Willem Elbers. October 9, 2015

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1 S N 1 and S N 2 reactivity of 3 alkyl bromides Willem Elbers ctober 9, Abstract n this experiment, we investigate the relative reactivities of three alkyl bromides with increasing steric bulk. We react the primary halide 1bromobutane, the secondary halide 2bromobutane, and the tertiary halide 2bromo2methylpropane with a solution of sodium iodide in and a solution of silver nitrate in ethanol. We wish to determine the mechanism behind these reactions. Results suggest that the reactions with Na follow the S N 2 mechanism, whereas the reactions with follow the S N 1 mechanism. 2 Theory Due to the electronegativity of bromine, the alkyl bromides contain an electrophilic carbon atom. Nucleophilic compounds, such as iodide and ethanol, may therefore attack the carbon atom, thereby replacing the bromine. The formation of a white precipitate (Na or Ag) indicates whether the reaction has occurred. This process of nucleophilic substitution can occur along two pathways: unimolecular (S N 1) and bimolecular (S N 2) nucleophilic substitution. n the S N 2 reaction, the nucleophilic attack and the breaking of the C bond occurs in a single step. See the transition state of the reaction of 2bromobutane with Na in figure 1. The process is therefore characterised by secondorder kinetics. [3, ] The S N 1 reaction occurs in three steps. First, the C bond undergoes heterolytic cleavage, forming a carbocation. Because opposite charges are separated, this step is slow and ratedetermining. Hence, the S N 1 process is characterised by firstorder kinetics. n the second step, the carbocation is attacked by the nucleophile, which is subsequently deprotonated in the final step. [3, ] See the reaction mechanism for 2bromobutane with in figure 2. 1

2 A number of structural factors determines which of the two pathways dominates. First of all, the presence of alkyl groups around the electrophilic carbon atom can hinder the nucleophile through steric repulsion. Because of this, the S N 2 reaction occurs much faster for primary bromides than for secondary bromides and not at all for tertiary bromides. n the contrary, the presence of alkyl groups stabilises the carbocation through hyperconjugacy, increasing S N 1 reactivity. ndeed, primary halides do not undergo the S N 1 reaction because the energy of the carbocation, which features little hyperconjugacy, is prohibitively high. A second factor is the polarity of the solvent. The first step of the S N 1 process involves separating opposite charges, which is eased by the presence of a polar solvent. n particular, S N 1 reactivity in protic solvents (ethanol) is significantly greater than in aprotic solvents (). For the S N 2 process, the effect is the opposite, because protic solvent molecules encapsulate the nucleophile, weakening its attack on the electrophile. This is not a factor in the S N 1 process, because the nucleophilic attack is not rate determining. This is also why the strength of the nucleophile does not influence the kinetics of the S N 1 process, unlike that of the S N 2 process. [3, , ] These considerations imply that the S N 2 reaction will dominate the S N 1 reaction for the alkyl bromide/na/ system, because iodide is a good nucleophile (which increases S N 2 reactivity), and is aprotic (decreasing S N 1 reactivity). See figure 3 for the reactions. n the other hand, for the alkyl bromide/ /ethanol system, the S N 1 reaction will dominate, because N 3 is a poor nucleophile (decreasing S N 2 reactivity) and ethanol is a protic solvent (increasing S N 1 reactivity, as well as decreasing S N 2 reactivity). See figure 4 for the reactions. Finally, we observe that the tertiary bromide will likely react very slowly with Na, because the S N 2 reaction is significantly hindered by steric effects. Likewise, unless additional energy is supplied, the primary bromide will react very poorly in ethanol, because the energy of the unstable carbocation formed in the S N 1 reaction is prohibitively high. 2

3 Na C Na + CH Na HC Figure 1: Reaction between 2bromobutane and Na/. The S N 2 reaction is a one step process: in the transition state, the C bond is broken as the C bond is forming. Note the backside attack of the nucleophile. CH N 3 + CH Ag H + N 3 HC N 3 2 HC Figure 2: Reaction between 2bromobutane and /ethanol. n the first step, the C bond is cleaved in the presence of silver, which then coordinates with the leaving group N 3. n the second step, the nucleophile ethanol attacks the carbocation. Finally, the product is deprotonated by ethanol. 3

