Organic Chemistry SN1 SN2 Reaction With Alkyl Halide
DATA SECTION
1. SN2 Conditions – Reaction with NaCN.
In this part of the experiment, different alkyl halides were treated with LiCN according to the general reaction shown below. The reaction was carried out in DMSO-d6 at 25°C, and after 10 min, a 1H NMR spectrum was taken and the peaks integrated.
(You may also find this reaction in Remote Learning Topic 5.ppt on D2L. Look for the red box.)
The spectra are shown on pages 1-2 in the data pages file. Note that the spectra are of the reaction mixture (without purification), so we should expect to see peaks from both the reactant – alkyl halide and the product – alkyl nitrile.
For the reactions with 1-halobutanes and 2-halobutanes, only the signals for the CH2X (from reactant) and CH2CN (from product) are shown, while for the tert-butyl halides, the methyls (from reactant and product, respectively) are shown.
Notice that So for each set of three reactions (Trials 1-3, Trials 4-6, and Trials 7-9), the same product is obtained despite which halogen was used.You should first identify the product structure, then find the product’s signal CH2CN, as that will be the signal that is at the same ppm in each of the 3 spectra.
The red decimal numbers above each peak is the corresponding integration.
For trial 10, you need look back in 1-9, and find the trend of the relative chemical shifts of CH2X and CH2CN. Make an “educated guess” of which peak is from the reactant and which from the product.
Here are your tasks:
1. Determine the percent conversion for the reaction, using the formula below (“S. mat.” Stands for “reactant”):
2. Calculate the percent conversion for each reaction, and enter it in Table 1. Summary of results 1-10 below.
3. Rank the reactivity based on the calculated percent conversion – highest percent conversion gets number 1 reactivity, whereas lowest percent conversion gets number 10 reactivity. Place your ranking in the last column of table 1.
4. complete the remaining columns in the table.
2. SN1 Conditions – Reaction with CH3OH.
In this part of the experiment, different alkyl halides were treated with CH3OH according to the general reaction shown below. The reaction was carried out in CH3OH at 25°C, and after 10min, a portion of the reaction mixture was analyzed by GC.
The chromatograms are shown on pages 4-5 in the data pages file.
Same scenario, notice that from the reaction above, the same product is obtained despite what type of alkyl halide was used for each set (Trials 20-22, Trials 23-25, and Trials 26-28). You should first identify the product structure, then the product signal, as that will be the signal that is at the same ppm retention time in the 3 chromatograms of the same set of trials (note that some peaks could be really, really tiny….).
Beside each chromatogram, a table of peak retention times and areas (aka integrations) are shown.
For trial 29, you need look back in 22-28, and find the trend of the relative retention times of RX and ROCH3. Make an “educated guess” of which peak is from the reactant and which from the product.
Here are your tasks:
1. Determine the percent conversion for the reaction, using the formula below (“S. mat.” Stands for “reactant”):
2. Calculate the percent conversion for each reaction, and enter it in Table 2. Summary of results 20-29 below.
3. Rank the reactivity based on the calculated percent conversion – highest percent conversion gets number 1 reactivity, whereas lowest percent conversion gets number 10 reactivity. Place your ranking in the last column of table 2.
4. Complete the remaining columns in the table.
3. Role of Nucleophile #1 – Reaction of 1-chlorobutane.
In this part of the experiment, 1-chlorobutane was treated with different nucleophiles according to the general reaction shown below. The reaction was carried out in acetone (similar to DMSO) at 25°C.
CH3CH2CH2Cl
+
Nu- Na+
CH
3
CH
2
CH
2
CH
2
Nu
+
Na+ Cl-
Since the byproduct NaCl is not soluble in acetone, a precipitate will form as the reaction occurs.
Here are your tasks:
Given below is a table that gives the time until a precipitate was observed, and from this data, rank the relative reactivity of nucleophiles (shortest time means fastest reaction, which gets number 1 reactivity, whereas longest time means slowest reaction, which gets number 10 reactivity).
| Nucleophile (Nu) | Time until precipitate observed | Relative reactivity |
| NaOCH3 | 0.52 sec | |
| HOCH3 | none observed | |
| NaSCH3 | 0.15 sec | |
| HSCH3 | 4.35 min |
4. Role of Nucleophile #2 – Reaction of tert-butyl p-nitrophenyl ether.
In this part of the experiment, tert-butyl p-nitrophenyl ether was treated with different nucleophiles according to the general reaction shown below. The reaction was carried out in ethanol (similar to CH3OH) at 25°C.
The byproduct -OC6H4NO2 has an intense yellow color, while the starting materials are colorless, so the appearance of a yellow color indicates the reaction is occurring.
