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Chapter 34. Carbonyl Compounds. 34.1 Introduction 34.2 Nomenclature of Carbonyl Compounds 34.3 Physical Properties of Carbonyl Compounds 34.4 Preparation of Carbonyl Compounds 34.5 Reactions of Carbonyl Compounds 34.6 Uses of Carbonyl Compounds. 34.1 Introduction (SB p.2).

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Carbonyl Compounds

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Chapter 34

Carbonyl Compounds

34.1Introduction

34.2Nomenclature of Carbonyl Compounds

34.3Physical Properties of Carbonyl Compounds

34.4Preparation of Carbonyl Compounds

34.5Reactions of Carbonyl Compounds

34.6Uses of Carbonyl Compounds


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34.1 Introduction (SB p.2)

Aldehydes and ketones :

carbonyl compounds, contain group

General formula of aldehydes:

Examples:


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34.1 Introduction (SB p.2)

General formula of ketones:

Examples:


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34.1 Introduction (SB p.3)

  • Carbonyl carbon is sp2-hybridized

  • The  bond is formed by the head-on overlap of an sp2 hybrid orbital of C and one p prbital of O

  • The  bond is formed by the side-way overlap of p orbitals of C and O


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34.1 Introduction (SB p.3)

  • The three atoms that are bonded to the carbonyl carbon forms a trigonal planar structure

  • The bond angles between three attached atoms are 120


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34.1 Introduction (SB p.3)

  • Oxygen is more electronegative

  • The carbonyl oxygen bears a partial negative charge and the carbonyl carbon bears partial positive charge


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34.2 Nomenclature of Carbonyl Compounds (SB p.3)

Aldehydes

Aldehydes are named by replacing the final “-e” of the name of the corresponding alkane with “-al”

Examples:


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34.2 Nomenclature of Carbonyl Compounds (SB p.4)

Ketones

  • Ketones are named by replacing the final “-e” of the name of the corresponding alkane with “-one”.

  • The parent chain is then numbered in the way that gives the carbonyl carbon atom the lowest possible number, and this number is used to indicate its position.

  • Examples:


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34.2 Nomenclature of Carbonyl Compounds (SB p.4)

Check Point 34-1

(a)Draw the structural formulae of all carbonyl compounds having the molecular formula C4H8O. Give their IUPAC names.

Answer


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34.2 Nomenclature of Carbonyl Compounds (SB p.4)

Check Point 34-1

(b)Draw the structural formulae of all straight-chain carbonyl compounds having the molecular formula C5H10O.

Answer


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34.2 Nomenclature of Carbonyl Compounds (SB p.4)

(c)Ketones are compounds with the group situated between two carbon chains. Therefore, the simplest ketone is the one with three carbon atoms. “Ethanone”, however, suggests that there are two carbon atoms in it and it does not exist.

Check Point 34-1

(c)Explain why there is no such a compound called “ethanone”.

Answer


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34.3 Physical Properties of Carbonyl Compounds (SB p.5)

  • Simple aldehydes and ketones are gases or liquids at room temperature

  • Aliphatic aldehydes have unpleasant and pungent smell

  • Ketones and benzaldehyde have a pleasant and sweet odour


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34.3 Physical Properties of Carbonyl Compounds (SB p.5)


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34.3 Physical Properties of Carbonyl Compounds (SB p.5)

Boiling Point and Melting Point

  • Carbonyl compounds have higher b.p. and m.p.than hydrocarbons of similar relative molecular masses∵ the presence of dipole-dipole interactions

  • Carbonyl compounds have lower b.p. and m.p. than the corresponding alcohols∵ dipole-dipole interactions are weaker than intermolecular hydrogen bonds


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34.3 Physical Properties of Carbonyl Compounds (SB p.6)

Density

  • The densities of aliphatic carbonyl compounds are lower than that of water at 20°C

  • Aromatic carbonyl compounds are slightly denser than water 20°C

  • Densities increase with increasing relative molecular masses


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34.3 Physical Properties of Carbonyl Compounds (SB p.6)

Solubility

  • Aldehydes and ketones of low molecular masses show appreciable solubilities in water∵carbonyl oxygen can form strong hydrogen bonds with water molecules

  • e.g. propanone and ethanal are soluble in water in all proportions


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34.3 Physical Properties of Carbonyl Compounds (SB p.7)

Example 34-1

(a)In each pair of compounds below, select the one you would expect to have a higher boiling point.

