The effect of solvent’s polarity on the rate of SN1 and SN2 reactions

By M. Morawski

1. Aim

Determining the effect of solvent polarity on the rate of SN1 and SN2 reactions

2. Research question

How does the rate of the reactions of 1-bromobutane and 2-bromo-2-methylpropane with aqueous sodium methoxide depend on the

polarity of the used solvent?

3. The experimental setup

The experimental setup consists of

1. Test tubes2. 25 cm3 measuring cylinder3. 100 cm3 volumetric flask4. 5 cm3 and 25 cm3 pipettes5. 200 cm3 beaker6. Thermometer7. Glass rod8. Hot plate (electric heater)9. Stopwatch10. Piece of black paper11. Balance12. The reactants – 95% sodium methoxide powder, AgNO3 crystals/powder and

liquid 99% ethanol, deionised water, 1-bromobutane and 2-bromo-2-methylpropane

4. The experiment

Safety measures: halogenoalkanes used in the experiment are highly toxic and flammable, all manipulations should be done under a fume hood. Silver (I) Nitrate (V) and sodium methoxide are dangerous when they come in contact with skin and eyes, appropriate protection (gloves and glasses) should be worn at all times.

The experiment can be divided into two parts:

1. Preparing the solutions

a. Ethanol solutionsi. Using a 25 cm3 measuring cylinder mix 5 cm3 of ethanol with 15 cm3 of

water.ii. Using a 25 cm3 pipette, transfer 10 cm3 of the solution to one test tube

and 10 cm3 to another. Stir bothiii. Repeat the steps i. and ii. with solutions of 7,5 cm3 of ethanol in12,5

cm3 of water, 10 cm3 of ethanol in 10 cm3 of water, 12,5 cm3 of ethanol in 7,5 cm3 and 15 cm3 in 5 cm3 of water.

b. 0.1 M AgNO3 solutioni. Pour a little (~5cm3) of water into a 100 cm3 volumetric flask

ii. Using the balance, weigh out accurately 1.70g of AgNO3

crystals/powderiii. Put the AgNO3 crystals/powder in the flask, stir the flaskiv. Fill the flask up to the 100 cm3 mark, stirring all the time.v. Using a 5 cm3 pipette, pour 3 cm3 of the solution into each of the test

tubes. Stir well.c. 0.05M NaOCH3 solution

i. Pour a little (~5cm3) of water into a 250 cm3 volumetric flaskii. Using the balance, weigh out accurately 0.675g of NaOCH3. Put the

NaOH beads in the flask, stir the flaskiii. Fill the flask up to the 100 cm3 mark, stirring a couple of times in the

process.iv. Using a 5 cm3 pipette, pour 2 cm3 of the solution into each of the test

tubes. Stir well.

2,5 cm3 C2H5OH7,5 cm3 H2O

3,75 cm3 C2H5OH6,25 cm3 H2O

5 cm3 C2H5OH5 cm3 H2O

6,25 cm3 C2H5OH3,75 cm3 H2O

I II III IV V VI VII VIII

Fig. 1 Experimental setup at this point in the preparation procedure

d. Preparing the water bathi. Take the beaker, fill it with water to approximately ¾ of its volume.

ii. Put a thermometer in the beakeriii. Put the beaker on a hot plate, adjust the heating power (or, ideally, use

a temperature controlled heater with the attached thermocouple) so that the temperature of water is constant at about 50 degrees Celsius.

2. The actual experimenta. SN2 reaction

i. Put test tube I in the water bath, wait for about 240s until it reaches the temperature of the bath.

ii. Place a black piece of paper behind the beakeriii. Using a 5 cm3 pipette, put 3cm3 of 1-bromobutane into the test tube,

start the stopwatch at the same moment.iv. While constantly stirring, observe the test tube and stop the time

measurement when the creamy precipitate which begins to form covers the paper completely. Record the time.

v. Repeat steps a., b., c. with test tubes III, V and VII.b. SN1 reaction

i. Put a black piece of paper behind the test tube.ii. Using a 5 cm3 pipette, put 3cm3 of 2-bromo-2-methylpropane into test

tube II, start the stopwatch at the same moment.iii. While constantly stirring, observe the test tube and stop the time

measurement when the creamy precipitate which begins to form covers the paper completely. Record the time.

