Factors Affecting the Relative rates of Nucleophilic Substitution Reactions

CHM145L Organic Chemistry 1 Laboratory 2nd Quarter SY 2012-2013

Factors Affecting the Relative rates of Nucleophilic Substitution Reactions 1

Estopace, Edgie , Tayag, Carlo A.

2

1Estopace,

Edgie, School of Chemical Engineering, Chemistry and Biotechnology, Mapua Institute of Technology; 2Tayag, Carlo A., CHM145L/A21, School of Chemical Engineering, Chemistry and Biotechnology, Mapua Institute of Technology

ABSTRACT In all of the organic reactions, nucleophilic substitution is one of the most studied because of its usefulness to the students who are taking up organic chemistry. Nucleophilic substitution reactions can occur by a range of mechanisms. There are two types of nucleophilic reactions, the unimolecular and bimolecular. Unimolecular reaction is also called as S N1 while bimolecular is also called as SN2. In an SN1 reaction, there are two steps, the fast step and the slow step which is the rate determining step. The first step is the formation of a carbocation after the departure of the leaving group. In the second step and fast step, the carbocation rapidly captures a nucleophile. In an SN2 reaction, there is only one step. This step is when nucleophile is attached as the leaving group leaves. Here, the reaction rate depends only on the concentration of the substrate. In both nucleophilic reactions, different factors affect their rates. These factors are the structure of the substrate, solvent used, concentration of the nucleophile, and the leaving group. In this experiment, the students were required to identify these factors affecting the relative rate of SN1 and SN2 reactions. Also, they were obliged to differentiate these two kinds of nucleophilic reactions. The apparatus used in this experiment are simple. These are the micro test tubes, pipets and corks. The reagents used in this experiment are the one listed in the laboratory manual and are discussed in the materials part of this paper. This is experiment is quite simple but it requires proper handling of chemicals since the chemicals used are toxic. Also, this experiment requires proper observation. The students had hard time in observing some of reactions since some reactions are fast. At the end, the students managed to answer all the questions needed and proved the theory behind. Keywords: nucleophilic substitution, SN1, SN2, substrate, nucleopile, substrate

INTRODUCTION Nucleophilic substitution is a reaction wherein a nucleophile attacks a partially positive charge or an atom called the electrophile. After attacking the substrate, atoms called the leaving group departs bringing with it an electron pair. There are two types of nucleophilic substitution. They are the unimolecular and bimolecular. Unimolecular substitution reaction is also called as SN1. In this type of reaction, there are two steps, the formation of a carbocation or slow step and the nucleophilic attack or the fast step. This reaction is found to be first order with respect to the substrate. This means that all other factors do not have any effect and only the concentration of the substrate affects the rate of the reaction. Also, the slow step determines the rate of the reaction. Regarding bimolecular substitution, two molecules which are the nucleophile and the substrate are involved in the rate determining step. Thus, its rate is second order overall. This type of reaction is also called as SN2. The Experiment 08│ Group No. 5│ 11-20-12

rates of these two types of reaction are affected by many factors. These factors are the substrate, nucleophile, leaving group and solvent. In bimolecular substitution, the nucleophile must be able to attack he electrophilic center substrate, resulting to the expulsion of the leaving group. If the substituents in the substrate are small atoms, there would be less steric hindrance. As a result, the ease of nucleophilic substitution is high. And if the substituents of the substrate are bulky, there would be a great amount of repulsion which diminishes the rate of reaction. This is because the bulky groups hider the backside attack of the nucleophile to the electrophile. However, in unimolecular substitution, it is the other way around. Next, the nucleophile’s strength affects the rate of reaction. The stronger the nucleophile, the faster the reaction can be. However, in unimolecular substitution, the nucleophile has no effect since it is not included in the rate law. The effect of

