Mapua Institute of Technology Organic Chemistry Laboratory 2
Final Report Factors Affecting the Relative Rates of Electrophilic Aromatic Substitution Reaction Justiniano, Priscilla Raiza N. School of Chemical Engineering and Chemistry, Mapua Institute of Technology, Intramuros, Manila, Philippines Experiment No. 1, Submitted on August 6, 2011 at N402.
EXPERIMENT NUMBER ONE IS ALL ABOUT THE ELECTROPHILIC SUBSTITUTION OF AROMATIC COMPOUNDS. AROMATIC COMPOUNDS ARE THOSE ORGANIC COMPOUNDS WHICH HAVE BENZENE RING (CYCLOHEXA-1,3,5-TRIENE).
AROMATIC COMPOUNDS ARE ALWAYS FOLLOWS THE SUBSTITUTION REACTION BECAUSE OF THE STABILITY OF THE BENZENE RING. IT WILL NOT PROCESS THE ELIMINATION, ADDITION OR REARRANGEMENT REACTION. THIS EXPERIMENT WILL DETERMINE THE FACTORS OF ELECTROPHILIC SUBSTITUTION ON DIFFERENT AROMATIC RINGS THUS IT WILL HAVE DIFFERENT REACTION RATES BECAUSE OF DIFFERENT STRUCTURE OF THE COMPOUNDS. DIFFERENT REACTION WILL YIELDS TO A CONCLUSION OF WHAT IS/ARE THE FACTOR/S OF ELECTROPHILIC SUBSTITUTION ON AN ORGANIC COMPOUND – AROMATIC COMPOUNDS.
There are three experiments in this chapter of the laboratory. First is the substitution by bromination. Second is the solvent effect which is dealing in the nature of the solvent. And third is the temperature test. Having this division in the experiment one, we can now conclude that substituent in substitution, the nature of the solvent polar or non-polar and the temperatures are the factors of electrophilic substitution on aromatic compounds.
The most common reaction of aromatic compounds is electrophilic substitution.
It is when an electron-poor reagent (an electrophile, E+) reacts with the electron-rich aromatic ring (a nucleophile) and substitutes for one of the ring hydrogen. Electrophilic Substitution has several factors that affect its relative rates and this includes the following: substituents, solvent and temperature. Substituent like –OH and Br has an effect in electrophilic aromatic substitutions. All activating groups are ortho- and para-directing, and all deactivating groups other than halogen are meta directing. The halogens are ortho- and para- directing deactivators.
The activation or deactivation of the benzene ring toward electrophilic attack, and electron withdrawing substituents deactivate the ring. Using the reagents benzene, chlorobenzene, aspirin, acetanilide, p-nitrophenol, anisole and phenol in ethyl acetate, we will observe the bromination rate and will classify as either activator or deactivator. In performing the experiment, 0.20 ml of bromine solution will be added in each solution. In general terms, the choice of solvent can have a significant effect on the performance of a reaction.
For an SN1 reaction, the polarity and ability of the solvent to stabilize the intermediate carbocation is of paramount importance. For an SN2 reaction, the effect of solvent polarity is usually much less, but the ability (or really lack thereof) of the solvent to solvate the nucleophile is the important. Using the two solvents cyclohexane, non-polar, and acetic acid, polar, we will determine the reaction rate of bromination in anisole. Another thing that must be considered is the temperature.
Typically heat will speed up a chemical reaction because it causes the actual atoms to move quicker and collide with each other causing a chemical reaction. In this experiment we are given three different temperature and we will going to determine the trend occurred as we increase the temperature during the bromination of acetanilide.
Materials The Reagents used are the following: 0.2 M solutions in ethyl acetate of the following: a. Benzene – is an organic chemical compound with the molecular formula C6H6. It is sometimes abbreviated Ph–H. Benzene is a colorless and highly flammable liquid with a sweet smell and a relatively high melting point. Because it is a known carcinogen, its use as an additive in gasoline is now limited, but it is an important industrial solvent and precursor in the production of drugs, plastics, synthetic rubber, and dyes. b. Chlorobenzene- is an aromatic organic compound with the chemical formula C6H5Cl. This colorless, flammable liquid is a common solvent and a widely used intermediate in the manufacture of other chemicals. . Aspirin- is a salicylate drug, often used as an analgesic to relieve minor aches and pains, as an antipyretic to reduce fever, and as an anti-inflammatory medication. d. Acetanilide- is an odorless solid chemical of leaf or flake-like appearance. It is also known as N-phenylacetamide, acetanil, or acetanilide, and was formerly known by the trade name Antifebrin. e. p-Nitrophenol- is a phenolic compound that has a nitro group at the opposite position of hydroxy group on the benzene ring. f. Anisole- is the organic compound with the formula CH3OC6H5. It is a colorless liquid with a smell reminiscent of anise seed, and in fact many of its derivatives are found in natural and artificial fragrances. The compound is mainly made synthetically and is a precursor to other synthetic compounds. g. Phenol- is an organic compound with the chemical formula C6H5OH. It is a white, crystalline solid. This functional group consists of a phenyl, bonded to a hydroxyl (-OH). The Apparatus/ materials used are the following:
a. Test tube rack – is used in a laboratory and it is used to hold/support test tubes containing chemicals waiting for further operations. b. Hot water bath- is when you put water in some container (usually a beaker) and heat it using a Bunsen burner or hot plate. The purpose of using a hot water bath is actually to heat something else c. 2-ml pipet- allow the user to measure a volume of solution extremely accurately and then add it so something else. They are commonly used to make laboratory solutions from a base stock as well as prepare solutions for titration. d.
