Experiment #3: Competition Studies of Substitution and

Elimination Reactions

Name:

Jordan Wilson-Morrison 6767554

Lab Partner:

Chloe Stewart 7745210

Demonstrator:

Samuel Oteng-Pabi

Date:

October 5, 2015

Laboratory of Organic Chemistry II

CHM2123

Section A1

Department of Chemistry

University of Ottawa

Introduction:

In the majority of chemical reactions, it is rare for the product to consist of just one compound.

Of these multiple products, the one to which the reaction is most favourable is known as the

major product, and will form in greater amounts. To decide the outcome of the major product,

the type of reaction - elimination or substitution - that the reagents undergo will be paramount.

This experiment explores SN2 reactions, which are one of the two types of substitution reactions,

as well as the E2 reaction, which conforms to one of the two types of elimination reactions.

Under very similar conditions, these two types of reactions produce different products; this

causes constant competition between the reactions. Subsequently, the other types of reactions are

the elimination E1 reactions, as well as the SN1 substitution reaction, however, their competing

reactions were not studied throughout this experiment. Schematics of both the E2 and SN2

reactions are shown below, respectively:

Schematic 1: E2 Reaction

Schematic 2: SN2 Reaction

As previously discussed, there are similar conditions between the E2 and SN2 reactions

that cause competing reactions between them. The first similar condition includes the fact that

most nucleophiles may act as the base, providing both the E2 and SN2 mechanisms a chance to

proceed with the formation of their respective major products. However, the experimenter's

choice of nucleophile or base can assist with a solution to the issue of competing reactions

between the two types. If one introduces a compound that is a strong base but a weaker

nucleophile, an elimination reaction would be more favourable over that of a substitution. In

contrast, the SN2 pathway demands use of a strong nucleophile, not one of a weaker nature. Thus,

E2 reactions can only proceed with a strong base rather than a weaker one, making this situation

more favourable towards an E2 mechanism, and vice versa.

Furthermore, the structure of the molecules that are undergoing the reaction is another

condition that determines the pathway the reaction will take. An SN2 reaction is most favoured

when the -carbon is in the primary position, or will form a primary carbocation. It is also

moderately favoured with a secondary-carbon. However, due to steric hindrance, the E2

mechanism will be more favourable for the reactants. Subsequently, if the -carbon is branched -

or substituted - then the E2 mechanism an increasingly favourable pathway for the secondary -

carbon. Furthermore, if the -carbon is of a tertiary nature, then the SN2 reaction will not be

favoured whatsoever, providing certainty that the reaction will adhere to an E2 mechanism. In

retrospect, SN2 pathways are not favoured when there is increased branching and steric

hindrance.

During this experiment, the competing reactions between the E2 and SN2 reaction

pathways will be observed with 1-bromobutane and 2-bromobutane. Under conditions which will

favour the elimination reaction, the two products that are to be expected include 1-butene and 2-

butene for each respective reaction. Furthermore, by measuring the amount of gas obtained by

the eudiometer during the reaction progress with 1-bromobutane at reflux, 1-bromobutane at

50℃, and 2-bromobutane at reflux, the ratio of products for SN2:E2 will be determined. This all

occurs while reacting with basic KOH.

Additionally, specific calculations must be performed in order to resolve each ratio. This

includes the pressure and moles of each product. Making use of the following formula, one can

deduce the pressure of the gas:

Pgas = Patm - Pwater - P*

Where Pgas is the pressure of the gas that is collected in the eudiometer tube, Patm is the

atmospheric pressure, Pwater is the vapour pressure of the water at a definitive temperature, and P*

is the pressure that is exerted by the height of the water column situated above the water level in

the beaker. To expand, one may resort to the Ideal Gas Law formula, PV=nRT, to conclude the

moles of the gas formed within the eudiometer tube. Factors such as temperature, pressure, and

volume are recorded experimental values. The following equation is conducive to the calculation

of the moles of solid KBr that is produced:

nKBr = ngas + nbutanol = nE2 + nSN2

Proceeding forward, the ratio of the E2:SN2 products can be consolidated using the integrated

information.

