iPREPARATION iPREPARATION OF 1-BROMOBUTANE FROM 1-BUTANOL VIA SN2 REACTION
ABSTRACT: The synthesis of 1-butanol with sodium bromide and sulfuric acid via nucleophilic substitution (SN2 mechanism) yielded 1-bromobutane. This reaction required a catalyst to convert the functional group -OH in butanol to a better leaving group in which sulfuric acid was used. For 1bromobutane to be synthesised, purification methods including refluxing, simple distillation and separation was performed. This resulted in a product of slightly impure 1-bromobutane with a yield of 76.49% (11.1 g). The possible impurities include 2-bromobutane, 1-bromobutane and dibutyl ether.
1. INTRODUCTION Alkyl halides or more commonly known as haloalkanes are a group of compounds composed of alkanes with one or more hydrogens substituted by a halogen atom i.e. bromine, chlorine, fluorine and iodine. These chemical compounds have desirable physical properties which make them suitable for a variety of industrial uses such as solvents and degreasing agents due to their good solubility in organic liquids, non-flammable inhalation anaesthetic as they resist combustion [1] as well as applications in herbicides and insecticides [2]. The versatility in haloalkane production is another benefit as they can easily be synthesised from many reactions including radical chain reactions, electrophilic additions of alkenes [3] (hydro halogenation) and nucleophilic substitution [4] with alcohols [ref lecture], with the latter method focused on in this experiment. Nucleophilic substitutions can happen via two different mechanisms; SN1 (unimolecular) and SN2 (bimolecular). SN1 reactions occur in two steps, first the leaving group dissociates from the carbon forming the carbocation intermediate, this then reacts with the nucleophile to form the haloalkane. To favour the SN1 pathway the carbocation intermediate needs to be stabilised by the presence of other bonding carbons i.e. reactivity increases as more carbons are bonded (tertiary > secondary > primary) [1]. On the other hand, SN2 reactions take place in only one step where the carbon has both the leaving group and nucleophile partially bonded at the same time, this produces what is known as the transition stage. SN2 reactions are more likely to happen when less carbons are bonded as steric hindrance is decreased i.e. reactivity increases as less carbons are bonded (primary > secondary > tertiary) [1]. Both SN1 and SN2 depend on good leaving groups (weakly basic) to occur. Consequently, these reactions do not naturally take place with alcohols as the strongly basic -OH group is a poor leaving group which prevents SN1 and SN2 reactions from producing alkyl bromide when heated with bromide salts. However, the -OH group can still leave as a weakly basic water molecule if it is activated in the presence of a strong acid [5]. This activation protonated the -OH group to H2O which is now a good leaving group and can hence undergo nucleophilic substitution via the SN2 mechanism. SN2 is the favourable pathway because once the nucleophile displaces the protonated hydroxyl the transition state produced is a primary carbon with little steric hindrance and is too unstable for SN1 to occur.
