8 Preparation of 2-Acetylcyclohexanone
When hydrogens are present on the ? -carbon of ketones (like the reaction used in this experiment), carbonyl compounds and aldehydes, they turn the compound slightly acidic1. These functional groups are removed by using a basic solution as shown in (i) below1. The product formed with water is stable only due to resonance, but does not form a very stable equilibrium.
Enamine reactions are used to avoid many problems usually associated with alkylating or acylating carbonyl compounds when they are reacted with aqueous sodium hydroxide1 as shown in reactions (ii) and (iv)1. The main problem being that the reaction results in a great number of crucial secondary side reactions, such as those in equations (iii), (iv) and (vi), and this in turn results in the main product formed in (i) being only available in small amounts because of the previously mentioned unstable equilibrium, while nucleophile OH- in equation (iii) is much higher1.
When the OH- reacts with alkyl halide (iii) or acyl halide (iv), the conjugate base will tend to react with the unreacted carbonyl compound by means of an aldol condensation reaction as shown in equation (vi)1.
Preparation of the enamine PyrrolidineCyclohexanone Molecular formula: C4H9NMolecular formula: C6H10O Volume: 4. 0 ml+. Volume: 5. 0 ml Density: 0. 87 g cm-3Density: 0. 95 g cm-3 Boiling point: 87? CBoiling point: 156? C Masses were worked out using densities: Mass cyclohexanone = 0. 947g/ml x 5ml = 4. 739g Therefore moles = mass/molar mass = 4. 739g/98. 15g mol-1 = 0. 483 moles Mass pyrrolidine = 0. 866g/mol x 4. 0 ml = 3. 464g Therefore moles = mass/molar mass = 3. 464 / 71. 11g mol-1 Addition of the acetic anhydride EnamineAcetic Anhydride Molecular formula: C10H9N Molecular formula: C4H6O3¬ (2) Volume: 4. 5 ml Density: 1. 08 g cm-3 (2) Molecular Weight: 102. 1 g/mol Boiling point: 139. 8? C(2) Moles : 0. 04760 moles Acetic anhydride is the limiting reagent The stoichiometric ratio of acetic anhydride to 2 acetylcyclohexanone is 1:1 Preparation of 2-Acetylcyclohexanone 1-pyrrolidino-1-cyclohexene2-Acetylcyclohexanone
Molecular Weight: 151. 25 g mol-1 Molecular Weight: 140. 18 g mol¬-1 Mass: 0. 14g Moles: 0. 04760 moles The stoichiometric ratio of acetic anhydride to 2 acetylcyclohexanone is 1:1, therefore there is 0. 04760 moles of 2-acetylcyclohexanone. Theoretical mass of 2-acetylcyclohexanone = 0. 04760mol x 140. 18g/mol = 6. 673g Percentage yield = Actual yield/ Theoretical yield x 100 = 0. 14g/6. 673g x 100 = 2. 10%
As previously mentioned, aldehydes and ketones will tend to react with primary amines in order to form an imine, as can be seen in step 2 below.
Enamines are very easily alkylated due to them being very nucleophillic1. The first step entails the reaction of pyrrolidine and cyclohexanone with p-Toluenesulfonic acid to form a toluene solution of the enamine as shown below. Step 1: Reaction of pyrrolidine and cyclohexanone with p-Toluenesulfonic acid to form a toluene solution of the enamine The second step involves the reaction of the toluene solution of the enamine with acetic anhydride to form 1-pyrrolidino-1-cyclohexene, which is the intermediate of the reaction.
This intermediate is not seen as it cannot be isolated due to it being very unstable. Step 2: Reaction of the enamine with the acetic anhydride to form 1-pyrrolidinocyclohexene (the intermediate) The third and final step is the reaction of the intermediate(1-pyrrolidino-1-cyclohexene) with water in order to form final product 2-acetylcyclohexanone. Step 3: 1-pyrrolidino-1-cyclohexene (the intermediate) reacts with water in order to form 2-acetylcyclohexanone. The product was isolated by means of using a separator funnel.
The solution was cooled till it was room temperature and transferred to a seperatory funnel. 10 ml water was also added to the funnel and the contents were mixed thoroughly by inverting the funnel several times and releasing the gas build-up pressure by releasing the valve on the stopcock. The aqueous layer was the bottom layer due to it being higher in density than the organic layer and was disposed of in a beaker. Similarly, the organic layer was rinsed with three portions of hydrochloric acid (3M) and finally rinsed again with water (10 ml).
