Substitution Reactions of Alcohols and Reaction Rate studies of an SN1 Reaction

Substitution Reactions of Alcohols and Reaction Rate studies of an SN1 Reaction

Abstract

The main objective of the experiment was to study the SN2 reaction by converting 1-butanol to 1-bromobutane using hydrobromic acid. This objective was achieved through a number of reactions, which included distillation, solvent extraction, heating at reflux, and yield calculation. The IR spectrum indicated the presence of SP3 hybridized C-H bond, alkane stretch, and a percentage yield of 71.9%. 

Objective

The main objective of the experiment was to study the SN2 reaction by converting 1-butanol to 1-bromobutane using hydrobromic acid. This objective was achieved using experimental techniques, which included calculation of the percentage yield, and IR spectrum analysis.

Synthetic Equation

Physical Properties and Hazards of Reagents and Products

Name	Structure	M/W (g/mol)	MP (C)	BP (C)	Density (g/mL	Notable Hazards
1-butanol		74.12 g/mol		116-118	0.81 g/mL	Flammable, irritant, toxic, hazardous to environment
1- Bromobutane		137.02 g/mol		100-104	1.276 g/mL	Flammable, toxic, irritant, hazardous to environment
Hydrobromic acid		80.91 g/mol		122	1.49 g/ml	corrosive, irritant,
Sulfuric acid		98.08 g/mol		290	1.840 g/mL	Corrosive, irritant
Sodium bicarbonate		84.01 g/mol	300			Mild irritant
Anhydrous sodium sulfate		142.04 g/mol	884			irritant

Spectra Data 

Figure 1: Before Reaction

Figure 2: After Reaction

Procedure

The experiment was carried out by first placing 6.2 mL of 1-butanol into a 100 mL round bottom flask. This was followed by addition of 10 mL of 48% hydrobromic acid (HBr) to the flask and swirling it. In the next step, and while still swirling, 4 mL of concentrated sulphuric acid was added to the flask. 1-2 boiling chips were added to the flask before being connected to a reflux condenser. Since some noxious fumes were likely to be generated during the heating period, the ground glass joint was left open and the gas outlet of the connecting tube was connected to a vacuum hose, which was connected to the sink aspirator. The aspirator water was then turned on full blast. The reaction mixture was heated at reflux for 45 minutes, before the flask and its contents were cooled to room temperature in an ice bath. 10 mL of deionized water and 1-2 new boiling chips were added to the mixture before the reflux apparatus was converted to a simple distillation apparatus. The contents of the flask were distilled into a 25 mL round bottom flask until the distillate reached 100ºC. The aqueous layer was removed from the distillate using a transfer pipet. 5 mL of deionized water was added to the organic layer of the distillate and mixed well with a transfer pipet. The aqueous layer was separated from the organic layer and combined with the original aqueous layer. The organic layer was washed with 5 mL 5% sodium bicarbonate solution, separating the layers and combining the aqueous layers into the waste flask. The organic layer was washed once more with 5 mL deionized water, separating the layers and combining the aqueous layers into the waste flask. The organic layer was dried over anhydrous sodium sulfate and decanted into a clean, dry, pre-weighed, capped sample bottle. The product was weighed, IR spectroscopy was performed, and the percentage yield was calculated.

Calculations                        

CH₃CH₂₃OH + HBr     CH₃CH₂₃Br + H₂O 

Starting reagents:  6.2 mL, 0.06775 mol

 Mass of 1-bromobutane distillate: 6.675 g 

6.2 mL 1-butanol×0.81gmLdensity of 1-butanol 

=5.022 g× 1 mol/74.12 g  = 0.06775 mol of 1-butanol  

10 mL HBr×1.49gmLdensity of HBr 

=14.9 g× 1 mol/80.91 g = 0.18416 mol of HBr 

Calculation for finding limiting reagent

0.06775 mol 1-butanol ×1 mol 1-bromobutane1 mol 1-butanol =0.06775 mol 1-bromobutane  

