Relative Reactivity of Alkyl Halides

Relative Reactivity of Alkyl Halides Introduction Nucleophilic interchange of alkyl halides can proceed by two different mechanisms the SN2 and the SN1. The purpose of the experimentation was to discover the make that the alkyl congregation and the halide- divergence congregation live with on the range of SN1 chemical reactions, and the effect that the solvent has on the localizes of SN1 and SN2 reactions. The SN1 mechanism is a two-step nucleophilic substitution, or unimolecular displacement. In the rootage step of the mechanism, the carbon copy-halogen chemical bond breaks and the halide ion leaving group leaves in a slow, rate-determining step to form a carbocation liaise.The carbocation intermediate is then immediately detained by the weak nucleophile in a fast, hour step to give the mathematical crossing. A solution of ethyl inebriant with some fluid medal nitrate may be added provided the weak nucleophile the alcohol. If an SN1 reaction occurs, the alky l halide go away dissociate to form a carbocation, which will then react with the neutral spirits to form an ether. Since there is non a unassai science lable nucleophile present, the cleavage of the carbon-halogen bond is encouraged by the formation and precipitation of silver bromide.The halide ion will unite with a silver ion from the silver nitrate to form a silver halide precipitate, which will advise that a reaction has occurred. + AgBr + NO3- Figure 1 The SN1 mechanism of 2-bromo-2-methylpropane and silver nitrate. The nucleophile would render been ethanol while the silver nitrate would have disassociated to form a silver halide precipitate. The more stable the carbocation, the quicker the reaction. Therefore, SN1 reactions liking third substrates most, followed by secondary, and lastly base.Because the strength of the nucleophile is unimportant, an ionizing solvent is needed. Water is the best(p) solvent, followed by methanol, ethanol, propanol, and lastly acetone. In experiment two, the tertiary 2-bromo-2-methylpropane was the most successful reactant followed by the secondary 2-bromobutane, the primary 1-bromobutane, and the primary 1-chlorobutane. This order is determined by whether the molecule is primary, secondary, or tertiary. 2-bromo-2-methlypropane + AgNO3 + (CH3)2CO AgBr + ethyl-t-butylether + isobutylene Figure 2 The SN2 mechanism of 2-bromo-2-methlypropane with AgNO3 in (CH3)2CO.The SN2 reaction mechanism is a one-step, bimolecular displacement in which the bond-breaking and bond-making processes occur simultaneously. The SN2 reaction requires a strong nucleophile. The order of reactivity is the arctic of the SN1 reaction because the nucleophile essential attack from the back, and is favored with the least steric hindrance. The halide attached to a primary carbon is easier to attack from the back. In experiment one, the 1-chlorobutane was the most favored reactant followed by the primary 1-bromobutane, the secondary 2-bromobutan e, and the tertiary 2-bromo-2-methylpropane.This order is determined by whether the molecule is primary, secondary, or tertiary. SN2 reactions are particularly sensitive to steric factors, since they are greatly retarded by steric hindrance (crowding) at the site of reaction. In general, the order of reactivity of alkyl halides in SN2 reactions is methyl 1 2. The 3 alkyl halides are so crowded that they do not generally react by an SN2 mechanism. 1 1-chlorobutane and NaI-acetone 1-iodobutane + NaCl (precipitate) In general, weaker bases make better leaving groups.SN1 and SN2 reactions show the homogeneous archs, but SN1 is more sensitive. The reactants favored in the SN2 mechanism are the opposite of the SN1 reaction. the primary 1-chlorobutane was most favored, followed by the primary 1-bromobutane, the secondary 2-bromobutane, and tertiary 2-bromo-2-methylpropane. remit 1 Table of Reagents with molecular weight, density, melting point, and boiling point. Name molecular Wei ght (g/mol) Density (M/V) Melting point (C) Boiling point (C) 2-bromo-2-methylpropane 137. 02 g/mol 1. 22 g mL? 1 -16C 73C 2-bromobutane 137. 02 g/mol 1. 55 g mL? 1 -113C 91C 1-bromobutane 137. 02 g/mol 1. 2676 g mL? 1 -112C 99-103C 1-chlorobutane 92. 57 g mol? 1 0. 89 g/mL -123. 1C 78C NaI 149. 89 g/mol 3. 67 g/cm3 661C 1304C AgNO3 169. 87 g mol? 1 5. 35 g/cm3 212C 444C Methanol 32. 04 g mol? 1 0. 7918 g/cm3 -97. 6C 64. 7C Ethanol 46. 07 g mol? 1 0. 789 g/cm3 ? 114C 78. 37C Propanol 60. 10 g mol? 1 . 803 g/mL -126C 97-98C acetone 58. 08 g mol? 1 0. 791 g cm? 3 -93C 56-57C, NaOH 39. 9971 g mol-1 2. 13 g/cm3 318C 1388C Phenolphthalein 318. 2 g mol? 1 1. 277 g cm? 3 260 C NA Experimental For part A, four clean discharge pipings were acquired and numbered 1-4. Five drops of from all(prenominal) one of the following reagents were added to the screen out tubings in numerical order 2-bromo-2-methylpropane, 2-bromobutane, 1-bromobutane, 1-chlorobutane. Twenty drops of NaI in acet one were added to for separately one campaign tube, the time of the first drop was recorded, and the tubes were mixed. The direct time of the first sign of sloppiness in from separately one turn up 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 body of pee bath. No color change was observed, barely a change in cloudiness of the solutions. For part B, the test tubes were rinsed with ethanol and then the same amount of each alkyl halide was placed into each test tube. Twenty drops of silver nitrate in ethanol was added to each of the four test tubes, and the exact time that the first drop was added to each test tube was noted. 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 pee 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 11 methanol and water was added to the first test tube 1mL of 11 mixture of ethanol/water was added to the second test tube 1mL of 11 mixture of 1-propanol/water was added the the tercet test tube and 1 mL of 11 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-bromob utane 71 seconds 1-chlorobutane 86. 