The organic layer was transferred to a clean 250 ml beaker and dried over MgSO4 – MgSO4 was added until it stopped clumping together at the bottom of the beaker in solution. The MgSO4 drying agent was then filtered off by vacuum suction. The melting point range obtained in the experiment was 110-119? C, which is not the same as the theoretical boiling point of 138? C. This could be due to impurities present in the sample, like water, which would lower the boiling point. The % yield was particularly low and could be attributed to insufficient reflux of the enamine solution that was supposed to have een refluxed for a minimum of half an hour, but due to time constraints, was only refluxed for 20 minutes. Another way to make 2-acetylcyclohexanone is by using a suspension of tellurium chloride and chloroform4
There are three steps in the mechanism to forming 2-acetylcyclohexanone. These are reacting pyrrolidine and cyclohexanone with p-Toluenesulfonic acid to form a toluene solution of the enamine, reaction of the enamine with the acetic anhydride to form 1-pyrrolidinocyclohexene, and reacting 1-pyrrolidino-1-cyclohexene (the intermediate) with water in order to form 2-acetylcyclohexanone.
The melting point range obtained experimentally was 110-119? C and is very different from the theoretical melting point range due to errors that have occurred during the procedure of the practical and are explained in the discussion above. The particularly low % yield of 2. 10% is due to insufficient reflux time.
Preparation of the toluene solution of the enamine A 100ml round bottomed flask was pre-weighed, after which cyclohexanone (5. 0ml), pyrrolidine (4. 0ml), toluene-4-sulfonic acid (0. 1g) and 40 ml toluene were added to it. boiling chips were also added to the reagent mixture so as to prevent loss of product due to splashing and boiling of the product. The Dean and Stark apparatus was set up individually by each student. The 100 ml round bottomed flask with reagent mixture was then fit to the Dean and Stark apparatus with a reflux condenser fit to the top of the apparatus. The reflux condenser was fit with a calcium chloride drying tube in order to prevent atmospheric moisture from reacting with the contents of the round bottomed flask, and also to serve as a means to relieve pressure formed during the reaction.
A heating mantle was used to heat the round bottomed flask till a vigorous reflux of the toluene, with vapour rising up the condenser was observable. It was observed that the water that formed as a result of the reaction of the toluene reflux collected in the trap portion of the Dean and Stark trap apparatus. Once the solution was boiling, reflux was continued for an hour. During this waiting period, a solution of acetic anhydride (4. 5 ml) in toluene (10 ml) was prepared.
Once reflux was complete after one hour, the solution was cooled till it could be handled comfortably and the Dean and Stark apparatus was removed. While waiting for the flask to cool, the distillation setup was prepared by re-positioning the condenser with both a receiver and receiver flask. The now cooled round bottomed flask was fit with a thermometer attached by means of a still head and fit to the distillation setup. The flask was once again heating using a heating mantle to distil pyrrolidine and water that was still present.
Distillation was continued until the temperature reached 108-110? C. Thereafter, the round bottomed flask was allowed to cool down to room temperature after removing it from the distillation setup and removing the still head with thermometer. Preparation of the 2-Acetylcyclohexanone The acetic anhydride solution that had been previously prepared was then added to the round bottom flask containing the enamine solution while stirring and was further stirred for 15 minutes. Water (5 ml) and 3 boiling chips was then added to the flask and fit with a reflux condenser.
This new reagent mixture containing intermediate product enamine was heated under reflux for a full 30 minutes once the solution came to a boil. This was the stopping point for day 1 of the experiment. The round bottomed flask was labelled and stoppered for continuation of the experiment on the following week. On day 2 of the experiment, the content of the round bottomed flask was transferred to a separating funnel. Water (10 ml) was used to rinse the round bottomed flask and this rinsing was added to the separating funnel.
The content of the funnel was mixed well and the gas was released by opening the stopcock. The aqueous layer was the lower layer and was separated from the organic layer, which remained in the funnel. Similarly, the organic layer was rinsed with three portions of hydrochloric acid (3M) and finally rinsed again with water (10 ml). The organic layer was transferred to a clean 250 ml beaker and dried over MgSO4 – MgSO4 was added until it stopped clumping together at the bottom of the beaker in solution. The MgSO4 drying agent was then filtered off by vacuum suction.
The vacuum distillation setup was then prepared and the filtered solution was transferred to this setup in order to distil off the toluene at its boiling point of 110? C at 760 mmHg pressure. The remaining residue was put in a very small round bottomed flask. This flask was attached to a short path condenser with 3 anti-bumping granules added to the flask. 3 very small receiver flasks were preweighed before the product was flame distilled. Flame distillation took place under reduced pressure by means of using the bench vacuum. different fractions of product during the distillation were collected in 3 different receiver flasks. The temperature range that each of the fractions were collected at were recorded. The added masses of the product collected in all three flasks were used.
1. http://www. courses. chem. psu. edu/chem36/Chem36H/IndivExpt1/435%20Acetylcyclohexanone. pdf – Accessed 11/08/08
2. http://en. wikipedia. org/wiki/Acetic_anhydride – Accessed 11/08/08
3. P. Y Bruice, Organic Chemistry, 4th Edition, Pearson Education Inc. , 2004, USA pgs 747-753 4. http://www. freepatentsonline. com/4355097. html