Lab Conclusion: Alkyl Halide Nucleophilic Substitution Experiment

Lab Report: Alkyl Halide Nucleophilic Substitution Experiment

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Lab Conclusion: Alkyl Halide Nucleophilic Substitution Experiment

  1. The actual results correlated somewhat to the predicted reactivity of the alkyl halides. In the SN1 reactions, all products were formed. It was predicted that three of the alkyl halides would not precipitate, namely 1-chlorobutane (1), 1-bromobutane (2), and 1-chloro-2-methylpropene. While seven of the predictions were correct, these three were wrongly predicted mainly because it was initially thought that the carbocation stability would have more of an effect on reactivity, while it ended up just taking longer for the reaction to take place. These three halides theoretically formed primary carbocations which were thought to be unstable, but apparently they were stable enough. Also, 1-chloro-2-methylpropene might have been able to rearrange to form a secondary carbocation, which would explain the relatively short reaction time of 1:00 (…)
  1.  With the SN2 reactions, the predictions were mostly correct in that all reactions predicted to not take place did not, but 3 halides that were predicted to react did not as well. Those were namely 1-chlorobutane (1), 2-chlorobutane (3), and 1-chloro-2-methylpropane (8). Since the leaving group of all three of these is the chloride anion, it can be determined that possibly chloride was not a strong enough leaving group for the SN2 reactions. Since these were SN2 reactions, and all the bromide reactions predicted took place, no carbocation intermediates were possible.
  2. SN1: 1-chlorobutane reacted in 2 minutes, whereas 1-bromobutane reacted in 10 minutes. This makes sense because Cl is a better leaving group than Br in SN1 reactions, according to the textbook. 2-chlorobutane reacted in 7 minutes using the AgNO3/ethanol solvent. 2-bromobutane reacted in just 20 seconds. This conflicts with the previous statement since 2-chlorobutane should have reacted faster. This could be because with 2-chlorobutane and 2-bromobutane, Br is a larger anion than Cl, and could have (since it formed a secondary carbocation) been easier to “pull off”.
    SN2: 1-chlorobutane did not react at all, whereas 1-bromobutane did react in 1 minute. This agrees with theory, in that Br is a better leaving group than Cl, but it was not predicted that 1-chlorobutane would not react at all. Leaving groups must play a larger part in reactions than previously predicted. The same was obtained for 2-chlorobutane and 2-bromobutane, in that 2-chlorobutane did not react at all.

i.     SN1: Compounds 1, 3, and 5 reacted faster with less substitution of carbocations, which conflicts with the theory that more stable carbocations react faster. However, compounds 2, 4, and 6 did follow this theory, in that the more highly substituted carbocation intermediate reacted faster.
SN2: Compounds 1, 3, and 5 did not react. Compounds 2, 4, and 6 did react, excluding compound 6. This agrees with the theory that compounds that are less substituted by the leaving group should react faster, because compound 2 reacted faster than compound 4, and compound 6 did not react at all.

ii.     SN1: 1º alkylhalides in this case reacted faster when there was less steric hindrance, which agrees with the theory. However, there was not much of a difference noted. Compound 1 reacted in 2 minutes, and compound 8 reacted in 3 minutes, so experimentally it did not make much of a difference.
SN2: No reactions of compound 1 or 8 took place.

iii.     SN1: Using simple vs. complex 3º did make a large difference in reaction time, with compound 5 taking 9 minutes and compound 10 taking 20 minutes. This makes sense because the complex 3º had more carbons attached, and therefore was a larger molecule, which may have made a difference in reactivity. Also, the carbocation intermediate could not have formed an sp2 hybridization because of the other bonds, and was less stable, slowing the reaction.
SN2: No reaction took place for compound 5 or compound 10.

iv.     S­N1: Compound 7 (the allyl halide) reacted in 1 minute. Compound 1 (the corresponding saturated alkylhalide) reacted in 2 minutes. This makes sense because compound 7 reacted faster with the allyl group, which is more stable due to resonance, and are more reactive in SN1 reactions.
SN2: Compound 7 did react, whereas compound 1 did not. This makes sense because allyl halides are comparable to methyl groups in SN2 reactivity.

v.     SN1: Compound 9 (the vinyl halide) did react, which is suprising given that the manual stated that vinyl halides normally do not react in SN1 or SN2 reactions. Compound 8 reacted as well, but took a longer time. An intermediate rearrangement could have been formed in this case, which may have allowed the reaction to take place faster.
SN2: Neither reaction took place.

  1. SN1 reactions take place better with polar solvents because a polar, protic solvent will stabilize a carbocation intermediate, which increases the S­N1 reaction rate.
  2. Reaction temperature played a role in some reactions. Higher temperature increases thermal energy, and the reaction can take place faster. For reactions that take longer but do take place nonetheless, increasing the temperature can move the reaction along faster.