AN INTRODUCTION TOSYNTHETIC ORGANIC CHEMISTRYKNOCKHARDY PUBLISHING

CONVERSIONSINTRODUCTIONThis 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...www.argonet.co.uk/users/hoptonj/sci.htm

Navigation is achieved by...either clicking on the grey arrows at the foot of each page orusing the left and right arrow keys on the keyboardKNOCKHARDY PUBLISHINGORGANIC REACTION SEQUENCES

ESTERSREACTIONS OF ORGANIC COMPOUNDSALKANESALKENESHALOGENOALKANESALCOHOLSAMINESALDEHYDESKETONESCARBOXYLIC ACIDSPOLYMERSNITRILESDIBROMOALKANESCONVERSIONS

KESTERSREACTIONS OF ORGANIC COMPOUNDSALKANESALKENESHALOGENOALKANESALCOHOLSAMINESALDEHYDESKETONESCARBOXYLIC ACIDSAPSTGTNRPOLYMERSENITRILESHJDIBROMOALKANESUUIBLDMQOFCV

CHLORINATION OF METHANEInitiation Cl2 > 2Cl radicals created

Propagation Cl + CH4 > CH3 + HClradicals used and Cl2 + CH3 > CH3Cl + Cl then re-generated

Termination Cl + Cl > Cl2radicals 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 productsACONVERSIONS

ELECTROPHILIC ADDITION OF HBrReagent Hydrogen bromide... it is electrophilic as the H is slightly positiveCondition Room temperature.Equation C2H4(g) + HBr(g) > C2H5Br(l) bromoethane

Mechanism

Step 1As 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 2The bromide ion behaves as a nucleophile and attacks the carbocation.Overall there has been addition of HBr across the double bond.BCONVERSIONS

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.CELECTROPHILIC ADDITION OF BROMINECONVERSIONS

DIRECT HYDRATION OF ALKENESReagent steamConditions high pressureCatalyst phosphoric acidProduct alcohol

Equation C2H4(g) + H2O(g) C2H5OH(g) ethanolUse ethanol manufactureComments 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.DCONVERSIONS

HYDROGENATIONEReagent hydrogenConditions nickel catalyst - finely dividedProduct alkanesEquation C2H4(g) + H2(g) > C2H6(g) ethaneUse margarine manufactureCONVERSIONS

POLYMERISATION OF ALKENESETHENEEXAMPLES OF ADDITION POLYMERISATIONPROPENETETRAFLUOROETHENECHLOROETHENEPOLY(ETHENE)POLY(PROPENE)POLY(CHLOROETHENE)POLYVINYLCHLORIDE PVCPOLY(TETRAFLUOROETHENE) PTFE TeflonFCONVERSIONS

AQUEOUS SODIUM HYDROXIDE

ReagentAqueous* sodium (or potassium) hydroxideConditionsReflux in aqueous solution (SOLVENT IS IMPORTANT)ProductAlcoholNucleophilehydroxide 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 HYDROLYSISNUCLEOPHILIC SUBSTITUTIONGCONVERSIONS

NUCLEOPHILIC SUBSTITUTIONAMMONIA

ReagentAqueous, alcoholic ammonia (in EXCESS)ConditionsReflux in aqueous, alcoholic solution under pressureProductAmineNucleophileAmmonia (NH3)

Equation e.g.C2H5Br + 2NH3 (aq / alc) > C2H5NH2 + NH4Br

(i) C2H5Br + NH3 (aq / alc) > C2H5NH2 + HBr(ii) HBr + NH3 (aq / alc) > NH4Br

Mechanism

NotesThe 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)HCONVERSIONS

NUCLEOPHILIC SUBSTITUTIONAMMONIA

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 doesnt 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

HCONVERSIONS

POTASSIUM CYANIDE

ReagentAqueous, alcoholic potassium (or sodium) cyanideConditionsReflux in aqueous , alcoholic solutionProductNitrile (cyanide)Nucleophilecyanide ion (CN)

Equation e.g. C2H5Br + KCN (aq/alc) > C2H5CN + KBr(aq)

Mechanism

Importanceit extends the carbon chain by one carbon atomthe CN group can then be converted to carboxylic acids or amines.

