new way chemistry for hong kong a-level book 3b1 syntheses and interconversions of organic compounds...
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New Way Chemistry for Hong Kong A-Level Book 3B
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Syntheses and Interconversions of Organic Compounds
37.137.1 Planning Organic SynthesesPlanning Organic Syntheses
37.237.2 Interconversions of Functional Groups Interconversions of Functional Groups of Organic Compoundsof Organic Compounds
37.337.3 Chain Lengthening or Shortening of Chain Lengthening or Shortening of CarCarbon Skeletonbon Skeleton
Chapter 37Chapter 37
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37.1 Planning Organic Syntheses (SB p.122)
The organic compounds that are now known:
• Only small fractions of them can be isolated from
natural resources
• All the remaining are synthesized by organic chemists
Reasons 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 pathways
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37.1 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 molecule
• A synthesis usually involves more than one step
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37.1 Planning Organic Syntheses (SB p.123)
There are usually more than one way to carry out a synthesis
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37.1 Planning Organic Syntheses (SB p.123)
The feasibility of an organic synthesis depends on a number of factors:
• Numbers of steps involved in the synthesis
• Availability of starting materials and reagents
• Duration of the synthetic process
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37.1 Planning Organic Syntheses (SB p.123)
• Most organic reactions are reversible and seldom proceed to completion
• As the backward reaction takes place, it is impossible to have a 100% yield of product from each step of the synthetic route
Numbers of Steps involved in the SynthesisNumbers of Steps involved in the Synthesis
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• The yield of desired product is 12.96%
∴ an efficient route of synthesis consists of a minimal number of steps
• Usually the number of steps is limited to not more than four
37.1 Planning Organic Syntheses (SB p.123)
e.g.
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37.1 Planning Organic Syntheses (SB p.123)
• There are often a restricted number of simple, relatively
cheap organic compounds available
e.g. simple haloalkanes, alcohols of not more than four
carbon atoms, simple aromatic compounds such as
benzene and methylbenzene
Availability of Starting Materials and ReagentsAvailability of Starting Materials and Reagents
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37.1 Planning Organic Syntheses (SB p.124)
• The time factor must be considered when planning the synth
etic pathway
∵ many organic reactions proceed at a relatively slow rate
e.g. acid-catalyzed esterification requires the reaction mixtur
e to be refluxed for a whole day
• Involvement of slow reactions in the synthetic route is impra
ctical as the reaction will be too long
Duration of the Synthetic ProcessDuration of the Synthetic Process
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37.2 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 content
• Common oxidizing agents used: KMnO4, K2Cr2O7 or H2
CrO4
• KMnO4 is the strongest oxidizing agent and can be used
in acidic, neutral or alkaline medium
• Other oxidizing agents: Tollens’ reagent, Fehling’s reagent, ozone
OxidationOxidation
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37.2 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 oxi
dized to diols
Potassium Manganate(VII)
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37.2 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 reaction
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37.2 Interconversions of Functional Groups of Organic Compounds (SB p.125)
• Alkylbenzenes are converted to benzoic acid by vig
orous oxidation of KMnO4
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37.2 Interconversions of Functional Groups of Organic Compounds (SB p.126)
• 1° alcohols are oxidized to carboxylic acids by vigorous oxidation of KMnO4
• The 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 formed
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37.2 Interconversions of Functional Groups of Organic Compounds (SB p.126)
• The most commonly used chromium(VI) reagent is H2Cr
O4 which is prepared by adding CrO3, Na2Cr2O7 or K2Cr2O
7 to aqueous H2SO4
• It oxidizes 1° alcohols or aromatic side chains to aldehydes but not in good yields
Potassium Dichromate(VI) or Sodium Dichromate(VI)
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37.2 Interconversions of Functional Groups of Organic Compounds (SB p.126)
• It is most often used to oxidize 2° alcohols to ketones in excellent yields
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37.2 Interconversions of Functional Groups of Organic Compounds (SB p.126)
• Tollens’ and Fehling’s reagents are weak oxidizing agent
s which are able to oxidize aldehydes to carboxylate ions
Tollens’ and Fehling’s Reagents
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37.2 Interconversions of Functional Groups of Organic Compounds (SB p.126)
• Ozonolysis is the most important oxidation reaction of alkenes
• This reaction provides a method for locating the double bond of an alkene
• Ozone reacts vigorously with alkenes to form ozonides and then reduced by treatment with Zn and H2O to give carb
onyl compounds
Ozone
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Check Point 37-1 Check Point 37-1
Show how each of the following transformations could be accomplished.
