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Halogenoalkane compounds and 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. 30.1.
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Halogenoalkane compounds and 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
30.1 Introduction
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: CnH2n+1X
30.1 Introduction (SB p.208) Haloalkanes • According to the number of alkyl groups attached to the carbon atom which is bonded to the halogen atom • haloalkanes are classified into primary, secondary or tertiary
30.2 Nomenclature of Halogeno-compounds
30.2 Nomenclature of Halogeno-compounds (SB p.209) Nomenclature of Halogeno-compounds • The IUPAC rules for naming halogeno-compounds are similar to those for naming alkanes
30.2 Nomenclature of Halogeno-compounds (SB p.209) Nomenclature of Halogeno-compounds • 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
30.2 Nomenclature of Halogeno-compounds (SB p.209) Nomenclature of Halogeno-compounds e.g.
30.2 Nomenclature of Halogeno-compounds (SB p.209) Nomenclature of Halogeno-compounds • When the parent chain has both a halogen and an alkyl substituent • the chain is numbered from the end closest to the first substituent
30.2 Nomenclature of Halogeno-compounds (SB p.209) Nomenclature of Halogeno-compounds • 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
30.2 Nomenclature of Halogeno-compounds (SB p.209) Nomenclature of Halogeno-compounds e.g.
30.3 Physical Properties of Halogeno-compounds
30.3 Physical Properties of Halogeno-compounds (SB p.211) Physical properties of some halogeno-compounds
30.3 Physical Properties of Halogeno-compounds (SB p.211) Physical properties of some halogeno-compounds
30.3 Physical Properties of Halogeno-compounds (SB p.211) Physical properties of some halogeno-compounds
30.3 Physical Properties of Halogeno-compounds (SB p.212) Boiling Point and Melting Point • CX 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
30.3 Physical Properties of Halogeno-compounds (SB p.212) Boiling Point and Melting Point • B.p. and m.p. of fluoro-, chloro-, bromo- and iodo- compounds increase in the order: • RCH2F < RCH2Cl < RCH2Br < RCH2I • Larger, more polarizable halogen atoms lead to • increase in dipole-dipole interactions between molecules
30.3 Physical Properties of Halogeno-compounds (SB p.212) Boiling Point and Melting Point • 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.
30.3 Physical Properties of Halogeno-compounds (SB p.212) Variation of boiling points with the number of carbon atoms of straight-chain haloalkanes
30.3 Physical Properties of Halogeno-compounds (SB p.212) 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 oC
30.3 Physical Properties of Halogeno-compounds (SB p.213) Solubility • Haloalkanes are immiscible with water • But dissolve in organic solvents
30.5 Reactions of Halogeno-compounds
30.5 Reactions of Halogeno-compounds (SB p.218) Reactions of Halogeno-compounds • CX bond is polar • halogens are more electronegative than carbon • The carbon atom bears a partial negative charge • The halogen atom bears a partial positive charge
30.5 Reactions of Halogeno-compounds (SB p.218) Reactions of Halogeno-compounds • One of the characteristic reactions of haloalkanes is nucleophilic substitution reactions:
30.5 Reactions of Halogeno-compounds (SB p.218) Reactions of Halogeno-compounds • In the reaction, • the nucleophileattacks the electropositive carbon centre • displaces a halide ion from the haloalkane • a kind of substitution reactions
30.5 Reactions of Halogeno-compounds (SB p.218) Reactions of Halogeno-compounds • The reaction is initiated by a nucleophile • called nucleophilic substitution reaction
30.5 Reactions of Halogeno-compounds (SB p.219) Reactions of Halogeno-compounds • Another characteristic reaction of haloalkanes is elimination reactions:
30.5 Reactions of Halogeno-compounds (SB p.219) Reactions of Halogeno-compounds • A baseremoves a hydrogen atom from the carbon atom adjacent to the CX bond • C=C bond is formed
30.6 Nucleophilic Substitution Reactions
30.6 Nucleophilic Substitution Reactions (SB p.219) Reaction with Sodium Hydroxide Consider the following reactions:
30.6 Nucleophilic Substitution Reactions (SB p.219) Reaction with Sodium Hydroxide • In both reactions, • the nucleophile (OH-) attacks the haloalkane • replacing the halogen atom with a hydroxyl group
30.6 Nucleophilic Substitution Reactions (SB p.221) 2. Stereochemistry of SN2 Reactions • The nucleophile attacks the electropositive carbon centre from the backside • the configuration of the carbon atom under attack inverts
30.6 Nucleophilic Substitution Reactions (SB p.226) The Concentration and Strength of the Nucleophile A negatively charged nucleophile (e.g. OH-) is always a stronger nucleophile than a neutral nucleophile (e.g. H2O)
30.6 Nucleophilic Substitution Reactions (SB p.227) The Concentration and Strength of the Nucleophile 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 > H2O
30.6 Nucleophilic Substitution Reactions (SB p.227) The Nature of the Leaving Group Bond enthalpies of carbon-halogen bonds
30.6 Nucleophilic Substitution Reactions (SB p.228) 6. Comparison of Rates of Hydrolysis of Haloalkanes Experiment 1: Comparison of the Rates of Hydrolysis of 1-Chlorobutane, 1-Bromobutane and 1-Iodobutane Three test tubes containing ethanol, silver nitrate solution and different haloalkanes
AgBr(s) AgI(s) AgCl(s) Test tube A Test tube B Test tube C 30.6 Nucleophilic Substitution Reactions (SB p.228) Results and Observation:
30.6 Nucleophilic Substitution Reactions (SB p.229) Discussion: • The ease of leaving of halide ionsdecreases 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)
30.6 Nucleophilic Substitution Reactions (SB p.233) 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
30.6 Nucleophilic Substitution Reactions (SB p.233) Reaction with Potassium Cyanide e.g.
30.6 Nucleophilic Substitution Reactions (SB p.233) Reaction with Potassium Cyanide • CN- acts as a nucleophile • Halobenzenes do not react with KCN
30.6 Nucleophilic Substitution Reactions (SB p.233) Reaction with Potassium Cyanide • Nitriles can be hydrolyzed to carboxylic acids 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
30.6 Nucleophilic Substitution Reactions (SB p.234) Reaction with Ammonia • Ammonia is a nucleophile • Presence of a lone pair of electrons on the nitrogen atom
30.6 Nucleophilic Substitution Reactions (SB p.234) Reaction with Ammonia • When a haloalkane is heated with an aqueous alcoholic solution of ammonia under high pressure • an amine is formed
30.6 Nucleophilic Substitution Reactions (SB p.234) Reaction with Ammonia e.g.
30.6 Nucleophilic Substitution Reactions (SB p.234) Reaction with Ammonia • Ethylamine will compete with ammonia as the nucleophile • a series of further substitution reactions takes place • a mixture of products is formed
30.6 Nucleophilic Substitution Reactions (SB p.234) Reaction with Ammonia • The reaction stops at the formation of a quaternary ammonium salt
Let's Think 2 30.6 Nucleophilic Substitution Reactions (SB p.234) Reaction with Ammonia • The competing reactions can be minimized by using excess ammonia • Not a satisfactory method for preparing amines • a mixture of amines is always formed
30.7 Elimination Reactions
30.7 Elimination Reactions (SB p.236) Formation of Alkenes • 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