preparation of cyclohexene from cyclohexanol

AMOGELANG MAGELE 24840572 MCHE 221 07-AUGUST-2015 EXPERIMENT 1 PREPARATION OF CYCLOHEXEN E FROM CYCLOHEXANOL.

Abstract: The objective in this experiment was to prepare cyclohexene by dehydrating cyclohexanol. Cyclohexanol was converted into cyclohexanol by heating it in the presence of phosphoric acid catalyst and dehydration was carried out in a liquid phase continuously removing cyclohexene by distillation. The purpose was also to synthesize cyclohexene from cyclohexanol which was accomplished through elimination of OH functionality from cyclohexanol by acid( Sulphuric or phosphoric) to yield cyclohexene and water. Test for purity and identification of the main product was done.

Introduction: Preparations of alkenes from alcohols involves dehydration reaction which is commonly known as elimination reaction whereby alcohol is heated at high temperatures in the presence of strong acids known as sulphuric acid or phosphoric acid. However the term elimination indicates that a small molecule is lost during a chemical reaction. During this reaction, both OH from one carbon and H from adjacent carbon are lost and yield the formation of water and formation of double bond of an alkene. The formation of water is acknowledged as dehydration. In some cases of dehydration reactions, more than one products of elimination are formed. The phosphoric acid is a catalyst and as such increases the rate of reaction but does not affect the overall stoichiometry. The major product is determined using Zaitsev’s rule in which a proton is removed from the adjacent carbon that has fewer hydrogen. The product has more stable transition state because of its stability hence it is referred as a major product of elimination reaction as shown below, it can be seen that the excess of H+ protonates the alcohol:

It can be seen from the balanced reaction below that 1 mole of alcohol produces 1 mole of alkene. The theoretical yield of alkene in moles is therefore equal to the number of moles of alcohol used:

The above reaction is most likely to reverse as far as hydration is concerned, the addition of water to an alkene in the presence of acid forms alcohol in a reaction is known as hydration. Furthermore, distillation method is used inhibit alkene formed in the dehydration reaction

from reforming back to alcohol because alkene has a lower boiling point than alcohol. The reaction will shift to the right as further extrapolated by le Chatelier’s principles. However this experiment entails information of cyclohexene from cyclohexanol. Straight chain alkenes can produce more than one product, while cyclic alkenes generally produce one product in high yield. The common design behind each and every dehydration +¿¿ reaction is that the –OH group in the alcohol donates two electrons to H from the acid reagent, forming alkyloxonium ion. This ion acts as a very good leaving group which leaves to form a carbocation and form a double bond. As the reaction takes place, fractional distillation will be used to separate the product from the starting materials. Fractional distillation is a physical separation process from a boiling mixture that uses vaporization and condensation cycles to separate a mixture. It is primarily used when trying to separate substances that have similar boiling points. It will be used for this lab because cyclohexene has a boiling point of approximately 80⁰C. Mechanism: One step of mechanism of elimination is denoted as E2 reaction and the two-step mechanism is known as E1 reaction. These numbers specify the kinetics of the reaction, bimolecular and unimolecular in that order. E2 stands for bimolecular elimination in which carbon-hydrogen and carbon-halogen bonds break to form double bond in one step mechanism.

Experimental procedure: Step 1: a 50ml round bottom flask was used to contain the measured 40ml of cyclohexanol. H 3 PO 4 3ml of 85% phosphoric acid; , boiling chips were added carefully and the flask was mounted for simple distillation. The mixture was then heated slowly until it came to a gentle boil. After 10 minutes of boiling the heat was increased to cause distillation, the distillate was cooled in a 25ml bottom flask that was used as a receiver. To the distillate 1ml of sodium carbonate solution was added to neutralize any traces of acid which might have been carried over. The liquid was then transferred to a separatory funnel, 5ml of cold water was added, the mixture was swirled gently and the lower aqueous layer was drained off. The

remaining upper organic layer was poured into a small, dry 50ml Elenmeyer flask; it was then dried over anhydrous calcium chloride for 5-10minutes with a cover. Step 2: the dried cyclohexene was decanted into a small distilling flask, a boiling stone was added; the flask was attached to a simple distillation assembly and was carefully distilled. The distilled material was collected at 80 to 85ºC. Step 3: the last step was the identification of whether the product was an alkene, this was done by adding 5-10 drops of the product in two small test tubes with drop wise bromine and drop wise potassium permanganate. Results:

1. 2. The weight of cyclohexene was obtained: Compound Cyclohexanol Phosphoric acid Cyclohexene

Molecular weight (g/mol) 100.16 98.00 82.14

Melting point(ºC) 25.93 42.35 -103.5

Boiling point (ºC) 160.84 158.00 82.98

Density (g/mol.L^-1) 0.962 1.885 0.8110

Volume of cyclohexanol used: 40ml The obtained number of moles of cyclohexanol is equal to that of cyclohexene since the ratio of the reaction is 1:1 Actual mass of cyclohexanol=d ×V = 40ml × 0,96g/ml = 38.4g m Number of moles of cyclohexanol= Mr 38.4 = 100.16

