Nomenclature of Halogeno-compounds

1

Halogenoalkane

Nucleophilic

Substitution

30.1

Introduction

30.2

Nomenclature of Halogeno-compounds

30.3

Physical Properties of Halogeno-compounds

30.4

Preparation of Halogeno-compounds

30.5

Nucleophilic Subsititution Reaction

30.6

Elimination Reactions

New Way Chemistry for Hong Kong A-Level 3A

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

1

按一下以編輯母片標題樣式

30.1

Introduction

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30.1 Introduction (SB p.208)

Haloalkanes


• Organic compounds having one or more halogen atoms replacing hydrogen atoms in alkanes

• If there is one halogen atom replacing a hydrogen atom

 General formula of haloalkanes:

C n

H

2n+1

X

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30.1 Introduction (SB p.208)

Haloalkanes attached to the carbon atom which is bonded to the halogen atom

 haloalkanes are classified into primary, secondary or tertiary

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5

Nomenclature of

Halogenocompounds



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30.2 Nomenclature of Halogeno-compounds (SB p.209)

Nomenclature of Halogeno-


• The IUPAC rules for naming halogenocompounds are similar to those for naming alkanes

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1. The halogens are written as prefixes: fluoro- (F), chloro- (Cl), bromo- (Br) and iodo- (I)

Numbers are assigned to the halogen substituent in the same way as the alkyl substituent

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2. When the parent chain has both a halogen and an alkyl substituent

 the chain is numbered from the end closest to the first substituent

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2. When the parent chain has both a halogen and an alkyl substituent

 the chain is numbered from the end closest to the first substituent

 all the prefixes are listed in alphabetical order



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30.2 Nomenclature of Halogeno-compounds (SB p.209) e.g.

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Physical Properties of Halogenocompounds



12

30.3 Physical Properties of Halogeno-compounds (SB p.211)

Physical properties of some halogeno-compounds

Name Molecular Boiling Melting Density at 按一下以編輯母片標題樣式

20 o C

(g cm -3 )

Chloroderivatives:

Chloromethane CH

3

Cl

Chloroethane CH

3

CH

2

Cl

1-

Chloropropane

CH

3

(CH

2

)

2

Cl

1-Chlorobutane CH

3

(CH

2

)

3

Cl

1-

Chloropentane

CH

3

(CH

2

)

4

Cl

3

(CH

2

)

5

Cl

-23.8

12.5

46.6

78.5

108

133

-97.7

-136

-123

-123

-99

-83

-

-

0.889

0.886

0.883

0.878

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20 o C

(g cm -3 )

Bromoderivatives:

Bromomethane CH

3

Br

Bromoethane CH

3

CH

2

Br

1-

Bromopropane

CH

3

(CH

2

)

2

Br

1-Bromobutane CH

3

(CH

2

)

3

Br

1-

Bromopentane

CH

3

(CH

2

)

4

Br

Br

3

(CH

2

)

5

3.6

38.4

70.8

101

129

156

-93.7

-119

-109

-113

-95

-85

-

1.460

1.354

1.279

1.218

1.176

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20 o C

(g cm -3 )

Iododerivatives:

Iodomethane CH

3

I 42.5

Iodoethane CH

3

CH

2

I 72.4

1-Iodopropane CH

3

(CH

2

)

2

I 102

1-Iodobutane CH

3

(CH

2

)

3

I 130

1-Iodopentane CH

3

(CH

2

)

4

I 155

-66.5

-108

-101

-103

-85.6

2.279

1.940

1.745

1.617

1.517

1-Iodohexane CH

3

(CH

2

)

5

I 181

(Iodomethyl)-

15 benzene

C

6

H

5

CH

2

I dec.

-

24.5



1.437

1.734

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Boiling Point and Melting Point


• C 

X bond is polar in nature

 Difference in electronegativity between carbon and halogens

• Molecules of haloalkanes are held together by dipole-dipole interactions

• Haloalkanes have higher b.p. and m.p. than alkanes of comparable relative molecular masses

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• B.p. and m.p. of fluoro-, chloro-, bromoand iodo- compounds increase in the order:

RCH

2

F < RCH

2

Cl < RCH

2

Br < RCH

2

I

• Larger, more polarizable halogen atoms lead to

 increase in dipole-dipole interactions between molecules

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• When the number of carbon atoms in the alkyl groups increases

 the molecular size also increases

 stronger dipole-dipole interactions between molecules

 higher b.p. and m.p.

