Ch37---Syntheses-and-interconversions-of-organic-compounds.ppt

New Way Chemistry for Hong Kong A-Level Book 3B1Syntheses and Interconversions of Organic Compounds37.1Planning Organic Syntheses37.2Interconversions of Functional Groups of Organic Compounds37.3Chain Lengthening or Shortening of Carbon SkeletonChapter 37New Way Chemistry for Hong Kong A-Level Book 3B237.1 Planning Organic Syntheses(SB p.122)The organic compounds that are now known:Only small fractions of them can be isolated from natural resourcesAll the remaining are synthesized by organic chemistsReasons for carrying out syntheses:e.g.To make a new medicine,dye,plastics,pesticide;To make a new compound for studying reaction mechanisms or metabolic pathwaysNew Way Chemistry for Hong Kong A-Level Book 3B337.1 Planning Organic Syntheses(SB p.122)The way to plan the synthesis is to think backwards From the desired product to simpler molecules that can act as the precursor for our target moleculeA synthesis usually involves more than one stepNew Way Chemistry for Hong Kong A-Level Book 3B437.1 Planning Organic Syntheses(SB p.123)There are usually more than one way to carry out a synthesisNew Way Chemistry for Hong Kong A-Level Book 3B537.1 Planning Organic Syntheses(SB p.123)The feasibility of an organic synthesis depends on a number of factors:Numbers of steps involved in the synthesisAvailability of starting materials and reagentsDuration of the synthetic processNew Way Chemistry for Hong Kong A-Level Book 3B637.1 Planning Organic Syntheses(SB p.123)Most organic reactions are reversible and seldom proceed to completionAs the backward reaction takes place,it is impossible to have a 100%yield of product from each step of the synthetic routeNumbers of Steps involved in the SynthesisNew Way Chemistry for Hong Kong A-Level Book 3B7The yield of desired product is 12.96%an efficient route of synthesis consists of a minimal number of stepsUsually the number of steps is limited to not more than four37.1 Planning Organic Syntheses(SB p.123)e.g.New Way Chemistry for Hong Kong A-Level Book 3B837.1 Planning Organic Syntheses(SB p.123)There are often a restricted number of simple,relatively cheap organic compounds availablee.g.simple haloalkanes,alcohols of not more than four carbon atoms,simple aromatic compounds such as benzene and methylbenzeneAvailability of Starting Materials and ReagentsNew Way Chemistry for Hong Kong A-Level Book 3B937.1 Planning Organic Syntheses(SB p.124)The time factor must be considered when planning the synthetic pathwaymany organic reactions proceed at a relatively slow ratee.g.acid-catalyzed esterification requires the reaction mixture to be refluxed for a whole dayInvolvement of slow reactions in the synthetic route is impractical as the reaction will be too longDuration of the Synthetic ProcessNew Way Chemistry for Hong Kong A-Level Book 3B1037.2 Interconversions of Functional Groups of Organic Compounds(SB p.124)Oxidation of an organic compound usually corresponds to increasing its oxygen content or decreasing its hydrogen contentCommon oxidizing agents used:KMnO4,K2Cr2O7 or H2CrO4KMnO4 is the strongest oxidizing agent and can be used in acidic,neutral or alkaline mediumOther oxidizing agents:Tollens reagent,Fehlings reagent,ozoneOxidationNew Way Chemistry for Hong Kong A-Level Book 3B1137.2 Interconversions of Functional Groups of Organic Compounds(SB p.125)1.Mild Oxidation by Potassium Manganate(VII)Under mild oxidation by alkaline KMnO4,alkenes are oxidized to diolsPotassium Manganate(VII)New Way Chemistry for Hong Kong A-Level Book 3B1237.2 Interconversions of Functional Groups of Organic Compounds(SB p.125)2.Vigorous Oxidation by Potassium Manganate(VII)Occur in acidic or alkaline medium Heating is to ensure the vigour of the reactionNew Way Chemistry for Hong Kong A-Level Book 3B1337.2 Interconversions of Functional Groups of Organic Compounds(SB p.125)Alkylbenzenes are converted to benzoic acid by vigorous oxidation of KMnO4New Way Chemistry for Hong Kong A-Level Book 3B1437.2 Interconversions of Functional Groups of Organic Compounds(SB p.126)1 alcohols are oxidized to carboxylic acids by vigorous oxidation of KMnO4The oxidation is difficult to stop at the aldehyde stage.The way to obtain aldehyde from oxidation is to remove the aldehyde by distillation as soon as they formedNew Way Chemistry for Hong Kong A-Level Book 3B1537.2 Interconversions of Functional