4 Na Na Na Na Na Na Figure 3: Reaction between the alkyl bromides and Na in. The third reaction is very slow. Ag N 3 H + Ag N 3 H + Ag N 3 H + Figure 4: Reactions between the alkyl bromides and /ethanol. The first reaction will go to completion only when energy is added. 4

5 3 Experimental nitially, two solution were prepared: 3 g of Na in 20 ml (solution A) and 0.34 g (0.1 M) in 20 ml ethanol (solution B). Test tubes labelled A1, A2, and A3 were each filled with 3 ml of solution A. Subsequently, 34 drops of the alkyl bromides were added: 1bromobutane (A1), 2bromobutane (A2), and 2bromo2methylpropane (A3) and carefully mixed with the solution. f no white precipitate had formed after 5 minutes, a heat gun was used to speed up the reaction. Next, test tubes labelled B1, B2, and B3 were each filled with 3 ml of solution B. Again, 34 drops of the alkyl bromides were added: 1 bromobutane (B1), 2bromobutane (B2), and 2bromo2methylpropane (B3) and mixed with the solution. f no precipitate formed within 5 minutes, the mixture was heated. nce the precipitate had formed, the ph of the solutions in B1, B2, and B3 was measured, as well as that of the initial solution B. [1, 2] 4 Results and discussion Within 3 minutes, a precipitate had formed in tube A1 containing the primary bromide 1bromobutane. After 5 minutes, no precipitate had formed in tubes A2 and A3, which had to be heated to induce the reaction. n tube B3, containing 2bromo2methylpropane, a white precipitate formed almost instantaneously. Tube B2, containing 2bromobutane, followed within the first minute. No precipitate formed in tube B1, until it was heated after 5 minutes. Due to time limitations, ph values were not measured, but another experimenter reported values of ph = 5 for solutions B and B1, ph = 4 for B2, and ph = 3 for B3. See the table below for an overview. Label Substrate Solution Duration ph A1 1bromobutane Na/ 3 minutes A2 2bromobutane Na/ >5 minutes A3 2bromo2methylpropane Na/ >5 minutes B /ethanol 5 B1 1bromobutane /ethanol >5 minutes 5 B2 2bromobutane /ethanol < 1 minute 4 B3 2bromo2methylpropane /ethanol instantaneously 3 Table 1: verview of experimental results. Duration indicates the time until a white precipitate (Na or Ag) has formed. The results clearly indicate that for the reactions with /ethanol 5

6 (solution B), the most reactive bromide is 2bromo2methylpropane, followed by 2bromobutane and finally 1bromobutane. This matches the theoretical prediction for S N 1 reactions, where increasing steric bulk improves reactivity due to hyperconjugacy. Moreover, the decrease in ph values in tubes B2 and B3 suggests that the deprotonation step of the S N 1 process has occurred (see figure 2). The negligible change in ph in tube B1 may be explained by the fact that the reaction occurred only slightly. Alternatively, the heat may still have induced an S N 2 reaction with the poor nucleophile N 3. For the reactions with Na/ (solution A) the picture is less clear, because only in tube A1 did precipitate form within the allotted 5 minutes. Although it is clear that 1bromobutane is the most reactive, we cannot clearly distinguish between 2bromobutane and 2bromo2methylpropane. Theory suggests that for the S N 2 process, where increasing steric hindrance decreases reactivity, 1bromobutane should be the most reactive, followed by 2bromobutane and finally 2bromo2methylpropane. This is consistent with our results. 5 Conclusion n this experiment, we have tested the reactivities of 3 alkyl bromides: 1 bromobutane, 2bromobutane and 2bromo2methylpropane with a solution of Na in and a solution of in ethanol. For the Na/ reactions, we found that 1bromobutane was the most reactive, although we could not distinguish between 2bromobutane and 2bromo2methylpropane. This is consistent with the secondorder nucleophilic substitution (S N 2) mechanism. For the /ethanol reactions, 2bromo2methylpropane was the most reactive, followed by 2bromobutane, and finally 1bromobutane. A measured decrease in ph values confirms that these reactions follow the firstorder S N 1 mechanism. References [1] Practical Manual, Molecules: Structure, Reactivity and Function, Faculty of Mathematics and Natural Sciences, University of Groningen, [2] J.W. Lehman. Multiscale perational rganic Chemistry: A Problemsolving Approach to the Laboratory Course. Prentice Hall, [3] K.P.C. Vollhardt and N.E. Schore. rganic Chemistry: Structure and Function. W. H. Freeman, New York, 6th edition,

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