Transcript Nomenclature of Halogeno-compounds
1
Halogenoalkane
30.1
30.2
30.3
30.4
30.5
30.6
Nucleophilic Substitution
Introduction Nomenclature of Halogeno-compounds Physical Properties of Halogeno-compounds Preparation of Halogeno-compounds Nucleophilic Subsititution Reaction Elimination Reactions New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
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30.1
Introduction
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30.1
Introduction (SB p.208) Haloalkanes
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• 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 3 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
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 4 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
5
Nomenclature of Halogeno compounds
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30.2
Nomenclature of Halogeno-compounds (SB p.209) Nomenclature of Halogeno-
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• The IUPAC rules for naming halogeno compounds are similar to those for naming alkanes 6 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.2
Nomenclature of Halogeno-compounds (SB p.209) Nomenclature of Halogeno-
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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 7 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.2
Nomenclature of Halogeno-compounds (SB p.209) Nomenclature of Halogeno-
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e.g.
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30.2
Nomenclature of Halogeno-compounds (SB p.209) 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 9 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.2
Nomenclature of Halogeno-compounds (SB p.209) 10 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 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.2
Nomenclature of Halogeno-compounds (SB p.209) e.g.
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12
Physical Properties of Halogeno compounds
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30.3
Physical Properties of Halogeno-compounds (SB p.211) Physical properties of some halogeno-compounds
Name Molecular Boiling Melting Density at
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20 o C (g cm -3 )
Chloro derivatives:
Chloromethane Chloroethane 1 Chloropropane CH 3 Cl CH 3 CH 2 Cl CH 3 (CH 2 ) 2 Cl -23.8
12.5
46.6
-97.7
-136 -123 1-Chlorobutane 1 Chloropentane CH 3 (CH 2 ) 3 Cl CH 3 (CH 2 ) 4 Cl 78.5
108 -123 -99 1-Chlorohexane CH 3 (CH 2 ) 5 133 -83 New Way Chemistry for Hong Kong A-Level Book 3A Cl 0.889
0.886
0.883
30.3
Physical Properties of Halogeno-compounds (SB p.211) Physical properties of some halogeno-compounds
Name Molecular Boiling Melting Density at
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20 o C (g cm -3 )
Bromo derivatives:
Bromomethane Bromoethane 1 Bromopropane CH 3 Br CH 3 CH 2 Br CH 3 (CH 2 ) 2 Br 3.6
38.4
70.8
-93.7
-119 -109 1-Bromobutane 1 Bromopentane CH 3 (CH 2 ) 3 Br CH 3 (CH 2 ) 4 Br 101 129 -113 -95 1-Bromohexane CH 3 (CH 2 ) 5 156 -85 New Way Chemistry for Hong Kong A-Level Book 3A Br 1.460
1.354
1.279
1.218
30.3
Physical Properties of Halogeno-compounds (SB p.211) Physical properties of some halogeno-compounds
Name Molecular Boiling Melting Density at
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20 o C (g cm -3 )
Iodo derivatives:
Iodomethane Iodoethane 1-Iodopropane 1-Iodobutane 1-Iodopentane CH 3 I CH 3 CH 2 I CH 3 (CH 2 ) 2 I CH 3 (CH 2 ) 3 I CH 3 (CH 2 ) 4 I 42.5
72.4
102 130 155 -66.5
-108 -101 -103 -85.6
2.279
1.940
1.745
1.617
1.517
1-Iodohexane (Iodomethyl) 15 benzene CH 3 (CH 2 ) 5 I 181 C 6 H 5 CH 2 I dec.
