Chemistry Extended Essay Essay Example for Free

Chemistry Extended Essay Essay To investigate the effect of 2-bromo-2-methyl propane concentration and temperature of the system on the rate of reaction of solvolysis of 2-bromo-2-methyl propane in 90% ethyl alcohol Done by: Habib Iscandar Hinn Friends Boys School June 22, 2007 To investigate the effect of 2-bromo-2-methyl propane concentration and temperature of the system on the rate of reaction of solvolysis of 2-bromo-2-methyl propane in 90% ethyl alcohol Introduction: The major product of the solvolysis of t -butyl chloride in 70 % water 30 % acetone is t-butyl alcohol, with a small amount of isobutylene being formed as a by product And this is with accordance of first order kinetic and suggests a two step mechanism in which the rate determining step consists of the ionization of t-butyl chloride, and in this mechanism a carbonium ion is formed as inter- mediate and this bonds immediately to near by nucleophile (in this case nucleophile is a neutral molecule) the initial product is t-butyl carbonium ion. Note1: if the nucleophile is neutral the product will be charged since the leaving group takes both bonding electrons away with it So chemists have proposed to general types of mechanism: 1- Nucleophilic substitution Sn1 The ionization step in a Sn1 reaction is endothermic and much slower than the exothermic neutralization of carbonium ion by a nucleophile. And so the rate determining step being the unimolecular ionization of the t-butyl chloride equation 4, and as a result, the overall rate of reaction is not affected by changes in the concentration or kinds of nucleophilic reagents present. Note2: the factor which determines the mechanisms employed is typically the nature of the substrate it self and not the particular nucleophile Note3: if the sum of the energy of the product is lower than the energy of the reactant the reaction is exothermic, and if the product have higher energy than the reactant the reaction is endothermic. 2- Elimination E1 (elimination unimolecular) And because t-butyl chloride acts as a Lewis acid (an electrophile) and combines with a nucleophile to give a substitution product, so the major product of the solvolysis of t-butyl chloride in water-acetone solvent is t-butyl alcohol. (Note4: electrophile: an electron deficient atom, ion or molecule that as affinity for an electron pair, and will bond to a base or nucleophile.) (Note5 : nucleophile: and atom, ion , or molecule that has an electron pair that may be donated in forming covalent bond to an electerophile.) Evaluating the mechanism: The only reactant that is undergoing change in the rate determining step is t-butyl chloride and so such reactions is a unimolecular and follow a first order equation (Sn1, E1). This means that the rate of the reaction varies directly with the concentration of t- butyl chloride. And since nucleophilic only participate in the fast second step, so their relative molar concentrations rather than their nucleiophilities are the primary product determining factor, and by using nucleophilic solvent like water, so its high concentration will assure that alcohols are the major product, and because water have a high dielectric constant (e=81) so water molecule tend to orient them-selves in such a way as to decrease the electrostatic forces between ions. And an important factor is the salvations which refer to water molecules ability stabilize ions by encasing them in a sheath of weakly bonded solvent molecules: 1- Anions are solvated by hydrogen bonding, 2- Cations are solvated by nucleophilic sites on water molecule (oxygen). And in this case of t-butyl carbonium ion the nucleophiles form strong covalent bond to carbon and converting the intermediate to a substitution product. The reaction mechanism is a sequential account of each transition state and intermediate in a total reaction, the over all rate of reaction is determined by the transition state of highest energy in the sequence, so the rate determining step is the rate determining step for both the Sn1 and E1 for t butyl chloride. (Note 6: the water soluble organic solvent acetone is used to keep a reasonable concentration of t-butyl chloride in solution) The balance equation for t-butyl chloride solvolysis in water-acetone solvent is: The effect of concentration on the solvolysis of t-butyl chloride in 70 %water 30 %acetone solvent. As the reaction proceeds the solution becomes increasingly acidic until all of the t -butyl chloride has reacted and all HCl that can form has formed. So we will monitor the reaction by allowing HCl formed to neutralize a predetermined amount of NaOH. An indicator dye (bromo-phenol blue) will change color when the NaOH has been neutralized, and clocking of the reaction should begin at the instant. So according to kinetic measurements: Rate of reaction = K [t butyl chloride] Where K is the specific rate constant in S -1 and [t butyl chloride] is the concentration of t-butyl chloride in M. Our kinetic measurement will depend on the determination of the amount of HCl produced by the reaction, so by monitoring the color change of the acid base indicator, we will determine the time required for 10% of t-butyl chloride to hydrolyze by having 10 % as much NaOH present as T-butyl