0.18416 mol HBr×1 mol 1-bromobutane/1 mol HBr = 0.18416 mol 1-bromobutane  

1-butanol is the limiting reagent

Calculation of theoretical yield

 0.06775 mol 1-bromobutane×137.02 g/mol = 9.2831 g 1-bromobutane

Calculation of percentage yield

% yield =actualtheoretical ×100  

= 6.675 g 1-bromobutane9.2831 g 1-bromobutane *100 = 71. 9 % 

Synopsis of the Results 

Several techniques were used in carrying out this experiment. The first technique used involved heating of the mixture at reflux. This procedure helped to ensure that the reaction proceeded without allowing any of the solvent used evaporating.  One of the major observations made during the heating was that the vacuum hose was not connected to the sink aspirator, and the water was not on full blast. Also, it was observed that there was a noxious fume. This was most likely due to the fact that the mixture of alcohol, hydrobromic acid, and sulphuric acid give off gaseous HBr when heated. Next, the reflux condenser was converted to a simple distillation apparatus, which was used to separate the components of the mixture. One observation made during this step was that the resulting distillate formed an organic layer on the bottom and the aqueous layer on top. This observation was because the organic layer containing 1-bromobutane (density = 1.276 g/mL¹) was denser than the aqueous layer containing water (density = 1.0 g/mL²); thus, the organic layer remained on the bottom. After distillation, the organic layer was washed with deionized water and sodium bicarbonate. The 1-bromobutane was then dried over anhydrous sodium sulfate. Finally, IR spectroscopy was performed on the product. The IR spectrum (attached) revealed three strong peaks with frequencies within the 2850-3000 cm^-1 range in the diagnostic region. The percentage yield of the reaction (calculated above) was 71.9%.

Discussion

A reaction is classified under S_N2 reaction (Second-order Nucleophilic Substitution) or a substitution reaction when a nucleophile is used to replaces a leaving group such as a halide, where the leaving group takes the lone pair of electrons with it (Meneses et al. 310).  As a result of a backside attack that occurs, an S_N2 reaction mechanism inverts the stereochemistry at the centre of the reaction, leading to steric effects, or crowdedness.  Due to steric hindrances, the nature of substrate used in an S_N2 reaction affects the rate of reaction.  In case a reaction is tertiary, the mechanism such a reaction undergoes is carbocation intermediate, and therefore it is said to have undergone S_N1 reaction, as opposed to S_N2 reaction due to carbocation stability and crowdedness. Increased amount of steric hindrance means that the nucleophile will not be able to attack and carry out the reaction in a concerted step. As a result, the rate of S_N2 reaction can also be said to be determined by the strength of nucleophile.  Generally, S_N2 reaction takes place by having a nucleophile donate a pair of lone electrons, resulting to a covalent bond (Bachrach and Debbie 233). It is however crucial to note that the strength of a nucleophile can be determined by considering three major factors including electronegativity, the charge, and the steric effects of the nucleophile. Due to this, the type of solvent used during an S_N2 reaction also plays a role in determining the strength of the nucleophile (Weldegirma 189). In other words, stronger nucleophiles are more capable of conducting a backside attack that can displace the leaving group, and therefore it’s true to say that the stronger the nucleophile, the faster the rate of S_N2 reaction.  Some examples of S_N2 reaction that are faster as a result of strong nucleophile include NaCN and NaOH, while example of weak S_N2 reaction as a result of weak nucleophile include H₂O and H₂S (Hallett et al. 1864). 

Another factor affecting the rate of S_N2 reaction is the nature of the leaving group.  Good leaving groups favor S_N2 reactions and are weak bases that are able to be stable after leaving (Hamlin et al. 1353). In addition, the rate of S_N2 reactions is also affected by the type of solvent used.  For instance, polar aprotic solvents are more suitable for S_N2 reactions. This is because such solvent do not form hydrogen bond to the nucleophile.  Generally, S_N2 reactions between reactants such as methyl chloride and sodium hydroxide will proceed at a faster rate due to the fact that sodium hydroxide nucleophile is capable of creating a bond with the carbon atom.  On the other hand, the reaction is also take place due to the nature of alkyl halide substrate that is unhindered, since it is methyl substrate.  Even more, the chloride leaving group is able to leave effectively because of the potential it has to withdraw electrons. 

Unlike S_N1 reaction, an S_N2 reaction occurs in a single step in which the nucleophile bonds and the leaving group leaves at the same time.  An S_N2 reaction prefers primary alkyl halides over tertiary. The reason as to why S_N2 reaction is not favorable on tertiary halides is due to the fact that the number of substituents increases with increase in the number of carbon, steric hindrances, or increases in the bulkiness at the electrophilic carbon. An S_N2 reaction favors strong nucleophile, and strong leaving group.  The rate of reaction for SN2 reaction is also determined by the concentration of both alkyl halides.  Therefore, the best halides to use for S_N2 reaction are the methyl halides, since their structure are sterically not hindered and comprises of less R group. 

