Relative Reactivity of Alkyl Halides

2270 words 10 pages
Relative Reactivity of Alkyl Halides

Introduction
Nucleophilic substitution of alkyl halides can proceed by two different mechanisms – the SN2 and the SN1. The purpose of the experiment was to identify the effects that the alkyl group and the halide-leaving group have on the rates of SN1 reactions, and the effect that the solvent has on the rates of SN1 and SN2 reactions. The SN1 mechanism is a two-step nucleophilic substitution, or unimolecular displacement. In the first step of the mechanism, the carbon-halogen bond breaks and the halide ion leaving group leaves in a slow, rate-determining step to form a carbocation intermediate. The carbocation intermediate is then immediately detained by the weak nucleophile in a fast, second
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The exact time of the first sign of cloudiness in each test tube was noted. The exact time of the first sign of precipitation was noted. If no reaction occurred within five minutes, the test tube was placed in a warm water bath. No color change was observed, only a change in cloudiness of the solutions.
For part C, four clean test tubes were acquired and numbered 1-4. 1 mL of 1:1 methanol and water was added to the first test tube; 1mL of 1:1 mixture of ethanol/water was added to the second test tube; 1mL of 1:1 mixture of 1-propanol/water was added the the third test tube; and 1 mL of 1:1 mixture of acetone/water was added to the fourth test tube. Three drops of .5M NaOH and three drops of 1% phenolphthalein were added to each test tube. Three drops of 2-bromo-2-methylpropane were added to each test tube, and the exact time that the first drop was added to each tube was noted. The tubes were swirled, and the disappearance of the pink color was observed. The final color of each solution was yellow.
Results
Table 2: Reaction times pertaining to SN2 reactions, SN1 reactions, and the solvent effects on SN1 reactions. Part A (SN2 reaction with NaI) | Time in seconds | 2-bromo-2-methylpropane | 97 seconds | 2-bromobutane | 95 seconds | 1-bromobutane | 71 seconds | 1-chlorobutane | 86.7 seconds | Part B (SN1 reaction with AgNO3 in ethanol) | Time in seconds | 2-bromo-2-methylpropane | 34.6 seconds | 2-bromobutane | 49 seconds |

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