new way chemistry for hong kong a-level book 3b1 carbonyl compounds 34.1introduction...

New Way Chemistry for Hong Kong A-Level Book 3B

1

Carbonyl Compounds

34.134.1 IntroductionIntroduction

34.234.2 Nomenclature of Carbonyl CompoundsNomenclature of Carbonyl Compounds

34.334.3 Physical Properties of Carbonyl CompoundsPhysical Properties of Carbonyl Compounds

34.434.4 Preparation of Carbonyl CompoundsPreparation of Carbonyl Compounds

34.534.5 Reactions of Carbonyl CompoundsReactions of Carbonyl Compounds

34.634.6 Uses of Carbonyl CompoundsUses of Carbonyl Compounds

Chapter 34Chapter 34


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34.1 Introduction (SB p.2)

Aldehydes and ketones :

carbonyl compounds, contain group

General formula of aldehydes:

Examples:


3


General formula of ketones:

Examples:


4


• 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


5


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

• The bond angles between three attached atoms are 120


6

• Oxygen is more electronegative

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



7

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

Examples:

34.2 Nomenclature of Carbonyl Compounds (SB p.3)

AldehydesAldehydes


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• 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:


KetonesKetones


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Check Point 34-1 Check Point 34-1

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

Answer



10


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

Answer



11


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

Answer


(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.


12

• 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

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

Carbonyl compound

FormulaBoiling

point (°C)Melting

point (°C)

Density at 20°C (g cm–

3)

Aldehydes:

Methanal

Ethanal

Propanal

Butanal

Methylpropanal

Benzaldehyde

HCHO

CH3CHO

CH3CH2CHO

CH3(CH2)2CHO

(CH3)2CHCHO

C6H5CHO

–21

20.8

48.8

75.7

64.2

179

–92

–124

–81

–99

–65.9

–26

—

0.783

0.807

0.817

0.790

1.046


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Carbonyl compound

FormulaBoiling

point (°C)

Melting point (°C)

Density at 20°C (g cm–

3)

Ketones:

Propanone

Butanone

Pentan-3-one

Pentan-2-one

3-Methylbutan-2-one

Hexan-2-one

Phenylethanone

CH3COCH3

CH3COCH2CH3

CH3CH2COCH2CH3

CH3CO(CH2)2CH3

CH3COCH(CH3)2

CH3CO(CH2)3CH3

C6H5COCH3

56.2

79.6

102

102

95

127

202

–95.4

–86.9

–39.9

–77.8

–92

–56.9

19.6

0.791

0.806

0.814

0.811

0.803

0.812

1.028


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• 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

Boiling Point and Melting PointBoiling Point and Melting Point


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• 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

DensityDensity


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

SolubilitySolubility

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


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Example 34-1Example 34-1(a) In each pair of compounds below, select the one you

would expect to have a higher boiling point.

(i) A: CH3CH2CHO B: CH3CH2CH2OH

(ii) C: D:

(iii)E: CH3CH2CH2CHO F: CH3CH2CH2CH3

(iv)G: H:

Answer


Solution:

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


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Example 34-1Example 34-1(b) Propanone, CH3COCH3, is completely soluble in wat

er, 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)

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

Dehydrogenation of AlcoholsDehydrogenation of Alcohols


20


Example of oxidizing agents: acidified K2Cr2O7

• Aldehydes are prepared by oxidation of 1° alcohols

Oxidation of AlcoholsOxidation of Alcohols

• Ketones are prepared by oxidation of 2 ° alcohols


21


• Ozone reacts with alkenes vigorously to from ozonides

• Ozonides are reduced by Zn and H2O to give aldehydes

and/or ketones

Oxidative Cleavage of Alkenes (Ozonolysis)Oxidative Cleavage of Alkenes (Ozonolysis)


22


Aldehydes can be prepared by heating a mixture of calciu

m methanoate and calcium carboxylate

e.g.

Decarboxylation of Acid SaltsDecarboxylation of Acid Salts


23


Ketones can be prepared by heating calcium carboxylate

e.g.


24


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

st and S

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

Reduction of Acyl ChloridesReduction of Acyl Chlorides


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34.5 Reactions of Carbonyl Compounds (SB p.9)

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

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

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

Nucleophilic Addition ReactionsNucleophilic Addition Reactions


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• 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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• 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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2. 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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• 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


30


Addition of hydrogen cyanide to the carbonyl group to form

2-hydroxyalkanenitriles (also known as cyanohydrins)

Addition of Hydrogen Cyanide


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Examples:


32


Mechanism for the nucleophilic addition of hydrogen

cyanide to the carbonyl group:


33


• 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 g

ives HCN:

2KCN + H2SO4 2HCN + K2SO4

2NaCN + H2SO4 2HCN + Na2SO4

• 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


34


• 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.


35


• With the use of reducing agents (e.g. LiAlH4),

2-hydroxyalkanenitriles are reduced to amines

e.g.


