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Computer-Interfaced Experiments - Absorbance
Measurement Bromination of reactive Aromatics - Pseudo First Order Reaction Peter Keusch |
IBK electronic + informatic |
Hazards and safety precautions:
Safety goggles and gloves must be worn when handling bromine, acetic acid, 4-nitrophenol and diphenylether. The preparation of the corresponding solutions and the experimental procedure are carried out in a fume hood! Theoretical background: Aromatics without activating substituents react with chlorine or bromine only in the presence of Lewis acids (FeCl3, AlBr3 and other). The role of the FeCl3 or AlBr3 is to complex the bromine to form a bromonium cation-like species (often simply referred to as Br+) which is the actual electrophilic agent. Reactive aromatics do not need catalysts. 
Fig. 1: Bromination of acetanilide
|  The rate of the reaction depends on the concentration of the bromine cBr and the aromatics cAr (reaction second order). Under the given conditions (see procedure - large excess of one of the reactants) the reaction is pseudo first order. Kinetic equations (Download PDF file) During the thermal equilibration the calibration of the photometer and the matching of the programm are carried out. Calibration of the photometer and matching of the program CHEMEX Spectronic 20 (Download) features an analog output on the bottom of the instrument. The analog output of the photometer is connected to the input Sensor1 of the CHEMBOX. The photometer has been designed so that when it displays 100 % transmittance, the analog signal at its output connector is 1 VDC; when the instrument displays 0 % transmittance, the output voltage is 0 VDC. Calibration of the photometer: Using the wavelength control knob the wavelength is set to 420 nm. After zero calibration is completed, a cuvette filled with a mixture of 0.02 M molar bromine solution and 0.5 M solution of the aromatic compound (2:1) is placed into the sample compartment. When the transmittance maximum has been reached, the meter needle is adjusted to "100" on the % transmittance scale (0 absorbance). Matching of the program: Via the menu item Options - Calibration the calibration dialog for the appropriate input is to activate. In the field of Ref1 is entered 0,0V and in the field of Ref2 is written 1,0V. The cuvette is removed from the sample compartment and the button Set of Ref1 is clicked. Afterwards the cuvette with the decolorized bromine solution is placed into the sample compartment and the button Set of Ref2 is clicked. The check box of the two buttons (Ref1, Ref2) must show a green tick. If the check mark does not appear, the calibration must be repeated.  Setup - Channel-Linking Under the menu item View the Setup dialog is activated.  The calibrated signal of the photometer is indicated with K1. K1 · 100 is for the transmittance T. The other channel uses likewise the signal K1, converts it however via the formula 2 - log(K1 · 100) into the value for the absorbance A. Experimental procedure:
Fig. 5: Real-time plots - bromination of acetanilide at 23.5 °C determination of half-life Appropriate cursor-positioning allows the determination of the half-life t 1/2 on the screen (Fig. 5 mouse cursor: nw-resize). In the graph above three successive half-life periods of 18 s are illustrated. The constancy of the half-life is proof of a reaction first order. Data analysis using Excel (Download) - determination of the rate constants and the activation parameters: An Excel function is used to convert the absorbances into -lnA values (Tab. 1). A plot of -lnA versus t is generated (Fig. 8). 
Tab. 1: Measured values A (t) calculation of -lnA 
Fig. 6: Temperature effect 1: 12.6 °C 2: 17.5 °C 3: 23.5 °C The pseudo-first order rate constant k' can be already determined from the measured data, by determining the half-life t1/2 and inserting the appropriate value into equation (11) Kinetic equations (Download PDF file). The half-life t1/2 is found in the table of the measured values (Tab. 1).
k' is also obtained from the plot of E against t, if a "best-fit" exponential curve is drawn through the plotted data points according to equation (7) Kinetic equations (Download PDF file). 
Fig. 7: Determination of the pseudo-first order rate constants k' Finally the pseudo-first order rate constant k' can be determined in accordance with equation (9) Kinetic equations (Download PDF file) from the plot -lnA versus t. 
Fig. 8: Determination of the pseudo-first order rate constants k'
(According to the reaction conditions: k = k' / 0.1875) If the reaction temperatures and the corresponding rate constants are entered into the table of them Excel file Activation parameter (Download), then all activation parameters (Tab. 4) will be calculated and the ARRHENIUS and the EYRING plot will be generated (Fig. 9). 
Tab. 4: Calculation of the activation parameters 
Fig. 9: ARRHENIUS (1) and EYRING plot (2)
Discussion: 
Fig. 10: Diphenylether (1) 4-nitrophenol (2) acetanilide (3) The different reactivity of the available aromatics (diphenylether << 4-nitrophenol < acetanilide) is based on the different substitutents. The total substituent effects are a combination of inductive effects and resonance effects. Therefore their quantitative interpretation is difficult. · In acetanilide the acetyl amino group has a strong electron-donating resonance effect which outweighs a weaker electron-withdrawing inductive effect. The electron density on the ring is increased. The resonance only allows electron density to be positioned at the ortho- and para- positions. The acetyl amino group is moderatly activating. The rate of substitution is accelerated. The electron density on the ring of 4-nitrophenol is not increased distinctly: · The hydroxyl group is electron-donating by resonance (weak electron-withdrawing inductive effect and strong electron-donating resonance effect) · On the other hand the NO2 group is electron withdrawing by both inductive and resonance effects. Therefore, the rate of substitution is decreased. Reference: Substituent Effects Index of Chembox Experiments 
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