Table of Reagents:

Table 1: Table of reagents use for the competition studies of SN2 and E2 reactions.

Compound Molar Mass

(g/mol)

Quantity

(g or mL)

Density

(g/mL)

Number of Moles

(mol)

KOH (for 2-

bromobutane)

56.1056 8.04 g 2.12 0.1433

95% EtOH 46.0684 45 mL 0.789 0.7707

2-bromobutane 137.028 0.70 mL 1.25 6.386x10-3

1-bromobutane 137.018 1.5 mL

(x2)

1.25 1.368x10-2 (x2)

KOH (for 1-

bromobutane)

56.1056 8.02 g 2.12 0.1429

Refluxed KBr 119.002 0.41 g 2.75 3.445x10-3

Experimental Procedure:

Refer to the experimental procedure for Experiment #3 of the Laboratory of Organic

Chemistry II Laboratory Manual posted on Blackboard Learn, pages 4-6.

Modifications: The water bath was heated to 57℃ rather than 50℃.

Observations:

Table 2: Table of experimental observations and key steps.

Organic Substance Key Step/Observation

1-bromobutane

(Observations cited

from Sarah Zhang

7618401)

The Tygon tube was attached only 30 seconds before adding

bromobutane to prevent EtOH vapours from condensing onto

the tube.

~0.01 mL of bubbles escaped into the eudiometer when the

Tygon tube was inverted.

Solution turned a yellow colour as reflux progressed.

The salt was white and powdery.

1-bromobutane in a

57℃ water bath

(Observations cited

from Sarah Zhang

7618401)

The KOH was not fully dissolved before the reaction had

begun.

Some KOH reacted with the air before being added to the flask.

The salt was white and powdery.

2-bromobutane Step 1: The initial KOH solid was oily, white, and oval-shaped.

Step 3: The solution turned clear and colourless once the 95% ethanol

was added to the KOH crystals.

Step 10: Large gas bubbles formed rapidly within the eudiometer tube

initially, and as equilibrium was reached, the gas bubbles became

smaller and more infrequent. Two eudiometer tubes were required for

the gas in order for the reaction to reach equilibrium.

Step 11: The solution turned a pale yellow, cloudy and opaque colour

once the 2-bromobutane was added.

Step 14: Bubbles began to form rapidly, requiring two eudiometer

tubes to capture the full volume of the gas bubbles, until the the gas

reached the water level of the 1L beaker.

Step 18: As the solution cooled, a white and powdery precipitate

formed on the bottom of the round-bottom flask. The liquid solution

surrounding it was clear and colourless.

Step 19: The KBr crystals retrieved were a dry white powder.

Results:

Table 3: Results of the competition studies of SN2 and E2 reactions with 1-bromobutane at

reflux, 1-bromobutane at 57℃, and 2-bromobutane at reflux. Results from

1-bromobutane at reflux and 1-bromobutane at 50℃ have been cited from Sarah Zhang,

student number 7618401.

Reaction

Conditions Pressure

of Gas

(atm)

Volume

of Gas

(mL)

Moles of

Butene

(mol)

Mass

of

KBr

(g)

Moles of

KBr (mol) Moles of

Butanol

(mol)

Conv-

ersion

(%)

Select-

ivity

(%)

SN2:E2

Value

1-

bromobutane

at reflux

0.968 23.6 9.397x10-

4 0.35 2.941x10-

3 2.001x10-

3 21.5 31.95 2.129

1-

bromobutane

at 57℃

0.945 0.45 1.749x10-

4 0.26 2.185x10-

3 2.01x10-3 15.97 8.00 11.49

2-

bromobutane

at reflux

0.982 71.2 0.002876 0.41 3.445x10-

3

0.000569 53.9 83.5 0.198

Sample Calculations

*all calculations pertain to the reaction of 1-bromobutane at reflux*

Pgas: P* = (500 mmH2O - 109 mmH2O - 250 mmH2O) 13.6 mmH20= 10.368 mmHg

Pgas = Patm - Pwater - P* = 767.313 mmHg - 21.0 mmHg - 10.368 mmHg

= 735.945 mmHg x 0.00131578947 atm1 mmHg

= 0.968

∴ the pressure of the gas was 0.968 atm.