36


Carbonyl compounds react reversibly with excess 40% aque

ous hydrogensulphate(IV) solutions at room temperature

Addition of Sodium Hydrogensulphate(IV)


37


Examples:


38


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

The reaction mechanism:


39

• This reaction is very sensitive to steric hindrance and is lim

ited to aliphatic aldehydes and sterically unhindered keto

nes

• This reaction can be used for the separation and purificatio

n of the aldehydes and ketones, as they can be regenerated

by treating the bisulphite addition product with aqueous alkal

is or dilute acids.



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Example 34-2Example 34-2Outline 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



41


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.


42


• 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

Addition – Elimination (Condensation) ReactionsAddition – Elimination (Condensation) Reactions


43


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

Reaction with Hydroxylamine


44


Examples:


45


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

Reaction with 2,4-dinitrophenylhydrazine


46


Examples:


47


• 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


48


Carbonyl compound

Formula Melting point of

2,4-dinitrophenylhydrazone (°C)

Aldehydes:

Methanal

Ethanal

Propanal

Butanal

Benzaldehyde

HCHO

CH3CHO

CH3CH2CHO

CH3CH2CH2CHO

C6H5CHO

167

146, 164 (2 forms)

156

123

237

Ketones:

Propanone

Butanone

Pentan-2-one

Pentan-3-one

Hexan-2-one

Phenylethanone

CH3COCH3

CH3CH2COCH3

CH3CH2CH2COCH3

CH3CH2COCH2CH3

CH3CH2CH2CH2COCH3

C6H5COCH3

128

115

141

156

107

250


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

Answer


(a) Carbonyl compounds always undergo nucleophilic addition r

eactions. As the carbon atom in the carbonyl group bears a partial

positive charge, the carbonyl group is susceptible to nucleophilic a

ttack.

In the case of alkenes, they always undergo electrophilic addition reactio

ns. As the bonding electrons of the carbon-carbon double bond are onl

y loosely held by the carbon atoms and are exposed, the carbon-carbon d

ouble bond is susceptible to electron-loving reagents (i.e. electrophiles)


50


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

Answer

(b) The two compounds can be distinguished by

determining the melting points of their

2,4-dinitrophenylhydrazone derivatives.



51


• Aldehydes can be oxidized to carboxylic acids by strong

oxidizing agents such as KMnO4 and K2Cr2O7, and als

o by mild oxidizing agents such as Tollen’s reagent a

nd Fehling’s reagent

OxidationsOxidations


52


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

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


53


• 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


54


• Tollens’ reagent contains Ag(NH3)2+

• Ag(NH3)2+ oxidizes aldehydes to carboxylic acids wh

ile it is reduced to metallic silver which deposits on the wall of the reaction vessel as silver mirror

Reaction with Tollens’ Reagent (Silver Mirror Test)


55


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

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

• This reaction can be used to distinguish aldehydes from ketones

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


56


• 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

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


57


• 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

Addition of an aliphatic

aldehyde

Fehling’s solution


58


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

ReductionsReductions


59


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

ent

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

Reaction with Lithium Tetrahydridoaluminate


60


• 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


61


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

Reaction with Sodium Tetrahydridoborate


62

• NaBH4 is a less powerful reducing agent than LiAl

H4

• NaBH4 reduces only aldehydes and ketones

• The reduction by NaBH4 can be carried out in water o

r alcohol solutions



63


Triiodomethane Formation (Iodoform Reaction)

Aldehydes or ketones having the group react

with iodine in aqueous sodium hydroxide solution to give a bright yellow precipitate of iodoform (CHI3)


64


• Ethanol and secondary alcohols with the group

also give a positive result of iodoform test

• Iodoform test is test for the presence of or

∵ the group is first oxidized to group and

further oxidized to give the carboxylate and the iodoform


65


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

(a) KCN/H2SO4 conc. HClCH3CH2CHO A B

20°C

(b) 2,4-dinitrophenylhydrazineCH3CH2CHO C

(c) 1. LiAlH4 / dry etherCH3CH2CHO D

2. H3O+

(d) NaBH4CH3CH = CHCH2CHO EH2O

Answer


(a) A: B:

(b) C:

(c) D: CH3CH2CH2OH

(d) E: CH3CH = CHCH2CH2OH


66


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

(e)

(f)

Answer


(e) F:

(f) G:

H: CHI3


67

34.6 Uses of Carbonyl Compounds (SB p.22)

As Raw Materials for Making PlasticsAs Raw Materials for Making Plastics

Urea-methanal

Urea-methanal is produced by condensation

polymerization of urea and methanal

under heat and pressure with the elimination of a water molecule


68


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


69


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

• excellent electrical insulator

• resistant to chemical attack

• used for moulding electrical sockets

Urea-methanal


70


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

Perspex

• Perspex is a dense, transparent solid

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


71


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

As SolventsAs Solvents


72

The END

new way chemistry for hong kong a-level book 3b1 carbonyl compounds 34.1introduction...

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