chloride. Rate = d [Rcl] dt ; Where Rcl = -dt [Rcl] = K [Rcl] dt Rearranging, d [Rcl] = -K dt [Rcl] And integrating for t=0 to t=t will give; = Ln [Rcl] t Ln [Rcl] 0 = Kt 2.303 Log [Rcl] 0 = Kt [Rcl] t 2.303 Log [Rcl] 0 = Kt [Rcl] t Where [Rcl] 0: is the molar concentration at time t = 0 [Rcl] t: is the molar concentration at time t = t Two methods to calculate K 1- since the equation Kt = 2.303 Log [Rcl] 0 [Rcl] t Is an equation of a straight line (y=mx+b) with slope k. and intercept =0, a plot of 2.303 log [Rcl] 0 / [Rcl] t versus t should yield a straight line with slope k. 2- if the solvolysis reaction run to 10% completion Then, [Rcl] = 0.90 [Rcl] 0 Kt = 2.303 Log [Rcl] 0 = 2.303 log (1.11) 0.90 [Rcl] 0 And therefore, K = 0.104 T So by finding the value of K and compensate it in the rate of reaction equation Rate = K[Rcl] where the concentration of Rcl is known we can calculate the value of the rate of reaction and we will see its effect on the solvolysis of t butyl chloride in 70% water 30 % acetone solution. The effect of temperature on the solvolysis of t -butyl chloride in 70%water 30%acetone solvent. In nearly every instance an increase in temperature causes an increase in the rate of reaction, because the total fraction of all of the t butyl chloride 1molecules having energies equal to or greater than activation energy (Ea) Corresponds to the shaded portion of the area under the curve increases by increasing the temperature and by comparing the area for two different temperature, we see that the total fraction of t- butyl chloride molecules with sufficient kinetic energy to undergo reaction increases with increasing temperature and consequently, so does the reaction rate. Note7: changing the concentration affects the rate of reaction changing the temperature affects the rate constant as well as the rate. By finding the values of reaction rate constant K for different concentration of t-butyl chloride and different reaction temperature, we will find the effect of temperature on the solvolysis of t-butyl chloride in water acetone solvent. Quantitatively, K (s-1) is related to Ea and T by the equation K1 = Ae-Ea/RT1 1 Ea is the activation energy, in joule / mole. (Jmol-1) A is a proportionality constant, in s-1 R is the gas constant = 8.314 Jmol-1K-1 e is the base of the natural logarithms. T is temperature in Kelvin. This relation ship is known as Arrhenius equation We measure Ea by taking the natural logarithm of eq.1 Ln K = ln A Ea RT Thus, a plot of ln k versus 1/T gives a straight line whose slope is equal to -Ea/R and whose intercept with coordinate is ln A Note8: Ea is the activation energy, a constant characteristic of the reaction We can calculate the rate constant at some specific temperature if Ea and K at some other temperature are known. For any temp. T1 (known), Ea (known), K1 (known) K1 = A e -Ea/RT1 For any other T2 (known); (K2 unknown) K2 = A e -Ea/RT2 By dividing K1 over K2 K1 = A e -Ea/RT1 K2 A e -Ea/RT2 Taking natural logarithm of both sides, we get Ln K1 = Ea (1/T2 1/T1). K2 R Or in common logarithms (base 10 logarithms) gives: Log K1 = Ea (1/T2 1/T1) K2 2.303 R And by finding the value of K2 we will be able to find the rate of reaction at T2 and we will find the effect of temperature on the rate of solvolysis of t butyl chloride in 70 % water 30 % acetone solution. By finding the values of reaction rate constant K for different concentration of t-butyl chloride and different reaction temperature, we will find the effect of concentration and temperature on the solvolysis of t-butyl chloride in water acetone solvent. Procedure: Part A: the effect of concentration on the rate of solvolysis of t butyl chloride in 70%water 30%acetone solvent. a- Experimental procedure: to measure the time necessary for 10 % solvolysis of t butyl chloride (0.1 M concentration) in 70 % water 30% acetone solvent at room temperature. A, a, I:- 1- Prepare 500 ml of 0.1 M t- butyl chloride in acetone only and put it in an Erlenmeyer flask and label it #1. 2- Prepare 100 ml of 0.1 M NaOH solutions (in water) and put it in an Erlenmeyer and label it #2. 3- Using a burette take 30 ml of the solution in flask #1 and put it in another Erlenmeyer and label it #3. 4- By a graduated pipette take 3 ml of sodium hydroxide 0.1 M in an Erlenmeyer flask and label it #4. 5- Using a graduated cylinder measure 67 ml of distilled water added to an Erlenmeyer flask #4. 6- Add two drops of Bromo-phenol blue indicator to flask #4. A, a, II:- 1- Add quickly the solution in Erlenmeyer flask #4 to solution in flask #3 and start the stop watch to count for time in seconds. 2- Swirl the mixture and after one or two seconds immediately pour the combined solutions back into Erlenmeyer flask #4 to minimize the errors in the results. 3- The color of the mixed solutions is blue, so continue swirling the solution in Erlenmeyer flask #4 till the instant color of the solution start changing to yellow, then we stop the stopwatch and record the time. 4- Repeat the procedure at least three times and calculate the average. 5- Tabulate the results in record A. b- Experimental procedure: to measure the time necessary for 10 % solvolysis of t butyl chloride (0.2 M concentration) in 70 % water 30% acetone solvent at room temperature. A, b, I:- 1- Prepare 500 ml of 0.2 M t- butyl chloride in acetone only and put it in an Erlenmeyer flask and label it #1. 