Groups of Organic Compounds(SB p.126)The most commonly used chromium(VI)reagent is H2CrO4 which is prepared by adding CrO3,Na2Cr2O7 or K2Cr2O7 to aqueous H2SO4It oxidizes 1 alcohols or aromatic side chains to aldehydes but not in good yieldsPotassium Dichromate(VI)or Sodium Dichromate(VI)New Way Chemistry for Hong Kong A-Level Book 3B1637.2 Interconversions of Functional Groups of Organic Compounds(SB p.126)It is most often used to oxidize 2 alcohols to ketones in excellent yieldsNew Way Chemistry for Hong Kong A-Level Book 3B1737.2 Interconversions of Functional Groups of Organic Compounds(SB p.126)Tollens and Fehlings reagents are weak oxidizing agents which are able to oxidize aldehydes to carboxylate ionsTollens and Fehlings ReagentsNew Way Chemistry for Hong Kong A-Level Book 3B1837.2 Interconversions of Functional Groups of Organic Compounds(SB p.126)Ozonolysis is the most important oxidation reaction of alkenesThis reaction provides a method for locating the double bond of an alkeneOzone reacts vigorously with alkenes to form ozonides and then reduced by treatment with Zn and H2O to give carbonyl compoundsOzoneNew Way Chemistry for Hong Kong A-Level Book 3B19Check Point 37-1 Show how each of the following transformations could be accomplished.(a)(b)Answer37.2 Interconversions of Functional Groups of Organic Compounds(SB p.127)(a)(b)New Way Chemistry for Hong Kong A-Level Book 3B20Check Point 37-1 Show how each of the following transformations could be accomplished.(c)(d)Answer37.2 Interconversions of Functional Groups of Organic Compounds(SB p.127)(c)(d)New Way Chemistry for Hong Kong A-Level Book 3B2137.2 Interconversions of Functional Groups of Organic Compounds(SB p.128)Reduction of an organic compound usually corresponds to increasing its hydrogen content or decreasing its oxygen contentExamples:ReductionNew Way Chemistry for Hong Kong A-Level Book 3B2237.2 Interconversions of Functional Groups of Organic Compounds(SB p.128)Hydrogen is added to the C=C and C C bonds in alkenes and alkynes in the presence of transition metal catalysts Hydrogen with Transition Metal CatalystNew Way Chemistry for Hong Kong A-Level Book 3B2337.2 Interconversions of Functional Groups of Organic Compounds(SB p.129)Nitriles or nitro compounds are reduced to amines by hydrogen in the presence of transition metal catalystsNew Way Chemistry for Hong Kong A-Level Book 3B2437.2 Interconversions of Functional Groups of Organic Compounds(SB p.129)LiAlH4 is a powerful reducing agentIt reduces carboxylic acids,esters,aldehydes,ketones,amides,nitriles and nitro compoundsLithium TetrahydridoaluminateNew Way Chemistry for Hong Kong A-Level Book 3B2537.2 Interconversions of Functional Groups of Organic Compounds(SB p.129)New Way Chemistry for Hong Kong A-Level Book 3B2637.2 Interconversions of Functional Groups of Organic Compounds(SB p.129)LiAlH4 cannot normally reduce unsaturated centres like C=C and C C bonds and benzene ringReduction with LiAlH4 must be carried out in anhydrous solutions LiAlH4 reacts violently with waterNew Way Chemistry for Hong Kong A-Level Book 3B2737.2 Interconversions of Functional Groups of Organic Compounds(SB p.130)NaBH4 is a less powerful reducing agent than LiAlH4It reduces aldehydes and ketones onlyIt can be used in water or alcoholsSodium TetrahydridoborateNew Way Chemistry for Hong Kong A-Level Book 3B28Check Point 37-2 Which reducing agent,LiAlH4 or NaBH4,would you use to carry out the following transformations?(a)(b)(c)37.2 Interconversions of Functional Groups of Organic Compounds(SB p.130)(a)LiAlH4(b)LiAlH4(c)NaBH4AnswerNew Way Chemistry for Hong Kong A-Level Book 3B2937.2 Interconversions of Functional Groups of Organic Compounds(SB p.130)Aromatic nitro group can be reduced to amines by treatment with HCl and Fe,Zn or Sn,or a metal salt such as SnCl2Zinc,Tin,Tin(II)Chloride or Iron with Hydrochloric acidNew Way Chemistry for Hong Kong A-Level Book 3B3037.2 Interconversions of Functional Groups of Organic Compounds(SB p.131)Free radical substitution of alkanes with halogens forms haloalkanesA mixture of mono-,di-and poly-substituted haloalkanes is formedRH+X2 RX+HXSubstitutionNew Way Chemistry for Hong Kong A-Level Book 3B3137.2 Interconversions of Functional Groups of Organic Compounds(SB p.131)Various aromatic compounds can be prepared from benzene by electrophilic substitution of a hydrogen atom by substituentsNew Way Chemistry for