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.

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Presentation transcript:

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.

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.

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.

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.

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.

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.

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.

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.