2. EXPERIMENTAL METHOD The method outlined in the chapter Experiment 4: halogen compounds #1 (nucleophilic substitution reactions) of the Organic Chemistry 1 Practical Manual was followed for this experiment. No alterations were made to the outlined method. Observations: ● During reflux, there was a noticeable phase separation ● After reflux, the distillate that was collected from simple distillation was cloudy but gradually became clear as distillation continued ● When this distillate was transferred to a separation funnel and added with water, again there was a noticeable phase separation seen
3. RESULTS PART A Preparation of 1-bromobutane 3.1 Physical Constants Table 1. Physical characteristics of reactants and products
Compound n-butanol 1-bromobutane
Molecular Weight (g/mol) 74.14 137.03
Refractive Index Lit. value: 1.4398 at 20°C 1.433 at 22.1°C
Sulfuric acid Sodium bromide
98.08 102.90
-
3.2 Quantities and physical description of reactants and products Table 2. Quantities and description of reactants and products
Reactants
Sodium bromide 1-butanol 18M Sulfuric acid Water
Quantities 13.01 g 7.87 g 12 mL 10 mL
Products
1-bromobutane
11.1 g
3.3 IR Spectras 3.31 IR Spectrum of 1-bromobutane
Physical description White powder Colourless liquid Colourless/slightly yellow liquid Clear/colourless liquid
Clear liquid
Figure 1. IR spectra of 1-bromobutane obtained from experiment 3.32 IR Spectrum of 1-butanol
Figure 2. IR spectra of 1-butanol obtained from demonstrators
3.4 GC Chromatogram of 1-bromobutane
Figure 3. GC chromatogram of 1-bromobutane from experiment Table 3. Area and area percentage of products from GC chromatogram
Products
Area
Area Percentage
1-bromobutane
2256.17700
87.96287
1-butanol
258.84048
10.09156
2-bromobutane
31.33878
1.22182
Dibutyl ether
18.56351
0.72375
% area = analyte peak area/ total sum of all peak areas x 100% % of 1-bromobutane = 2256.17700/2564.91977 x 100% = 87.96% 3.5 Theoretical yield
Molar mass 1-butanol = 4(12) + 10(1.008) + 16 = 74.08 Molar mass 1-bromobutane = 4(12) + 9(1.008) + 79.9 = 136.972 Molar mass NaBr = 22.99 + 79.90 = 102.89
Moles of 1-butanol = mass of 1-butanol used/molar mass 1-butanol = 7.87/ 74.08 = 0.1062365011 mol Moles of NaBr = mass of NaBr used/molar mass NaBr = 13.01/ 102.89 = 0.1264457187 mol Therefore, the limiting reagent is 1-butanol 0.1062365011 mol 1-butanol react with 0.1062365011 mol Br- gives 0.1062365011 mol bromobutane Hence moles 1-butanol = moles 1-bromobutane Theoretical yield of 1-bromobutane = moles 1-butanol x molar mass 1 -bromobutane = 0.1062365011 x 136.972 = 14.55 g 3.6 Percentage of actual yield Amount of 1-bromobutane obtained = 11.1 g
% yield = actual/theoretical x 100% = 11.1/14.55 x 100% = 76.3% PART B SN1 and SN2 reactions of halohydrocarbons 3.7 Reaction with silver nitrate in ethanol Table 4. Observation and reaction time for silver halide precipitation
Compound tested
Time for formation of Observation silver halide precipitate
1-chlorobutane
no reaction
n/a
2-chlorobutane
no reaction
n/a
2-chloro-2methylpropane
5 seconds
white precipitate formed
chlorobenzene
no reaction
n/a
benzyl chloride
2 minutes
white precipitate formed, after 12 minutes solution became more cloudy
3-chloropropene
30 seconds
white precipitate formed, after 13 minutes solution became more cloudy
3.8 reaction with sodium iodide in acetone Table 5. Observation and reaction time for sodium bromide precipitation
Compound tested
Time for formation of Observation sodium bromide precipitate
1-bromobutane
3 minutes
solution almost immediately turned yellow, after 3 minutes white precipitate formed
2-bromobutane
no reaction
n/a
2-bromo-2methylpropane
no reaction
solution turned light yellow and cloudy after 2 minutes, no precipitate
bromobenzene
no reaction
n/a
4. DISCUSSION PART A This experiment’s aim was to synthesise a primary haloalkane, 1 -bromobutane from a primary alcohol, 1-butanol through nucleophilic substitution SN2 [6]. The reaction of 1butanol with sodium bromide, sulfuric acid and water resulted in the production of 1bromobutane, sodium sulphate and water. To acquire a product with high purity/yield 1 bromobutane had to undergo purification processes which included refluxing, distilling and separating [4]. Sulfuric acid’s presence in this reaction was to protonate 1 -butanol to make it a better leaving group [3] as well as producing HBr, the nucleophile so that once the oxonium ion left 1-bromobutane could form [7].
Noting that HBr is produced as a gas and hence is corrosive therefore a gas trap was necessary.
