Analysis of Nucleophilic and Electrophilic Substitution Reaction of Tertiary Butanol and Bromobenzene

ANALYSIS OF NUCLEOPHILIC AND ELECTROPHILIC SUBSTITUTION REACTION OF TERTIARY BUTANOL AND BROMOBENZENE

By

Ni Luh Gede Enik Karnila Yanti

Chemistry Education Department, Faculty of Mathematics and Natural Sciences, Ganesha University of Education

email: enikkarnilayanti@yahoo.co.id

Abstract

The aims of the experiment was to (1) undertand the electrophilic substitution reaction of bromobenzene and nuchleophilic substitution reaction of tert-butyl alcohol (2) identify the products of those reactions. The compound used are tertiary buthanol and bromobenzene were prepared by the laboratory assistant as the subjects of this experiment. The mechanism and the product of the substitution reaction were as the object in this experiment. The procedure was divided into two steps are electrophilic and nuchleophilic substitution reaction. Then, identification of products was done by comparing the physical and chemical properties of the products with theory. Based on comparison, it was known that this experiment produced exactly the same compound as the theory. The electrophilic substitution reaction produced result of 4– bromonitrobenzene while the nucleophilic substitution reaction produced result of tert-butyl chloride.

Key words: Nucleophilic substitution reaction, electrophilic substitution reaction, 4–bromonitrobenzene; tert-butyl chloride.

INTRODUCTION

Substitution reactions can occur in the positively charged carbon substrate (carbonium ions) with the excess electron species / negative charge (nucleophiles), so called nucleophilic substitution reactions (SN). In addition, substitution reactions can also occur on the negatively charged carbon substrate (electron source) with shortage electron species / positive charge (electrophiles), so called electrophilic substitution reactions (SE) ​​(Frieda & Suja, 2004).

Nucleophilic Substitution Reaction

Nucleophile is a species that likes to be the core because negatively charged or electron-rich. If the substitution reaction involving nucleophiles organic compounds, the reaction is called nucleophilic substitution (SN), which S states substitution and N state the nucleophilic. General reaction is as follow

Nu: + R-X → R-Nu + X:

There are two kind of nucleophiles, are negative nucleophile (Nu :-) and neutral nucleophiles (Nu :). Negative nucleophiles have not bonded pairs of electrons and negatively charged, for example, chloride ions (Cl^-), carbanion ion (R – CH₂:^-), hydroxyl ion (OH^-), etc. Neutral nucleophiles (Nu :) have not bonded pairs of electrons and uncharged, like alcohol (R-OH), tioalcohol (R - SH), ether (R - O - R).

One example of nucleophilic substitution reactions of alkyl halides is the hydrolysis reaction by alkaline solution to form alcohol.

R – X   +  OH^-  →  R- OH + X^-

X is called group apart (group away)

Nucleophilic substitution reactions can be divided into two, namely: unimolecular nucleophilic substitution reactions (S_N1) and bimolecular nucleophilic substitution reaction (S_N2).

The rate of nucleophilic substitution reactions that depend on the substrate concentration and does not depend on the concentration of nucleophile called unimolecular S_N1 nucleophilic substitution reactions. Reaction rate equation is as follows.
The reaction rate = k [substrate]

Nucleophilic substitution reactions that occur in tertiary butyl alcohol belonging to the S_N1 reaction. This is because the structure is the tertiary reagent and solvent used is polar solvent (HCl). Tertiary structure has a big steric hindrance that cannot undergo S_N2 reactions. S_N1 reaction rate does not depend on the nucleophile concentration, but depends only on concentration alkyl halide. Ionization stage is the slowest step in the overall reaction sequence, so it is a rate-determining step. S_N1 reaction that occurs in tertiary butyl alcohol, the presence of halogen will replace hydroxyl groups. Ion hydroxyl group is going bad, so there must be bonded to H⁺ and OH^- released as H₂O. It can be described in nucleophilic substitution reactions that occur in tertiary butyl alcohol with HBr (McMurry and Simanek, 2007).

If the rate of nucleophilic substitution reactions depends on the concentration of substrate and nucleophile, the reaction considered two levels, and expressed as S_N2. Reaction rate equation is follow:

The reaction rate = k [substrate] [nucleophile]

In the S_N2 reaction, the nucleophile attacking from the opposite direction to the group will be separated, resulting in the inverse configuration. If the alkyl halide is actively hydrolyzed, the resulting product is also optically active. Most primary alkyl halides undergo hydrolysis reaction by S_N2 mechanism. S_N2 reaction is contrast to S_N1 reaction, in terms of new bond formation and breaking old ties. For example, the reaction of methyl bromide with hydroxide ions is S_N2 reaction because of the increased concentration of methyl bromide causes quickened reaction, as well as to increase the concentration of hydroxyl ions. S_N2 reaction called simultaneous reactions cause break the old ties and new bond formation occur simultaneously.

S_N2 and S_N1 reaction mechanism is influenced by several factors, namely the structure of the substrate, the properties nucleophile, the polarity of the solvent and the properties group away.