2- Prepare 100 ml of 0.1 M NaOH solutions (in water) and put it in an Erlenmeyer flask and label it #2. 3- Using a burette take 30 ml of the solution in Erlenmeyer flask #1 and put it in another Erlenmeyer flask and label it #3. 4- By a graduated pipette take 3 ml of sodium hydroxide 0.1 M in an Erlenmeyer flask and label it #4. 5- Using a graduated cylinder measure 67 ml of distilled water added to an Erlenmeyer flask #4. 6- Add two drops of bromo-phenol blue indicator to Erlenmeyer flask #4. A, b, II:- 1- Add quickly the solution in an Erlenmeyer flask #4 to solution in flask #3 and start the stop watch to count for time in seconds. 2- Swirl the mixture and after one or two seconds immediately pour the combined solutions back into an Erlenmeyer flask #4 to minimize the errors in the results. 3- The color of the mixed solutions is blue, so continue swirling the solution in Erlenmeyer flask #4 till the instant color of the solution start changing to yellow, then we stop the stopwatch and record the time. 4- Repeat the procedure at least three times and calculate the average. 5- Tabulate the results in record A. Part B: the effect of temperature on the rate of solvolysis of t butyl chloride in 70%water 30%acetone solvent. a- Experimental procedure: to measure the time necessary for 10 % solvolysis of t butyl chloride (0.1 M concentration) in 70 % water 30% acetone solvent at zero Celsius degree. B, a, I:- 1- Prepare 500 ml of 0.1 M t- butyl chloride in acetone only and put it in an Erlenmeyer flask and label it #1. 2- Prepare 100 ml of 0.1 M NaOH solutions (in water) and put it in an Erlenmeyer flask and label it #2. 3- Using a burette take 30 ml of the solution in Erlenmeyer flask #1and put it in an Erlenmeyer flask and label it #3. 4- By a graduated pipette take 3 ml of sodium hydroxide 0.1 M in an Erlenmeyer flask and label it #4. 5- Using a graduated cylinder measure 67 ml of distilled water added to Erlenmeyer flask #4. 6- Add two drops of bromo-phenol blue indicator to Erlenmeyer flask #4. B, a, II:- 1- Suspend the Erlenmeyer flasks in a water bath full with ice and water, allowing the temperature of the Erlenmeyer flasks and their contents to equilibrate for ten minutes. 2- Adding quickly the solution in Erlenmeyer flask #4 to solution in Erlenmeyer flask #3 and start the stop watch to count for time in seconds. 3- Swirl the mixture and after one or two seconds immediately pour the combined solutions back into Erlenmeyer flask #4 to minimize the errors in the results. 4- The color of the solution after that will become blue, so continue swirling the solution in Erlenmeyer flask #4 till the instant color of the solution start changing to yellow we stop the stop watch and record the time 5- Repeat the procedure at least three times and calculate the average. 6- Tabulate the results in record B. b- Experimental procedure: to measure the time necessary for 10 % solvolysis of t butyl chloride (0.1 M concentration) in 70 % water 30% acetone solvent at a temperature greater than room temperature by ten degrees. B, b, I:- 1- Prepare 500 ml of 0.1 M t- butyl chloride in acetone only and put it in an Erlenmeyer flask and label it #1. 2- Prepare 100 ml of 0.1 M NaOH solutions (in water) and put it in an Erlenmeyer flask and label it #2. 3- Using a burette take 30 ml of the solution in Erlenmeyer flask #1 and put it in an Erlenmeyer flask and label it #3. 4- By a graduated pipette put 3 ml of sodium hydroxide 0.1 M in an Erlenmeyer flask and label it #4. 5- Using a graduated cylinder measure 67 ml of distilled water added to Erlenmeyer flask #4. 7- Add two drops of bromo-phenol blue indicator to flask #4. B, b, II:- 1- Suspend the flasks #3 and #4 in a water bath full with ice and water, allowing the temperature of the flasks and their contents to equilibrate for ten minutes.(to reach the temperature of the water bath) 2- Adding quickly the solution in flask #4 to solution in flask #3 and start the stop watch to count for time in seconds. 3- Swirl the mixture and after one or two seconds immediately pour the combined solutions back into flask #4 to minimize the errors in the results. 4- The color of the mixed solutions is blue, so continue swirling the solution in flask #4 till the instant color of the solution start changing to yellow we stop the stopwatch and record the time 5- Repeat the procedure at least three times and calculate the average. 6- Tabulate the results in record B. Record A Run number Temperature Time of 10 % reaction Average time / seconds Record B Run number Temperature Time required for 10% reaction Average time/seconds Average time/ seconds References; * E. Brady, James. E. Humiston, Gerard., General Chemistry Principles and Structure, second edition, SI version, john Willy and sons, Inc. * Brewester, Vaderwerf and McEwen. Unitized Experiments in Organic Chemistry, 3rd Ed. * Streitwieser, Andrew. H. Heathcock, Clayton. Introduction to Organic Chemistry. * H. Reusch, William. An Introduction to Organic Chemistry. * J. Laidler, Keith. Chemical kinetics. 2nd ed. * Search engines that where used: o www.google.com o www.yahoo.com * Goldwhite, Harold. R. Spielman, John. College Chemistry, 1984