Download alcohols, alkyl halides, and nucleophilic substitutions

Table of Physical Properties for Investigation of SN1 and SN2 Reactions
M.W.
M.P.
B.P.
DEN.
SOL.
HAZARD
1-butanol
74.12
-89.5
117.6
0.81
Ac, Bz,EtOH,Et2O
1
2-butanol
74.12
-114.7
99.5
0.808
W,Al,eth
1
t-butylalcohol
74.12
25.6
82.41
0.781
W,Al
t-amylalcohol
88.15
-12
102
0.805
W,Al
1
Benzylalcohol
108.14
-15.19
204.7
1.042
W,eth Al,eth,petr eth
2
Phenol
99.11
40.85
182
1.071
W,Bz,Al,Chlor,eth,petr
2
1-chlorobutane
92.57
-123
78.5
0.8865
Al,eth
2
2-chlorobutane
92.57
-140
68
0.871
W,Al,eth
2
t-amylchloride
106.59
-74
83
0.865
W,Al,eth
2
3
0
3
0
3
0
1
3
0
1
0
3
0
3
0
3
0
3
0
3
t-butylchloride
92.57
-26.5
51.0
0.847
W,Al,eth
2
Benzylchloride 126.59
-43
179
1.10
Al, chl,eth
2
Chlorobenzene
-45
131
1.107
Al,Bz,Chl,eth
2
1-bromobutane 137.02
-112
101.3
1.269
Al, Eth
2
2-bromobutane 137.02
-112
91.2
1.253
Al,eth
2
t-butylbromide
-16.3
73.3
1.2125
Org.solvent
Benzylbromide 171.04
-3.9
198
1.4380
Et2O,eth
Bromobenzene
-31
156
1.491
Chl,Bz,Al,eth,petr
carbons
0
2
0
3
112.56
0
3
0
3
0
3
137.02
0
2
3
157.01
0
3
2
2
0
Investigation of SN1 and SN2 Reactions
If a positive result does not occur in 1 min, continue timing but start a test on a new
compound (i. e. do experiments in parallel).
PART A: REACTIVITY OF BROMIDES TOWARD SODIUM IODIDE.
Sodium Iodide in acetone. In this part of the experiment you will test the reactivity of
several alkyl halides under SN2 conditions. In acetone solution, other alkyl halides can be
converted to alkyl iodides. Although we might expect such a reaction to be reversible, it can be
driven forward by using anhydrous acetone as the solvent. Sodium iodide is soluble in this
solvent, whereas sodium chloride and sodium bromide are not. If a reaction occurs, a precipitate
of sodium bromide or sodium chloride will form.
Directions: Use scrupulously dry test tubes for this experiment. Add 2 drops of the liquid to be
tested to about 2 mL of a 15% solution of sodium iodide in anhydrous acetone. Mix the contents
and note the time required for a precipitate to form up to 15 minutes. Test each of the following
organic bromides: 1-bromobutane, 2-bromobutane, t-butyl bromide, benzyl bromide, and
bromobenzene. Report your results in tabular form.
I-
Br
I
*
Br
I
Br-
PART B: REACTION OF ALCOHOLS WITH LUCAS REAGENT
Lucas's Test: Of the various methods available for preparing alkyl halides, the most
common is replacement of the hydroxyl group of an alcohol. The ease with which this can be
done depends on the structure of the alkyl group and on the mechanism of the reaction. In this
experiment you will test the relative ease of conversions of several alcohols into alkyl chlorides
with Lucas reagent (HCI + ZnCl2). The reaction may occur by one of two mechanisms, SN1 or
SN2, depending on the structure of the R group. In either mechanism, the first step is a rapid
protonation of the alcohol to form an oxonium ion. What happens next depends on the nature of
R. If R is a group that readily forms a carbocation, then the slow, rate-determining step will be
the loss of a water molecule from the oxonium ion. The carbocation then reacts rapidly with a
halide ion to form the alkyl halide.