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solvent on the two types of reaction also varies. Polar aprotic solvents are used in bimolecular substitution and protic solvents are used in unimolecular substitution. Lastly, the effect of the leaving group on both reactions is the same. The weaker the leaving group is as a conjugate base and thus the stronger its corresponding acid, the better the leaving group. Examples of good leaving groups are therefore the halides. The purposes of this experiment are to identify the factors affecting the rates of unimolecular and bimolecular substitution and to differentiate SN1 from SN2 reactions. MATERIALS AND METHODS In this experiment, the students used the following apparatus: micro test tubes, 5.0 mL pipet and corks. The reagents that were used are 1-Bromobutane, 2Bromobutane, Benzyl bromide, 1-Chlorobutane, Absolute ethanol, 1.0 M 1-Bromobutane, 2.0 M 1-Bromobutane, 2Bromo-2-methylpropane, 7.5% NaI in acetone, 15% NaI in acetone, 1-Bromo-2,2-dimethylpropane, 0.1 M silver nitrate in absolute ethanol, 0.1 M silver nitrate in 5% ethanol 95% acetone, 0.1 M tert-butyl bromide in ethanol, 0.2 M tert-butyl bromide in ethanol, and 2-Chloro-2-methylphropane. This experiment is easy to perform. It only requires proper observation and timing because time of every reaction should be noted and the reaction happens in small span of time. The students just followed the simple steps stated in their laboratory manuals. Since they are two in each group, one member did the transferring of liquids in the micro test tubes and the other one recorded the time. After the time of the reaction was recorded, one of them observed what will happen to the solution after 5 to 10 minutes. While doing the observation, the other member prepared for the next part of the experiment to save time and for them to accomplish the work on time.

2-bromobutane

28 s

2-bromo-2-methylpropane

58 s

benzyl bromide

4.5 s

Yellow cloudy with precipitate Yellow solution with precipitate Yellow cloudy with precipitate

B. Steric effect Compound 1-bromobutane 1-bromo-2,2dimethylpropane

Reaction time 3s 18 s

Observations Yellow white supernatant liquid Yellow precipitate formed

C. Leaving group effect Compound 1-bromobutane

Reaction time 10 s

1-chlorobutane

16 s

Observations Yellow solution with precipitate Yellow solution with precipitate

D. Concentration of nuchleophile effect Compound

Reaction time N/A

Observations

2.09 s

Yellow solution

6s

Cloudy

N/A

Clear

RESULTS

15 % NaI in acetone + 1 M bromobutane 15 % NaI in acetone + 2 M bromobutane 7.5 % NaI in acetone + 1M bromobutane 15% NaI in acetone + 1M bromobutane Rate Law Expression

The tables below show the data gathered in the first part of the experiment, factors affecting the rate of nucleophilic bimolecular substitution reaction.

The tables below show the data gathered in the second part of the experiment, factors affecting the rate of nucleophilic unimolecular substitution reaction.

A. Structure of the substrate effect

A. Structure of the substrate effect

Compound 1-bromobutane

Time for reaction 19.5 s

Experiment 08│ Group No. 5│ 11-20-12

Observations

Compound

Cloudy solution with precipitate

1-bromobutane 2-bromobutane

Clear

R = k[Bromobutane][NaI]

Time for reaction 82 s 24 s

Observations Cloudy Cloudy

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2-bromo-2-methylpropane

4.5 s

benzyl bromide

11 s

White liquid with yellow precipitate White liquid with light green precipitate

Nucleophilic unimolecular substitution reaction: R----X  R+ + X- (Slow step – rate determining step) R++ + Nu-  R ---- Nu (Fast step)

B. Leaving group effect Rate = k[R----X] Compound 2-Bromo-2-methylpropane

Reaction time 2s

2-Chloro-2-methylpropane

2s

Observations Cloudy with grey precipitate Cloudy with violet precipitate

2-Bromo-2-methylpropane + silver nitrate in absolute ethanol 2-Bromo-2-methylpropane + silver nitrate in 5% ethanol 95% acetone

Reaction time 1.35 s 1.82 s

Observations White liquid with light yellow precipitate White liquid with more light yellow precipitate

D. Concentration of nuchleophile effect Compound

R----X + Nu-  R----Nu + XRate = k[X-][R----X]

C. Solvent polarity effect Compound

Nucleophilic bimolecular substitution reaction:

