Analysis Of Elimination Reaction Of Cyclohexanol 593h2q

ANALYSIS OF ELIMINATION REACTION OF CYCLOHEXANOL Putu Pratiwi Surya Rahayu (1013031003) Chemistry Department of Education, Faculty of MIPA, Ganesha University of Education ABSTRACT Eliminaton reaction is a chemical reaction in which element of the starting material are loss and a bond is formed. In an elimination reaction, two bonds are broken and a bond is formed between adjacent atoms (Solomons, 2000). This experiment used laboratory method, the objective are synthesizing the cyclohexene from cyclohexanol, determining the physical properties consist of boiling point and refractive index of cyclohexene, determining the result of cyclohexene. The result of this experiment Cyclohexene produces through the reaction of cyclohexene and phospate acid. The boiling point of cyclohexene is 82oC and the refractive index is 1.4435. The cyclohexene that gotten is 61.04 % and the error is 38.96 %. Key word: Elimination reaction, Cyclohexene, Cyclohexanol. Introduction Elimination reaction is usually done to produce double bond. Two methods of elimination reaction are dehydrohalogenation and dehydration (Anwar, Purwono, Pranowo, & Wahyuningsih, 1994). Dehydrohalogenation is the elimination of a hydrogen and a halogen from an alkyl halide to produce an alkene (L.G. Wade, 2010). Dehydration is omission of water molecule from alcohol to result alkenes (Anwar, Purwono, Pranowo, & Wahyuningsih, 1994). Elimination reaction is reaction which is occurs through omission of leaving group (X) and hydrogen which is bonded to the C atom in β position toward the C atom which is bonded the leaving group, as shown in the following reaction (Anwar, Purwono, Pranowo, & Wahyuningsih, 1994). X H C



H

H C H



H2C

CH2

H

X

While, elimination reaction tend to attacked by string base nucleophile. The structure of substrate also influences this competition reaction. Tertiary alkyl halide tends to produce more elimination product. Elimination reaction can be classified into two, namely one-order elimination (E1) and second-order elimination (E2). 2.1 One-order elimination (E1) To force second-order kinetics, one-order elimination (E1) usually conducted in a good ionizing solvent (such as an alcohol or water), without a strong nucleophile or base (L.G. Wade, 2010). Firstly, ionization should be done to form a carbocation, and then generally continued by SN1 substitution. SN1 substitution occurs because the nucleophilic solvent can also attack the carbocation directly to form the substitution product. The mechanism of elimination of alcohol is as follows: Step 1: Protonation of the hydroxyl group

H

Both of elimination and nucleophilic substitution reaction occur in the same way; that is formation of carbocation (Anwar, Purwono, Pranowo, & Wahyuningsih, 1994). Therefore, competition reaction may occur. Substitution reaction tends to attacked by strong nucleophile such as Br- and Cl-.

O H H3C

+

H O H

CH

CH2

H2SO4

CH3

H3C

CH

CH2

CH3

HSO4-

Step 2: Ionization to a carbocation (slow; rate limiting) +

H O H H3C

CH

CH2

CH3

H3C

+

C H

CH2

CH3

H2O

Step 3: Deprotonation to give the alkenes (fast) H H3C

+

C

C

H

H

CH3

H2O

CH3

H3C CH

CH

H3O

+

major product (cis and trans)

reaction, especially if the alkyl halide is a poor SN2 substrate. The reactions takes place in one step, where a strong base abstracts a proton from one carbon atom as the leaving group leaves the adjacent carbon, as the following mechanism. -

HO

OH H

Or

H

C

C

H -

H H

H

C

-

C

H

H H Br

Br

H

transition state H H

H +

C

C

C

H

H

H

CH3

H2O

CH3

H CH

CH

H

H H3O

minor product

Alcohol dehydration follows an order of reactivity according to its carbocation stability: 3o alcohols react faster than 2o alcohols, and 1o alcohols are the least reactive (L.G. Wade, 2010). Rearrangements of the carbocation intermediate are common in alcohol dehydration, it will produce a lot of mixture of alkenes product (Anwar, Purwono, Pranowo, & Wahyuningsih, 1994). Therefore, 3o alcohols are good in produce alkenes. 2o alcohols can result two kinds or more of alkenes because proton – b came from two kinds of carbon which bond OH- group. For example 2-methyl-2butanol will produce 2-methyl-butene and/or 2-methyl-2-butene. The major product is 2-methyl-2-butene (Zaitsev’s rule: the major product is usually the one with the most substituted double bond). Dehydration of alcohol needs catalyst from strong acid, such as H2SO4 and H3PO4 (Anwar, Purwono, Pranowo, & Wahyuningsih, 1994). The catalyst has function to give proton to OH- group so it will become leaving group; H2O. By releasing of H2O group, carbocation will be resulted. Carbocation will release proton-b to produce alkenes. 2.1 Second-order elimination (E21) L.G. Wade (2010) in his book Organic Chemistry stated that Secondorder elimination is a reliable synthetic

