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Nucleophilic Substitution Reactions

Nucleophilic Substitution Reactions

Nucleophilic Substitution Reactions

The purpose of this lab is to compare the nucleophilic abilities of the addition of chloride and bromide ions to the compounds of n-butyl alcohol and tertiary butyl alcohol, in SN1 and SN2 environments. This will determine which of the anions the better nucleophile, in each mechanism is.

To better understand the reactivity of competing nucleophiles, the mechanisms of their reactions must be observed. In the lab, both of the basic nucleophilic mechanisms of unimolecular and bimolecular substitution reactions take place. In both of these reactions, the reactivity is based on the substrates structure, the nucleophile’s basicity, and the reaction conditions, such as the solvent used and the temperature (Smith 389). In the substitution bimolecular reaction (SN2), the reaction is bimolecular and takes place in a one-step process. The rate is dependent on both the concentration of the substrate and the nucleophile. This makes the mechanism a second-order reaction. The rate law is described as Rate = k [substrate][nucleophile] (Carey 331). If a large excess of the nucleophile is present then the law would be a pseudo-first order reaction (false-first order) (Smith 390). But in this experiment, this will not be observed. SN2 reactions occur mostly with the substrates being a methyl structure or a primary and sometimes a secondary structure. These structures allow for a backside attack of the nucleophile to take place. The mechanism rarely proceeds with a tertiary structure. This is due to the steric hindrance that does not allow the back attack of the nucleophile, because of the decreased availability to attack the carbon atom (390). Usually a good leaving group on the substrate, such as a halogen, is present for substitution, but in this experiment the halogens will be acting as the nucleophiles. The nucleophiles of SN2 reactions must act as a Lewis base. The stronger the Lewis base, the faster the nucleophile will attack the substrate. For the SN2 reaction to occur, the use of a polar aprotic solvent should be used. Ideally, the substrate should remain depronated or else a front side attack of the nucleophile may take place. This is why the non-polar solvent is used. The temperature of the mechanism is low (Carey 322). Once the environment is established, the reaction proceeds as follows:

H H H H Nu + H C X Nu—–C——X Nu C H + X

H H HThe rate-determining step is when the nucleophile is partially bonded to the carbon atom, while the leaving group is partially being pushed off. This intermediate is called a pent coordinate (309). The substitution from the opposite side causes inversion of the product to invert from its starting configuration of dextrorotary (+) or levorotary (-) to its opposite enantiomer. Walden inversion is the name of this process. The product produced is one hundred percent of the inversion of the reactant (308).

In the substitution unimolecular reaction (SN1), the rate is also based on the substrate’s structure, the nucleophile’s basicity, and the reaction conditions. The mechanism is unimolecular and it is a two-step process. The first-order rate law is described as Rate = k [substrate] (Smith 394). The concentration of the substrate is the sole component in the rate law. This is from the slow step of the substrate’s ionization to form the carbocation. This must occur first. Because the formation of the carbocation, caused by the removal of the leaving group, requires less energy in tertiary and secondary structures, SN1 reactions never occur with methyl or primary substrates (393). A good leaving group is also needed, such as a halide. But again, the experiment uses the halides as the nucleophile. Opposite of the SN2 reaction, the nucleophile must be a weaker Lewis base. The use of a polar protic solvent is needed for the mechanism to take place. The polar solvent allows for the required pronation of the carbon atom to occur, so the weaker nucleophile can begin to attack. The temperature of the SN1 mechanism also remains fairly low (Carey 331). The reaction will proceed as follows:

CH CH CH

H C C X + Nu H C C + + Nu + X H C C Nu + X

CH CH CHSince the nucleophile can add to either the front side or the backside of the carbocation intermediate, a racemic mixture of the product is produced. However, slightly more of the inverted enantiomer is produced (319).

The main reagents used in this experiment are n-butyl alcohol, tertiary butyl alcohol, ammonium chloride, and ammonium bromide. The substrates are the two alcohols and the nucleophiles are equal amounts of the chloride and bromide ions from the ammonium salts. Consequently, alcohols do not readily react by simple nucleophilic substitution as halides do. For this reason, of the need to remove the strong hydroxide ion base of the alcohol, the use of an acidic medium must be used. Such an acid is concentrated sulfuric acid. The acid causes a rapid pronation of the alcohol, creating water, then the stable water molecule dissociates from the nucleophile. This allows the chloride and bromide ions to attack the carbocation. Depending on the structures of normal or tertiary, the ions will react with an SN1 or SN2 mechanism (Handout). Because equal molar concentrations of the ammonium salts are added, the conclusion of which is the better nucleophile, the one that can readily attack the substrate at a faster rate, can be attained. The analysis of the percent of alkyl halide products can be achieved with the use of a refractometer.