7 secondsPart B (SN1 reaction with AgNO3 in ethanol) Time in seconds 2-bromo-2-methylpropane 34. 6 seconds 2-bromobutane 49 seconds 1-bromobutane 80 seconds 1-chlorobutane 905 seconds Part C (Solvent effects on SN1 reaction with. 5M NaOH and phenolphthalein) Time in seconds Methanol/water 11 129 seconds Ethanol/water 11 97 seconds Propanol/water 11 93 seconds Acetone/water 11 90 seconds Discussion In experiment A, the results followed the trend hypothesized for SN1 reactions in the introduction. The primaries stainless first, followed by secondary, and lastly tertiary.The primary 1-bromobutane completed first, followed by the primary 1-chlorobutane. The secondary 2-bromobutane completed third, and the tertiary 2-bromo-2-methylpropane terminate last. The 1-bromobutane spotless forwards the 1-chlorobutane because bromine was a better leaving group than centiliter. This is because bromine is bigger and less electronegative than chlorine. Therefo re the bromine did not form as tight of bonds with carbon as chlorine, and could more easily leave the group. In experiment B, the results followed the trend hypothesized for SN2 reactions in the introduction.The tertiary finished first, followed by secondary, followed by the primaries. 1-bromo-1-methylpropane finished first because of its tertiary nature. The secondary 2-bromobutane finished second. The primary 1-bromobutane finished third, followed by primary 1-chlorobutane. The rate-limiting formation of the carbocation decided this pattern. The 1-bromobutane finished forrader the 1-chlorobutane because bromine was a better leaving group than chlorine. In experiment C, the results did not follow the trend hypothesized for the solvent effects on the SN1 reaction.Methanol should have finished first, then ethanol, then propanol, and lastly acetone. The results were the exact opposite acetone finished first, then propanol, then ethanol, and lastly methanol. The boiling points of the solutions were responsible for this result. The bottles containing the solutions must have been left open for a prolonged period of time before use to allow evapo symmetryn. The acetone, propanol, and ethanol must have been mainly water, and so finished earlier than methanol. The first error pertained to experiment C. The results were backwards, and had to have been due to the boiling points of the solutions.The solutions that evaporated the most before the experiment finished quickest because of the ratio of water to the specific solution. The error could have been corrected by make the solutions before lab. The second error pertained to experiment B. The experiment was performed twice because the test tubes were not floped with ethanol between uses. The use of water to wash the test tubes sped up the reactions because water was the best solvent for the SN1 reactions. The use of ethanol to wash the test tubes had little effect on the rate of the reactions because it was the th ird best solvent after water and methanol.Conclusion The purpose of the experiment was to identify the effects that the alkyl group and the halide leaving group had on the rates of SN1 and SN2 reactions, and the effect that the solvent had on the rates of SN1 reaction. The purpose was achieved because the tertiary alkyl group sped up the SN1 reaction most, while the primary alkyl group sped up the SN2 reaction most. The effect of the halide-leaving group was established because bromine was bigger and less electronegative than chlorine, which allowed bromine to leave the group sooner than chlorine due to the less strong bonds of bromine.The solvents effect on the rates of the SN1 reaction was due to the encouragement of the cleavage of the carbon-halogen bond, instead of the use of a strong nucleophile. An improvement for the experiment would have been to mix the solvents just before lab to reduce the amount of solvent that boiled off at means temperature. More accurate measurements such as 1 mL mensural in the graduated cylinder would have been more precise and consistent than the addition of a specific number of drops. References The SN2 Reaction. University of Texas Educational Website. 11 family 2012. Web. 1 March 2013. http//iverson. cm. utexas. edu/courses/old/310N/spring2008/ReactMoviesFl05%20/SN2text. html Questions 1. What would be the major product if 1-bromo-3-chloropropane was to react with NaI in acetone? Explain. 1-iodo-3-chloropropane would be the major product. This substitution reaction works best for primary alkyl halides. The bromide ion is a better leaving group than chloride ion. 2. In the reaction of 1-chlorobutane with CN-, the reaction rate is increased by the addition of catalytic amount of NaI. Explain the result. The I- ion replaces the Cl to form 1-iodobutane.The iodo group is a better leaving group than the chloro group and hence the reaction goes faster. 3. What would be the major product if 1,4-dibromo-4-methylpentane was allo wed to react with a. ) unmatchable equivalent of NaI in acetone? Iodide is a strong nucleophile but a weak base, so SN2 is the preferred reaction. Only the bromine on C1 is legal to undergo SN2, so it will be replaced by iodide. b. ) One equivalent of silver nitrate in ethanol? Silver ion tends to receive a halide ion and leave a carbocation, which means E1 and SN1. If theres only one equivalent, then the tertiary bromide on C4 is the one that will go.The resulting carbocation can give E1 products 5-Br-2-Me-2-pentene (major, trisubstituted) and 5-Br-2-Me-1-pentene (minor, disubstituted). SN1 product 5-bromo-2-ethoxy-2-methylpentane. 4. What causes the color change in part C? The formation of the hot after the addition of the base NaOH and an acid-base indicator may be visibly observed when the reaction has completed. As the reaction proceeds, for every molecule of alcohol produced, there is one molecule of acid produced as well. This means that as the reaction proceeds the acid begins to neutralize the base, once enough product and