Preparation of 2-Acetylcyclohexanone
When hydrogens are present on the α-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
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: 87ºCBoiling point: 156ºC
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 = 0.0483 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
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: 139.8ºC(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
Molecular Weight: 151.25 g mol-1 Molecular Weight: 140.18 g mol¬-1
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
Percentage yield = Actual yield/ Theoretical yield x 100
= 0.14g/6.673g x 100
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...