This mechanism, called SN1
ZnCl2
(substitution, nuceleophilic,
H2O R
+ ZnCl3
HO R
+ H Cl
unimolecular in the rate-determining
step), is followed when R is a tertiary
H2O R
H2O + R
alkyl group and may also be involved
when R is a secondary group or when
R can form a resonance-stabilized
ZnCl3- + R
Cl R + ZnCl2
carbocation, such as an allyl or benzyl
ion. If R is a primary alkyl group,
then substitution occurs by an SN2 mechanism and is usually not observed with Lucas reagent at
room temperature, but may be seen if the solution is heated.
Directions: Place about 2 mL of Lucas reagent in each of five test tubes (Note: Lucas reagent
is a contact and vapor hazard. Avoid contact and do not breathe it’s vapors). Test each
alcohol by adding about 5 drops of the alcohol to the reagent. “Thump” the tube and note the
length of time it takes for the mixture to become cloudy or separate into two layers (a positive
test). Test 1-butanol, 2-butanol, t-butyl alcohol, t-amyl alcohol, benzyl alcohol, and phenol. If
no reaction is observed after one hour, warm the test tube in bath of boiling water for a few
minutes and allow to cool to room temperature. Report the results in tabular form.
PART C: REACTIVITY OF HALIDES TOWARD SILVER NITRATE.
Silver nitrate in ethanol. In this part of the experiment you will test the reactivity of several
alkyl halides under SN1 reaction conditions. Organic halides may react with ethanol to form
ethyl ethers. If the ethanol contains silver ion, the reaction is speeded up because the silver ion
acts as an electrophile toward the halogen and helps to break the carbon-halogen bond. Alkyl
chlorides yield a discernible silver chloride precipitate, which is insoluble in ethanol and thus
provides a signal that a reaction has occurred.
C Cl + AgNO3
R
+ AgCl(s) + NO3
R
R
+ HOCH2CH3
R C OCH2CH3
R H
C OCH2CH3
C OCH2CH3 + H
H
Directions: Add 2 drops of the organic chloride to be tested to 2 mL of 2% ethanolic silver
nitrate solution (Avoid skin contact with the silver nitrate solution, as it can form a dark,
hard to remove stain.), “Thump” to mix and record the time required for a silver chloride
precipitate to form. Test each of the following chlorides: 1-chlorobutane, 2-chlorobutane, t-butyl
chloride, t-amyl chloride, benzyl chloride, and chlorobenzene. Report your results in tabular
form.
Use the following type of table for reporting your observations in parts A, B, and C.
R group
Benzyl
n-butyl
2-butyl
t-butyl
t-amyl
phenyl
OBSERVED REACTION TIMES
Time for reaction to be observed
HCl/ZnCl2/water
AgNO3/ethanol
NaI/acetone
1. Arrange the alcohols tested in order of decreasing experimental reactivity toward Lucas
reagent. Consider the aliphatic alcohols first and then consider the aromatic compounds
separately. What structural change correlates with reactivity? Assuming this to be an
SN1 reaction (see scheme in part B), and that the reaction is faster for more stable cation
intermediates, indicate the order of stability of all the carbon cations formed from each
alcohol. Examine the cations and explain what makes some cations more stable than
others. Hint: Draw structures of all carbon cations.
2. Arrange the chlorides tested in order of decreasing experimental reactivity toward silver
ion. Consider the aromatic compounds separately. Assuming this to be an SN1 reaction,
how do these results compare with that observed in the Lucas test.
3. Arrange the bromides tested in order of decreasing experimental reactivity toward iodide
ion. Consider the aromatic compounds separately. Does the reactivity follow the same
order as before? What structural change correlates with reactivity? Assuming this to be
an SN2 reaction (see scheme in Part A), the lower the energy of the transition state the
faster the reaction. Draw the transition state for each compound and suggest a reason for
the observed order of reactivity.
Dr. Mohammad A. R. Ismaiel / College of Science for Women
Babylon University