=0.38 mol Mass of cyclohexene= n × Mr =0.38 mol × 82 g/mol =31.16g Mass of beaker= 116.36g Mass of beaker+ cyclohexene solution= 122.08g Mass of cyclohexene solution= 122.08g -116.36g =5.72g actual mass × 100 Percentage yield= theoretical yield 5.72 g ×100 = 31.16 g =18.36% Step 3 results:  Bromide: the solution decolourized  Potassium permanganate: purple colour disappeared and changed into brown precipitate. Discussion: In the preparation of an alkene, cyclohexanol was dehydrated by an acid catalyzed elimination reaction. Routinely this reaction is carried out with a strong acid H2SO4 as a catalyst. 85% H3PO4 was substituted in the experiment as this acid is slightly less harmful. The result yields cyclohexene as a product with minimal amounts of water as a product. Fractional distillation was used to maximize the yield of the products. High temperature, strong acid, and distillation of cyclohexene were used in the suppression of the substitution. Separation of this manner allows for the distillation flask to be gently heated ensuring an even separation. However strong anions from strong acid and high temperature favour the elimination reaction. Preparation of cyclohexene from cyclohexanol is achieved through dehydration reaction of alcohol. This elimination reaction in which two groups or atoms were removed from the starting molecule. The fractional distillation leads to a cleaner separation of the components in the distillation pot, because the added surface area results in multiple cycles of evaporation

and condensation. So it leads to a cleaner, more complete separation and purer product distilling over into your receiving flask. It's not exactly about temperature control. Elimination reactions can occur as either a one-step(E2) or a two-step(E1) mechanism. Due to steric hindrance, this reaction will proceed primarily via E1 as shown below:

Alcohol was protonated by phosphoric acid on the most electronegative atom know as oxygen and the protonated alcohol then lost water to give the positively charged species denoted to give alkene which is cyclohexene in our case. The testing and purification of cyclohexene was proficient using bromide and potassium permanganate. The presence of alkene was obtained by the depolarization of colour (disappearance of the purple colour) by bromide test and potassium permanganate tests was performed to determine the purity of cyclohexene. The phosphoric acid is added as catalyze as such increase the rate of reaction in dehydration without affecting the particular chemical reaction. The boiling chips are added into the solution in order to prevent over boiling of the solution. The boiling chips are small, insoluble, and porous stones made of calcium carbonate or silicon carbide. There are a lot of pores inside the boiling chips which provide cavities both to trap air and to provide spaces to allow bubbles of solvent can be form. When boiling chips are heated, it will release tiny bubbles which can prevent boiling over. Boiling over of solvent will cause lost of solution which may lead to inaccurate result to be obtained. The temperature of mixture is reached to maximum at 107°C. This temperature is known as the activation temperature which the cyclohexanol start to be dehydrated. The temperature of mixture drops to 83°C and remains constant. This phenomenon takes place because the cyclohexene with lowest boiling point will tends to be distilled first before the higher one. The temperature remains unchanged because the heat is being absorbed to break down the bond between cyclohexene molecules. In all the distillation process, some of the product will be lost since it is hold up in the apparatus which reduce the product yield. In order to maximize the yield, the mixture is continued to be heated at higher temperature range which more than the boiling point of cyclohexene. When the mixture is heated at 90°C-100°C, the water in the mixture will push over the products into the receiving flask along the condenser. The products produced are collected in the same receiving flask. Two layers liquid are present in the receiving flask, one drop of distilled water is added into flask in order to determine the location of aqueous layer. Since the water droplet mix with the lower layer, so the upper layer is determined as cloudy/organic solution while the lower layer is aqueous layer. The upper cloudy layer is cyclohexene with some impurities and water

inside. The lower aqueous layer is removed and discarded. But, that is not easy to remove all the water in the receiving flask. So, sodium carbonate solution was added. The purpose of adding sodium carbonate was to neutralize any traces of acids which may have been carried over. As already calculated above, the cause of lower percentage yield might have been caused by the wrong calibration of the apparatus or some small amounts of cyclohexene was lost during the collection after the reaction(distillation). The rapid temperature increase could have prevented all of the cyclohexanol from being converted into cyclohexene.

Conclusion: Heating the reaction mixture at too high a temperature caused the vapor to collect in the top of the distillation adapter, solidify and drop back into the round flask. Once the temperature was reduced proper distillation occurred. However, the initial error could have possibly reduced the amount of product recovered due to collection in the distillation adapter. Another possible error was a small amount of water in the collection flask that was not removed before the distillation process commenced. This could have also been from the cyclohexanol OH group that was not properly removed during the distillation process due to the rapid temperature increase at the initial portion of the experiment. The product was confirmed to be an alkene by bromide test. Low calibration of apparatus may be a result of low percentage found to be 18.36% and some of cyclohexene may have been lost during the experiment. The effect of boiling chips could have also been decreased as they seemed affect the boiling of cyclohexanol and caused the cyclohexanol to distillate back into the distilling flask. References: 1. Experiment 9-alkene synthesis from alcohol, 20 Nov 2012, available from (10Aug2014) 2. University of the West Indies, Preparatn of cyclohexen from cyclohexanol, available fromwww.chem.uwimoa.edu.jm/labmanuals/c10expt8.html(8Aug2014) 3. Foster and Smith 2013,Bright Hub, Pet education.com, South Africa, viewed 05 August 2015,< http://www.chem.ucla.edu/-bacher/faqs/LabWriteup/image1.gif> (05 Aug 2015)