With most organic reactions unwanted by-products are unavoidable, in this case it is 1butanol, 2-bromobutane and dibutyl ether; 1-butanol could possibly be due to unreacted starting material, 2- bromobutane’s presence is almost negligible and the dibutyl ethe r could
have easily co-distilled along with water and 1-bromobutane after refluxing, as a consequence of this 1-bromobutane must be extracted by work-up with water in order to dissolve as much 1-butanol as possible, following this is the addition of cold (to avoid charring of organic layer) sulfuric acid and sodium hydroxide (removes sulfuric acid that clings to side of separating funnel). Finally, 1-bromobutane is dried using anhydrous calcium chloride and distilled. To confirm the purity of 1-bromobutane several tests were carried out [8], this included IR spectroscopy, RI indexing [9] and GC chromatography. The absence of O-H and C-O stretches in graph 1 indicate that 1-butanol was successfully converted to 1-bromobutane, as further highlighted with the C-Br stretch around the 700 cm -1 and 1000 cm-1 respectively. The comparison of graph 1 and 2 clearly shows the presence and absence of these bonds, therefore the aim of this experiment was achieved. The GC chromatogram analysis from table three further confirms the successful conversion of 1-bromobutane as its retention time and peak is the highest. This once again illustrates a high yield with only a small presence of 1-butanol, 2-bromobutane and dibutyl ether. In this case, the percentage of 1-bromobutane was calculated to be 87.96 %. Finally, the similarities between the RI values obtained versus the literature value solidifies the presence of 1-bromobutane’s high yield. With the value obtained being 1.433 at 22.1°C and literature value being 1.4398 at 20°C, there is only a mere difference of 0.0068, it can be concluded that 1-bromobutane was successfully synthesised from 1-butanol via SN2 mechanism [6]. Loss of yield in this experiment was possibly from the unreacted starting material which then impacted the equilibrium as it could not completely react to form 1-bromobutane. In addition to this further loss can be accounted for during the purification and isolation processes. Improvements to this experiment include using lower temperatures during refluxing to keep elimination side reactions at a minimal as well as slower addition of acid. In conclusion, based on the results from IR, GC chromatography and RI the aim of this experiment, to synthesise 1-bromobutane from 1-butanol via SN2 mechanism, is achieved. The data collected supports that the 1-bromobutane obtained contains little to no impurity. PART B
From table four only 2-chloro-2-methylpropane, 3-chloropropene and benzyl chloride react to form silver halide precipitate with silver nitrate in ethanol. This reaction favoured the SN1 mechanism hence 1-chlorobutane (forms primary carbocation), 2-chlorobutane (forms secondary carbocation therefore takes longer than the timed 20 minutes to react) and chlorobenzene (has benzene ring attached hence cannot undergo SN1 or SN2 due to very unstable carbocation) did not react since they form unstable carbocation intermediates. Their reaction times are different as some occur faster than others i.e. 2-chloro-2methypropane forms very stable tertiary carbocation in comparison to 3-chloropropene which forms a secondary carbocation and benzyl chloride is only stabilised by its resonance structure. Thus, their respective reaction times are in this order.
From table five only 1-bromobutane reacted with sodium iodide in acetone to form a sodium bromide precipitate. This is due to the reaction conditions favouring the SN2 pathway. 1-bromobutane reacts to form a minimally steric hindered primary carbocation during the transition stage whilst the other compounds form much more steric hindered carbocations i.e. 2-bromobutane forms secondary carbocation hence reaction time increased, 2-bromo-2-methylpropane forms extremely steric hindered tertiary carbocation and bromobenzene is heavily substituted and hindered by its be nzene ring.
5. CONCLUSION
The aim of synthesising 1-bromobutane from 1-butanol via an SN2 reaction [5] is achieved. This synthesis process is carried out with two reagents heated alongside the primary alcohol thus producing an aqueous solution of water and alko-halide with a good yield of 87.96% and little to no impurities as seen from the results and analysis of GC chromatography. 6. REFERENCES
1. Ung, A. 2017, ‘Halohydrocarbons’, UTS Online Subject 65202, lecture notes, UTS, Sydney, viewed 15 May 2017, < https://online.uts.edu.au/bbcswebdav/pid-1581698dt-content-rid-9011623_1/courses/65202-2017-AUTUMN-CITY/Lec5%20halogen%20compounds-2hr-17.pdf>. 2. ChemgaCrew 2016, Introduction to the chemistry of alkyl halides, Berlin, viewed 14 May 2017, .
