Lab 7 Preparation of Alkyl Halides by Substitution Reactions PDF

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Lab 7: Preparation of Alkyl Halides by Substitution Reactions. NaI and AgNO3 Tests for Alkyl Halides. Katja Gonzalez Lab Partner: Kyle Leonida 2018/04/02 Methods and Background The goal of experiment seven was to produce primary alkyl halides by SN2 reactions and alkyl halides by SN1 reactions. Simple distillation was utilized to isolate a liquid product by used of a separatory funnel. NaI and AgNO3 tests were completed in order to differentiate between tertiary, secondary and primary alkyl halides. Synthesis reaction was used to calculate percent yield of the seperated product. A major part of this lab involve understanding nucleophilic aliphatic substitution reactions. This reactions includes the certain conversions of differing functional groups. In the example below, nucleophile(Nu), leaving group(L) and carbon(C) are present. Nucleophiles are either negative or neutral in charge, regardless the pair of electrons that are non bonding are given to the electrophilic atom in the reaction. This addition will end in the formation of a new covalent bond through the process known as a substitution reaction. Similar to the Lewis acid-base reaction, the electrophilic carbon atom show itself as the Lewis acid while the nucleophile will represent itself as the Lewis base. Whether the leaving group is negative or neutral doesn’t change the absolute need to accept the pair of electron coming from the carbon atom due to the C-L bond breakage. It is concluded that the rate in which the substitution reaction occurs is mainly dependant on the ability for the leaving group to actually leave.

Figure 1: Nucleophilic Aliphatic Substitution Reaction

Figure 2: Reactions in this lab

As demonstrated above, the nucleophilic substitution reaction being utilized for this lab are classified into two variants that are dependant on the mechanistic pathways of the substance in question. One type, known as SN 1, is classified as an endothermic process meaning the very first step is noticeably longer than the second step. This will also involve a heterolytic cleavage or the ionization of the C-L bond in order to develop a desired unstable carbocation. Due to the slowness of the first step it is considered to be the rate determining step of the reaction. The SN1 reaction is considered to be a unimolecular reaction due to the rate step only have one molecule involved, the substrate R-L. The equation rate = k1[R-L] explains the reasoning behind the rate where the k1 represents the first rate constant. In accordance to Le Chatelier’s principle, when equilibrium is disrupted the reaction conditions will alter to maintain equilibrium. It is concluded that when reactant concentrations increase the reaction shifts to the right producing more products.

Figure 3: Example of SN 1 versus E1

The competing reaction with the SN1 is known as the unimolecular elimination reaction or E1. The nature of the nucleophiles is the determining factor of the reaction. The substitution can occur in both weak basic and high polarizable conditions. In the other type known as the SN2 reaction in order to obtain the primary halide primary alcohols are used to react with the hydrogen halide. SN2 reactions only have one step, the rate determining step and this does not have a carbocation formation. For this reaction the nucleophile will attach itself to the carbon making the leaving group to cleave resulting in the carbon to take a bonding pair of electrons. The rate determination is based on the concentrations not only the substrate but also the nucleophile due to the presence of the bimolecular elimination reaction. The equation Rate = k2 [R-L][Nu:] explains this concept. In this equation k2 shows the second rate constant. In accordance with Le Chatelier's principle, this reaction is favored toward the incline of the reactants concentrations. With this incline, the amount of compound present in the transition state will also raise.

An SN2 reaction is in competition with the bimolecular elimination reaction in order to produce alkenes. As for this lab for the primary alkyl halide of 1-bromobutane is completed through SN2 reaction. It accomplishes this by using 1-butanol with sodium bromide and sulfuric acid. Due to -OH being a fairly poor leaving group is easily protonated with the sulfuric acid which ends in the formation of the oxonium ion. H2 O is a viable leaving group so the reaction is completed how desired with they nucleophilic chloride ion.

Figure 4: SN2 Mechanism

To verify primary, secondary or tertiary alkyl halides presence sodium iodide and silver nitrate test are completed. The silver nitrate test is considered an SN1 reaction due to one molecule determining the rate of the step. As for the sodium iodide test it is considered and SN 2 reaction due to the iodide ion being the nucleophile. This test will require time as in order to view primary and secondary halides should be heated in order to view any precipitate formations.

Figure 5: Silver Nitrate Test

Figure 6: Sodium Iodide Test

Experimental Procedure

Figure 7: Reflux apparatus (left) and simple distillation apparatus (right)

● SN2 Reaction Procedure Roughly 11.1 g of sodium bromide is collected in a 100 mL round bottom flask. 10 mL of H2O and 10 mL of 1-butanol is placed into the same flask. To mix solutions a gentle swirl is needed. Note that the entire product may not dissolve but it will when the boil begins. After the mixing is complete the flask is placed into an ice bath in order to safely add in 10 mL of concentrated sulfuric acid as this will heat the flask due to its chemical reaction. Again, the mixture should be gently swirled. The flask is placed either back into the ice bath or in a safe position so it doesn't tip over. The reflux apparatus demonstrated above is now assembled together. Next place the round bottom flask into the apparatus as the picture entails and heat until boil begins. Upon presence of slight boil begin 45 minutes collection process. Note SN 1 reaction procedure is completed during this 45 minute wait period. As soon as the 45 minutes is ended the apparatus is switched immediately to the simple distillation apparatus. On the collection side of this apparatus the 25 mL flask is placed into an ice bath during the collection process. This process will continue until roughly 115°C. The distillate is then placed into a separatory funnel and washed with 10 mL of H2O. Note that the organic layer is at the bottom of the funnel. These layer are then separated and the organic layer is placed back into the funnel. Next a series of extractions is done, they include: 8 mL of concentrated sulfuric acid, 5 mL of 2 M NaOH (2 times), 10 mL of H2O,and finally 10 mL of saturated NaCl (brine). These extractions are completed in the order above. In every separation except the sulfuric acid the layer to be extracted will be at the bottom of the funnel. Once all extractions are completed, the obtained 1-bromobutane is transferred to an erlenmeyer flask and dried with Na2SO4 and filtered out. After pre-weighing a watch glass the dried 1-bromobutane is placed upon it and weighed. Percent yield is calculated from these weights. ● SN1 Reaction Procedure