The Abbe refractometer is an important tool in analyzing liquid samples. It helps to acquire their identity and purity. It is used to measure the refractive idecies of the compounds being analyzed (Ault 161). An index of refraction is the ratio of the speed of light in a vacuum to the speed of light in the sample. The critical angle helps to distinguish this ratio. The angle is found by the use of Snell’s Law. It is described as the sin of a lights angle change through an air or vacuum media divided by the sin of the angle change through water, glass, or a prism. The refractive indices depend on the temperature as well as the frequency of the light (Palleros 238). For this reason, the refractive indices are usually standardized with a sodium lamp. This is the D line of sodium that has the wavelength of 5,890 nanometers. With the conditions, organic compound’s refractive indices range from 1.35 to 1.50 with added .00035 to .00055 for every degree above twenty degrees centigrade. The refractometer finds the critical angle of the sample between two glass prisms with a white light. The waves of light are refracted, or the speed and direction the light goes, below angles of critical value. The light is then adjusted by an adjustable reflector, until the critical boundary between the light and dark region shows in the eyepiece. The reading of the refractive index with a plus or minus 0.0002 error is taken (Ault 162).

Starting the lab begins with an assembly of a reflux apparatus. This is needed to carry out a reaction at high temperatures, constantly for long periods of time. The apparatus does not allow any solvent to escape into the atmosphere while boiling (Garner 123). The use of an acid trap is also needed due to the gas created by the heated sulfuric acid. Next, the solvent-nucleophile medium is made. It is composed of concentrated sulfuric acid, ice water, and equal molar amounts of ammonium chloride and ammonium bromide. Since ammonium chloride is only 28.3% soluble at twenty-five degrees Celsius, the solid salts are heated to dissolve, then half of the medium is transferred to a separatory funnel.

The n-butyl alcohol is then added down the condenser of the reflux apparatus, and a slight boiling is achieved. It is then boiled for seventy-five minutes. During the refluxing, the reactions with the tertiary butyl alcohol are then ran.

To begin the reactions, the t-butyl alcohol is added to the separatory funnel containing the solvent-nucleophile medium. The solvent and the alcohol are then mixed for two minutes, and the pressure is equalized in the funnel. After mixing, the alkyl halide layers should be formed by allowing the separatory funnel to sit for about two minutes. The lower layer, which is the acid and water, is drained and the less dense alkyl halide top layer is poured into a beaker containing solid anhydrous sodium bicarbonate. This is used as a drying agent to remove any contamination of water in the alkyl halide layer. Once the drying process is complete, the clear product is transferred into a vial with Saran wrap under the cap and saved for analysis on the refractometer.

Once the t-butyl alcohol procedure has been completed, the reflux period should be finished. The refluxed solution containing sulfuric acid and the alkyl halides is cooled and swirled to complete the reactions. The cooled solution is then transferred to the separatory funnel, and extracted with sodium bicarbonate. The sodium bicarbonate is used as an extracting liquid because it is insoluble with alcohols, and it is a very low polar molecule (Garner 131). After it extracts the organic layer, the lower dense layer is drained into a beaker containing solid anhydrous sodium sulfate. The sodium sulfate is also used as a drying agent to help to take out the contaminating water from the alkyl halide layer. The solution should be clear, and be placed in a vial with Saran wrap and saved for analysis on the refractometer (Handout).

The refractive indices for the SN2 mechanism products of pure 1-chlorobutane and pure 1-bromobutane are 1.4280 and 1.4398 respectively at twenty degrees Celsius. The SN1 mechanism’s pure products of 2-chloro-2-methyl propane and 2-bromo-2-methylpropane have the refractive indices of 1.3877 and 1.4280 at twenty degrees Celsius. Since each of the mechanisms products retain a percentage of each product and no amount of pure product, the refractive index of the SN 2 reactions should read between the pure readings of the 1-chlorobutane and the 1-bromobutane. Also the index of the SN 1 reactions should read between the pure readings of 2-chloro-2-methylpropane and 2-bromo-2-methylpropane (handout). Since this is true, the use of the following graphs could be used to estimate the percentages of each product:

To have a more accurate composition of the percentages, the following math equation can be used:

X= % Br (1.4398)(X) + (1.4015)(1- X)= refractometer reading

Once the percentages are found, the average molecular weight, theoretical yield, and percent yield can all be attained and calculated (Handout).