ReductionC2H5CN + 4[H] > C2H5CH2NH2HydrolysisC2H5CN + 2H2O > C2H5COOH + NH3

NUCLEOPHILIC SUBSTITUTIONIJKCONVERSIONS

ELIMINATIONReagentAlcoholic sodium (or potassium) hydroxideConditionsReflux in alcoholic solutionProductAlkeneMechanismEliminationEquationC3H7Br + 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.LCONVERSIONS

ELIMINATION OF WATER (DEHYDRATION)Reagent/catalystconc. sulphuric acid (H2SO4) or conc. phosphoric acid (H3PO4)Conditionsreflux at 180CProductalkeneEquation e.g. C2H5OH(l) > CH2 = CH2(g) + H2O(l)Mechanism

Step 1protonation of the alcohol using a lone pair on oxygenStep 2loss of a water molecule to generate a carbocationStep 3loss of a proton (H+) to give the alkene

NoteAlcohols 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...MCONVERSIONS

OXIDATION OF PRIMARY ALCOHOLSPrimary 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)NAldehyde has a lower boiling point so distils off before being oxidised furtherOXIDATION TOALDEHYDESDISTILLATIONOXIDATION TOCARBOXYLIC ACIDSREFLUXAldehyde condenses back into the mixture and gets oxidised to the acidCONVERSIONS

OXIDATION OF ALDEHYDESAldehydes 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 includeTOLLENS REAGENTand FEHLINGS SOLUTION

TOLLENS REAGENTReagentammoniacal silver nitrate solutionObservationa silver mirror is formed on the inside of the test tubeProductssilver + carboxylic acidEquationAg+ + e- > Ag

FEHLINGS SOLUTIONReagenta solution of a copper(II) complex Observationa red precipitate forms in the blue solution Productscopper(I) oxide + carboxylic acidEquationCu2+ + e- > Cu+OCONVERSIONS

OXIDATION OF SECONDARY ALCOHOLSSecondary 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.PCONVERSIONS

REDUCTION OF CARBOXYLIC ACIDSQReagent/catalystlithium tetrahydridoaluminate(III) LiAlH4Conditionsreflux in ethoxyethaneProductaldehyde

Equation e.g. CH3COOH(l) + 2[H] > CH3CHO(l) + H2O(l)CONVERSIONS

REDUCTION OF ALDEHYDESRReagentsodium tetrahydridoborate(III) NaBH4Conditionswarm in water or ethanol Productprimary alcohol

Equation e.g. C2H5CHO(l) + 2[H] > C3H7OH(l)CONVERSIONS

REDUCTION OF KETONESSReagentsodium tetrahydridoborate(III) NaBH4Conditionswarm in water or ethanolProductsecondary alcohol

Equation e.g. CH3COCH3(l) + 2[H] > CH3CH(OH)CH3(l)CONVERSIONS

ESTERIFICATIONReagent(s)carboxylic acid + strong acid catalyst (e.g conc. H2SO4 )ConditionsrefluxProductester

Equation e.g.CH3CH2OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l)

NotesConcentrated H2SO4 is also a dehydrating agent, it removeswater as it is formed causing the equilibrium to move to the rightand thus increasing the yield of ester.

Uses of estersEsters are fairly unreactive but that doesnt make them uselessUsed as flavourings

Naming estersNamed from the alcohol and carboxylic acid which made them... CH3OH + CH3COOH CH3COOCH3 + H2O

from ethanoic acid CH3COOCH3 from methanol METHYL ETHANOATETCONVERSIONS

HYDROLYSIS OF ESTERSUReagent(s)dilute acid or dilute alkaliConditionsrefluxProductcarboxylic acid and an alcohol

Equation e.g.CH3COOC2H5(l) + H2O(l) CH3CH2OH(l) + CH3COOH(l)

NotesIf alkali is used for the hydrolysis the salt of the acid is formed CH3COOC2H5(l) + NaOH(aq) > CH3CH2OH(l) + CH3COO-Na+(aq)CONVERSIONS

BROMINATION OF ALCOHOLSReagent(s) conc. hydrobromic acid HBr(aq) or sodium (or potassium) bromide and concentrated sulphuric acidConditionsrefluxProducthaloalkane

EquationC2H5OH(l) + conc. HBr(aq) > C2H5Br(l) + H2O(l)MechanismThe 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 1protonation of the alcohol using a lone pair on oxygenStep 2loss of a water molecule to generate a carbocation (carbonium ion)Step 3a bromide ion behaves as a nucleophile and attacks the carbocationVCONVERSIONS

2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHINGTHE ENDAN INTRODUCTION TOSYNTHETIC ORGANIC CHEMISTRY