Neha Gandra
Nucleophilic Substitution Reactions
Introduction (Purpose):
It is possible to tell whether a given compound favors a SN1 reaction, SN2 reaction, neither, or both because of the differing factors that affect each of these mechanisms. The purpose of this lab was to use two reagent mixtures to test a variety of halogenated hydrocarbons, where one reagent favors SN2 reactivity and the other SN1 reactivity. Furthermore, we explained the results of our tests by rationalizing them in terms of the electronic and/or steric factors involved for each compound.
Procedure:
Procedure was followed as outlined in List, Adam. Chemistry 2221L Organic Chemistry Laboratory. Fall 2018.
Data:
Compound
NaI/acetone (Time in Minutes)
NaI/acetone (Notes)
AgNO3/Ethanol (Time in Minutes)
AgNO3/Ethanol (Notes)
bromobenzene
Immediately
White
NR

bromocyclopentane
Immediately
Yellowish
Immediately
Greenish white, foamy
bromocyclohexane
Immediately
White
Immediately
Cloudy
2-bromobutane
11
White
Immediately
Cloudy
2-chlorobutane
NR

NR

1-chloropropan-2-one
Immediately
Thick, yellowish
NR

1-chlorobutane
NR

NR

2-chloro-2-methylpropane
NR

Immediately
White, milky
benzyl chloride
Immediately
Cloudy Yellow
4 minutes
Cloudy
* Purple Fill = heated in water bath
Discussion/Conclusion:
In general, if a compound precipitated immediately or quickly in the NaI/acetone reagent, it reacted mainly via a SN2 mechanism because Iodide is a good nucleophile and acetone is a polar aprotic solvent. If Iodide displaces Bromide or Chloride, NaBr or NaCl will precipitate, and this is what we see. If a compound precipitated immediately or quickly in the AgNO3/Ethanol reagent, it reacted mainly via a SN1 mechanism because ethanol is a poor nucleophile and polar protic solvent. The silver ion coordinates with the halide ion in the alkyl halide and enhances carbocation formation. Here is an example scheme of a SN1 versus SN2 reaction (Image found at https://chemistry.stackexchange.com).

Below, I will discuss each individual halogenated hydrocarbon and explain why each one reacted via the mechanism it did.
Bromobenzene did not undergo a SN1 or SN2 reaction and therefore did not precipitate because structurally, its ring form does not facilitate a backside attack, which is necessary for a SN2 reaction, or the formation of a carbocation, which is a necessary for a SN1 reaction. Additionally, this compound is stable and its allylic/vinylic cannot act as good nucleophiles.
Bromocyclopentane undergoes both mechanisms, SN1 faster than SN2. The bromine in this compound is secondary and has more steric strain because it is a cyclic compound. More steric strain results in more difficulty on the nucleophile’s part to attack the leaving group from appropriate, opposite sides.
Bromocyclohexane undergoes a SN1 mechanism because excessive steric strain does not allow a nucleophilic attack from the opposite side of the leaving Bromine group. Thus a SN2 mechanism will not occur. A disubstituted carbon center and good leaving group make a SN1 mechanism possible.

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2-bromobutane undergoes both mechanisms, SN1 faster than SN2. The disubstituted carbon center allows for both of these mechanisms depending on what solvent is present. Bromine’s decently large diameter and low electronegativity makes it an efficient leaving group.
2-chlorobutane undergoes a SN1 mechanism. The disubstituted carbon center allows for both SN1 and SN2 mechanisms depending on what solvent is present but chlorine’s smaller diameter and higher electronegativity makes its efficiency as a leaving group slightly lower.
1-chloropropan-2-one favored a SN1 and SN2 reaction, SN2 much more prominent and faster than SN1. This is because of the compound’s mono-substituted carbon center. SN1 is possible but not probably because the carbocation may be stabilized due to resonance.
1-chlorobutane can favor a very slow SN1 and SN2 reaction, SN2 being slightly more likely because of the compound’s mono-substituted carbon center. SN1 occurs much slower because with this compound, a primary carbocation is formed. SN1 mechanisms don’t occur readily with primary carbocations.
2-chloro-2-methylpropane occurs via the SN1 mechanism. It cannot/won’t occur via the SN2 mechanism because of excessive steric strain.
Benzyl chloride can occur with a SN1 or SN2 mechanism, SN2 happening faster and more readily. This is because Benzyl chloride is a primary alkyl halide. Moreover, the primary carbocation that is formed due to chloride leaving is balanced by electrons in the ring.
My data did not necessarily represent all the ideal data as explained above for a multitude of probable reasons. I should have paid more attention to timing accurately. Sometimes, I got caught up doing multiple steps of the lab at once, heating some test tubes, while waiting for others to precipitate, etc. Thus, some of my data could have been inaccurate do to this error.
Sources:

Comparing the SN1 and SN2 Reactions


https://web.iit.edu/sites/web/files/departments/academic-affairs/academic-resource-center/pdfs/SN1_SN2.pdf
http://courses.chem.indiana.edu/c341/documents/c341s2010ch9ns.pdf

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