The reaction between 1-butanol and hydrobromic acid under the presence of concentrated sulphuric acid as a catalyst is SN2 reaction.  The experiment was successful in converting 1-butanol to 1-bromobutane using hydrobromic acid and concentrated sulphuric acid. The fact that 1-butanol is a primary alcohol made it possible for it to undergo S_N2 reaction with hydrobromic acid, to produce 1-bromobutane.  Also, being a primary alcohol, the reaction between 1-butanol and hydrobromic acid was a substitution bimolecular reaction (S_N2). A small amount of concentrated sulphuric acid was added to the reaction to act as a catalyst and to protonate the –OH group (hydroxyl group) on 1-butanol to form a better leaving group, which in this case was water because it is neutral. In a concerted reaction, the bromide ions (nucleophile) from the hydrobromic acid attacked the primary alcohol through a backside attack, which resulted in the inversion of the configuration as the water left as a leaving group, which is a characteristic of a SN2 reaction. The products of this reaction included 1-bromobutane and water. In addition, the heating of the mixture during the reaction at reflux helped to increase the rate of reaction; however, whether the reaction was successful or not could not be determined until the distillation was completed.  Since the boiling point of 1-butanol ranges between 116 and 118 degree Celsius, and that of 1-bromobutane is ranges between 100 and 100 degree Celsius, the most effective method of separating the two is through distillation. This was done by heating the contents of the flask until the temperature reached 100 degree Celsius. According to the distillation theory and the content of the flask, if the distillate produced when temperature reaches 100 degree Celsius should be 1-bromobutane and water, which was produced as by-products of the SN2 reaction.  After removing as much water as possible through distillation, more water was added in an attempt to remove the remaining water in the organic layer.  The acidic impurities from the products were removed through addition of 5% sodium bicarbonate.  The product was then dried by adding 3 large scoops of anhydrous sodium sulfate, which helped to absorb any water that may have been left in the product.  Ensuring total removal of water from the product was crucial, as the success of the study was to be determined using IR spectrum.  Presence of OH-group in the product would results to large peas in the IR spectrum.  After an IR was done on the final product, the spectrum obtained was consistent with what was expected for the conversion of 1-butanol to 1-bromobutane. As shown in the Figure 2, no OH-group was observed in the spectrum (peak of about 3200 cm-1).  The three peaks observed in the spectrum had peaks that ranged from 2873-2933cm-1, which corresponded to SP3 hybridized C-H bonds (alkane stretches). 

Theoretically, the range for SP3 hybridized C-H bonds, alkane stretches is 2800-3000 cm-1. Compared to the experimental value obtained, there were some errors that may have resulted to the variation.  The first error may have resulted from the fact that the mixture that remained in the flask after heating during distillation might have reacted with unreacted 1-butanol molecules, and as a result, could not have been accounted for in the distillate flask.  Also, some 1-bromopropane could have been lost during the drying process over anhydrous sodium sulfate, and this could have resulted to possibility of becoming trapped between the clumps and not coming out into the sample bottle. Likewise, the transfer of compounds from one flask to another could also contribute to sample loss. 

The fact that the IR spectrum of the 1-bromobutane product did not include an O-H alcohol stretch suggests that the experiment successfully converted 1-butanol (a primary alcohol) into 1-bromobutane (an alkyl halide). The percentage yield of 1-bromobutane was 71.9%, which suggests that 28.1% of the product was lost. This loss could have occurred in any step of the experiment in which the sample was manipulated. For instance, the transfer of compounds from one flask to another could have greatly contributed to sample loss. Incomplete extractions also could have led to sample loss.

Conclusion

In conclusion, although the percentage yield was not as expected (100%), the experiment accomplished what it was set out to do. For instance, reaction proceeded as expected, and none of the results obtained failed to agree that steric hindrance, the identity of leaving group, or the strength of the nucleophile affects the rate of S_N2 reactions.

Works Cited

Bachrach, Steven M., and Debbie C. Mulhearn. “Nucleophilic Substitution at Sulfur: SN2 or Addition− Elimination?.” The Journal of Physical Chemistry 100.9 (1996): 3535-3540.

Hallett, Jason P., et al. “Charge screening in the SN2 reaction of charged electrophiles and charged nucleophiles: an ionic liquid effect.” The Journal of organic chemistry 74.5 (2009): 1864-1868.

Hamlin, Trevor A., Marcel Swart, and F. Matthias Bickelhaupt. “Nucleophilic Substitution (SN2): Dependence on Nucleophile, Leaving Group, Central Atom, Substituents, and Solvent.” ChemPhysChem 19.11 (2018): 1315-1330.

Lenz, Robert W., and Philippe Guerin. “Functional polyesters and polyamides for medical applications of biodegradable polymers.” Polymers in Medicine. Springer, Boston, MA, 1983. 219-230.

Meneses, Lorena, et al. “Computational study of vicarious nucleophilic substitution reactions.” Journal of molecular modeling 23.10 (2017): 301.

Weldegirma, S. Experimental Organic Chemistry, 8^th ed.; University of South Florida: Tampa, 2018.