Factors Affecting the Relative rates of Electrophilic Aromatic Substitution Reactions

CHM145L Organic Chemistry 1 Laboratory 2nd Quarter SY 2014-2015

Factors Affecting the Relative rates of Electrophilic Aromatic Substitution Reactions Miranda, Marilyn1, Monta, Xyrell Claude D.2 Professor, School of Chemical Engineering, Chemistry and Biotechnology, Mapua Institute of Technology; 2Student , CHM145L/A21, School of Chemical Engineering, Chemistry and Biotechnology, Mapua Institute of Technology 1

ABSTRACT Aromatic Compounds are those organic compounds which have a benzene ring. These compounds always follows the substitution reaction due to the stability of the benzene ring, thus it will not process the elimination, addition or the rearrangement reaction. One of its common reactions is the electrophilic aromatic substitution, wherein, an electrophile reacts with an aromatic ring and substitutes a hydrogen atom. The experiment was done in three parts that determined the factors of electrophilic substitution on different aromatic rings, that involved the comparison of different reaction rates and structure of the compounds. The purpose of this experiment is to visualize the effects of the substituents in the relative rates of electrophilic aromatic substitution; to know the effect of solvent on reaction rates of reaction; and to know the effect of temperature on relative rates of reaction. The difference between the reaction rates and identifying and understanding the structure of the said compounds leads to the conclusions that a substituent that is more reactive than bezene is termed as an activator and are classified as ortho- para- directors, while a substituent that is less reactive than benzene is termed as deactivator and are classified as meta- directors. In terms of the solvent, the more polar the solvent is, the faster the rate of reaction would likely occur, whereas, this goes the same for temperature, the higher the temperature means the faster the rate of electrophilic aromatic substitution reaction. In doing this experiment, students are advised to be strict in observing the decolorization reaction of each compounds to avoid errors in gathering the data. Keywords: : aromatic compounds, substituents, solvents, orho- para- directors, meta- directors

INTRODUCTION

Electrophilic Aromatic Substitution Reactions are one of the significant processes in synthesizing a certain organic compound. For this reaction allows the introduction of many different functional groups onto an aromatic ring system, wherein, the overall reaction involves the substitution of a hydrogen on the aromatic ring system by an electrophile. The said reaction then, most commonly occur in aromatic compounds. In the electrophilic substitution, there are several factors that affect its relative rates, such as the substituent, solvent and temperature. The presence of a substituent on the aromatic ring affects its reactivity, wherein, it could either activate or

Experiment 09│ Group No. 03│ 11 – 25 – 14 ‘

deactivate the ring, this can make the ring to be more or less reactive than benzene, respectively. In addition, the presence of a substituent does also affect the orientation of a particular reaction. It is said that these substituents can be divided into three groups, namely, the ortho- and para- directing activators; and deactivators, and meta-directing deactivators. It should be noted though, that there are no meta-directing activators.

1 of 5

CHM145L Organic Chemistry 1 Laboratory 2nd Quarter SY 2014-2015

0.05M bromine in cyclohexane Bromine in acetic acid Apparatus: Micro test tubes Test tube rack 2 milliliter pipet

Fig. 1 otho- para- directing activators; and deactivators metadirecting activators

As stated earlier, the presence of the substituents affects the reactivity and orientation of the ring, these two factors are said to be controlled by the inductive and resonance effects. By definition, inductive effect is the withdrawal or donation of electrons through a sigma bond due to electronegativity and the polarity of bonds in functional groups, the resonance functions the same as the inductive effect, yet this time the donation of electrons occurs in a pi bond reaction for the reason that, the p orbital overlaps the substituent on the aromatic ring. The purpose of this experiment is to visualize the effects of the substituents in the relative rates of electrophilic aromatic substitution; to know the effect of solvent on reaction rates of reaction; and to know the effect of temperature on relative rates of reaction. MATERIALS AND METHODS