Hong Kong A-Level Book 3B3237.2 Interconversions of Functional Groups of Organic Compounds(SB p.131)Haloalkanes can be converted into alcohols,nitriles or amines by substitution reactionsR X+OH R OH+XNew Way Chemistry for Hong Kong A-Level Book 3B3337.2 Interconversions of Functional Groups of Organic Compounds(SB p.131)Addition of alkenes and alkynes with various reagents can produce haloalkanes,haloalcohols,alcohols,alkanes and polymersAddition(X=Cl or Br)New Way Chemistry for Hong Kong A-Level Book 3B3437.2 Interconversions of Functional Groups of Organic Compounds(SB p.132)New Way Chemistry for Hong Kong A-Level Book 3B3537.2 Interconversions of Functional Groups of Organic Compounds(SB p.132)Carbonyl compounds undergo addition reaction with hydrogen cyanide for form hydroxyalkanenitrilesAddition of hydrogen to nitriles yields aminesNew Way Chemistry for Hong Kong A-Level Book 3B3637.2 Interconversions of Functional Groups of Organic Compounds(SB p.132)Haloalkanes can be converted to alkenes by elimination with alcoholic KOH or NaOHEliminationDihaloalkanes(with one halogen atom on each of two adjacent carbon atoms)can be converted to alkynes by eliminationNew Way Chemistry for Hong Kong A-Level Book 3B3737.2 Interconversions of Functional Groups of Organic Compounds(SB p.132)Alcohols undergo dehydration to give alkenes by treatment with conc.H2SO4New Way Chemistry for Hong Kong A-Level Book 3B3837.2 Chain Lengthening or Shortening of Carbon Skeleton(SB p.133)The carbon chain is lengthened by one when haloalkanes react with NaCN to form nitrilesHydrolysis of nitriles gives carboxylic acids,while hydrogenation of nitriles gives 1 aminesMethods of Chain LengtheningNew Way Chemistry for Hong Kong A-Level Book 3B3937.2 Chain Lengthening or Shortening of Carbon Skeleton(SB p.133)The carbon chain is also lengthened by one when carbonyl compounds react with HCN to form 2-hydroxyalkanenitrilesHydrolysis of the 2-hydroxyalkanenitriles yields 2-hydroxycarboxylic acidsNew Way Chemistry for Hong Kong A-Level Book 3B4037.2 Chain Lengthening or Shortening of Carbon Skeleton(SB p.133)In Hofmann degradation of amides,the carbon chain is reduced by one carbon atomMethods of Chain ShorteningNew Way Chemistry for Hong Kong A-Level Book 3B4137.2 Chain Lengthening or Shortening of Carbon Skeleton(SB p.134)In the triiodomethane formation reaction(iodoform reaction)of alcohols containing the group and aldehydes or ketones containing the group,the carbon chain is also degraded by one carbon atomNew Way Chemistry for Hong Kong A-Level Book 3B4237.2 Chain Lengthening or Shortening of Carbon Skeleton(SB p.134)New Way Chemistry for Hong Kong A-Level Book 3B4337.2 Chain Lengthening or Shortening of Carbon Skeleton(SB p.134)In ozonolysis,alkenes react with ozone to from ozonide which is reduced by using Zn and H2O to produce a mixture of carbonyl compounds resulting from the cleavage of the C=C bondThe cleavage of the C=C bond results in the degradation of carbon chainNew Way Chemistry for Hong Kong A-Level Book 3B44Check Point 37-3 By means of simple chemical equations,indicate how you would carry out the following conversion,which may involve more than one step.Give the reagents for each step and indicate the major product formed.(a)37.3 Chain Lengthening or Shortening of Carbon Skeleton(SB p.133)Answer(a)New Way Chemistry for Hong Kong A-Level Book 3B45Check Point 37-3 By means of simple chemical equations,indicate how you would carry out the following conversion,which may involve more than one step.Give the reagents for each step and indicate the major product formed.(b)37.3 Chain Lengthening or Shortening of Carbon Skeleton(SB p.133)Answer(b)New Way Chemistry for Hong Kong A-Level Book 3B46Check Point 37-3 By means of simple chemical equations,indicate how you would carry out the following conversion,which may involve more than one step.Give the reagents for each step and indicate the major product formed.(c)37.3 Chain Lengthening or Shortening of Carbon Skeleton(SB p.133)Answer(c)New Way Chemistry for Hong Kong A-Level Book 3B47Check Point 37-3 By means of simple chemical equations,indicate how you would carry out the following conversion,which may involve more than one step.Give the reagents for each step and indicate the major product formed.(d)37.3 Chain Lengthening or Shortening of Carbon Skeleton(SB p.133)Answer(d)New Way Chemistry for Hong Kong A-Level Book 3B48