iv. Repeat steps a., b., c. with test tubes IV, VI and VIII.3. For a good measure, steps 1. and 2. can be repeated to increase the reliability of the

obtained values, however, due to the qualitative nature of the experiment, too many

2,5 cm3 C2H5OH7,5 cm3 H2O

3 cm3 AgNO3

2 cm3 NaOH

2,5 cm3 C2H5OH7,5 cm3 H2O

3 cm3 AgNO3

2 cm3 NaOH

2,5 cm3 C2H5OH7,5 cm3 H2O

3 cm3 AgNO3

2 cm3 NaOH

2,5 cm3 C2H5OH7,5 cm3 H2O

3 cm3 AgNO3

2 cm3 NaOH

I II III IV V VI VII VIII

Fig. 2 Experimental setup at this point in the preparation procedure

iterations of the procedure will not be very helpful. The aim is not absolute precision, but simple observation. Two repetitions would be therefore enough.

5. Theoretical investigation and hypothesis.

a. SN2 reactionWhen exposed to a nucleophile (Nu:) such as the methoxide ion, 1-bromobutane, being a primary halogenoalkane, will react mainly by the single-step SN2 mechanism. CH3O- approaches the halogenoalkane from side, and bonds to the electron deficient carbon atom, while the halogen atom X (in this case – bromine) is released from the carbon skeleton.CH 3O(aq)

−¿ +Br−CH 2 –CH 2 –CH 2−CH 3(aq)→CH 3 O❑−CH 2 – CH2 –CH 2−CH 3(aq )+Br (aq)−¿ ¿¿

The bromine ion then reacts with Ag+ ions from AgNO3 to form AgBr - a creamy precipitate which is insoluble in water:

Ag(aq)+¿ +Br (aq )

−¿ → AgI (s)↓¿¿This reaction proceeds very slowly under normal conditions, so the solution is heated up to increase its rate. A good qualitative measurement of such rate is the time needed for the white precipitate to form. The shorter the time, the faster the rate. Among the many things other than temperature which can influence it is the type of used solvent.However, when the halogenoalkane is placed in a polar solvent, the solvent molecules will most likely bond to it in a process called solvatation. They surround the molecule and make it harder for the nucleophile to approach it, which decreases the rate at which the reaction proceeds. The more polar the solvent, the higher the degree of solvatation and the slower the rate of reaction. A quantitative measurement of the polarity of a molecule is its dielectric polarisation. The higher this parameter, the more polar the molecule. For water its 80.4 C·m-2, for ethanol 24.3 C·m-2 1. Therefore, by manipulating the concentration of ethanol in water, one can manipulate how solvated the molecule will be (the ‘overall polarity’ of the solution) and hence the rate of this reaction. The higher the concentration, the smaller the overall polarity of the solution, the faster it should be. So if one was to arrange the test tubes in which SN2 occurred in the order of increasing rate of reaction, it would be:

VII > V > III > Ib. SN1 reaction

The situation is, however, different for 2-bromo-2-methylpropane. As a tertiary halogenoalkane, it will react with OH- mostly by the SN1 mechanism, that is, through a carbocation intermediate.

CH 3 – C (CH 3 ) (Br )−CH 3(aq)→ [CH 2 – C (CH 3 )−CH 3 ](aq)

+¿+Br (aq)

−¿ ¿¿

[CH 2 – C (CH 3 )−CH 3 ](aq)

+¿+CH 3O( aq)

−¿ → CH 2 – C (CH 3 )(CH 3O❑)−CH 3(aq )¿¿

1 Values obtained from Organic Chemistry 5th ed. written by J. McMurry; Brooks/Cole, 2000

The first step is the slow, rate determining one. The bromine ions produced in it again react with silver ions which serves as an indicator of the rate of reaction.In this mechanism, the polarity of the solvent will have a radically different effect than in the previous one. According to the Hammond postulate, any factor that stabilizes a high-energy intermediate should also stabilize the transition state leading to that intermediate. (...) we would expect the reaction to be favored whenever a stabilized carbocation intermediate is formed. (...) The more stable the carbocation intermediate, the faster the SN1 reaction. (McMurry, 2000). And one of the ways to stabilize the carbocation is solvatation, when the polar solvent molecules surround it, neutralizing some of its charge. Therefore, the higher the degree of solvatation, the faster the rate of reaction. Applying the previously outlined line of reason leads us to formulate the hypothesis that in the reaction of 2-bromo-2-methylpropane with sodium methoxide, the higher the concentration of ethanol, the slower the reaction. So if one was to arrange the test tubes in which SN1 occurred in the order of increasing rate of reaction, it would be:

II > IV > VI > VIII

Also, the rates of SN1 reactions are usually much higher than the rates of SN2 reactions (which is why the test tubes containing 2-bromo-2-methylpropane don’t need to be heated), so the overall order would be:

II > IV > VI > VIII > VII > V > III > I

An analogous analysis can be carried out for any pair of primary/tertiary halogenoalkanes, so the final hypothesis is:

For primary halogenoalkanes (such as 1-bromobutane), increasing the polarity of the used solution increases the rate of

their reaction with nucleophiles (such as the methoxide ion), while for tertiary halogenoalkanes (such as 2-bromo-2-

methylpropane) it decreases this rate.

The exact nature of this relationship is, however, very complex and no mathematical description of the relationship can be provided.

This analysis leads us to formulate the following list of variables.

6. List of variables

Independent Concentration of ethanol in the solution & Halogenoalkane used

DependentTime needed for the precipitate to cover the piece of paper

Controlled Initial temperature of the solutions

Concentrations of the reactantsDistribution of reactants in the mixture

Volume of the mixture Shape of the reaction vessel

a. Independent variablesDuring the experiments two radically different halogenoalkanes are used in order to determine how the two possible substitution reaction mechanisms are affected by the polarity of the solvent. For each, the amount of ethanol added to the mixture is varied, which, as the overall volume of the mixture is kept constant, corresponds to the concentration of ethanol in the mixture and, hence, to the overall polarity of the solvent. The more ethanol is added, the less polar it becomes.

b. Dependent variableTime required to produce enough precipitate to make the mixture totally non-transparent is measured. Combined with the constant stirring motion applied to it, this makes a good indicator of the rate at which the reaction proceeds.The shorter the time, the higher the rate of reaction (less time is required to produce the same amount of product.

c. Controlled variablesi. Initial temperature of the solutions

Most reactions proceed faster at increased temperatures, so to accurately measure the rate of two different reactions, one must keep the mixtures at the same temperature. However, in a school laboratory, this is impossible to do, as both SN1 and SN2 reactions are exothermic, so the temperature of the mixture increases as they proceed. This, due to positive feedback, strengthens the observed relationships. Reactions which have faster initial rates produce more heat, which increases their temperature, which increases their rate and so on. But this does not distort the picture in any way, so it is only logical to settle on providing steady initial temperatures. SN2 reaction mixture is heated and SN1 isn’t, but this also doesn’t influence the overall result of the experiment (which effectively consists of two separate investigations, so varying the conditions is either doesn’t affect it so long as they are kept constant within each of the individual investigations), and is merely a practical measure to decrease the time needed to carry out the experiment.

ii. Concentration of the reactantsConcentration of the reactants determines the frequency of collisions between them and hence the rate, so it is kept constant during the experiments (except, of course, for the dependent variable).

iii. Distribution of reactants in the mixture

Fig. 3 Table of variables

Distribution of reactants in the mixture is crucial not only because it assures that the reaction takes place at the same rate in all of the volume of the reaction vessel, but also because it directly affects the way the dependent variable is measured. When the precipitate is formed, it doesn’t rest on the bottom of the test tube, but is kept ‘suspended’ in all of the mixture, which, at some point allows it to make it non-transparent. It is assured by constant stirring.

iv. Volume of the mixtureVolume of the mixture is kept constant at 18 cm3 to assure the (relatively) most uniform heat distribution to the surroundings from the reaction vessel and ensure the stirring has the same effect everywhere in the mixture.

v. Shape of the reaction vesselShape of the reaction vessel is important for the same reasons as the ones outlined in iv.