Reagents: 0.2 M solutions in ethyl acetate of the ff: Benzene Acetanilide Phenol Chlorobenzene P-nitrophenol Aspirin Anisole Bromine Solution

Experiment 09│ Group No. 03│ 11 – 25 – 14 ‘

As stated earlier, this experiment aims to determine the effects of the substituents, solvent and temperature on the relative rates of the electrophilic aromatic substitution, hence, this experiment was done in three parts. At the first part, in determining the substituent effect, a 0.20 milliliter of each solutions were transferred into separated test tubes. This was then followed by the addition of a 0.20 milliliter of bromine solution to each tube that contains the benzene, chlorobenzene, and aspirin. Afterwards the tubes were set aside and observed until decolorization occurs, the process was then repeated to the phenol and anisole. This was then followed by the addition of the bromine solution to the tubes that contains the acetanilide and p-nitrophenol, decolorization was then observed. At the second part, in determining the solvent effect, a 0.20 milliliter of the anisole solution was first transferred in a test tube, which was then followed by the addition of the 0.20 milliliter of freshly prepared 0.05M bromine in cyclohexane. On another test tube, a 0.20 milliliter of anisole solution was mixed with 0.20 milliliter bromine in acetic acid. The reactivity between the two solvents were then compared after its completion for two hours. At the last part, in determining the temperature effect, a 0.20 milliliter of 0.2 M in ethyl acetate was first transferred into as test tube, this was then followed by the addition of the 0.20 milliliter of bromine in acetic acid. A water bath was then prepared, wherein, a 300 milliliter of water in a 400 milliliter beaker was preheated, whereas, the water bath was heated 1 degree Celsius above the desired temperature. The test tube was then heated in a 50oC water bath. The same procedure was done in a 70oC and 90oC water bath, the gathered results were then plotted with 1/t sec as the ordinate and the absolute temperature as the abscissa.

2 of 5

CHM145L Organic Chemistry 1 Laboratory 2nd Quarter SY 2014-2015

RESULTS

layer and was transparent in color.

l. Substituent Effect Table 2.1

COMPOUND

Benzene

Chlorobenzene

Aspirin

Acetanilide

p-nitrophenol

Anisole

Phenol

TIME OF DECOLORIZ ATION

REMARKS

25 mins. 32 sec.

From red orange color, the solution contained an immiscible layer and turned yellow in color.

II. Solvent Effect

Compound

45 mins. 13 sec.

The red orange color remained the same and produced an immiscible layer.

Anisole + bromine in cyclohexane

32 sec

The color changed from orange to yellow and contained an immiscible layer.

Anisole + bromine in acetic acid

22 sec

The color of the solution became transparent with an immiscible layer

28 sec

There was no color change

13 sec

The solution became transparent and contained an immiscible layer.

11 sec

Remarks

2 hrs. and 15 mins.

From the red orange color, the solution tuned into pale yellow .

1 hr. and 35 mins

The orange color of the solution turned pale yellow.

Table 2.2

III. Temperature Effect Compound

o

90 C

o

70 C

Time of decolorization

Remarks

170 sec

The time of decolorization was fast.

192 sec

The time of decolorization was faster

215 sec

The time of decolorization of 5 was the3 fastest

The solution have an immiscible o

50 C Experiment 09│ Group No. 03│ 11 – 25 – 14 ‘

Time of Decolorization

CHM145L Organic Chemistry 1 Laboratory 2nd Quarter SY 2014-2015

Table 2.3

Fig. 2

DISCUSSION

Based on the gathered data, as observed from the first part, the time of the decolorization of cholorobenzene is much slower than that of benzene, due to the fact that chlorine is less reactive than benzene. This then makes the chlorine act as a deactivator for the aromatic ring, thus, making them ortho-para directors. Again, as seen on table 2.1, the time of decolorization of aspirin is much faster than the benzene and chlorobezene, this then makes the aromatic ring less reactive than benzene, which makes aspirin a metadirecting deactivator. This means that, the destabilization of ortho- and para- intermediates is caused by the formation of resonance at the positive charges of the carbocation intermediate directly on the ring carbon atom that contains the deactivating group,