(i)A: CH3CH2CHOB: CH3CH2CH2OH

(ii)C:D:

(iii)E: CH3CH2CH2CHOF: CH3CH2CH2CH3

(iv)G:H:

Solution:

(a)(i)B(ii)D(iii)E(iv)H

Answer


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34.3 Physical Properties of Carbonyl Compounds (SB p.7)

Example 34-1

(b)Propanone, CH3COCH3, is completely soluble in water, but octan-4-one, CH3CH2CH2COCH2CH2CH2CH3, is almost insoluble in soluble in water. Explain their difference in solubility.

Answer

Solution:

(b)This is because the solubility in water decreases as the hydrophobic hydrocarbon portion lengthens


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34.4 Preparation of Carbonyl Compounds (SB p.7)

Dehydrogenation of Alcohols

Industrially, lower members of aldehydes and ketones are prepared by passing alcohol vapour over hot silver catalyst


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34.4 Preparation of Carbonyl Compounds (SB p.8)

Oxidation of Alcohols

  • Example of oxidizing agents: acidified K2Cr2O7

  • Aldehydes are prepared by oxidation of 1° alcohols

  • Ketones are prepared by oxidation of 2 ° alcohols


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34.4 Preparation of Carbonyl Compounds (SB p.8)

Oxidative Cleavage of Alkenes (Ozonolysis)

  • Ozone reacts with alkenes vigorously to from ozonides

  • Ozonides are reduced by Zn and H2O to give aldehydes and/or ketones


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34.4 Preparation of Carbonyl Compounds (SB p.8)

Decarboxylation of Acid Salts

Aldehydes can be prepared by heating a mixture of calcium methanoate and calcium carboxylate

e.g.


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34.4 Preparation of Carbonyl Compounds (SB p.9)

Ketones can be prepared by heating calcium carboxylate

e.g.


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34.4 Preparation of Carbonyl Compounds (SB p.9)

Reduction of Acyl Chlorides

  • Aldehydes can be prepared by reducing acyl chlorides by treatment with H2 in the presence of Pd / BaSO4 catalyst and S

  • The purpose of adding sulphur is to poison the catalyst, so that the reduction does not proceed to produce alcohols


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34.5 Reactions of Carbonyl Compounds (SB p.9)

Nucleophilic Addition Reactions

  • Carbonyl group is susceptible to nucleophilic attack∵ carbonyl carbon bears a partial positive charge

  • Nucleophiles use its lone pair electronsto form a bond with carbonyl carbon

  • One pair of bonding electrons of the carbon-oxygen bond shift out to the carbonyl oxygen


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34.5 Reactions of Carbonyl Compounds (SB p.10)

  • The electron-rich oxygen transfers its electron pair to a proton addition of Nu – H to the carbonyl group

  • The carbonyl carbon changes from a trigonal planar geometry (i.e. sp2-hybridized) to a tetrahedral geometry (i.e. sp3-hybridized)


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34.5 Reactions of Carbonyl Compounds (SB p.10)

  • Aldehydes are more reactive than ketones∵ inductive effect and steric effect

1.The inductive effectThe carbonyl carbon in ketones is less electron-deficient because two alkyl groups release electrons whereas only one present in aldehydes


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34.5 Reactions of Carbonyl Compounds (SB p.10)

  • The steric effect

  • Aldehyde molecules are relatively open to the attack of nucleophiles∵one group being attached to the carbonyl carbon is a small hydrogen atom

  • In ketones, the two alkyl or aryl substituents cause a greater steric hindrance to the nucleophiles


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34.5 Reactions of Carbonyl Compounds (SB p.11)

  • Due to the above 2 factors, the general order of reactivity of carbonyl compounds:

  • The delocalization of  electrons from the benzene ring reduce the electron deficiency of the carbonyl carbon atom and makes aromatic carbonyl compounds even less reactive than aliphatic ketones


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34.5 Reactions of Carbonyl Compounds (SB p.11)

Addition of Hydrogen Cyanide

Addition of hydrogen cyanide to the carbonyl group to form 2-hydroxyalkanenitriles (also known as cyanohydrins)


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34.5 Reactions of Carbonyl Compounds (SB p.11)

Examples:


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34.5 Reactions of Carbonyl Compounds (SB p.11)

Mechanism for the nucleophilic addition of hydrogen cyanide to the carbonyl group:


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34.5 Reactions of Carbonyl Compounds (SB p.11)

  • HCN is a poor nucleophile while CN– is much stronger the reaction can be catalyzed by a base stronger than CN–, as the base can increase the concentration of CN–

  • HCN + OH– CN– + H2O

  • As HCN is very toxic and volatile, it is safer to generate it in the reaction mixture

  • Mixing KCN or NaCN with dilute H2SO4 at 10 – 20°C gives HCN:2KCN + H2SO4 2HCN + K2SO42NaCN + H2SO4 2HCN + Na2SO4


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34.5 Reactions of Carbonyl Compounds (SB p.12)

  • 2-Hydroxyalkanenitriles are useful intermediates in organic synthesis

  • On acid hydrolysis, 2-hydroxyalkanenitriles are converted to 2-hydroxycarboxylic acids or 2-alkenoic acids

  • e.g.


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34.5 Reactions of Carbonyl Compounds (SB p.12)

  • With the use of reducing agents (e.g. LiAlH4), 2-hydroxyalkanenitriles are reduced to amines

  • e.g.


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34.5 Reactions of Carbonyl Compounds (SB p.13)

Addition of Sodium Hydrogensulphate(IV)

Carbonyl compounds react reversibly with excess 40% aqueous hydrogensulphate(IV) solutions at room temperature


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34.5 Reactions of Carbonyl Compounds (SB p.13)

Examples:


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34.5 Reactions of Carbonyl Compounds (SB p.13)

The reaction mechanism:

  • The reaction is initiated by the attack of nucleophile, HSO3–


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34.5 Reactions of Carbonyl Compounds (SB p.13)

  • This reaction is very sensitive to steric hindrance and is limited to aliphatic aldehydes and sterically unhindered ketones

  • This reaction can be used for the separation and purification of the aldehydes and ketones, as they can be regenerated by treating the bisulphite addition product with aqueous alkalis or dilute acids.


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34.5 Reactions of Carbonyl Compounds (SB p.14)

Example 34-2

Outline how you are going to separate a mixture of butanone (b.p. 79.6°C) and 1-chlorobutane (b.p. 78.5°C) in diethyl ether.

Answer


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34.5 Reactions of Carbonyl Compounds (SB p.14)

Solution:

The mixture of butanone and 1-chlorobutane cannot be separated by distillation as their boiling points are too close. However, they can be separated through the nucleophilic addition reaction of sodium hydrogensulphate(IV). With the addition of sodium hydrogensulphate(IV) to the mixture, only butanone reacts to give the bisulphite addition product which is soluble in water. Then the organic later (containing 1-chlorobutane) and the aqueous layer (containing the bisulphite addition product of butanone) are separated using a separating funnel. 1-Chlorobutane can be obtained by distilling off the ether. On the other hand, with the addition of a dilute acid, the bisulphite addition product is converted to the carbonyl compound (i.e. butanone) which dissolves in diethyl ether. The organic layer (containing butanone) is separated from the aqueous layer by means of a separating funnel. Butanone is obtained after distilling off the ether.