Here are your tasks:
Given below is a table that gives the observed results, and from this data, rank the relative reactivity of nucleophiles. (shortest time means fastest reaction, which gets number 1 reactivity, whereas longest time means slowest reaction, which gets number 10 reactivity). Note that we could have “ties”.
| Nucleophile (Nu) | Data observed | Relative reactivity |
| NaOCH3 | Yellow after 0.52 sec | |
| HOCH3 | Very bright yellow immediately | |
| NaSCH3 | Yellow after 0.52 sec | |
| HSCH3 | Faint bright yellow immediately |
Table 1. Summary of results 1-10
|
Trial |
Alkyl Halide |
Line Structure of
Compound |
Type of Alkyl Halide
(1°, 2°, or 3°) |
Percent Conversion | Relative
Reactivity |
| 1 | 1-chlorobutane | ||||
| 2 | 1-bromobutane | ||||
| 3 |
1-iodobutane
|
||||
| 4 |
2-chlorobutane
|
||||
| 5 | 2-bromobutane | ||||
| 6 | 2-iodobutane | ||||
| 7 | tert-butyl chloride | ||||
| 8 | tert-butyl bromide | ||||
| 9 | tert-butyl iodide | ||||
| 10 | Chlorocyclobutane |
Table 2. Summary of results 20-29
|
Trial |
Alkyl Halide |
Line Structure of
Compound |
Type of Alkyl Halide
(1°, 2°, or 3°) |
Percent Conversion | Relative
Reactivity |
| 20 | 1-chlorobutane | ||||
| 21 | 1-bromobutane | ||||
| 22 |
1-iodobutane
|
||||
| 23 |
2-chlorobutane
|
||||
| 24 | 2-bromobutane | ||||
| 25 | 2-iodobutane | ||||
| 26 | tert-butyl chloride | ||||
| 27 | tert-butyl bromide | ||||
| 28 | tert-butyl iodide | ||||
| 29 | allyl chloride |
DATA ANALYSIS
SN2 Reactions.
4. Answer each of the following questions completely but briefly. Be sure to use your data and to note any discrepancies in your data.
a. Looking at trials 1, 4, and 7, what conclusion can you draw about the type of alkyl chloride (1o, 2o, or 3o) employed and the rate at which the SN2 reaction occurs? Explain why this might be.
b. Looking at trials 2, 5, and 8, is the trend the same or different with the alkyl bromides (so compare with trials 1, 4, 7, in terms of the halide type)? Also look at trials 3, 5, and 9 – is the trend the same or different with the alkyl iodides (again, in terms of the halide type, please)? Does these results make sense?
c. Comparing the trend in trials (1 to 2 to 3), does the type of halogen have an effect on the reaction? If so, what is the trend? Do you see the same trend in comparing trials 4 to 5 to 6 (in terms of the halogen type)? How about trials 7 to 8 to 9 (in terms of the halogen type)? Explain the effect, or lack of effect, of the alkyl halide.
d. Comparing trials 4 and 10, is there a difference in the halide type? And is there a difference in the rates? If there is, explain why there is a difference in the rate. (Hint: think about the mechanism and molecular structures of 4 and 10)
e. Compare the chemical shift of the signal for the alkyl halides in trials 1, 2, and 3 (from the data file). Is there a trend? If so, what is it and does it make sense?
SN1 Reactions.
5. Answer each of the following questions completely but briefly. Be sure to use your data and to note any discrepancies in your data.
a. Looking at trials 20, 23, and 26 what conclusion can you draw about the type of alkyl chloride (1o, 2o, or 3o) employed and the rate at which the SN1 reaction occurs? Explain what conclusion you can draw about the stability of carbocations from this data.
f. Looking at trials 21, 24, and 27, is the trend the same or different with the alkyl bromides (so compare with trials 1, 4, 7, in terms of the halide type)? Also look at trials 22, 25, and 28 – is the trend the same or different with the alkyl iodides (again, in terms of the halide type, please) (again, in terms of the halide type, please)? Does these results make sense?
g. Comparing the trend in trials (20 to 21 to 22), does the type of halogen have an effect on the reaction? If so, what is the trend? Do you see the same trend in comparing trials 23 to 24 to 25 (in terms of the halogen type)? How about trials 26 to 27 to 28 (in terms of the halogen type)? Explain the effect, or lack of effect, of the alkyl halide.
h. Comparing trials 21 and 29, is there a difference in the halide type? And is there a difference in the rates? If there is, explain why there is a difference in the rate. (Hint: think about the mechanism and the intermediate structures)
i. Compare the retention times in trials 21, 22, and 23. Is there a trend (in terms of halogen), and if so, can you explain it?
j. Compare the retention times in trials 21, 24, and 27. Is there a trend (in terms of halide type), and if so, can you explain it?
Comparison of SN1 and SN2 Reactions.
6. Is the trend in the reactivity between leaving groups (Br vs Cl) the same or different between the SN1 and SN2 experiments? Does this make logical sense to you? Explain.
7. a. Is the trend in the reactivity with different degrees of substitution (1°, 2°, and 3°) the same or different between the SN1 and SN2 experiments? Does this make logical sense to you? Explain.
b. Based on your answer to 7a, for which type of alkyl halide (1°, 2°, or 3°) will it be the most difficult to control the type of substitution reaction (SN1 and SN2) it undergoes?
Effect of the Nucleophile.
8. Explain the relationship between the strength of the nucleophile (see also in RLT 5.ppt on D2L) and the reactivity based on the results in the table in part 3 Role of Nucleophile #1 – Reaction of 1-chlorobutane.
9. Explain the relationship between the strength of the nucleophile (see also in RLT 5.ppt on D2L) and the reactivity based on the results in the table in part 4 Role of Nucleophile #2 – Reaction of tert-butyl p-nitrophenyl ether.
RX + LiCN RCN
RX + CH
3
OH ROCH
3
(CH
3
)
3
COC
6
H
4
NO
2
+ Nu (CH
3
)
3
CNu +
–
OC
6
H
4
NO
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