Substitution vs. Elimination
Substitution vs. Elimination SN2 and E2 reactions both are favoured by a high concentration of strong nucleophile or base. Substitution occurs when the nucleophile attacks the carbon which has the leaving group. Elimination occurs when the base attacks a β-hydrogen. Industries that prefer one reaction over the other often need to control the reaction as to produce the favoured reaction.1 Mechanisms + OH–
+ H2O
+ CH3CH2I
+ H2O
+ CH3CH2I
Data Table 1: Chemical Quantities2 Name Formula
+ CH2=CH2
Ethyl iodide
C2H5I
Molecular weight g/mol 155.97
Ethyl Ether Ethanol 2-naphthol 2-ethoxynaphthalene
C4H10O CH3CH2OH C10H7O– C12H10O
74.12 46.04 143.00 172.22
Amount (g)
Amount (mL)
# moles
n/a
8.0x10-6 (800μl) 10 2 5 n/a
n/a
Table 2: Chemical and Physical Properties2 Name Formula M.P (°C)
B.P. (°C)
Ethyl iodide Ethyl Ether Ethanol 2-naphthol 2-ethoxynaphthalene
72 35 78 285 282
C2H5I C4H10O CH3CH2OH C10H7O– C12H10O
Melting point of 2-naphthol: 122°C
-108 -123 -114 122 37
Density (g/cm3) 1.95 0.71 1.59 1.22 1.06
Solubility
Safety
Slightly -corrosive 6.9g/100mL -flammable Miscible -irritant -negligible in water
Melting point of 2-ethoxynaphthalene: 37°C Melting point of product: 32-34°C Yield in grams: 0.220g Discussion This experiment is a synthesis by nucleophilic substitution (SN2) since the melting point of the product obtained was fairly close to that of 2-ethoxynaphthalene. Since it was slightly lower and not exact, this indicates slight traces of impurities in the obtained sample. Also the exact amount of 2-naphthol was never given so the percent yield cannot be calculated. SN2 reaction was favoured because the leaving group was attached to a 2° carbon, and a strong nucleophile, OH-, was used. Also the nucleophile was not bulky, allowing the backside attack of the nucleophile in the SN2 reaction to occur. A catalyst could also be used to speed up the SN2 reaction by creating a strong nucleophile. Although the SN2 reaction was favoured, there probably was an E2 side reaction (very minor), because a 2° alkyl halide and strong nucleophile yields a mixture of both reactions. If a higher temperature was maintained throughout the reaction, then it would have pushed it towards the elimination reaction. Another way to force an E2 reaction is to use a strong, sterically hindered base, such as t-butoxide ion. Since the t-butoxide ion is very bulky, it inhibits the backside attack that occurs in the substitution reaction, allowing elimination to take place.3 Sources of error that occurred during the experiment could have been during the refluxing process where some of the solvent vapour could have evaporated. Although refluxing tries to minimize the amount of vapour escaped, it is not completely efficient, therefore a loss of some product. To improve this, a longer column could be use to make sure more of the vapour is cooled to prevent solvent vapour from escaping. Also since the separation of the ether layer and aqueous layer was done by approximation by the eye and hand, it was difficult to have an exact separation. Some aqueous layer could have still remained in the ether layer. To fix this a micropipette could be used to separate the two layers when separation becomes difficult to distinguish. Lastly drying the ether in an open flask was quite inefficient. It took quite a bit of time to completely dry to ether. To improve the efficiency and promptness of drying a rotary evaporator can be used. The rotary evaporator is connected to a water aspirator, which greatly reduces the pressure in the closed system, allowing evaporation to occur faster. Also there is a speed motor which allows rotation of the flask allowing a more even distribution of suction, and therefore speeds up the evaporation rate. Lastly the coils of the water condenser increase surface area and increases condensation of the ether vapours.1 References 1. 2. 3.