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Variation of boiling points with the number of carbon atoms of straight-chain haloalkanes


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Density


• When the relative molecular mass of haloalkanes increases

 the densities of haloalkanes decrease

• Closer packing of the smaller molecules in the liquid phase

• Bromoalkanes and iodoalkanes are denser than water at 20 o C

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• Haloalkanes are immiscible with water

• But dissolve in organic solvents

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Reactions of

Halogenocompounds



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30.5 Reactions of Halogeno-compounds (SB p.218)

Reactions of Halogeno-compounds


 halogens are more electronegative than carbon

• The carbon atom bears a partial negative charge

• The halogen atom bears a partial positive charge

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• One of the characteristic reactions of haloalkanes is nucleophilic substitution reactions:

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• In the reaction,

 the nucleophile attacks the electropositive carbon centre

 displaces a halide ion from the haloalkane

 a kind of substitution reactions



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•  The reaction is initiated by a nucleophile

 called nucleophilic substitution reaction



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• Another characteristic reaction of haloalkanes is elimination reactions:

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• A base removes a hydrogen atom from the carbon atom adjacent to the C



X bond

• C=C bond is formed



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Nucleophilic

Substitution

Reactions



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30.6 Nucleophilic Substitution Reactions (SB p.219)

Reaction with Sodium Hydroxide

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Reaction with Sodium Hydroxide


• In both reactions,

 the nucleophile (OH ) attacks the haloalkane

 replacing the halogen atom with a hydroxyl group

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2. Stereochemistry of S

N

2 Reactions carbon centre from the backside

 the configuration of the carbon atom under attack inverts

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The Concentration and Strength of the


A negatively charged nucleophile (e.g. OH ) is always a stronger nucleophile than a neutral nucleophile (e.g. H

2

O)

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The Concentration and Strength of the

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In a group of nucleophiles in which the nucleophilic atom is the same, the order of nucleophilicity roughly follows the order of basicity e.g. oxygen compounds show the following order of reactivity:

RO > OH >>ROH > H

2

O



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The Nature of the Leaving Group

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C



F

C



Cl

C



Br

C



I

(kJ mol -1 )

+484

+338

+276

+238

Bond enthalpies of carbon-halogen bonds



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6. Comparison of Rates of Hydrolysis of

Experiment 1: Comparison of the Rates of

Hydrolysis of 1-Chlorobutane, 1-Bromobutane and 1-Iodobutane

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Three test tubes containing ethanol, silver nitrate solution and different haloalkanes



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AgCl(s)

Test tube A

AgBr(s)

Test tube B

AgI(s)

Test tube C



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Discussion:

按一下以編輯母片標題樣式 decreases in the order:

I > Br > Cl -

• The order of precipitates appeared follows the order of ease of leaving of the halide ions

Ag + (aq) + X (aq)



AgX(s)

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Reaction with Potassium Cyanide


• When a haloalkane is heated under reflux with an alcoholic solution of KCN or KCN in ethanol

 a nitrile is formed

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Reaction with Potassium Cyanide


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• CN acts as a nucleophile

• Halobenzenes do not react with KCN

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Reaction with Potassium Cyanide which can be reduced to alcohols

• Provides a useful way of introducing a carbon atom into an organic molecule

 length of carbon chain can be increased

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• Ammonia is a nucleophile

 Presence of a lone pair of electrons on the nitrogen atom

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Reaction with Ammonia


• When a haloalkane is heated with an aqueous alcoholic solution of ammonia under high pressure

 an amine is formed

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• 

按一下以編輯母片標題樣式 as the nucleophile

 a series of further substitution reactions takes place

 a mixture of products is formed

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• The reaction stops at the formation of a quaternary ammonium salt

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• The competing reactions can be minimized by using excess ammonia

• Not a satisfactory method for preparing amines

 a mixture of amines is always formed

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30.7

Elimination

Reactions

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30.7 Elimination Reactions (SB p.236)

• Heating a haloalkane with a strong base

(e.g. sodium hydroxide in ethanol)

 causes the elimination of HX from adjacent carbon atoms of a haloalkane

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Formation of Alkenes


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Formation of Alkenes dehydrohalogenation

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Formation of Alkenes haloalkanes yields more than one product

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• The major product will be the more stable alkene

 The more stable alkene has the more highly substituted double bond

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Formation of Alkenes


• The order of stabilities of alkenes is:

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• Whenever an elimination reaction occurs to give the more highly substituted alkene as the major product

 the elimination reaction follows the

Saytzeff’s rule

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The END

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57

Nomenclature of Halogeno-compounds

Halogenoalkane

Nucleophilic

Substitution

Introduction

Nomenclature of

Halogenocompounds

Physical Properties of Halogenocompounds

Reactions of

Halogenocompounds

Nucleophilic

Substitution

Reactions

Elimination

Reactions

The END

Related documents

Products

Support

Nomenclature of Halogeno-compounds

Halogenoalkane

Nucleophilic

Substitution

Introduction

Nomenclature of

Halogenocompounds

Physical Properties of Halogenocompounds

Reactions of

Halogenocompounds

Nucleophilic

Substitution

Reactions

Elimination

Reactions

The END

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