(a)
(b)
Answer
37.2 Interconversions of Functional Groups of Organic Compounds (SB p.127)
(a)
(b)
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Check Point 37-1 Check Point 37-1
Show how each of the following transformations could be accomplished.
(c)
(d)
Answer
37.2 Interconversions of Functional Groups of Organic Compounds (SB p.127)
(c)
(d)
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37.2 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 content
Examples:
ReductionReduction
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37.2 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 Catalyst
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37.2 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 catalysts
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37.2 Interconversions of Functional Groups of Organic Compounds (SB p.129)
• LiAlH4 is a powerful reducing agent
• It reduces carboxylic acids, esters, aldehydes, ketones, amides, nitriles and nitro compounds
Lithium Tetrahydridoaluminate
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37.2 Interconversions of Functional Groups of Organic Compounds (SB p.129)
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37.2 Interconversions of Functional Groups of Organic Compounds (SB p.129)
• LiAlH4 cannot normally reduce unsaturated centres lik
e C = C and C C bonds and benzene ring
• Reduction with LiAlH4 must be carried out in anhydrous
solutions ∵ LiAlH4 reacts violently with water
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37.2 Interconversions of Functional Groups of Organic Compounds (SB p.130)
• NaBH4 is a less powerful reducing agent than LiAlH4
• It reduces aldehydes and ketones only
• It can be used in water or alcohols
Sodium Tetrahydridoborate
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Check Point 37-2 Check 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) NaBH4
Answer
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37.2 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 SnCl2
Zinc, Tin, Tin(II) Chloride or Iron with Hydrochloric acid
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37.2 Interconversions of Functional Groups of Organic Compounds (SB p.131)
• Free radical substitution of alkanes with halogens forms haloalkanes
• A mixture of mono-, di- and poly-substituted haloalkanes is formed
RH + X2 RX + HX
SubstitutionSubstitution
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37.2 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 substituents
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37.2 Interconversions of Functional Groups of Organic Compounds (SB p.131)
• Haloalkanes can be converted into alcohols, nitriles or a
mines by substitution reactions
R – X + OH– R – OH + X–
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37.2 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 polymers
AdditionAddition
(X = Cl or Br)
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37.2 Interconversions of Functional Groups of Organic Compounds (SB p.132)
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37.2 Interconversions of Functional Groups of Organic Compounds (SB p.132)
• Carbonyl compounds undergo addition reaction with hydr
ogen cyanide for form hydroxyalkanenitriles
• Addition of hydrogen to nitriles yields amines
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37.2 Interconversions of Functional Groups of Organic Compounds (SB p.132)
• Haloalkanes can be converted to alkenes by elimination with alcoholic KOH or NaOH
EliminationElimination
• Dihaloalkanes (with one halogen atom on each of two adjacent carbon atoms) can be converted to alkynes by elimination
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37.2 Interconversions of Functional Groups of Organic Compounds (SB p.132)
• Alcohols undergo dehydration to give alkenes by
treatment with conc. H2SO4
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37.2 Chain Lengthening or Shortening of Carbon Skeleton (SB p.133)
• The carbon chain is lengthened by one when haloalkanes react with NaCN to form nitriles
• Hydrolysis of nitriles gives carboxylic acids, while hydrogenation of nitriles gives 1° amines
Methods of Chain LengtheningMethods of Chain Lengthening
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37.2 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-hydroxyalkanenitriles
• Hydrolysis of the 2-hydroxyalkanenitriles yields 2-hydroxycarboxylic acids
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37.2 Chain Lengthening or Shortening of Carbon Skeleton (SB p.133)
• In Hofmann degradation of amides, the carbon chain i
s reduced by one carbon atom
Methods of Chain ShorteningMethods of Chain Shortening
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37.2 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 atom
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37.2 Chain Lengthening or Shortening of Carbon Skeleton (SB p.134)
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37.2 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 mix
ture of carbonyl compounds resulting from the cleavage of the C = C bond
• The cleavage of the C = C bond results in the degradation of carbon chain
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Check Point 37-3 Check 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)
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Check Point 37-3 Check 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)
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Check Point 37-3 Check 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)
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Check Point 37-3 Check 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)