Generally, primary alkyl halides react with S_N2 mechanism, tertiary alkyl halides S_N1 mechanism and secondary alkyl halides can react with S_N2 and S_N1 mechanisms. This caused by the electron density on the carbon atom that binds halide. At the tertiary alkyl halides are three alkyl groups. When alkyl halides are ionized, the carbonium ion formed is more stable, due to the induction effect of the three groups were tied. In primary alkyl halides there is only one alkyl, alkyl halide when ionized, the resulting carbonium ion is not stable. The more stable carbonium ions generated, the S_N1 reaction mechanism dominates.

Strong nucleophiles such as alcoxide ions and hydroxide ions will undergo S_N2 reaction mechanism, whereas weak nucleophiles such as water and alcohol are the dominant mechanism of S_N1 reaction. In big nucleophile concentration is the dominant mechanism of S_N2 reactions, and the small nucleophile concentration is the dominant mechanism of S_N1 reaction.

If the solvent polarity has the great, tendency of alkyl halides undergo S_N1 substitution reaction with the larger mechanism. This is due to the polar solvent to facilitate substrate and stabilize ionized ions generated. Conversely, if the solvent polarity has small or non-polar and the possibility of a small ionization, so that the dominant mechanism is S_N2.

The properties of the group away, for example halogen atom, do not affect the mechanism of the reaction, but the effect on the rate of reaction.

Electrophilic Substitution Reaction

The most important reaction of aromatic compounds is Electrophilic substitution reaction. Described as an electrophilic (E +) which will react with the aromatic ring by replacing one hydrogen atom, the reaction is as follow

ArH + E⁺ → Ar – E + H⁺

Many substituents’ can react with aromatic compounds through electrophilic substitution reaction. Depends on the reagent, aromatic can react with halogen, nitrate, sulfonate, alkyl and acyl. By using a few simple materials, the reaction can produce thousands of substituted aromatic compounds.

Benzene is very easy to get electrophilic attack because benzene π electron rich. Benzene acts as an electron donor (a Lewis base or a Nucleophilic), so it will be easy to react by accepting electrons (Lewis acid or electrophile).

In general, there is electrophilic substitution reaction mechanism: first, if a positive ion electrophilic X+. Two of the delocalized electrons in the systems of interest towards X+ and form a bond. Resulting in termination of delocalization, although not entirely.

Ions are formed at this stage is not the result. This stage is only an intermediate step. The intermediate result is

In the intermediate results still occurs delocalization, but only in some areas of the ions. Ion in the intermediate results is the positively charged as a result of a merger of positive ions and neutral molecules. Positive charge then spread along the delocalized on the ring.

            Second steps will be introducing a new ion, Y-. Impossible to obtain only positive ions in a chemical system. So the Y-ion is an ion that binds to the X + previously.

Unpaired electrons in Y- form bonds with hydrogen atoms on the top of the ring. This means that the pair of electrons that connects hydrogen with the ring is not required anymore. The section then moves down and fill the empty spaces in area of ​​electron delocalization and electron delocalization returns its original. Therefore, the stability of benzene was back.

METHOD

Equipments and Materials

Experiment was done at organic chemistry laboratory Ganesha University of Education. This experiment need time approximately 12 hours to finish all steps. To conduct the experiment need some equipments and materials. The equipments were used are one volumetric flask 100 mL, two spatulas, one set statif and clamp, one funnel, one of distillation equipment, one beaker glass 100 mL, one beaker glass 200 mL, one beaker glass 500 mL, one graduated cylinder  mL, one graduated cylinder 10 mL, one graduated cylinder 25 mL, 12 test tubes, one test tube rack, two watch glass, three petri dish, three drop pipette, one cycle flask 100 mL, one thermometer, one claisen adaptor, one Erlenmeyer flask 100 mL, one electrical balance, three capillary pipette, one heater, one metal block, one holder, one stirrer and one separatory funnel. The materials were used are 15 mL of concentrated HCl, 5 mL of tertiary butyl alcohol, a sufficient t-butanol, 10 mL of sodium bicarbonate, 1 L of aquades, 5 mL of anhydrous substrate, 5 mL of concentrated HNO₃, 5 mL of H₂SO₄, a sufficient ice, a sufficient bromobenzene, 20 mL ethanol 95%, and filter paper.

Nucleophilic Substitution Reaction

First, 15 mL of concentrated HCl were cooled in the ice bath and it was poured into separatory funnel. After that, 5 mL of tertiary butyl alcohol were added drop by drop then it was shaked. Shaked was continued until 20 minutes and it was let until form two layers. Bottom layer was separated as HCl and up layer was washed by using 5 mL of aquades then 10 mL of sodium bicarbonate. The product was dried by anhydrous substance. Finally the product was collected at temperature 49-52⁰C as tertiary butyl chloride and it has nD=1.386. Since the product is volatile, so distillation for determining the boiling point was not conducted. Only refractory index of product was checked.