+

C H

C

H

H2O

HBr

Almost all of cyclohexane is most stable in chair conformations. All of carbon-carbon bonds are staggered and any two adjacent carbon atoms have axial bonds in an anti-coplanar conformation, already ideally oriented for E2 reaction. In two adjacent carbon atoms, one has its axial bond pointing up and the other has its axial bond pointing down. They are Trans to each other: geometrically called transdiaxial (L.G. Wade, 2010). In this experiment, cyclohexanol will be conduct elimination reaction to produce cyclohexene. The boiling point of cyclohexanol is 1600C, boiling point of posphate acid is 1580C, boiling point of cyclohexene is 83oC- 85oC (ChemSpider: Search and Share, 2013), and boiling point of xilena for o-xilena is144,40C while for p-xilena is 138.350C. Materials, Equipments and Methods The method as well as the equipment and materials is used in conducting this experiment was adopted from an Praktikum Kimia Organik book developed by Frieda Nurlita and I Wayan Suja (2004:24-25). The procedures of analysis elimination reactions in this use some materials namely: Cyclohexanol (C6H12O) solution, phosphate acid 85%, xylene, aquades, anhydrous substance, ice. The equipments use to these experiments are boiling stone, round flask 150 mL, graduated cylinder 5 mL and 10 mL, drop pipette, spatula, watch

glass, fractionation column, distillation adapter, thermometer, heater, beaker glass 100 mL and 200 mL, stirred rod, separatory funnel, funnel, Erlenmeyer 50 mL and 100 mL, filter paper, fractional distillation equipment, statif and clamp. Some procedures were done in conducting the elimination of Cyclohexanol reaction and identifying its product. First, 10,64 mL of siklohexanol, 5 mL phosphate acid 85% and boiled stone were entered into erlenmeyer 100 mL, then shaked to mix it. The mixture was distilated by using fractination tools until the residue become 2,5 mL-5 mL in erlenmeyer, the distilate was cooled in ice bath to decrease the eaporation.10 mL of xilene was entered into reaction result. Then, The mixture was distilated again until the top layer decrease in half. The distilate was washed with 10 mL distilate water in separated funnel then the cyclohexane layer was dried with anhydrous substance. Destilate was distilated. And the last, destilate was weight and its refraction index was tested also observed its boiling point. Results Elimination reaction of Cyclohexanol produces colorless solution which has boiling point of 83oC as much as 6.0 mL.

Step 2: Ionization to a carbocation and deprotonation to give the alkenes

Elimination Reaction of Cyclohexanol Based on the mechanism of the elimination reaction above the product resulted is cyclohexene. Cyclohexene has boiling point of 83-85oC (ChemSpider Search and Share, 2013). Cyclohexanon is colorless liquid. The elimination reaction of cyclohexanol conducted in this experiment result 4,70 mL colorless solution as the result of fractional distillation process in which the drop of the liquid are in 79oC which is considered as cyclohexene.The difference of boiling point between experiment and theory is caused by the pressure when conducting experiment is below 1 atm. Cyclohexene has lowest boiling point so cyclohexene will firstly drop as distillate. The volume of product obtained in this experiment is 6 mL or 4,866 gram. Theoretically, the volume of product should be 9,83mL or 7,9712 gram according to following equation: C6H11OH → C6H10 + H2O

Discussion

V cyclohexanol used = 10,6 mL

Reaction Mechanism

Mass cyclohexanol V. cyclohexanol  

Theoretically, the elimination reaction of cyclohexanol that is secondary alcohol produces an alkene namely cyclohexene. The elimination reaction of cyclohexanol are included in one order elimination reaction (E1). The reaction mechanism are as follow: Step 1: Protonation of the hydroxyl group

=

= 10,6 mL x 0,94 g/mL = 9,964 g Mol cyclohexanol =

=

massa Mr cyclohexan ol

9,964 g 100 g/mol

= 0,09964mol

Mol cyclohexanol = mol cyclohexene= 0,09964 mol Mass cyclohexene = mol cyclohexene x Mrcyclohexene = 0,09964mol x 80 g/mol = 7,9712 g V cyclohexene = Mass / ρ = 7,9712 g / (0,811 g/ml) = 9,83 mL Experimentally, the volume obtained is 6 mL, so the mass is Mass cyclohexene = ρ x V = 0,811 g/mL x 6 mL = 4,866 gr So the percent purity of the cyclohexene from experiment can be calculated as follow: % yield 



experimental mass x 100% theoritical mass

4,866 x 100%  61,04 % 7,9712

Conclusion Based on the result and data analysis, it can be concluded that 1) elimination reaction of Cyclohexanol result Cyclohexene; (2) the Cyclohexene resulted is 6.0 mL with percent purity of 61,04%.

References Anwar,

Chairil,dkk. 1994. Pengantar Praktikum Kimia Organik. Yogyakarta: UGM. Chang, R. 2007. Chemistry: Seventh Edition. New York: Van Hoffman Press. Fessenden, Ralph J and Jean S. DasarDasar Kimia Organik. Jakarta:Binarupa Akasara. Frieda Nurlita and I Wayan Suja. 2004. Buku Ajar Praktikum Kimia Organik.Singaraja: IKIP Negeri Singaraja. Furniss, Brian S., Antony J. Hannaford, Peter W.G. Smith, Austin R. Tatchell. 1989. Vogel’s Textbook of Practical Organic Chemistry. New York : The Bath Press Solomons, Graham and Craig Fryhle. 2002. Organic Chemistry Seventh Edition. United State of America: Van Hoffmann Press. (2013). Retrieved November 10, 2013, from ChemSpider: Search and Share: http://www.chemspider.com/Chemi cal-Structure.10054.html Acknowledgement The authors first say thanks to the God “ Ida Sang Hyang Widhi Wasa” and also big thanks to my lecturer that teach this subject, he is Mr. I Nyoman Tika, M.Si. Don’t forget to say thanks for Laborant Mr. Lasia and assistant of lecture (Miss Dewi) as guidance in conducting practice.