Reactio n time 4.2 s

Observations

0.1 M 2-Bromo-2White precipitate methylpropane in ethanol + was observed. 0.1 M silver nitrate solution in ethanol 0.2 M 2-Bromo-23s White precipitate methylpropane + 0.1 M was observed silver nitrate solution in ethanol 0.1 M silver nitrate in 2.3 s Cloudy with ethanol + 0.1 M 2-Bromo-2yellow recipitate methylpropane in ethanol 0.1 M silver nitrate solution 4.5 s Cloudy with in absolute ethanol and 1 yellow precipitate mL absolute ethanol + 0.1M 2-Bromo-2-methylpropane in ethanol Rate Law Expression R = k[2-Bromo-2-methylpropane]


DISCUSSION In this experiment, nucleophilic substitutions and factors affecting their rates were observed. The different factors that affect the rate of the reactions are substrate, concentration of the nuchleophile, leaving group and the solvent. In the first part of the experiment, the effect of the structure of the substrate was shown. The benzyl bromide is the fastest to react while 2-bromo-2-methylpropane is the slowest. This shows that primary reacts faster because there is a less amount of hindrance and so the nuchleophile can attack easily. Considering the two alkyl halides, 1bromobutane and benzyl bromide, the benzyl bromide has faster rate. The explanation behind this is that since carbons are electron donating, they hinder the attack which hinders the SN2 reaction. Since 1-bromobutane has one electron donating group, this group slows down the attack. In the leaving group effect experiment, 1-bromobutane has faster rate than 1-chlorobutane which means that bromide ion is the better leaving group. Bromide ion is better because it is a weaker base than chloride ion. It is classified as weak base because its conjugate acid is strong. In part D of the first part of the experiment, it is clear seen that higher concentration yields faster reaction. In part two, letter A of the experiment, the explanation is just the opposite of part one, letter A. In part B, the table shows that the reaction times are the same which means it is incorrect since the compound with bromine should react faster. In part C, the solution with the absolute ethanol reacted faster because of the pure protic solvent involved in the reaction.

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Lastly, the concentration of the nucleophile in part D has no effect to rate and doubling it also has no effect because it is not included in the rate law. Only the concentration of the substrate has an effect to the rate of the reaction. CONCLUSIONS AND RECOMMENDATIONS Based on the data gathered, factors affecting the rate of reactions vary. In nucleophilic unimolecular substitution reaction, the effect of the substrate is that tertiary compounds react faster than primary and secondary. Then, It reacts faster in polar protic solvent. However, the nuchleophile has no effect since it is not included in the rate law. In nucleophilic bimolecular substitution, the effect of the nucleophile is that stronger nuchleophile will result to faster reaction. The primary substrate will react faster than the secondary and tertiary since larger group of substituents hinder the attack of the nucleophile. In this type of reaction, polar aprotic solvents are used. With this information, I conclude that the two reactions are different. They are different in terms of how they happen and what factors affect their rates. To the next students who will perform this experiment, I recommend that they must always be observant because some reactions are fast. Also, they must always follow the safety precautions since many reagents used in this experiment are toxic and can cause irritation to the eyes.


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REFERENCES 1. Baluyut, J.Y., and De Castro, K. Organic Chemistry Laboratory Manual for Chemistry Students, Part 1, 59-62 2. Klein, D. (2012). Organic Chemistry. United States: John Wiley & Sons, Inc. 3. Nucleophilic Substitution. Retrieved from http://en.wikipedia.org/wiki/Nucleophilic_substitution 4. Effect of Nuchleophile. Retrieved from http://chemwiki.ucdavis.edu/index.php?title=Organic_Chemi stry/Reactions/SN1_Reaction/Effects_of_Solvent%2C_Lea ving_Group%2C_and_Nucleophile_on_Unimolecular_Subst itution 5. Unimolecular Substitution. Retrieved from http://www.cartage.org.lb/en/themes/Sciences/Chemistry/Or ganicchemistry/CommonReaction/Unimolecular/Unimolecul ar.htm 6. Bimolecular Substitution. Retrieved from http://en.wikipedia.org/wiki/SN2_reaction


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Factors Affecting the Relative rates of Nucleophilic Substitution Reactions

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