a 2° amine
(C2H5)2NH + C2H5Br ——> HBr + (C2H5)3N triethylamine, a 3° amine
(C2H5)3N + C2H5Br ——> (C2H5)4N+ Br¯ tetraethylammonium bromide, a 4° salt
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POTASSIUM CYANIDE
Reagent Aqueous, alcoholic potassium (or sodium) cyanideConditions Reflux in aqueous , alcoholic solutionProduct Nitrile (cyanide)Nucleophile cyanide ion (CN¯)
Equation e.g. C2H5Br + KCN (aq/alc) ———> C2H5CN + KBr(aq)
Mechanism
Importance it extends the carbon chain by one carbon atomthe CN group can then be converted to carboxylic acids or amines.
Reduction C2H5CN + 4[H] ——> C2H5CH2NH2
Hydrolysis C2H5CN + 2H2O ——> C2H5COOH + NH3
NUCLEOPHILIC SUBSTITUTIONNUCLEOPHILIC SUBSTITUTIONII
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ELIMINATIONELIMINATION
Reagent Alcoholic sodium (or potassium) hydroxide
Conditions Reflux in alcoholic solution
Product Alkene
Mechanism Elimination
Equation C3H7Br + NaOH(alc) ———> C3H6 + H2O + NaBr
Mechanism
the OH¯ ion acts as a base and picks up a protonthe proton comes from a C atom next to the one bonded to the halogenthe electron pair moves to form a second bond between the carbon atomsthe halogen is displaced; overall there is ELIMINATION of HBr.
With unsymmetrical halogenoalkanes, a mixture of products may be formed.
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ELIMINATION OF WATER (DEHYDRATION)ELIMINATION OF WATER (DEHYDRATION)
Reagent/catalyst conc. sulphuric acid (H2SO4) or conc. phosphoric acid (H3PO4)
Conditions reflux at 180°C
Product alkene
Equation e.g. C2H5OH(l) ————> CH2 = CH2(g) + H2O(l)
Mechanism
Step 1 protonation of the alcohol using a lone pair on oxygenStep 2 loss of a water molecule to generate a carbocationStep 3 loss of a proton (H+) to give the alkene
Note Alcohols with the OH in the middle of a chain can havetwo ways of losing water. In Step 3 of the mechanism, a proton can be lost from either side of the carbocation. This gives a mixture of alkenes from unsymmetrical alcohols...
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OXIDATION OF PRIMARY ALCOHOLSOXIDATION OF PRIMARY ALCOHOLS
Primary alcohols are easily oxidised to aldehydes
e.g. CH3CH2OH(l) + [O] ———> CH3CHO(l) + H2O(l)
it is essential to distil off the aldehyde before it gets oxidised to the acid
CH3CHO(l) + [O] ———> CH3COOH(l)
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Aldehyde has a lower boiling point so distils off before being oxidised further
OXIDATION TOALDEHYDES
DISTILLATION
OXIDATION TOCARBOXYLIC ACIDS
REFLUX
Aldehyde condenses back into the mixture and gets oxidised to the acid
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OXIDATION OF ALDEHYDESOXIDATION OF ALDEHYDES
Aldehydes are easily oxidised to carboxylic acids
e.g. CH3CHO(l) + [O] ———> CH3COOH(l)
• one way to tell an aldehyde from a ketone is to see how it reacts to mild oxidation• ALDEHYES are EASILY OXIDISED• KETONES are RESISTANT TO MILD OXIDATION• reagents include TOLLENS’ REAGENT and FEHLING’S SOLUTION
TOLLENS’ REAGENTReagent ammoniacal silver nitrate solutionObservation a silver mirror is formed on the inside of the test tubeProducts silver + carboxylic acidEquation Ag+ + e- ——> Ag
FEHLING’S SOLUTIONReagent a solution of a copper(II) complex Observation a red precipitate forms in the blue solution Products copper(I) oxide + carboxylic acidEquation Cu2+ + e- ——> Cu+
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OXIDATION OF SECONDARY ALCOHOLSOXIDATION OF SECONDARY ALCOHOLS
Secondary alcohols are easily oxidised to ketones
e.g. CH3CHOHCH3(l) + [O] ———> CH3COCH3(l) + H2O(l)
The alcohol is refluxed with acidified K2Cr2O7. However, on prolonged treatment with a powerful oxidising agent they can be further oxidised to a mixture of acids with fewer carbon atoms than the original alcohol.
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REDUCTION OF CARBOXYLIC ACIDSREDUCTION OF CARBOXYLIC ACIDSQQ
Reagent/catalyst lithium tetrahydridoaluminate(III) LiAlH4
Conditions reflux in ethoxyethane
Product aldehyde
Equation e.g. CH3COOH(l) + 2[H] ———> CH3CHO(l) + H2O(l)
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REDUCTION OF ALDEHYDESREDUCTION OF ALDEHYDESRR
Reagent sodium tetrahydridoborate(III) NaBH4
Conditions warm in water or ethanol
Product primary alcohol
Equation e.g. C2H5CHO(l) + 2[H] ———> C3H7OH(l)
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REDUCTION OF KETONESREDUCTION OF KETONESSS
Reagent sodium tetrahydridoborate(III) NaBH4
Conditions warm in water or ethanol
Product secondary alcohol
Equation e.g. CH3COCH3(l) + 2[H] ———> CH3CH(OH)CH3(l)
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ESTERIFICATIONESTERIFICATION
Reagent(s) carboxylic acid + strong acid catalyst (e.g conc. H2SO4 )
Conditions reflux
Product ester
Equation e.g. CH3CH2OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l)
Notes Concentrated H2SO4 is also a dehydrating agent, it removes
water as it is formed causing the equilibrium to move to the rightand thus increasing the yield of ester.
Uses of esters Esters are fairly unreactive but that doesn’t make them uselessUsed as flavourings
Naming esters Named from the alcohol and carboxylic acid which made them...
CH3OH + CH3COOH CH3COOCH3 + H2O
from ethanoic acid CH3COOCH3 from methanol
METHYL ETHANOATE
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HYDROLYSIS OF ESTERSHYDROLYSIS OF ESTERSUU
Reagent(s) dilute acid or dilute alkali
Conditions reflux
Product carboxylic acid and an alcohol
Equation e.g. CH3COOC2H5(l) + H2O(l) CH3CH2OH(l) + CH3COOH(l)
Notes If alkali is used for the hydrolysis the salt of the acid is formed
CH3COOC2H5(l) + NaOH(aq) ———> CH3CH2OH(l) + CH3COO-Na+(aq)
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BROMINATION OF ALCOHOLSBROMINATION OF ALCOHOLS
Reagent(s) conc. hydrobromic acid HBr(aq) or sodium (or potassium) bromide and concentrated sulphuric acid
Conditions reflux
Product haloalkane
Equation C2H5OH(l) + conc. HBr(aq) ———> C2H5Br(l) + H2O(l)
Mechanism The mechanism starts off in a similar way to dehydration(protonation of the alcohol and loss of water) but the carbocation(carbonium ion) is attacked by a nucleophilic bromide ion in step
3.
Step 1 protonation of the alcohol using a lone pair on oxygen
Step 2 loss of a water molecule to generate a carbocation (carbonium ion)
Step 3 a bromide ion behaves as a nucleophile and attacks the carbocation
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© 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING© 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING
THE THE END
AN INTRODUCTION TOAN INTRODUCTION TO
SYNTHETIC ORGANIC SYNTHETIC ORGANIC CHEMISTRYCHEMISTRY