KinPrtIIfall04

KinPrtIIfall04 - A Kinetic Study of a Solvolysis Reaction...

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A Kinetic Study of a Solvolysis Reaction Part II (by Jyotsna Pradhan Ph.D. updated Oct. 1998) In today’s experiment, we shall investigate some effects on the solvolysis of t-butyl chloride in water-acetone solvent, a representative example of a simple but important class of reactions in organic chemistry. The major product of the solvolysis of t-butyl chloride in water-acetone is t- butyl alcohol, with a small amount of isobutylene being formed as a by-product. The overall reactions involved are represented as follows: (1) (CH 3 ) 3 C-Cl + 2H 2 O (CH 3 ) 3 C-OH + H 3 O + + Cl - (2) (CH 3 ) 3 C-Cl + H 2 O (CH 3 ) 2 C=CH 2 + H 3 O + + Cl - The first of these is a nucleophilic substitution of OH for Cl in the t-butyl chloride, the second an elimination of the elements of hydrogen chloride. You will observe that hydronium ion is formed as a product in both reactions. This will enable you to follow the rate of the combined reactions by noting the time required for neutralization of a fixed amount of sodium hydroxide added to the reaction mixture. On the basis of a vast array of evidence, both experimental and theoretical, chemists have proposed two general types of mechanisms for nucleophilic substitution and elimination reactions of this type. In the first of these, the initial step is a slow solvent-aided ionization of the halide into a carbocation and chloride ion: (3) (CH 3 ) 3 C-Cl (CH 3 ) 3 C + + Cl - (slow) This is followed by rapid reaction of the carbocation with water, to form the corresponding alcohol in the case of the substitution reaction, and isobutylene in the case of the elimination reaction. For t-butyl carbocation these two reactions are as follows: (4) (CH 3 ) 3 C + + 2H 2 O (CH 3 ) 3 C-OH + H 3 O + (fast) (5) (CH 3 ) 3 C + + H 2 O (CH 3 ) 2 C=CH 2 + H 3 O + (fast) It is a well-established principle of kinetics that if a reaction goes through several steps and one of these is very much slower than the others, then the rate of the slowest step represents the overall rate of the reaction. The slowest single step, whose rate determines the overall rate of a stepwise reaction, is called the rate-determining step. Thus, in this first general mechanism proposed for nucleophilic substitution and elimination reactions in alkyl halides, equation (3) represents the rate-determining step for both the nucleophilic substitution and the elimination reaction, as applied to t-butyl chloride. Any reaction in which the rate-determining step involves the making or breaking of a covalent bond in only one particle is designated a unimolecular reaction. In both the nucleophilic substitution reaction and the elimination reaction only a single particle, a molecule of the halide, is undergoing a change in covalence in the rate-determining step (3). Hence, both reactions are said to be unimolecular reactions. A nucleophilic substitution reaction of this type is called S
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KinPrtIIfall04 - A Kinetic Study of a Solvolysis Reaction...

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