Butyl Chloride

8/3/2019 Butyl Chloride

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The Hydrolysis of Tertiary-Butyl ChlorideA Chemical Kinetic Study

The field ofchemicalkinetics is concerned with the rate or speed at which a chemicalreaction occurs.

Knowledge of a chemical reaction and the integrated rate law (an expression relatingconcentration with time) for the reaction allows us to accurately predict how much of a givenreactant or product exists at any time during the reaction. The rate law can also provide usefulinsight into which variables control a reaction (temperature, reactant concentration, catalysts)and how these variables can be used to maximize the amount of product(s) formed in thereaction or minimize the time involved to obtain product(s). The experimental kinetic dataobtained during the reaction is used to help understand the mechanism of a reaction. Thereaction mechanism describes how a reaction proceeds through single (elementary) steps onthe molecular level.

Although a significant amount of data has been collected in the field of kinetics, chemical ratelaws and reaction orders cannot be predicted from the stoichiometry of any chemical reaction.Thus, to obtain reach conclusions about a process, only experimentation will provide us withreaction kinetics.

Several factors influence the rate of a chemical reaction. In particular, the following should beexamined:

The inherent reactivity of the participating molecules,

The initial concentration of all reactants and their effects,

Changes in the rate caused by fluctuations in reaction temperature,

The effect of catalysts in the reaction.

The Theory Behind the Experiment

One of the primary goals in this experiment is to measure the rate constant kfor this reaction,performed at three different temperatures (temperature is held constant during each reaction).Once the rate constants are found, determining the half-life at each temperature and theactivation energy (Ea) should be straightforward.

A. The Reaction Mechanism

In this experiment we will investigate the effects ofconcentration and temperature on thehydrolysis oftertiarybutyl chloride ort-butyl chloride (2-chloro-2-methylpropane). Notice fromthe reaction sequence below, water replaces chlorine on the molecule to make an alcohol. Theterm "hydrolysis" refers to this process, as a water molecule is broken apart to form the alcoholproduct.


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The reaction oft-butyl chloride with water proceeds as shown in the two-step reaction below:

Combining steps (1) and (2), we find the overall reaction shown below. Notice that t-butyl

alcohol and hydrochloric acidare the two products.

The reaction rate for this process can be expressed as either the disappearance oft-BuCl orthe appearance oft-BuOH (see equation below). Referring back to the overall net equation,HCl is also a product. Because of our familiarity with measuring acid strength (pH) in the lab,

monitoring the appearance ofHCl during the reaction would be the most convenient methodto study kinetic changes during the reaction. Measuring the pH of the reaction can be doneeasily with apH meter residing in the flask during reaction (recall thatpH= - log [H+]).

B. The Rate Law for t-BuOH

Because step (2) in the reaction mechanism is faster than step (1), step (1) determines the

rate of reaction (step (1) is the RDS or rate determining step). Step (1) is unimolecular,meaning the rate for that reaction depends only upon the concentration of the t-BuCl. Anyunimolecularreaction is also a first-orderreaction. Thus, the rate law for a first-orderreaction is: (notice the last expression is a form of first order integrated rate law)


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As the overall reaction indicates, for every mole oft-BuCl consumed in the reaction, one moleof HCl is produced (i.e. 1 mole of H+ and 1 mole Cl-). Therefore, the number of moles of HClproduced during the course of the reaction is a measure of the number of moles of t-BuClreacting. Understand that not every molecule oft-BuCl reacts in this process, so monitoring

[HCl] is crucial to determining [t-BuCl]t. Using the integrated rate law for the first order process,we can easily find the initial concentration oft-BuCl in the reaction. Experimentally, things arenot quite this simple.