Hot plate- is an adjustable heating source which is ideal for heating beakers, Erlenmeyer flasks, hot water baths, and other flat-bottomed containers. It is essentially an electric stove top that is used in the laboratory. e. Micro test tubes with cork stopper – clear, cylindrical glass tube usually open at one end and rounded at the other, used in laboratory experimentation f. Thermometer- measures temperature and the device has been calibrated so that a given height of the liquid inside will correspond to a certain temperature. Methods The experiment was divided into three parts.
For the first part which is Substituent Effect, using separate test tubes we prepared seven reagents namely: benzene, chlorobenzene, aspirin, acetanilide, p-nitrophenol, anisole and phenol, they are in 0.2 M solutions of ethyl acetate. Then, we added 0.20 ml of bromine solution to each test tube. Upon the addition of bromine solution, we shook it and observed them occasionally for evidence of reaction. We are after the change of color and the rate of color change so it is necessary to list down the rates of reaction. It is necessary to ensure that they are kept away from sunlight.
If loss of color is not immediate, set the test tube aside. For the second part which is the Solvent Effects, we transferred 0.20 ml of the anisole solution in a test tube and then added 0.20 ml of freshly prepared 0.05 M bromine in cyclohexane. We repeated the process but this time we added acetic acid. Like the first experiment we are asked to observe the color change for it indicates the reactions and we are asked to compare the reactivity of the anisole in the two solvents. And for the last part which is the Temperature Effect, we transferred 0.0 ml of 0.2 M acetanilide in ethyl acetate into the test tube then we heated the test tube in a 50C water bath prepared by preheating a 400 ml beaker with 300 ml water. After that we added 0.20 ml of bromine in acetic acid. Given three temperature pointers we repeated the procedures for 70C and 90C. It is important to note the time it consumed for the plotting of information.
3. RESULTS AND DISCUSSION
Substituent Effect For this part of experiment, Substituent effect in the relative rates of electrophilic aromatic substitution is tested.
We used eight reagents and added bromine solution. Upon transferring the solution, we as well observed the change of color and the time of decolorization on each reagent. The result is shown on Table 1. |COMPOUND |TIME OF DECOLORIZATION |REMARKS | |Benzene |More than 30 min |Immiscible layer | |Cholorobenzene |More than 30 min |Immiscible layer | |Aspirin |16. 93 sec |Light yellow to | | | colorless | |Acetanilide |4mins 53 sec |Partially | | | |immiscible | |p-nitrophenol |1 min 35 sec |Dark red to dark | | | |orange | |Anisole | |Dark red to dark | | |1min 11 sec |orange | | | |Dark red to orange| |Phenol |2 sec | |
Table 1 Substituent Result The result of this experiment can be explained by considering that Electrophilic Aromatic Substitution (EAS) reactions are among the most common and most important in organic synthesis. Preexisting substituents on an aromatic ring have both kinetic and directive effects on electrophilic aromatic substitution reactions. That is: 1. substituent will either increase or decrease the rate of an EAS reaction (relative to benzene); 2. a substituent will “steer” incoming electrophiles onto particular locations of the aromatic ring, relative to the original substituent itself (ortho, meta, or para). From the result we got, Phenol has the least time of decolorization while benzene and chlorobenzene got the most. This means that phenol increase the rate of an EAS reaction.
