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Published byLisbeth Paramore Modified over 9 years ago
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Physical Properties of Haloalkanes 6-1 The bond strength of C-X decreases as the size of X increases. A halogen uses a p orbital to overlap an sp 2 orbital on a carbon atom. As the size of the halogen p orbital increases (F < Cl < Br < I), the percentage overlap with the smaller sp 2 carbon orbital is less and the bond strength decreases.
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The C-X bond is polarized. Because halogens are more electronegative that carbon, carbon- halogen bonds are polarized. The halogen atom possesses a partial negative (δ - ) and the carbon atom a partial positive (δ + ) charge. The electrophilic δ + carbon atom is subject to attack by anions and other nucleophilic species. Cations and other electron-deficient species attack the halogen atom.
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Haloalkanes have higher boiling points than the corresponding alkanes. Boiling points of haloalkanes are higher than those of the parent alkanes mainly due to dipole-dipole interactions between the haloalkane molecules: As the size of the halogen increases there are also larger London forces between the haloalkane molecules. Larger atoms are more polarizable and interact more strongly through London forces.
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Nucleophilic Substitution 6-2 Haloalkanes can react with nucleophiles at their electrophilic carbon atom. The nucleophile can be charged, as in :OH - or neutral, as in :NH 3. In nucleophilic substitution of haloalkanes, the nucleophile replaces the halogen atom.
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Nucleophilic Substitution 6-2 Nucleophiles attack electrophillic centers. Nucleophilic substitution of a haloalkane can be described by two general equations: In both cases, the leaving group is the halide anion, X -. In describing reactions, the organic starting material is called the substrate of the reaction. Here, the substrate is being attacked by a nucleophile.
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Nucleophililc substitution exhibits considerable diversity. Rxn 1: OH - (KOH) displaces Cl - to produce an alcohol. Rxn 2: OCH 3 - displaces Cl - to produce an ether. Rxn 3: I - displaces Cl - to produce a different haloalkane. Rxn 4: CN - (NaCN) displaces Cl - to form a new C-C bond.
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Rxn 5: The S analog of Rxn 2 forming a thioether. Rxn 6: Neutral :NH 3 produces a cationic ammonium salt Rxn 7: Neutral :PH 3 produces a cationic phosphonium salt.
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Halides can serve as nucleophiles and as leaving groups in nucleophilic substitution reactions. These reactions are reversible. Strong bases, such as HO - and CH 3 O -, however do not serve as good leaving groups. Substitution reactions involving these species are not reversible.
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