C. Adjusting for Water - Finding [t-BuCl]t

Water is used as a reactant and part of the solvent system in this reaction (its reaction order iszero, meaning [H2O] has no effect on the rate and is essentially constant). As time approachesinfinity, the moles of t-BuCl approaches zero and the moles of HCl produced approaches amaximum. By titrating the resulting solution after 48 hours, a good approximation of the initialamount of t-butyl chloride can be made. Experimentally time starts when the t-BuCl is added to

the solvent mixture. At any time, the number of moles of HCl produced in the reaction equal tothe total number of moles oft-BuCl consumed in the reaction.

reactedBuCltproducedHCl nn =

Reaction Mechanism

The hydrolysis involves 3 steps:

1. the ionization of the t-butyl chloride to form a carbocation intermediate;

2. the formation of a high energy transition state between the carbocation and water;

3. the formation of product, t-butyl alcohol by a hydrogen ion leaving the transition statecomplex.


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Virtual Models

The reactants and intermediates related to the kinetics experiment, showing the various

geometries of the molecules involved the hydrolysis oft-butyl chloride, (C=gray,H=white, Cl=green, O=red), Molecules can be viewed on-line at the following link.http://fp.academic.venturacollege.edu/doliver/chem12B/lab/kinetics_molecules.htm

To see other formats such as space-filling models, right click your mouse button whileon the picture. Choose another view under the Display submenu.

Half-Life

A useful parameter of rate equations, particularly first-order reactions, is the half-life ort1/2.The half-life is simply the time it takes for the reactant concentration in a reaction to reduce by50%.

For example, if we begin a reaction with a 0.20 M solution, then in one half-life the reaction willreduce to 0.10 M over a specific time frame, t1/2. In a second half-life, the reaction reducesanother 50% to 0.05 M.

Half-life for a reaction process is dependent on the reaction order. First the first order process,the t1/2 is independent on reactant concentration but dependent on the rate constant k:

Note:kis temperature-dependent, and so the half-life for a reaction will change with thetemperature.

You should obtain 3 half-lives for this kinetic study.

Activation Energy - The Arrhenius Equation

The rate of a chemical reaction varies directly with temperature. This is understandable interms of the frequency and productivity of collisions between the species in a reaction. Thehigher the kinetic energy (directly related to T), the greater the number of collisions. SvanteArrhenius proposed the following mathematical relationship for this phenomenon:
http://fp.academic.venturacollege.edu/doliver/chem12B/lab/kinetics_molecules.htmhttp://fp.academic.venturacollege.edu/doliver/chem12B/lab/kinetics_molecules.htm


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k= the rate constant of the reaction

A = the frequency factor (unitless)

Ea = activation energy (usually in kJ/mol)

R= ideal gas constant (usually 8.314 x 10 -3 kJ/mol K)

T = temperature in kelvins

Taking the natural logarithm of both sides of the equation gives you:

The bottom equation allows you to compare the data from reactions run at differenttemperatures in order to determine values forA and Ea. Yes, you should be thinkinggraphically.

Procedure

Set up the reaction vessel as shown at right. Toprepare a temperature-controlled reaction flask,place a small Erlenmeyer Flask (reaction flask)into a large beaker (temperature bath) as shown inthe figure. Accurately measure 50 mL of the 50%water/50% isopropyl alcohol v/v mixture and placeinto the flask.

You will perform one run at 10C and one run at40C. It is imperative that the temperature remainsconstant during the run, since kis extremelytemperature dependent (see theArrheniusequation).

Add 5 drops of phenolphthalein to the flask.
http://chemweb.calpoly.edu/chem/125/125LabExp/kinetics/Arrhenius.htmlhttp://chemweb.calpoly.edu/chem/125/125LabExp/kinetics/Arrhenius.htmlhttp://chemweb.calpoly.edu/chem/125/125LabExp/kinetics/Arrhenius.htmlhttp://chemweb.calpoly.edu/chem/125/125LabExp/kinetics/Arrhenius.html


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Allow the temperature bath (beaker) and reaction flask (Erlenmeyer) to come to thermalequilibrium (about 10 minutes) Temperature control is essential for this experiment.Toensure a constant temperature during the reaction, the solution level in the reaction flaskshould be well BELOW the water level in the temperature bath (beaker). You will need to addice or hot waterin small quantities to control the bath temperature within 1C. Use a magneticstirrer with stir bars in the outer beaker and in the reaction flask. It is often useful to onemember of the pair to maintain the constant temperature bath and record times, while the othermember titrates the reaction mixture and notes times of each color change.