These observations, and many others like them, have led chemists to formulate an empirical classification of the various substituent groups commonly encountered in aromatic substitution reactions. Thus, substituents that activate the benzene ring toward electrophilic attack generally direct substitution to the ortho and para locations. With some exceptions, such as the halogens, deactivating substituents direct substitution to the meta location. The following table summarizes this classification. Activating Subsituents|Deactivating Subsituents|Deactivating Subsituents| |Ortho & para- |Meta-Orientation |Ortho & para-Orientation| |Orientation | | | |–O(–) |–NO2 |–F | |–OH |–NR3(+) |–Cl | |–OR |–PR3(+) |–Br | |–OC6H5 |–SR2(+) |–I | |–OCOCH3–NH2 |–SO3H |–CH2Cl | |–NR2 |–SO2R –CO2H |–CH=CHNO2 | |–NHCOCH3 |–CO2R | | |–R |–CONH2 | | |–C6H5 |–CHO | | | |–COR | | | |–CN | | Reactivity of Ring Substituents The information summarized in the above table is very useful for rationalizing and predicting the course of aromatic substitution reactions, but in practice most chemists find it desirable to understand the underlying physical principles that contribute to this empirical classification. The following table (Table 2) shows the reactions involved during the bromination of each reagents. REACTIONS INVOLVED | |[pic] | |[pic] | |[pic][pic][pic] +[pic] | |[pic][pic][pic] + [pic] | |[pic] | |[pic][pic][pic]+ [pic] | |[pic][pic] | Table 2 Substituent Effect Result Solvent Effects |Compound |Time of decolorization |remarks | |Anisole + bromine in |25 min |Cloudy white to | |cyclohexane | |clear transparent | |Anisole + bromine |53 secs |Color changes from| |acetic acid | |red to yellow |
Table 3 Solvent Effect Result Based on the above data, the reaction of Anisole with bromine in cyclohexane is less favored than that of Anisole with bromine in Acetic acid. The scientific reason behind this is that acetic acid is more polar than cyclohexane thus the reaction of anisole with bromine using acetic acid as the solvent took place faster than the latter. Solvent polarity helps to increase the rate of reaction due to the fact that electrophilic aromatic substitution reaction is a two- step polar reaction. 3.3 Temperature Effect In this part of the experiment, the effect of temperature in the rate of electrophilic aromatic substitution was tested.
According to the data gathered when acetanilide in ethyl acetate was made to react with bromine in different temperatures, as shown in Table 4, the rate of reaction increases as the temperature increases. As a rough approximation based on what happened in the experiment, the rate of reaction doubles for every 10°C rise in temperature for the reactions that happened at around room temperature. |Compound |Time of Decolorization |Remarks | | | |Color changes | |90°C |59 sec |rapidly from | | | |light yellow to | | | |colorless. | | | |Color changes | |70°C |316 sec. moderatey from | | | |light yellow to | | | |colorless. | |50°C |543 sec. |Color changes | | | |slow. | Table 4 Temperature Effect Result The explanation behind this result can be interrelated with the collision theory that particles can only react when they collide. If heat is applied in a substance, the particles move faster and so collide more frequently that will speed up the rate of reaction. Collisions only result in a reaction if the particles collide with enough energy to get the reaction started. This minimum energy required is called the activation energy for the reaction.
By looking at the diagram below that is based on Maxwell-Boltzmann distribution further explains the relevance of the increase in temperature in the increase in rate of reaction. [pic] Figure 1: Activation energy diagram Only those particles represented by the area to the right of the activation energy will react when they collide. The great majority doesn’t have enough energy, and will simply bounce apart. To speed up the reaction, it is a need to increase the number of the very energetic particles – those with energies equal to or greater than the activation energy. Increasing the temperature has exactly that effect – it changes the shape of the graph. In the next diagram, the graph labeled T is at the original temperature. The graph labeled T+t is at a higher temperature. [pic] Figure 2: Activation energy diagram
It is still visible, although the curve hasn’t moved very much overall, that there has been such a large increase in the number of the very energetic particles that many more now collide with enough energy to react. [pic] Figure 3: Activation energy diagram On the last diagram, the area under the higher temperature curve to the right of the activation energy looks to have at least doubled – therefore at least doubling the rate of the reaction.
This experiment shows the effects of the substituent, solvent polarity and temperature on the relative rates of electrophilic aromatic substitution (EAS) reactions. Substituents on the benzene ring affect both the reactivity of the ring toward further substitution and the orientation of that further substitution.
Substituents that are classified as activators increase the rate of the reaction because it lowers the activation energy that is required for the reaction to occur. On the other hand, deactivators do otherwise; they increase the activation energy needed for the reaction to take place. Another factor is solvent polarity that affects the rate of reaction of EAS in a positive manner, it increases the reaction rate. Meaning, the more polar the solvent, the more the reaction will take place faster. Similarly, an increase in temperature, increases reaction rates because of the disproportionately large increase in the number of high energy collisions.
As future chemical engineers, this experiment is helpful in providing information on what factors may or may not favor the reactions or organic synthesis that we will be conducting. Furthermore, this experiment can serve as a guide as to what reagents must be used in order to maximize the production of the desired products. references  J. McMurry, Organic Chemistry fifth edition, 149, 2003 http://www2. chemistry. msu. edu/faculty/reusch/Virt TxtJml/benzrx1. htm http://www. personal. psu. edu/faculty/t/h/the1/chem/EASSubEffects/index. html#1 http://en. wikipedia. org/wiki/Electrophilic_aromatic_substitution http://www. chem. ucalgary. ca/courses/351/Carey5th/Ch12/ch12-0. html ———————– School of Chemical Engineering and Chemistry