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34.5 Reactions of Carbonyl Compounds (SB p.14)

Addition – Elimination (Condensation) Reactions

  • Addition – elimination reactions involve the first addition of two molecules to form an unstable intermediate followed by the spontaneous elimination of the elements of water

  • e.g.reaction of aldehydes or ketones with the derivatives of ammonia


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34.5 Reactions of Carbonyl Compounds (SB p.14)

Reaction with Hydroxylamine

  • Carbonyl compounds react with hydroxylamine (NH2OH) to form oximes


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34.5 Reactions of Carbonyl Compounds (SB p.15)

Examples:


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34.5 Reactions of Carbonyl Compounds (SB p.15)

Reaction with 2,4-dinitrophenylhydrazine

  • Carbonyl compounds react with 2,4-dinitrophenylhydrazine to form 2,4-dinitrophenylhydrazones


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34.5 Reactions of Carbonyl Compounds (SB p.15)

Examples:


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34.5 Reactions of Carbonyl Compounds (SB p.16)

  • The oximes and 2,4-dinitrophenylhydrazones are used to identify unknown aldehydes and ketones

  • They are insoluble solids and have sharp characteristic melting points

  • The products are purified by recrystallization from ethanol and then filtered and washed under suction

  • Their melting points are determined and compared with that in data book to identify the original aldehyde or ketone


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34.5 Reactions of Carbonyl Compounds (SB p.16)


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34.5 Reactions of Carbonyl Compounds (SB p.17)

(a)Carbonyl compounds always undergo nucleophilic addition reactions. As the carbon atom in the carbonyl group bears a partial positive charge, the carbonyl group is susceptible to nucleophilic attack.

In the case of alkenes, they always undergo electrophilic addition reactions. As the  bonding electrons of the carbon-carbon double bond are only loosely held by the carbon atoms and are exposed, the carbon-carbon double bond is susceptible to electron-loving reagents (i.e. electrophiles)

Check Point 34-2

(a)Compare the addition reactions of carbonyl compounds and alkenes.

Answer


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34.5 Reactions of Carbonyl Compounds (SB p.17)

(b)The two compounds can be distinguished by determining the melting points of their 2,4-dinitrophenylhydrazone derivatives.

Check Point 34-2

(b)Describe briefly how you can distinguish between two carbonyl compounds having similar boiling points.

Answer


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34.5 Reactions of Carbonyl Compounds (SB p.17)

Oxidations

  • Aldehydes can be oxidized to carboxylic acids by strong oxidizing agents such as KMnO4 and K2Cr2O7, and also by mild oxidizing agents such as Tollen’s reagent and Fehling’s reagent


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34.5 Reactions of Carbonyl Compounds (SB p.18)

Reaction with Potassium Manganate(VII) and Potassium Dichromate(VI)

  • Aldehydes are oxidized readily by common oxidizing agents such as KMnO4/H+ and K2Cr2O7/H+


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34.5 Reactions of Carbonyl Compounds (SB p.18)

  • Generally, ketones do not undergo oxidation as their oxidation involves the cleavage of the strong carbon-carbon bond

  • More severe conditions are required to bring about the oxidation


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34.5 Reactions of Carbonyl Compounds (SB p.18)

Reaction with Tollens’ Reagent (Silver Mirror Test)

  • Tollens’ reagent contains Ag(NH3)2+

  • Ag(NH3)2+ oxidizes aldehydes to carboxylic acids while it is reduced to metallic silver which deposits on the wall of the reaction vessel as silver mirror


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34.5 Reactions of Carbonyl Compounds (SB p.19)

  • Aldehydes are mixed with Tollens’ reagent in a clean test tube and placed in water bath kept at 60°C

  • If the wall of the reaction vessel is not clean enough, a silver mirror cannot be formed and a black precipitate is deposited instead

  • All ketones give a negative result of the silver mirror test

  • This reaction can be used to distinguish aldehydes from ketones


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34.5 Reactions of Carbonyl Compounds (SB p.18)

Reaction with Fehling’s Solution (Fehling’s Test)

  • Fehling’s solution is an alkaline solution of copper(II) tartrate. It is a blue solution

  • Aliphatic aldehydes reduce the Cu2+ ion in Fehling’s solution to form a brick-red precipitate of Cu2O


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34.5 Reactions of Carbonyl Compounds (SB p.18)