While waiting for thermal equilibrium, clean a burette and fill with .100M NaOH. Add 1.00ml ofthe NaOH to the flask containing the mixture of isopropyl alcohol and water. Prepare a datatable in you notebook with two columns; ml Base Added and clock time for colordisappearance.

If you dont know how to use a pipette bulb, PLEASE ASK before you mess up!!! Place1.00ml of t-BuCl into the flask and record the time. This marks t=0. Record the clock timerequired for the pink color to disappear. As soon as the pink color disappears, add another1.00 ml of base, and again, note the time required for the pink color to disappear. Continue theaddition of base and timing until the color remains pink. Stopper and label the bottle as 10C.Set aside for the next lab period.

Repeat the process for 40C. Have a Bunsen burner, hot water and some ice available tomaintain the temperature at 401C. When finished, stopper and label the bottle as 40C. Setaside for the next lab period. (At the instructors option a third run of 25C will be run.)

During the next lab period, titrate to a phenolphthalein end point with what ever isrequired (acid or base) to verify the initial moles of t-butyl chloride, and do lab calculations.

Important things to note:

1. Record your CLOCK times to the nearest second. You will have time to convert to seconds

after the data collection period.

2. Position the thermometer in the water bath so that the magnetic stir bar does not strike itwhile spinning.

3. Make sure that you are ready to take data before adding t-BuCl. When everything is ready,use a pipet to add 1.00 mL of the t-BuCl. Note that the total volume of solution will be 51.0 mL.

4. As soon as the first drops of t-BuCl enters the reaction vessel, the reaction has beeninitiated. This is time t=0 and you shouldimmediatelybegin collecting data. If you miss a colorchange, QUICKLY add an additional 1.00 ml of base and make note of the volume change.

Data Processing and Calculations

The values to be determined from this experiment are:

the rate constants, k, for each temperature performed the corresponding half-life, t1/2, for each temperature the Arrhenius-derivedA and Ea for the reaction.


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You will be producing several plots ([t-BuCl] vs. time andln [t-BuCl] vs. time for bothreaction temperatures + Arrhenius plot). You may wish to overlay the concentration plots onone graph to save space. It also affords a direct comparison between the graphs.

You will use Excel to process and plot the data. Your spread sheet can be set up somethinglike this. NOTE: The initial moles of t-butyl chloride must be calculated based on the totalmoles of base added at t = infinity or a couple of days, which ever comes first. This iscalculated using the volume of base added during the timing portion of the experiment plus thevolume of base used to titrate to the end point multiplied by the molarity of the base.

Clocktime tocolor

change

Time insecondsto colorchange M Base

mLbase

TotalVolumein liters

Molesbase

added

Molest-butyl

chloridereacted

Molest-butyl

chlorideremaining

[t-butyl Cl]in molarityln[t-butyl Cl]

7:55:20 0 0.1 0 0.051 0 0 0.009103 0.178484 -1.7232543

7:58:26 126 0.1 1 0.052 0.0001 0.0001 0.009003 0.173129 -1.7537189

8:02:01 etc 0.1 2 0.053 0.0002 0.0002 0.008903 0.167976 -1.783937

etc etc 0.1 3 0.054 0.0003 0.0003 0.008803 0.163013 -1.8139252

etc etc 0.1 4 0.055 0.0004 0.0004 0.008703 0.158231 -1.8436996

To make a graph, hold the control key down and click the columns you with to graph such astime and ln[t-BuCl]. Plot the data as a Scatter Plot. Add a trendline to each data set,displaying the equation and linear regression value (r2).

From the plot, obtain the rate constant k. With the plots handy for different reactiontemperatures, you should have two or three different values ofkfor the t-BuCl reaction.

Construct an Arrhenius plot, using your k values. You must include the data from the othermembers of your cluster to increase the number of data points for statistical validity. Yourinstructor may ask you to use more. Once the Arrhenius plot is complete, determine theactivation energyEa (in kJ/mole) and frequency factorA (a unit-less quantity).

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Butyl Chloride