Experiment 09│ Group No. 03│ 11 – 25 – 14 ‘

whereas, simultaneously, the resonance of the withdrawal of the electrons is felt at the ortho- and para- postion. Hence, the reaction in aspirin along with an electrophile occurs at the meta position. Moving down the table, the time of decolorization of acetanilide is 22 seconds which is faster, when compared to aspirin and p-nitrophenol. However, in beteween the acetanilide and the p-nitrphenol, the acetanilide must have a faster time of decolorization, due to the fact that acetanilide has strong, electron donating resonance effect that is most likely observed at the ortho- and para- positions. In the case of pnitrophenol, despite of the fact that it has a more reactive activator compared to acetanilide, this compound will still react slower because of its deactivator, particularly is the –NH2. As we move down further, the decolorization time of phenol, basing on the gathered results, is much faster when compared to the time of decolorization of anisole, for the reason that the phenol has an –OH substituent which then, makes it more reactive than benzene, and since they are more reactive than benzene, we could say that phenols can be classified as ortho- para- directing activators. Anisole on the other hand, do also have a substituent that makes the aromatic ring more reactive than benzene, this substituent is the –OCH 3 and is as well classifeid as an ortho- para- directing activators, Yet –OH is more reactive than –OCH 3, which will then make the phenol react faster than the anisole. At the second part of the experiment, the effect of solvent on the reaction rates of the aromatic substitution reaction via bromination was exhibited, whereas, the anisole was brominated in two different solvent, particularly the cycloexane and acetic acid. In the end of the experiment it was observed that the bromination of anisole in acetic acid transpired faster than that of cylcohexane. The last part of the experiment involves the bromination of acetanilide in ethyl acetate, which was tested on a 90°C , 70°C, and 50°C temperature. Table 2.3 clearly suggests that as the temperature is increased the rate of reaction is also increased, this then corresponds to the plotted graph, wherein, an

4 of 5

CHM145L Organic Chemistry 1 Laboratory 2nd Quarter SY 2014-2015

exponential increase in the reaction rate was observed as the temperature increases. Moreover, we can then say that the graph is linear with respect to the two variables, which is the temperature and the rate of reaction, which is identified as 1/t. This then suggests that the involved reaction is in a first order reaction. CONCLUSIONS AND RECOMMENDATIONS

Upon this experiment, firstly, I would like to recommend the students who will perform this experiment, should be strict in observing the time of decolorization of a certain compound in order to avoid errors in recording and gathering data. In doing this experiment, it is important to determine the effects of the substituents, solvent, and temperature, whereas, it has come to my conclusion, that the presence of a substituent affects the reactivity and orientation of the ring, whereas, a substituent more reactive than benzene is termed as an activator and are classified as ortho- para- directors, while a substituent less reactive than benzene is termed as deactivators and are classified as meta-directors. In terms of solvents, the more polar the solvents, the faster rates of reaction would occur, the same goes for temperature, whereas, the higher the temperature, the rates of electrophilic aromatic substitution reaction woukd react faster. REFERENCES : 1. Klein, David (2012). Organic Chemistry. Danvers. John Wiley & Sons, Inc., 2. Mendoza, Estrella (1990). Chemistry. Quezon City, Phoenix Press, Inc., 3. Wilbraham, Anthony (1997).Chemistry (4th Ed.). San Juan, Addison - Wesley Publishing Company, Inc., 4. Masterton, William (2009). Chemistry (Principles and Reactions).Belmont, Brooks/Cole Cengage Learning., 5. McMurry, John. Publications. 2004

Organic

Chemistry.

Experiment 09│ Group No. 03│ 11 – 25 – 14 ‘

Thomson

5 of 5