24.5
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1.734
30.3
Physical Properties of Halogeno-compounds (SB p.212) Boiling Point and Melting Point
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• 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 16 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.3
Physical Properties of Halogeno-compounds (SB p.212) Boiling Point and Melting Point
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• B.p. and m.p. of fluoro-, chloro-, bromo and 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 17 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.3
Physical Properties of Halogeno-compounds (SB p.212) Boiling Point and Melting Point
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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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30.3
Physical Properties of Halogeno-compounds (SB p.212) Variation of boiling points with the number of carbon atoms of straight-chain haloalkanes
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30.3
Physical Properties of Halogeno-compounds (SB p.212) Density
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• 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 20 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.3
Physical Properties of Halogeno-compounds (SB p.213) • Haloalkanes are immiscible with water • But dissolve in organic solvents 21 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
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Reactions of Halogeno compounds
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30.5
Reactions of Halogeno-compounds (SB p.218) Reactions of Halogeno-compounds
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halogens are more electronegative than carbon • The carbon atom bears a partial negative charge • The halogen atom bears a partial positive charge 23 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.5
Reactions of Halogeno-compounds (SB p.218) Reactions of Halogeno-compounds
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• One of the characteristic reactions of haloalkanes is nucleophilic substitution reactions: 24 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.5
Reactions of Halogeno-compounds (SB p.218) Reactions of Halogeno-compounds
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25 • In the reaction, the nucleophile attacks the electropositive carbon centre displaces a halide ion from the haloalkane a kind of substitution reactions New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.5
Reactions of Halogeno-compounds (SB p.218) Reactions of Halogeno-compounds
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26 • The reaction is initiated by a nucleophile called nucleophilic substitution reaction New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.5
Reactions of Halogeno-compounds (SB p.219) Reactions of Halogeno-compounds
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• Another characteristic reaction of haloalkanes is elimination reactions: 27 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.5
Reactions of Halogeno-compounds (SB p.219) Reactions of Halogeno-compounds
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28 • A base removes a hydrogen atom from the carbon atom adjacent to the C X bond • C=C bond is formed New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
29
Nucleophilic Substitution Reactions
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30.6
Nucleophilic Substitution Reactions (SB p.219) Reaction with Sodium Hydroxide 30 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.6
Nucleophilic Substitution Reactions (SB p.219) Reaction with Sodium Hydroxide
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• In both reactions, the nucleophile (OH ) attacks the haloalkane replacing the halogen atom with a hydroxyl group 31 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.6
Nucleophilic Substitution Reactions (SB p.221) 2. Stereochemistry of S N 2 Reactions carbon centre from the backside the configuration of the carbon atom under attack inverts 32 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.6
Nucleophilic Substitution Reactions (SB p.226) The Concentration and Strength of the
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A negatively charged nucleophile (e.g. OH ) is always a stronger nucleophile than a neutral nucleophile (e.g. H 2 O) 33 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.6
Nucleophilic Substitution Reactions (SB p.227) 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 34 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.6
Nucleophilic Substitution Reactions (SB p.227) The Nature of the Leaving Group 35 C F C Cl C Br C I
(kJ mol -1 )
+484 +338 +276 +238 Bond enthalpies of carbon-halogen bonds New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.6
Nucleophilic Substitution Reactions (SB p.228) 6. Comparison of Rates of Hydrolysis of
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Experiment 1: Comparison of the Rates of Hydrolysis of 1-Chlorobutane, 1-Bromobutane and 1-Iodobutane 36 Three test tubes containing ethanol, silver nitrate solution and different haloalkanes New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.6
Nucleophilic Substitution Reactions (SB p.228) 37 AgCl(s) Test tube A AgBr(s) Test tube B AgI(s) Test tube C New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.6
Nucleophilic Substitution Reactions (SB p.229)
Discussion:
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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) 38 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.6
Nucleophilic Substitution Reactions (SB p.233) Reaction with Potassium Cyanide
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• When a haloalkane is heated under reflux with an alcoholic solution of KCN or KCN in ethanol a nitrile is formed 39 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.6
Nucleophilic Substitution Reactions (SB p.233) Reaction with Potassium Cyanide
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e.g.
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30.6
Nucleophilic Substitution Reactions (SB p.233) • CN acts as a nucleophile • Halobenzenes do not react with KCN 41 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.6
Nucleophilic Substitution Reactions (SB p.233) 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 42 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.6
Nucleophilic Substitution Reactions (SB p.234) • Ammonia is a nucleophile Presence of a lone pair of electrons on the nitrogen atom 43 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.6
Nucleophilic Substitution Reactions (SB p.234) Reaction with Ammonia
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• When a haloalkane is heated with an aqueous alcoholic solution of ammonia under high pressure an amine is formed 44 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.6
Nucleophilic Substitution Reactions (SB p.234) Reaction with Ammonia
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e.g.
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30.6
Nucleophilic Substitution Reactions (SB p.234) Reaction with Ammonia •
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as the nucleophile a series of further substitution reactions takes place a mixture of products is formed 46 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.6
Nucleophilic Substitution Reactions (SB p.234) Reaction with Ammonia
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• The reaction stops at the formation of a quaternary ammonium salt 47 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.6
Nucleophilic Substitution Reactions (SB p.234) • The competing reactions can be minimized by using excess ammonia • Not a satisfactory method for preparing amines a mixture of amines is always formed 48 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
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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 50 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.7
Elimination Reactions (SB p.236) Formation of Alkenes
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e.g.
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30.7
Elimination Reactions (SB p.236) Formation of Alkenes dehydrohalogenation 52 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.7
Elimination Reactions (SB p.236) Formation of Alkenes haloalkanes yields more than one product 53 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.7
Elimination Reactions (SB p.236) • The major product will be the more stable alkene The more stable alkene has the more highly substituted double bond 54 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.7
Elimination Reactions (SB p.237) Formation of Alkenes
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• The order of stabilities of alkenes is: 55 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
30.7
Elimination Reactions (SB p.237) • Whenever an elimination reaction occurs to give the more highly substituted alkene as the major product the elimination reaction follows the Saytzeff’s rule 56 New Way Chemistry for Hong Kong A-Level 3A New Way Chemistry for Hong Kong A-Level Book 3A
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The END
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