Addition of an aliphatic aldehyde

Fehling’s solution

  • Ketones and aromatic aldehydes give a negative result of Fehling’s test

  • This reaction can be used to distinguish aliphatic aldehydes from ketones and aromatic aldehydes


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34.5 Reactions of Carbonyl Compounds (SB p.20)

Reductions

  • Aldehydes and ketones undergo reduction reactions forming 1° and 2° alcohols respectively


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34.5 Reactions of Carbonyl Compounds (SB p.20)

Reaction with Lithium Tetrahydridoaluminate

  • Lithium tetrahydridoaluminate (also called lithium aluminium hydride, LiAlH4) is a powerful reducing agent

  • It reduces aldehydes to 1° alcohols and ketones to 2° alcohols


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34.5 Reactions of Carbonyl Compounds (SB p.20)

  • LiAlH4 is able to reduce carboxylic acid and esters to give alcohols

  • LiAlH4 reacts violently with water, therefore the reaction must be carried out in anhydrous solutions, usually in dry ether


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34.5 Reactions of Carbonyl Compounds (SB p.21)

Reaction with Sodium Tetrahydridoborate

In practice, the reduction of aldehydes and ketones to alcohols is carried out by sodium tetrahydridoborate (also called sodium borohydride, NaBH4)


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34.5 Reactions of Carbonyl Compounds (SB p.21)

  • NaBH4 is a less powerful reducing agent than LiAlH4

  • NaBH4 reduces only aldehydes and ketones

  • The reduction by NaBH4 can be carried out in water or alcohol solutions


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34.5 Reactions of Carbonyl Compounds (SB p.21)

Aldehydes or ketones having the group react with iodine in aqueous sodium hydroxide solution to give a bright yellow precipitate of iodoform (CHI3)

Triiodomethane Formation (Iodoform Reaction)


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34.5 Reactions of Carbonyl Compounds (SB p.21)

  • Ethanol and secondary alcohols with the group also give a positive result of iodoform test

∵ the group is first oxidized to group and further oxidized to give the carboxylate and the iodoform

  • Iodoform test is test for the presence of or


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34.5 Reactions of Carbonyl Compounds (SB p.22)

  • (a)A:B:

  • (b)C:

  • D: CH3CH2CH2OH

  • E: CH3CH = CHCH2CH2OH

Check Point 34-3

Draw the structural formulae of the major organic products A to H in the following reactions:

(a)KCN/H2SO4conc. HCl

CH3CH2CHO  A  B

20°C

(b)2,4-dinitrophenylhydrazine

CH3CH2CHO  C

(c)1. LiAlH4 / dry ether

CH3CH2CHO  D

2. H3O+

(d)NaBH4

CH3CH = CHCH2CHO  E

H2O

Answer


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34.5 Reactions of Carbonyl Compounds (SB p.22)

(e) F:

(f)G:

H: CHI3

Check Point 34-3

Draw the structural formulae of the major organic products A to H in the following reactions:

(e)

(f)

Answer


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34.6 Uses of Carbonyl Compounds (SB p.22)

Urea-methanal is produced by condensation

polymerization of ureaand methanal

under heat and pressure with the elimination of a water molecule

As Raw Materials for Making Plastics

Urea-methanal


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34.6 Uses of Carbonyl Compounds (SB p.23)

In the presence of excess methanal, cross linkages will be formed


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34.6 Uses of Carbonyl Compounds (SB p.24)

Urea-methanal

  • thermosetting polymer (cannot be softened and insoluble in any solvents)

  • excellent electrical insulator

  • resistant to chemical attack

  • used for moulding electrical sockets


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34.6 Uses of Carbonyl Compounds (SB p.24)

Perspex

Propanone is converted to methyl 2-methylpropenoate, which is the monomer for the production of perspex

  • Perspex is a dense, transparent solid

  • Used to make safety goggles, advertising signs and carside light protectors


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34.6 Uses of Carbonyl Compounds (SB p.24)

As Solvents

  • Propanone

  • Liquid with a boiling point of 56.2°C

  • Can dissolve a variety of organic compounds

  • Important solvent used in industry and in the laboratory


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The END


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