Syntheses and Interconversions of Organic Compounds

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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• 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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There are usually more than one way to carry out a synthesis


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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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• 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


e.g.


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• 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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• 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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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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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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• Alkylbenzenes are converted to benzoic acid by vig

orous oxidation of KMnO4


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• 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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• 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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• It is most often used to oxidize 2° alcohols to ketones in excellent yields


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• 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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• 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


(a)

(b)


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Show how each of the following transformations could be accomplished.

(c)

(d)

Answer


(c)

(d)


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Reduction of an organic compound usually corresponds to increasing its hydrogen content or decreasing its oxygen content

Examples:

ReductionReduction


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• 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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• Nitriles or nitro compounds are reduced to amines by hydrogen in the presence of transition metal catalysts


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• LiAlH4 is a powerful reducing agent

• It reduces carboxylic acids, esters, aldehydes, ketones, amides, nitriles and nitro compounds

Lithium Tetrahydridoaluminate


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• 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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• 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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Which reducing agent, LiAlH4 or NaBH4, would you use to carry out the following transformations?

(a)

(b)

(c)


(a) LiAlH4

(b) LiAlH4

(c) NaBH4

Answer


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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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• 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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• Various aromatic compounds can be prepared from benzene by electrophilic substitution of a hydrogen atom by substituents


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• Haloalkanes can be converted into alcohols, nitriles or a

mines by substitution reactions

R – X + OH– R – OH + X–


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• 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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• Carbonyl compounds undergo addition reaction with hydr

ogen cyanide for form hydroxyalkanenitriles

• Addition of hydrogen to nitriles yields amines


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• 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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• 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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• 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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• 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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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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• 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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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)


Answer

(a)


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(b)


Answer

(b)


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(c)


Answer

(c)


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(d)


Answer

(d)


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Syntheses and Interconversions of Organic Compounds