Electrophilic Substitution Reaction

   Five mL of concentrated HNO₃ and 5 mL of H₂SO₄ were mixed in boiling flask and then it was cooled. After that boiling flask, claisen adaptor and thermometer were connected. 0.025 mole of bromobenzene were added thought tip on the top of cooler in 15 minutes and then it were shaked at the temperature 50-55⁰ C. after perfect addition occur, the temperature was keep bellow 50⁰ C until 30 minutes. Next boiling flask was cooled at room temperature, and then the mixture was poured into beaker glass that was contained 50 mL of aquades. Nitro-bromobenzene was filtered, and then it was washed by using cold water, than the crystal was dried. After that, the crystal was moved into Erlenmeyer by using 20 mL ethanol 95%. The crystal was heated until all crystal dissolve. It mixture was cooled until room temperature, and then the crystal of 4-nitrobromobenzene was separated by filter paper. The crystal (I) was washed by using cold alcohol. After that both of the mixture were mixed and then it were heated until the volume is a one third, the mixture was cooled until room temperature. Finally, the crystal was weighed. Since there was no capillary pipette in organic laboratory, so melting point could not be checked. While only shape and colour of solid were observed to determine the product.

RESULT AND DISCUSSION                

In theoretical, the electrophilic substitution reaction of bromobenzene  produces 4 – bromonitrobenzene. While the nucleophilic substitution reaction of tert-butyl alcohol is tert-butyl chloride. The reaction mechanism is as follow.

Formation of electrophile

Reaction of electrophile

Releasing of H⁺

Nucleophilic substitution is like follow :

Nucleophilic Substitution Reaction

According to Oxford University Chemical Safety Data, tert-butyl chloride have some properties are colorless liquid, stable and extremely flammable. It has melting point of 50 – 52^oC and refractive index of 1.3828. It is soluble in most organic solvents; almost insoluble in water.

In this experiment, the result has similar physical properties of tert-butyl chloride. It is colorless liquid, soluble in alcohol but slightly soluble in water. Therefore, the product of the nuchleophilic substitution reaction is tert-butyl chloride. The volume of product obtained is 2.8 mL. Theoretically, it should be 5.0 mL.

 

The volume is less than theory. It may because the reaction did not occur perfectly as the mixture of tert-buthyl alcohol and HCl was not shaken well.

Eletrophilic Substitution Reaction

According to database of CAS (Chemical Abstact Service) , 4– bromonitrobenzene is a yellow powder, insoluble in water and it has melting point of 124 – 126^oC.

In this experiment, the result has similar physical properties of 4– bromobenzene. It is pale yellow powder and insoluble in water. While chemical properties was also observed. Smoke test was conducted and produced white smoke. It indicated that the product is aromatic. Thus, the product of the electrophilic substitution reaction is 4-bromonitrobenzene. The mass of product obtained is 3.6 gram. In theoretical, the mass of product should be 5.075 gram according to following equation:

Mole 4-bromonitrobenzen = mol bromobenzen = 0,025 mol.

Mass 4-bromonitrobenzen = 0,025 mol x 203 g/mol = 5.075 gram.

            

The percentage of error in this experiment can be calculated by

        

The error was occurred may be caused by imperfectly cooling which cause decreasing of the amount of electrophile and product as well as the mixture of bromobenzene and NO₂⁺ was not shaken well.

CONCLUSION

Based on the experiment it can be concluded that nucleophilic reactions of tertiary butanol resulted colorless tertiary buthyl chloride. Electrophilic reaction of bromobenzene resulted 4-bromonitrobenzene. Tertiary-butyl chloride result is 2.8 mL, the crystal of 4-bromonitrobenzene resulted from substitution reactions of bromobenzene is 3.6 gram with % of rendement is 70.94% and percentage of error is 29.06%

ACKNOWLEDGMENT

Firstly, writer is grateful to the God, for the unlimited knowledge was created. Then, thanks for lecture Mr. I Nyoman Tika, M.Si who teach Organic Experiment subject. Thank for the laboratory assistant, Mr. Lasia who guides the writer in conducting experiment and give information how to work safely. Also thanks, the lecture assistant, Mrs. Dewi who guides the writer in conducting experiment. Moreover, the last for all members of VA class who give great motivation to the writer.

REFERENCES

L.G. Wade, J. (2010). Organic Chemistry. Upper Saddle River: Pearson Eduation, Inc.

Muderawan, I Wayan dan I Wayan Suja. 2008. Praktikum Kimia Organik. Singaraja :Universitas Pendidikan Ganesha

Nurlita,Frieda dan I Wayan Suja.2004.Buku Ajar Praktikum Kimia Organik.Singaraja : IKIP Negeri Singaraja

Ratcliff et al. 2004. AS Level and A Level Chemistry. UK: Cambridge University Press.

Anonymous. Tert-butyl Chloride. Accessed at 1^st October 2013 from

http://www.chemspider.com/Chemical-Structure.10054.html

Anonymous. Tert-butyl Chloride Accessed at 1^st October 2013 from http://chemindustry.ru/tert-Butyl_Chloride.php

Anonymous. 1-Bromo-4-Nitrobenzene. Accessed at 2^nd October 2013 from http://www.sigmaaldrich.com/catalog/product/aldrich/167150?lang=en&region=ID