Nucleophilic+Substitution+Reactions

=Introduction=

Nucleophilic substitution reactions occur as either a one (S N 2) or two (S N 1) step process. An S N 2 reaction is one in which a nucleophile makes a new bond to an electrophile while the electrophile's existing bond to a leaving group (LG) is simultaneously broken. Because both the nucleophile and electrophile are simultaneously involved in the reaction, the rate of the reaction is determined by the concentrations of both molecules and is expressed as: rate=k[nucleophile]x[electrophile]. An S N 1 reaction is one in which an LG leaves behind an electrophile that forms a carbocation intermediate.Once this carbocation is formed, a nucleophile makes a new bond to it and the substitution is complete. Because the carbocation must be formed before the nucleophile can attach, the rate of the reaction is determined only by the concentration of the electrophile and is expressed as: rate=k[electrophile]. However, the rate of reaction can be effected by whether it is performed in a polar protic or polar aprotic solvent. The purpose of this lab is to observe the effects of these two solvents on the reaction between tert-Butyl chloride and NaOH. Whoopsie! WATER is the nucleophile, not hydroxide. In this reaction, the LG (chlorine) leaves creating a tertiary carbocation. The nucleophile (OH - ) attaches to the carbocation and forms tert-Butanol. The mechanism is illustrated below with OH - playing the part of the nucleophile (NU - ).



In this lab, the solvents that will be used to observe the effects of a polar protic solvent and a polar aprotic solvent on an S N 1 reaction are acetone and isopropanol. For this experiment, the polar aprotic solvent acetone will be tested and compared to the results of other technicians using the polar protic solvent isopropanol for differences. As a byproduct of the reaction, hydrochloric acid will be produced and the pH of the mixture will become more acidic as the reaction proceeds. The rate of the reaction will be measured by using an acid-base indicator in the reaction mixture. The indicator will change color when the reaction generates enough acid. This will allow rate of reactions to be compared between samples. Generally very nicely explained--with that misunderstanding of the nucleophile I note above.

=Procedure=

The procedure for this experiment can be found here.

Relevant Compounds:

 * Name:** Acetone
 * Cas. #:** 67-64-1
 * Molecular Formula:** C 3 H 6 O


 * Name:** Hydrochloric acid
 * Cas. #:** 7647-01-0
 * Molecular Formula:** HCl


 * Name:** Isopropanol
 * Cas. #:** 67-63-0
 * Molecular Formula:** C 3 H 8 O


 * Name:** Sodium hydroxide
 * Cas. #:**1310-73-2
 * Molecular Formula:** NaOH


 * Name:** tert-Butanol
 * Cas#:** 75-65-0
 * Molecular Formula:** C 4 H 10 O


 * Name:** tert-Butyl chloride
 * Cas. #:** 507-20-0
 * Molecular Formula:** C 4 H 9 Cl


 * Name:** Water
 * Cas. #:** 7732-18-5
 * Molecular Formula:** H 2 O

=Data=

Data for individual tests is as follows:

Results from other lab groups were collected for comparison and to aid in the inference of trends and are as follows:



Results for the relative concentrations of flask contents at the addition of tert-Butyl chloride were found using the following equations:



=Analysis=

Individual:
As expected, as the concentration of acetone (polar aprotic) decreases and water (polar protic) increases (Figure 1 & 2) the S N 1 reaction rate increases. It appears that for every 10% decrease in acetone and 10% increase in water the reaction time is decreased by a factor of approximately 4.

Class:
Compiled class data (Figure 6) shows consistency between each run of the experiment with each solvent. Consistent with individual results, the class average time taken to complete the reaction in acetone solvent decreases as acetone decreases and water increases. This is due to the polar aprotic characteristics of acetone that hinder the S N 1 reaction decreasing while the polar protic characteristics of water that facilitate the S N 1 reaction increasing.

The data for the two recorded lab groups using isopropanol as a solvent is also consistent (Figure 6). The same trend as observed with acetone as a solvent is seen here. As isopropanol (polar protic) concentration decreases and water (polar protic) concentration increases, the rate of reaction increases.

When comparing the class averages for the rate of reaction for each solvent (Figure 6), a major inconsistency is observed. Aside from flask #1 (control) in which HCl was added to get the indicator to change color, each reaction proceeded more quickly with acetone as the solvent as opposed to isopropanol. This is unexpected because isopropanol is polar protic and increases the rate of an S N 1 reaction. One would expect that the combination of two polar protic solvents would yield a greater reaction rate and this is not the case. As a result, there may be some unforeseen interaction between water and isopropanol that lowers its effectiveness as a polar protic solvent.

This is suprising, isn't it? If you look at the dipole moments for acetone and isopropanol, however, the fact that acetone is more polar than isopropanol could suggest an explanation. But it's an interesting twist in the data.

The clarity in your explanation is notable. Good job!

Wouldn't a graph here showing the trends be absolutely helpful? I find myself wishing you had included one.

=Conclusion=

In this lab, the effects of a polar protic and a polar aprotic solvent on the rate of a uni-molecular substitution reaction were observed. The polar protic solvent used was isopropanol, and the polar aprotic solvent used was acetone. Isopropanol was expected to be most effective solvent for this reaction because polar protic solvents lower the potential energy of the carbocation intermediate; therefore, lowering the initial activation energy of the reaction and increasing its rate. However, the data collected in class does not support this hypothesis. All of the reactions using isopropanol as a solvent had a slower rate than the reactions using acetone. This could be due to some unforeseen interaction between isopropanol and water. In every case, the addition of more water resulted in faster reaction times. Water is a polar protic solvent so this effect is expected. These inconsistencies are significant because it implies that there are other factors at work when preforming an S N 1 reaction than just the type of solvent used.

A possible source of error that could have occurred during this lab is an inconsistency in timing methods. For some of the slower reactions, determining when the color had completely changed was a judgement call and could account for some of the minor discrepancies between data sets. Also, some of the glassware that was used by the class could have had traces of water in them leading to inconsistent reaction rates. However, this fact was addressed prior to class and steps were taken to avoid it. Furthermore, minor discrepancies between the methodology between groups could also lead to minor inconsistencies between data sets. Therefore, It is important to keep all of the methods consistent between the groups for the most accurate results.

References:
Carrol, H. Or maybe, Higginbotham, C.? (2010). //Ch242/338 organic chemistry ii laboratory nucleophilic substitution reactions//. Unpublished manuscript, Chemistry, Retrieved from https://bb.cocc.edu/bbcswebdav/pid-422989-dt-content-rid-2549319_1/courses/CH242-10406_WI12/SN1RxnslabPOGIL_FULL.pdf

Degel, C. M. (Producer). (1997). //Sn1 and e1 reactions//. [Web Graphic]. Retrieved from http://www.personal.psu.edu/the1/sn1ande.htm