Daoust, et al. “Organic Chemistry”, New Ed, McGill University, 2011. IPSC, CEC. (2005). IPC INCHEM. Retrieved from http://inchem.org on May 22, 2012. Solomons, T.W., Fryhle, C.B. Organic Chemistry, 10th Ed. New Jersey: John Wiley & Sons, Inc.; 2011.
+ H2O
+ CH3CH2I
+ H2O
+ CH3CH2I
Data Table 1: Chemical Quantities2 Name Formula
+ CH2=CH2
Ethyl iodide
C2H5I
Molecular weight g/mol 155.97
Ethyl Ether Ethanol 2-naphthol 2-ethoxynaphthalene
C4H10O CH3CH2OH C10H7O– C12H10O
74.12 46.04 143.00 172.22
Amount (g)
Amount (mL)
# moles
n/a
8.0x10-6 (800μl) 10 2 5 n/a
n/a
Table 2: Chemical and Physical Properties2 Name Formula M.P (°C)
B.P. (°C)
Ethyl iodide Ethyl Ether Ethanol 2-naphthol 2-ethoxynaphthalene
72 35 78 285 282
C2H5I C4H10O CH3CH2OH C10H7O– C12H10O
Melting point of 2-naphthol: 122°C
-108 -123 -114 122 37
Density (g/cm3) 1.95 0.71 1.59 1.22 1.06
Solubility
Safety
Slightly -corrosive 6.9g/100mL -flammable Miscible -irritant -negligible in water
Melting point of 2-ethoxynaphthalene: 37°C Melting point of product: 32-34°C Yield in grams: 0.220g Discussion This experiment is a synthesis by nucleophilic substitution (SN2) since the melting point of the product obtained was fairly close to that of 2-ethoxynaphthalene. Since it was slightly lower and not exact, this indicates slight traces of impurities in the obtained sample. Also the exact amount of 2-naphthol was never given so the percent yield cannot be calculated. SN2 reaction was favoured because the leaving group was attached to a 2° carbon, and a strong nucleophile, OH-, was used. Also the nucleophile was not bulky, allowing the backside attack of the nucleophile in the SN2 reaction to occur. A catalyst could also be used to speed up the SN2 reaction by creating a strong nucleophile. Although the SN2 reaction was favoured, there probably was an E2 side reaction (very minor), because a 2° alkyl halide and strong nucleophile yields a mixture of both reactions. If a higher temperature was maintained throughout the reaction, then it would have pushed it towards the elimination reaction. Another way to force an E2 reaction is to use a strong, sterically hindered base, such as t-butoxide ion. Since the t-butoxide ion is very bulky, it inhibits the backside attack that occurs in the substitution reaction, allowing elimination to take place.3 Sources of error that occurred during the experiment could have been during the refluxing process where some of the solvent vapour could have evaporated. Although refluxing tries to minimize the amount of vapour escaped, it is not completely efficient, therefore a loss of some product. To improve this, a longer column could be use to make sure more of the vapour is cooled to prevent solvent vapour from escaping. Also since the separation of the ether layer and aqueous layer was done by approximation by the eye and hand, it was difficult to have an exact separation. Some aqueous layer could have still remained in the ether layer. To fix this a micropipette could be used to separate the two layers when separation becomes difficult to distinguish. Lastly drying the ether in an open flask was quite inefficient. It took quite a bit of time to completely dry to ether. To improve the efficiency and promptness of drying a rotary evaporator can be used. The rotary evaporator is connected to a water aspirator, which greatly reduces the pressure in the closed system, allowing evaporation to occur faster. Also there is a speed motor which allows rotation of the flask allowing a more even distribution of suction, and therefore speeds up the evaporation rate. Lastly the coils of the water condenser increase surface area and increases condensation of the ether vapours.1 References 1. 2. 3.
Daoust, et al. “Organic Chemistry”, New Ed, McGill University, 2011. IPSC, CEC. (2005). IPC INCHEM. Retrieved from http://inchem.org on May 22, 2012. Solomons, T.W., Fryhle, C.B. Organic Chemistry, 10th Ed. New Jersey: John Wiley & Sons, Inc.; 2011.
