CHEM 2261 Chapter : Notes2261Chapter6

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15 Mar 2019

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Compounds with an sp3 carbon bonded to a electronegative atom or group (e. g. as in alkyl halides) can undergo 2 types of reactions: substitution and/or elimination: The rate of a nucleophilic substitution reaction depends upon: (1) the structure of the alkyl halide (2) the leaving group (3) the nucleophile (4) the solvent. The rate of an sn2 reaction depends on the concentration of both the alkyl halide and the nucleophile, therefore it is a bimolecular reaction (second order). In an sn2 reaction the reactivity order of alkyl halides (due to steric effects) is: Sn2 reactions take place in a single step, without intermediates. The nucleophile attacks the substrate from a direction 180o away from the leaving group, causing inversion of configuration of the molecule. The leaving group in the sn2 reaction. The leaving ability of a group is a measure of its ease of displacement.

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Questions:

1) In the tests with sodium iodide in acetone and silver nitrate in ethanol, why should 2-bromobutane react faster than 2-chlorobutane?

2) Why is benzyl chloride reactive in both tests, whereas bromobenzene is unreactive?

3) When benzyl chloride is treated with sodium iodide in acetone, it reacts much faster than 1-chlorobutane, even though both compounds are primary alkyl chlorides. Explain this rate difference.

4) 2-Chlorobutane reacts much more slowly than 2-chloro-2-methylpropane in the silver nitrate test. Explain this difference in reactivity.

5) Bromocyclopentane is more reactive than bromocyclohexane when heated with sodium iodide in acetone. Explain this difference in reactivity.

CHEM 211 Biological Chemistry Lab #8-Factors effecting the reactions of alkylhalides A. Experimental objectives Examine the effects of leaving group, alkyl group structure and solvent polarity on the reaction of alkyl halides Examine the effects of solvent polarity on the reaction of a tertiary alkyl halide with sodium hydroxide To examine SN1 and SN2 reaction conditions To learn the most two important factors that effects the reactivities of alkyl halides in nucleophilic substitution reaction B. Introduction The reactivities of alkyl halides in nucleophilic substitution reactions depend on two important factors: reaction conditions and substrate structure. The reactivities of the several substrate types will be examined under both SN1 and SN2 reaction conditions in this experiment A reagent composed of sodium iodide or potassium iodide dissolved in acetone is useful in classifying alkyl halides according to their reactivity in an SN2 reaction. Iodide is an excellent nucleophile, and acetone is a nonpolar solvent. Thetendency to form a precipitate increases the completeness of the reaction. Sodiumiodide and potassium iodide are soluble in acetone, but the corresponding bromidesand chlorides are not soluble. Consequently, as bromide ion or chloride ion is produced, the ion is precipitated from the solution. According to LeChatelier' s Principle, the precipitation of a product from the reaction solution drives the equilibrium toward the right such is the case in the reaction described here R-Cl Na RI NaCl (s) R-Br Na RI-NaBr (s) A reagent composed of silver nitrate dissolved in ethanol is useful in classifying alkyl halides according to their reactivity in an SN1 reaction. Nitrate ion is a poor nucleophile and ethanol is a moderately powerful ionizing solvent. The silver ion, because of its ability to coordinate the leaving halide ion to form a silver halide precipitate ,greatly assists the ionization of the alkyl halide. Again a precipitate as one of the reaction products also enhances the reaction C2H5OH R OC2H5 CI Ag AgCl (s) Page 1 of 5

The rate-determining step also involves the nucleophile. What is the nucleophile in the given SN2 reactions? Why does this nucleophile favor an SN2 reaction?

CHEM 2261 Chapter : Notes2261Chapter6

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