Chemistry · Ch 10 — Halogen Derivatives
Nucleophilic substitution reactions of haloalkanes
Nucleophilic substitution reactions of haloalkanes
When one group bonded to a carbon in a substrate is replaced by another group, with no change in the hybridisation state of that carbon, the reaction is called a substitution reaction. The C-X bond in an alkyl halide is a polar covalent bond in which the carbon is positively polarised -- in other words, the carbon of the C-X bond is an electrophilic centre, and it therefore has a natural tendency to react with a nucleophile (Class 11 Chemistry, Chapter 14). Alkyl halides react with a wide variety of nucleophiles to give nucleophilic substitution reactions (SN), represented in general as Nu(-) + C-X -> Nu-C + X(-). A substrate that reacts fast is described as reactive, and the reactivity of an alkyl halide in an SN reaction depends on two independent factors: the substitution state of the carbon carrying the halogen (primary, secondary or tertiary) and the identity of the halogen itself, with the overall order tertiary > secondary > primary for substitution state and R-I > R-Br > R-Cl for the halogen (because a weaker, more polarisable C-X bond, per Table 10.2, breaks more easily). Table 10.3 catalogues eight important nucleophilic substitution reactions of alkyl halides -- with hydroxide, alkoxide, carboxylate, ammonia, cyanide, silver cyanide, nitrite and silver nitrite -- giving alcohols, ethers, esters, amines, nitriles, isocyanides, alkyl nitrites and nitroalkanes respectively. Cyanide and nitrite are both examples of ambident nucleophiles, capabl …
Sr. No, reagent, substitution product (all reactions start from R-X). 1. NaOH(aq) or KOH, heat -> R-OH (alcohol) + NaX/KX. 2. Sodium alkoxide NaOR', heat -> R-O-R' (ether) + NaX. 3. Silver carboxylate R'-C(=O)-OAg, heat -> R'-C(=O)-OR (ester) + AgX(down-arrow). 4. Ammonia NH3 (alcoholic, excess, heat, pressure) -> R-NH2 (primary amine) + HX. 5. KCN (alcoholic), heat -> R-CN (nitrile / alkyl cyanide) + KX. 6. AgCN (alcoholic), heat -> R-NC (isocyanide) + AgX(down-arrow). 7. Potassium nitrite KO-N=O, heat -> R-O-N=O (alkyl nitrite) + KX. 8. Silver nitrite Ag-O-N=O -> R-NO2 (nitroalkane) + AgX(down-arrow). The table shows the same alkyl halide substrate R-X reacting with eight different nucleophiles to give …
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Worked out. An ambident nucleophile is one capable of attacking through more than one atom (site). Cyanide ion, C(triple bond)N- in resonance with C=N-, can attack an alkyl halide through either its carbon or its nitrogen atom. Potassium cyanide, KCN, is predominantly ionic and supplies a 'naked' cyanide ion in which both atoms are free to donate an electron pair; because the C-C bond that would form is stronger than the C-N bond that would form, attack occurs mainly through the carbon atom, giving an alkyl cyanide (nitrile) as the major product. Silver cyanide, AgCN, by contrast, is mainly covalent (Ag-C(triple bond)N), which ties up the carbon's lone pair in the Ag-C bond and leaves the nitrogen free to donate its electron pair instead, so attack occurs through nitrogen, giving an isocyanide. The nitrite ion, O-N=O in resonance with O=N-O, is a second ambident nucleophile that can attack through either its oxygen or its nitrogen atom: alkyl halides treated with alcoholic silver nitrite (covalent, nitrogen free to attack) give a nitroalkane, while alkyl halides tre …