Moles of Butene:

n = PVRT, where R = 0.0821 L atm/(mol K), P is in atm, V is converted to L:

n = (0.968 atm)(0.0236 L)(0.0821)(296.1K)= 9.397x10-4 mol butene

∴ the moles of butene are equivalent to 9.397x10-4 mol, or 9.397 mmol.

Moles of Butanol:

nKBr = ngas + nbutanol

nbutanol = nKBr - ngas = 2.941x10-3 mol KBr - 9.397x10-4 mol butene = 2.001x10-3 mol butanol

∴ the moles of butanol are calculated to be 2.001x10-3 mol, or 2.001 mmol.

Conversion:

C4H9Br(l) + KOH(s) → C4H10O(g) + KBr(s)

Limiting Reagent:

1 mol C4H9Br0.01368 mol = 1 mol KOHx → x = 0.01368 mol KOH needed

1 mol C4H9Brx = 1 mol KOH0.1429 → x = 0.1429 mol 1-bromobutane needed; ∴ it is limiting

because we

do not have 0.1429 mol of 1-bromobutane for a complete

reaction.

Conversion = nc + ndn limiting reagentx 100 = n butene + n butanoln 1-bromobutanex 100 =

2.941x10-3 mol 0.01368 molx100

= 21.5%

Selectivity:

Selectivity = ncnc + ndx 100 = n butenen butene + n butanolx100 = 9.397x10-4 mol2.941x10-3

molx100

= 31.95%

SN2:E2 Ratio:

nSN2nE2 = n butanoln gas = 2.001x10-3 mol butanol9.397x10-4 mol butene= 2.129

Discussion: The objective of this experiment was to determine the ratio of the competing reaction products

resulting from the SN2 and E2 reactions involving 1-bromobutane at reflux, 1-bromobutane at

57℃, and 2-bromobutane at reflux. For each experiment, approximately 8g of solid KOH was

used and placed in a 100 mL flask, mixed with 95% ethanol. The solution began to reflux as it

was heated and stirred, subsequently forming bubbles within the eudiometer tube until the

solution had reached equilibrium. The purpose of refluxing during the experiment is to allow the

solution to reach a gentle boil, which permits the vapour to circulate up the condenser and cool,

forcing it into a descent down the reflux condenser. Thus, if a molecule or compound has a lower

boiling point than the others within the mixture, that particular compound will circulate up the

condenser and travel through the Tygon tube and into the eudiometer tube, creating a pressure

that forces the water inside of it downward, allowing for a measurement of the gas volume. Once

the reagent - being either 1-bromobutane or 2-bromobutane - was added, the reaction begins and

the product with the lowest boiling point travels up and into the eudiometer tube. In each

corresponding experiment, the gaseous product was butene. Due to the presence of double bonds

within its alkene structure, butene has a lower boiling point, which allows it to react faster than

the other products consisting of butanol and KBr because of the highly electronegative halogen

group (Br) in the KBr product, and the alcoholic functional group in butanol (OH). The higher

electronegativity of those molecules make it more difficult for those atoms to interact and move

due to strong bonds caused by high electronegativity.

Also, suction filtration was performed for each experiment to isolate the KBr precipitate

from the liquid butanol. From here, the mass of the KBr product was measured. This allowed the

conversion - or the ratio of all products produced to the total that could have been produced - to

be determined. For the trial of 1-bromobutane at reflux, the conversion was 21.5%. For 1-

bromobutane at 57℃ and 2-bromobutane, the conversion values were 15.97% and 53.9%,

respectively. Furthermore, the selectivity - or the ratio of the desired product among all products

- was also calculated. For 1-bromobutane at reflux, 1-bromobutane at 57℃, and 2-bromobutane

at reflux, the respective selectivity values were 31.95%, 8.00%, and 83.5% respectively. With 1-

bromobutane and 2-bromobutane at reflux, errors in the trial that may have caused these lower

values could be that the solution did not reach proper reflux conditions. Furthermore, some of the

butene vapours could have escaped from the tubes during the reaction. Also, the fact that the

water bath was not exactly 50℃ may have caused the reaction to not proceed optimally. Also, of

all three experiments, the experiment with 2-bromobutane at reflux has the highest conversion

value, as well as the highest selectivity. Of all three reactions, this reaction produced as close to

the maximum amount of products as possible, and also produced the most desired product

among the ones produced.

