Lab6 - Lab#6 1-Bromobutane from 1-Butanol Chen Zhou...

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Unformatted text preview: Lab #6 1-Bromobutane from 1-Butanol Chen Zhou February 27 th , 2008 Purpose: To carry out simple kinetic studies on S N 1 and S N 2 displacements. Introduction: S N 1 is another type of nucleophilic substitution. Unlike S N 2 reactions, the rate determining step (r.d.s) is unimolecular, as it only depends on the electrophile. S N 1 reaction also involves a carbocation, which is a positively charged stable intermediate after the departure of a leaving group. Its gets “attacked” by the nucleophile to form an cation. A present reaction mixture then acts as a base and deprotonates the cation to finish the reaction. One example of this reaction is the hydrolysis of tert-butyl chloride. The balanced chemical equation is below, followed by the mechanism and a general reaction coordinate. (CH 3 ) 3 C-Cl + H 2 O (CH 3 ) 3 C-OH 2 (+) + H 2 O + Cl- (CH 3 ) 3 C-OH + H 2 O + HCl There are three major factors that actually affect S N 1 reactions. The first is the leaving group (LG). Similar to S N 2 reactions, without a good leaving group such as Br, I, Cl and weak bases, the reaction will not proceed. The second factor is the -carbon ( C). Unlike S α α N 2 reactions, the best C is one which is α resonance stabilized or a 3° C. This is because the charge from the carbocation will be α delocalized and the structure will be stabilized; this is known as hyperconjugation. Having a more stable C will result a lower transition state, and therefore a faster reaction. α The last factor is the solvent. As opposite to S N 2 reaction, the best solvent would be polar and protic. It would form dipole-dipole interaction with the carbocation to stabilize it, at the same time, forming hydrogen bond with the leaving group, stabilizing it as it leaves. Nonpolar, aprotic solvents would have the opposite effect and it would raise the transition state of the rate determining step, resulting a slower reaction. Unlike S N 2 reactions, the rate of formation of product in S N 1 reaction only depends on the concentration of the substrate, as this is a first order reaction. This indicates the more substrate present, the greater the amount will react at a given time. However, the percentage of material that reacts at any given time will be the same. As the rate is expressed as dP/dt = k [substrate], the rate constant k can be determined mathematically through manipulation of equations to find the slope of a linear line. Another method, allows the calculation of k, by determining the time it takes for a reaction to go to a given extent of completion. Procedure and Observations: Experiment A: SOLUTION # 1 Trial # 1 Trial # 2 0.10 M t-BuCl in Acetone (mL) 3.0 3.0 SOLUTION # 2 Trial # 1 Trial # 2 0.01 M NaOH (mL) 3.0 3.0 Distilled Water (mL) 4.0 4.0 Bromothymol Blue (drops) 3 3 Water Bath Temperature (°C) 22 22 Addit ion of Solution # 1 to Solution # 2 Time for Color Change (s) Trial # 1 42 Trial # 2 43- Four burettes were set up on the bench, with Burette 1 filled with 0.1M Four burettes were set up on the bench, with Burette 1 filled with 0....
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Lab6 - Lab#6 1-Bromobutane from 1-Butanol Chen Zhou...

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