The following tables are from the experimentation done in the laboratory:

SN2 SN1

Weight of n-alcohol 4.22 g Weight of t-alcohol 5.25g

Weight of product 1.46g Weight of product 1.40g

Refractometer reading 1.4353 Refractometer reading 1.3984

Percent n-bromobutane 88.2% Percent of t-bromobutane 23.6%

Percent n-chlorobutane 11.8% Percent of t-chlorobutane 76.4%

Theoretical Yield 7.50g Theoretical yield 7.29g

Percent yield 19.4% Percent yield 19.2%

The purpose of this lab was to compare the nucleophicility of the competing chloride and bromide ions in SN1 and SN2 mechanism environments, when reacting with t-butyl alcohol and n-butyl alcohol. For the comparison of the nucleophilic abilities of the two halide ions in SN1 and SN2 mechanisms to be observed, equal molar concentrations of the soluble ammonium salts of ammonium chloride and ammonium bromide were added to both the t and n-butyl alcohols. Usually, alkyl halides are used as the substrate, because of the halide leaving group, and water as the nucleophile to produce an alcohol. Since the leaving group and nucleophile are reversed, the use of concentrated sulfuric acid is needed. This allows the hydroxyl group, which is a strong base, to pronate off of the carbon atom that in turn creates the reactive carbocation. The procedures for the SN1 and SN2 reactions proceeded and the chlorobutane and bromobutane products were produced. The percentages of each of the products were discovered by analysis with the refractometer and simple calculations. The average molecular weights for the normal and tertiary halides were calculated, along with the theoretical yield and percent yield.

The data of the lab showed that the bromide ions in SN2 reactions have greater nucleophilicity than that of the chloride ions. Adversely, the chloride ions have greater nucleophilicity than bromide ions in SN1 reactions. In the SN2 reactions, bromide is a better nucleophile because it is a stronger Lewis base than chloride. This is due to the increased distance of the bromine’s electrons from its nucleus (Carey 313). Since the bromide is a larger molecule and there are no side chains on the n-butyl alcohol to cause steric hindrance, the bromide attacks the carbocation at a faster rate than the chloride. These occurrences are evident from the data found from the refractometer reading.

In the SN1 reactions, the chloride reacts with the t-butyl alcohol at a faster rate than that of the bromide. The chloride ion is a weaker Lewis base than bromide, making it more suitable for the SN1 reaction. This is from the electrons being closer to the nucleus in the ion (313). The tertiary structure of the butyl alcohol causes steric hindrance on the attacking nucleophiles. Since the bromide ion is larger than the chloride ion, the hindrance does not allow the larger ion to attack as easily as the chloride. This allows the chloride to add at a faster rate. Again, the evidence is found in the refractometer readings.

The data also shows a small percent yield of product. This implies that there were many places for error to occur. Possible errors include the following: Not dissolving the ammonium salts by not heating the solution enough, pouring the n-butyl alcohol into the condenser and allowing some to run down the outside (actual occurrence), having the heat too high when refluxing; this may cause the solution to be lossed from rapid boiling, not allowing the layers to separate completely in the separatory funnel, not venting the separatory funnel while mixing; too much pressure could build up causing the stopper to pop off (actual occurrence), confusing the organic layer with the water layers, not using enough of the drying agents, or not sealing the storage vial tight enough to allow the alkyl halides to evaporate (actual occurrence).

Errors on the refractometer reading may also occur. This could be from the ignorance of cleaning the prisms on the refractometer, or by incomplete covering of the prism with the alkyl halide solutions.

Refractometry has many uses. Not only can it be used to evaluate reactivity of certain compounds, it can also be used to find the identity of a compound, the density of a compound, or the concentration of a compound. In practical situations, such as winemaking, the refractometer can be used to find the amount of the ingredients in the wine. One could possibly find the alcohol content or even the sugar content. Other uses could be in identifying the oils, fats, or proteins in food and other places. It can also be used to find the chemical makeup of an unknown substance, such as drug analysis. Refractometry is one of the experimental methods that has many uses. It is used for many different situations and it is a valuable technique for any organization that requires the analysis of any chemical compounds.

Bibliography:

Ault, Addison. Techniques and Experiments for Organic Chemistry. Sausalito,

California: University Science Books, 1998.

Carey, Francis. Organic Chemistry: Fourth Edition. Boston: McGraw Hill, 2000.

Garner, Charles. Techniques and Experiments for Advanced Organic Laboratory.

New York: John Wiley and Sons, Inc., 1997.

Handout. Experiment 24: Nucleophilic Substitution Reactions: Competing Nucleophiles

Palleros, Daniel. Experimental Organic Chemistry. New York: John Wiley and Sons Inc.,

2000.

Smith, Michael and Jerry March. March’s Advanced Organic Chemistry: Reactions,

Mechanisms, and Structure. Fifth Edition. New York: John Wiley and Sons Inc.,

2001.

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