Finally, the last goal of this experiment was to determine the SN2:E2 ratios. The reaction

of 1-bromobutane at reflux yielded a ratio of 2.129; the reaction of 1-bromobutane at 57℃

yielded a ratio of 11.49, and 2-bromobutane at reflux gave a ratio of 0.198. Of all three reactions,

the 1-bromobutane at 57℃ yielded the highest ratio of SN2 products, and thus was most selective

towards the SN2 product over the E2. This is an error however, because heat promotes

elimination reactions, even though this R-X group is primary. KOH is a strong base, thus this

rule should apply to this reaction. Errors that may have caused this include rounding errors in

calculation, as well as possible escaping of gas product, which would lower the amount of moles

of gas, altering the ratio. The reaction of 1-bromobutane also yielded more SN2 product than E2,

as is expected of a primary R-X group. Also, the reaction of 2-bromobutane at reflux 0.198,

which indicates that the elimination product was favoured. This is also expected, as the product

of a secondary R-X group will promote the substitution.

Conclusion: The overall objective of all three experiments, respectively, was to determine the ratio of

products - SN2:E2 - as well as the conversion and selectivity of each reaction. The reaction of 1-

bromobutane at reflux yielded a conversion of 21.5%, a selectivity of 31.95%, and an SN2:E2

ratio of 2.129, thus favouring a substitution reaction. Next, the reaction of 1-bromobutane at

57℃ gave a conversion value of 15.97%, a selectivity value of 8.00%, and an SN2:E2 ratio of

11.49, also favouring the substitution reaction. Finally, the reaction of 2-bromobutane at reflux

gave a conversion of 53.9%, a selectivity of 83.5%, and an SN2:E2 ratio of 0.198, promoting the

elimination product.

Questions: 1. a) An experimental method that can ensure the production of 1-butene from the reaction

of 2-bromobutane in its entirety would include the introduction of a strong, bulky base;

thus, KOC(CH3)3 would one qualified base. To justify this, a strong and bulky base would

avoid the deprotonation of the most substituted region in 2-bromobutane due to steric

hindrance. Thus, use of a strong, bulky base would favour the deprotonation of the least

substituted hydrogen, yielding a final product of 1-butene.

b) One experimental method that ensures the entire production of 2-butene from the

reaction of 2-bromobutane is the use of a strong - yet small - base. A base such as

NaOCH3 would satisfy these parameters. Since the base is small and strong, steric

hindrance will not be a factor in determining the method of attack; thus, deprotonation of

the hydrogen located in the most substituted region would be more favourable. This

yields a final alkene product of 2-butene.

2. Vmax butane = ? 1-bromobutane + KOH → butene

T = 298.15 K n1-bromobutane = (1.5 mL)(1.25 g1 mL)(1 mol137.018 g)

P = 101.325 kPa = 0.01368 mol

1-bromobutane → butene n1-bromobutane = nbutene = 0.01368 mol

V1-bromobutane = 0.0015 L Vmax = nRTP = 0.335 mL

∴ the maximum volume at standard temperature and pressure is 0.335 mL, or 335 L.

3.

4. a) Elimination, as the haloalkane is tertiary.

b) Substitution, as the haloalkane is primary.

c) Substitution, as the R-X group is a methyl.

d) Elimination, as the R-X group is secondary.

The first experimental modification one can make to facilitate the production of 1-pentene over

1-pentanol would be to use a weaker nucleophile but a stronger base than that of NaOH. If the

base has a pKaH value greater than 10, then the reaction will favour an elimination reaction over

the substitution pathway; thus, forming 1-pentene will be favoured over 1-pentanol. The second

modification you could make would be to heat 1-bromopentane, since E2 reactions are more

commonly favoured when for a primary 𝛼-carbon when heat is added to the reacti