This procedure is begun during 45 minute wait time in the procedure above. To begin, 10 mL of 2-methyl-2-butanol and 25 mL of concentrated hydrochloric acid are placed into a separatory funnel. In order to help stop funnel combustion light swirls are completed without the stopper reaction is not prevalent. The stopper is now placed tightly into its position and inversions are begun. Pressure is released every 3 inversions by twisting the stopcock open and closed. After a proper of moment of inversion are completed the solution should be left to sit in the funnel in order for separation to become visible. The layers on then separated and the organic layer is determined. The determined organic layer is washed sequentially with 10 mL of saturated aqueous sodium chloride and cold saturated aqueous sodium bicarbonate. The reaction is very carbonated or contains a lot of bubbling agents, this means separatory needs to be swirled without the stopper until the bubbles are no longer forming. This will decrease the risk of funnel combustion. The stopper is then added and inversions are begun with intermittent stops to open and close stopcock. The organic layer is then separated and washed with H2O and sodium chloride in a sequential pattern. The obtained 2-chloro-2-methylbutan layer is carefully separated into an erlenmeyer flask. This product is then dried with Na2SO4. Note most students had two small of an amount so the simple distillation step after drying with the salt was skipped. On a pre-weighed flask the producted was placed and weighed in order to calculate percent yield. ● Sodium Iodide and Silver Nitrate Test The products of the procedures above will undergo sodium iodide and silver nitrate testing. Sodium iodide was the first test completed. Three test tubes were obtained and 1 mL of sodium iodide reagent in acetone is placed into each of the tubes. One test tube will contain 1-bromobutane obtained and is shaken well. This test tube it then to be left for 3 minutes to incubate. If there is a notable precipitate formation this is an indication of a positive test. This test is repeated with both the secondary alkyl halide of 2-chlorobutane and tertiary alkyl halide of 1-chloro-2-methylbutane. Data Acquisition I. Relevant equations Percent yield = (actual yield)/(theoretical yield)×100% II.

Product and Masses

Product

1-bromobutane

2-chloro-2-methylbutane

Mass (g)

4.19

0.17

III. Calculations Percent yield 1-bromobutane = (4.19 g)/(11.1 g)×100% = 37.7% yield Density of 2-methyl-2-butanol = 0.815 g/cm³

Theoretical yield = mass of 2-methyl-2-butanol = 10 mL × 0.815 g/mL = 8.15 g Percent yield 2-chloro-2-methylbutane = (0.17 g)/(8.15 g)×100% = 2.1% yield IV.

Tests for Alkyl Halides

Sodium Iodide Test

Silver Nitrate Test

1-bromobutane 2-chlorobutane 2-chloro-2-methylbutane 1-bromobutane 2-chlorobutane

2-chloro2-methylbutane

Positive

Negative

Negative

Positive

Positive

Positive

~3 minutes

No reaction

No reaction

Slow, heat required

A few minutes

Immediate reaction

Conclusion The end goal of this lab was to create various alkyl halides utilizing substitution reactions. The presence of theses halides were tested with the either the SN1 or SN2 test method. Even though the lab is considered successful only limited amounts were recovered from the extractions. With this concern in place however, the results ended with the appearance of the presence the alkyl halides. When examining the sodium iodide test using 1-bromobutane is noted that is was precipitated within the three minutes. No other alkyl showed after the three minute time frame, however. This is a demonstration of how 1-bromobutan has the fastest reaction rate, which in turn proves the presence of the primary alkyl halide. When viewing the silver nitrate testing all three of the tubes were observed to have precipitate. The variant however was time as some precipitates took longer to form than others. It is noted that the tertiary alkyl group reacted immediately while the secondary and primary took a few minutes for the reaction to occur. Due

to the silver nitrate test being and SN1 reaction it is correct in viewing the tertiary alkyl halide to have the fastest reaction time. This is the followed by the secondary and finally primary alkyl halide groups. A problem that could have accounted for the non reactiveness of the tertiary and secondary alkyl groups is the amount of time alloted for the reaction to take place. If more time were to be given for this lab the perfect extraction or better extractinon can be obtained making it clear as to which alkyl halide groups are present based on precipitation rates. References Gilbert, J.C., and Martin, S.M., Experimental Organic Chemistry , 5th Edition, Cengage Learning, Boston, MA, 2011. Landrie, C.L., and McQuade, L.E., Organic Chemistry: Lab Manual and Course Materials , 3rd Edition, Hayden-McNeil, LLC, Plymouth, MI, 2013....