Chemistry 242 Experiment #2
Nucleophillic Substitution Reactions
Prior Reading: Bruice, chapter 8
http://www.chemguide.co.uk/mechanisms/nucsubmenu.html
http://www.chem.ucalgary.ca/courses/351/Carey/Ch08/ch8-0.html
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The goal of this experiment is to determine how the structure and nature of the electrophile affects the rate
in each of these mechanisms. For example, the rate of an SN1 reaction depends on the carbocation stability
and you know that the trend of stability is tertiary > secondary > primary > methyl. You also know that the
rate of the SN2 reaction is sensitive to steric hindrance. Do these trends show up in your data? What else
does your data show?
The alkyl halides to be tested are:
2-bromopropane
1-chlorobutane
2-bromobutane
2-chlorobutane
2-chloro-2-methylpropane
iodoethane
For the PreLab, you only have to do 2 balanced eqns, one for the SN1 rxn and one for SN2 rxn (any
reactant).
Procedure
Work in pairs, in the hood. One person will mix the reactants and the partner will record data and
observations. Before beginning you should give some thought to organizing a table for your work. Draw
the molecular structure for each alkyl halide.
All test tubes should be clean and dry. If you have to wash them they should be rinsed with a small amount
of the reaction solvent. A designated waste container will be provided for the reaction solutions and rinses.
Set up a warm water bath (~50 degrees) in the hood using a 100 mL beaker on a hotplate set to low.
To follow the rates, it may be easier to react only four test tubes at a time.
SN2 reactions using NaI/acetone:
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This reagent provides an excellent nucleophile, I , and a nonpolar solvent, which favors a bimolecular (SN2)
mechanism. If a reaction occurs, the iodide substitutes for the chloride or bromide producing acetoneinsoluble NaCl or NaBr. Hence the appearance of a white precipitate indicates a positive reaction.
Iodoethane should not give a positive reaction here (why not?).
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Assemble and label six clean, dry test tubes. Add to each the different alkyl halides (0.1 mL). To each test
tube add 1 mL of a 1 M solution of NaI in acetone and mix thoroughly. For each tube, note how long after
the addition of the NaI solution the first trace of precipitate appears. Watch the tubes to a maximum time of
10 minutes after the addition. If a precipitate does not appear, heat the tube in a warm water bath for an
additional 10 minutes and watch for any change. Cover the tubes with aluminum foil. Be careful not to
allow the acetone to evaporate.
SN1 reactions using AgNO3/ethanol:
This reagent contains a polar solvent and a coordinating silver cation, which supports ionization of the
halide from the alkyl halide, facilitating a unimolecular (SN1) pathway. The formation of a precipitate
indicates a positive reaction between the alkyl halide and ethanolic AgNO 3 due to the formation of ethanolinsoluble AgCl or AgBr. In other words, the leaving group leaves, and is immediately precipitated out with
Ag ion.
Assemble and label six clean, dry test tubes. To each add a different alkyl halide (0.1 mL). To each test
tube add 1 mL of a 0.1 M solution of AgNO3 in ethanol and mix thoroughly. (Avoid skin contact! Silver
salts produce long-lived brown stain on your skin). For each tube, note how long after the addition of the
AgNO3 solution the first trace of precipitate appears. Watch the tubes to a maximum time of 10 minutes
after the addition. If no change occurs after that time, place the tube in the warm water bath and watch for
any precipitate formation over the next 10 minutes. Cover the tubes with aluminum foil. Be careful not to
allow the ethanol to evaporate. When you are finished, put the empty test tubes in the hood.
Be careful to avoid contact and minimize exposure to the alkyl halides. Also avoid getting the silver
nitrate solution on your skin. All used solutions to the Halogenated Waste Jar.
Post Lab Questions
1. Based on your results, predict how the reaction rates of the following compounds
will compare in an SN1 rxn to those that were tested. The best way to answer this
is to rank all thirteen molecules from most reactive (#1) to least reactive (#13).
There can be ties. For instance, if 2 or more don’t react in SN1, they would be
tied for last.
1-bromobutane
1-chloro-2-methylpropane
2-bromo-2-methylpropane
1-chloro-2-methyl-1-propene
2-iodo-2-methylpropane
benzylbromide (C7H7Br)
bromobenzene (C6H5Br)
2. From the question one list, predict the fastest SN1 reactant and the fastest SN2
reactant. Then sketch the NMR of the two products. If you predict that the same
product will be generated for both the fastest SN2 and SN1, sketch the NMR of
the second fastest SN2. In other words, no matter what your prediction, you have
two NMR sketches. Clearly indicate approximate chemical shift, integration,
splitting pattern, and which H’s on the molecule correspond with which peaks on
the NMR.
Good experimental procedures can be found at
http://www.bradley.edu/las/chm/Course/250/Substitution.pdf
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