Relative Rates of Electrophilic Aromatic Substitution

Relative Rates of Electrophilic Aromatic substitution:

Phenol, Anisole, Diphenyl Ether, Acetanilide, 4-Bromophenol, 1-Naphthol

Organic Chemistry VWCC

Lanette Upshaw

1/29/2014

Introduction:

Recordingthespeed of the electrophilic substitutionto evaluate the relative reactivates of different areneswill demonstrate the effects of electrophilic aromatic bromination.The reaction taking place in this procedure will be semiquantitative and quantitative measures of differentareneswith the bromoniumion, which will create a corresponding aryl bromide. As seen in Figure 1.isan arene in a solution with bromine to form an aryl bromide[1].

Figure 1. Aryl bromide reaction.

The experimentalconditions of an arene and bromine will allow for direct comparison of the relative reaction rates. These studies help determine if a substituent is a ring activator or deactivator which state whether the substituted is faster than benzene or one that reacts slower than benzene. The activators may also encourage other carbon groups to add on to the aromatic ring at the ortho-, para- or meta- locations. Ortho- and para- tend to be weak to strong activators and meta- will normally be mild to strong deactivators. If one of the major atoms is a nitrogen or oxygen and is directly attached to the carbon ring then the result is probably activation. A deactivating group is a functional group that removes electron density from the benzene ring [2]. This makes the benzene ring electrophilic aromatic substitution reactions more difficult than they would be on benzene by itself. Nitrogen groups are all very strongly activating. Nitrogen is more electronegative than carbon and will share a pair of electrons.-OH and -O- groupsare more electronegative than nitrogen and havetwo pairs of electrons to share.Both groups electrons will negate any withdrawing effects that might occur in the reactions. The resonance structure of the ortho- attack results in a positive charge with the carbon and hydroxyl group on an OH-aromatic ring such as phenol. Ortho- is the most stable of formation because the oxygen and the two negative electron pairs will stabilize the charge of the carbon. Para- is similar but leaves a less stable hydrogen atom with the positive charge. Para- is more common with reactions that have bulky compounds on aromatic rings. The resonance meta-attacksform a positive charge on a carbon with only a hydrogen attached at position 4. This will be the least stable if a hydroxyl group is involved and is the least common of the three reactions. Table 1. shows variables that go into determining strong activators and deactivators[1].

Table 1. Ortho-, Para-, and Meta- Reaction Guide

Although the ortho- and para- will be more common among the reactions used there will still be a mix of both that take place in the reaction. By comparing Se2 reactions the magnitude can be calculated of difference between substituents. In Figure 3. are the monosubstitutedarenecompounds phenol, anisole, diphenyl ether, acetanilide, 4-bromophenol, and 1-naphthol which will be used in this experiment.

Figure 3. Compounds phenol, anisole, diphenyl ether, acetanilide, 4-bromophenol, and 1-naphthol. These compounds will all go through bromination with 1.5 mL of bromine solution in each test tube of the solution[2].

IR spectra of the original and the new compounds through bromination have been labeled Figure 4. through Figure 9. respectively and are in order from most reactive to least reactive [3]. Analysis of the data from quantitative measurements the rate expression in Figure 2. will be used.

K1t = ln [Br2]0/[Br2]t

Figure 2. Se2 Rate Expression.

This rate order will be considered pseudo first order and excess of the aromatic substrate will be used so that the concentration does not change during the reaction. By plotting a natural log of the concentrations ratios a straight line will confirm a first order rate reaction.

Procedure:

Pasteur pipets were used to obtain 1.5 mL of each compound which was prepared in a concentrated solution of acetic acid . Each was individual added to 1.5 of bromine in separate test tubes. Each test was kept at 35 C n a water bath. The times were recorded of how long it took for the reaction to complete once the bromination was added. The experiment was repeated at 0 C in an ice bath with the same compounds and the data was recorded. Table 2. the total time results of each bromination reaction at 35 C has been recorded. Table 3. the total time results of each bromination reaction at 0 C have been recorded.

Data and Discussion:

Phenol bromination creates an ortho-2-bromophenol or a para-4-bromophenol under the monobrominationconditions of the experiment. Figure 4. Shows a large spike around the 3000 - 3100 cm-1 area which indicates a large amount of C-H bonds in aromatic rings. The IR spectra of 2-bromophenol doesn't have the presence of this spike due to the forming of a C-Br bond. The chance of aortho- group formin is 40 % for most bromination reactions that involve an -OH group and 60 % for para- reactions [5]. The absence of the C-H 2900 spike would confirm that the reaction took place and that the bromophenol was aortho- para- directing group.

Anisole in Figure 5. could become para-4-bromoanisole or meta-3-bromoanisole. Due to the low chance of meta- director that could be formed most likely the para- directing compound occurred. There was a decreased intensity around the 2100 cm-1 spike from anisole to bromoanisole indicating a decrease in C-H bonds. Since the reaction was fairly quick meta- can also be ruled out due to the slower reaction time meta- deactivators.

Acetanilide forms para-4-bromoacetanilide due to its bulky N-R group.Of the compounds used acetanilide has the largest chance of having a meta- directing group. Figure 6. Shows a change in abundance of waves from 2500 to 3500. This demonstrates the presence of a substitution in compounds and a greater intensity of C=C bonds on aromatic rings. Due to the small chance of meta- because of less stability it can be assumed that the substitution was para- directing. Unless the use of UV-vis was used it cannot be determined how much of the reaction took place as para- or meta-, of course there was likely a mix of both.

4-Bromophenol when monobrominated can become either ortho-2 or ortho -6. There is very little chance of meta- holding a place due to the low stability

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