Competitive Nucleophiles

Competitive Nucleophiles
By: Doug Kruse

Abstract:

The competitive nucleophilic reaction of two nucleophiles, chloride and bromide ion with tert-butyl alcohol have been studied in this experiment. Nucleophilic substitution at the tertiary carbon to form a haloalkane was observed. During nucleophilic substitution, a nucleophile replaces a leaving group from a carbon atom, using its lone pair electrons to form a new bond to the carbon atom.¹ Nucleophilic substitution can occur through two pathways namely S_N¹ and S_N² reaction mechanism. The former reaction mechanism is a unimolecular reaction while the later mechanism is a bimolecular reaction.² These two mechanisms are designated unimolecular and bimolecular because of the number of molecules involved in the rate determining step. This experiment was carried out in a polar protic solvent in which the nucleophiles were dissolved. Tert-butyl alcohol, a tertiary alcohol was used as the substrate, and because of the nature of the substrate, S_N¹ reaction mechanism was favored over S_N². A competitive reaction E₁ which indicates elimination, unimolecular reaction can also occur.³ E₁ reaction is favored when the nucleophiles are poor and solvent is polar protic. The probability of E₁ reaction occurring was reduced by using good nucleophiles in this experiment. The product of reaction was a mixture of tert-butyl chloride and tert-butyl bromide. The percent composition of the two haloalkanes was calculated based on adjusted refractive index at 22.2^oC which are 1.3887 for tert-butyl chloride and 1.4290 for tert-butyl bromide. The result was 35.5% tert-butyl bromide and 64.5% tert-butyl chloride. The refractive index of the mixture was 1.4030.

RESULTS & DISCUSSION

Table 1: Experimental Data

Table 2: Literature Data

The anionic nucleophiles studied in this experiment have different chemical properties. Cl^– has electronegativity 3.0 while Br^– has 2.8.⁴ Because of the high electronegativity Cl^–, Cl^– is more basic and reacts faster than Br^– in a polar aprotic solvent. On the other, Br^– is easily polarized and reacts faster in a polar protic solvent. The nucleophilic reaction that was studied in this experiment was carried out in a polar protic solvent containing sulfuric acid and two halide ions (Cl^– & Br^–). This reaction can occur through two reaction mechanism but would result in different products. The first mechanism is E₁ which involves dehydrating the alcohol to form an alkene. This reaction mechanism would dominate when concentrated acid and weak nucleophiles are used. The other mechanism is S_N¹ which involves the addition of a halide ion to form a haloalkane. S_N¹ would dominate when strong nucleophiles are used. Both reaction mechanism forms a carbocation as the intermediate. The free energies of activation for these two reactions of carbocation are not very different from one another.⁵ Thus, not all the carbocation react with nucleophiles; some were dehydrated. The reaction mechanism that dominated in this study was S_N¹ because strong nucleophiles were used. The products of this reaction were analyzed using a refractomer to measure the refractive index of the mixture of the two haloalkanes produced. The refractive index of the mixture was 1.4030. Refractive index of the individual compound was obtained by adjusting their literature refractive indices at 20^oC based on temperature differences. This was done by adding 0.00045 index to the observed reading for each degree above 20^oC or subtract 0.00045 for each degree below 20^oC. The room temperature was 22.2^oC when the reading was taken. After the appropriate adjustment was made, the refractive indices of tert-butyl bromide, and chloride were 1.4290 and 1.3887 respectively. The percent composition was calculated using the following method.

Let x = %comp. of t-butyl cl     % in fractions

y = %comp. of t-butyl br

x + y = 1, x = 1 – y

1.3887x + 1.4290y = 1.4030

Substitute 1 – y for x

1.3887(1 – y) + 1.4290y = 1.4030

0.0403y = 0.0143

y = 0.3548

x = 1 – 0.3548 = 0.6452

therefore, 35.5% t-butylBr and 64.5% t-butylCl was produced in this experiment. This result is not consistent with the theoretical prediction. The assumption was made that some of the

t-butyl bromide could have reverted back to alcohol through the reaction of t-butyl bromide with surrounding moisture and the acid in the mixture. This was because Br^– was a better leaving group than Cl^–. Table 1 summarizes the experimental findings while table 2 contains the literature refractive index of the compounds at 20^oC.

Question:

2) Bromine is the better nucleophile in a protic solvent because Br^–ion is easily polarized than Cl^– ion.

3) The principal organic by-product is 2-methyl-1-propene.

4) There would less t-butylCl than t-butylBr because t-butylCl is more volatile than t-butylBr base on molecular weight.

5) If all solids did not dissolve, there will be lower surface area for the reaction to occur; therefore fewer products will be made.

6) More of tert-butyl chloride would be made because chlorine is a better nucleophile in aprotic solvent due to its high electronegativity.

8) Mole percent because the refractive index of an atom is dependent on the number of moles of the atom present.

REFERENCES

(1)   Wade, L. Organic Chemistry, 7^th ed.; Pearson: Upper Saddle River, 2010, 228.

(2)   Brown, W.; Foote, C etal. Organic Chem. Hybrid, 6^th ed.; Cengage: Belmont, 2012, 285-286.

(3)   Structure and Preparation of Alkenes.

http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch05/ch5-5.html (2/19/14)

(4)   Burdge, J. Chemistry, 2^nd ed.; McGraw Hill: New York, 2011, 309.

(5)   ) Graham, S.; Craig, F.  Organic Chem., 9^th  ed.; Wiley Plus: Hoboken, 2008, 482