3. Chemguide 2000, The Nucleophilic Substitution Reactions Between Halogenalkanes and Water , U.K., viewed 13 May 2017, 4. Anderson, M.M. 1987, ‘Two working models for the SN2 mechanism’, J. Chem. Educ., vol. 64, no. 12, pp. 1023. 5. Microscale Synthesis of 1-Bromobutane n.d., viewed 14 May 2017, . 6. Preparation of 1-bromobutane via nucleophilic substitution n.d., viewed 14 May 2017, www.chemistry.mtu.edu/pages/courses/organic/manuals/2411/SN2/SN209.pdf . 7. Experiment 7: Preparation of 1-bromobutane n.d., viewed 16 May 2017, . 8. Bonner, O.D. & Choi, Y.S. 1975, ‘A spectroscopic investigation of the structure of alcohol-water solutions’, Journal of solution Chemistry, vol. 4, no. 5, pp. 457-469. 9. Smith, B.C. 2015, Infrared Spectral Interpretation: A Systematic Approach, 2nd edn, Taylor & Francis, U.K.
DISCUSSION QUESTIONS
1. Mechanisms of by-products A) Alkenes via E2 elimination, e.g. 1-butene
B) Dibutyl ether
2. Dibutyl ether formed co-distills along with water and 1-bromobutane after reflux, hence remains in upper layer as organic by-products. Alkenes specifically 1-butene, a gas is lost during reflux and the work-up processes. 3.
4. A) Silver halide precipitate (silver chloride) B) Sodium bromide precipitate 5. Pure 1-butanol
Pure 1-bromobutane
6. Sample spectra:
From above purity/yield of 1-bromobutane is quite high since there is no presence of 1-butanol detected (no O-H stretch on spectra band 3600-3300 cm -1) To distinguish if O-H stretch is due to water or alcohol look for the scissoring of two O-H bonds at 1630 cm -1, this vibration at this band is unique to water. Another note is the presence of O-H bends and C-O stretch are more likely to be alcohols rather than water. 7. Silver nitrate in ethanol 1-chlorobutane > 2-chlorobutane > 2-chloro-2-methylpropane SN1 reactions occur via two steps; first the slow C-X bond dissociates to form a carbocation intermediate; followed by the fast step forming the C-Nucleophile bond. There is a formation of a carbocation hence the rate of the reaction will depend on the stability of the carbocation. Tertiary being the most stable and hence fastest followed by secondary and finally primary carbocation. This is reflected in the observed results with 2-chloro-2-methylpropane being the most reactive/fastest since it forms a tertiary carbocation as the C-Cl bond dissociates. Following this mechanism, it would make sense that the next substrate to react would be 2-
chlorobutane then 1-chlorobutane (primary carbocation is unstable and therefore rearranges to a more stable carbocation to undergo the reaction). 8.
Sodium iodide in acetone 2-bromo-2-methylpropane > 2-bromobutane > 1 -bromobutane SN2 reactions occur via single step; the electrophilic carbon centre opposite to the leaving group is attacked (back-side attack, therefore inversion of stereochemistry i.e. Walden inversion) by the incoming nucleophile. This mechanism happens through a back-side attack thus the more ‘crowded’ the leaving group is the slower the reaction due to steric hindrance. Because of this the rate of the reaction follows primary > secondary > tertiary. Looking at the observed results 1-bromobutane occurred the fastest since the Br- ion was attached to a primary carbon hence the back-side attack could occur quite easily whereas the Br- ions in 2-bromobutane and 2-bromo-2-methylpropane were respectively secondary and tertiary.
9. There was a difference in reactivity between 1-chlorobutane and 3-chlropropene, this is due to one being a haloalkane and the other a haloalkene.
