CH5008 Assessment Details and Practical Procedures

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CH5008 Assessment Details and Practical Procedures

Introduction & assessment.

The three practicals in this module illustrate some of the more important reaction types and mechanisms covered in the lecture course. The practical portfolio comprises 20% of the overall assessment of the module.

Full details of the work required in the portfolio will be posted later in the semester. It will require submission your original notes taken while carrying out the experiments in the laboratory. Ensure that a demonstrator signs your laboratory notes before leaving the lab each week and do not loose them – failure to submit them along with the completed coursework will entail a substantial marking penalty. These notes do not need to be neat or contain mechanisms, but must be contemporary – made as you are doing the experiment, not after you have completed it. Record all observations, weights, etc, as you will need these to complete your portfolio.

EXPERIMENT 1: HYDROLYSIS OF PHENYL BENZOATE

Introduction

The experiment involves the use of base catalysis in the hydrolysis of phenyl benzoate and the isolation and purification of the resulting acid and alcohol. The reaction involves nucleophilic addition to a carbonyl group, followed by elimination (covered in lecture 5 during Semester 1). The new technique encountered in this experiment is solvent extraction – all students have done this experiment in the first year (4001). Solvent extraction techniques are explained in this RSC protocol for separating benzoic acid from benzil (http://www.rsc.org/learn-chemistry/wiki/index.php?title=Expt:Separation_of_an_Acid_from_a_Neutral_by_Base_Extraction&oldid=1825)

Reading:

1. Practical Organic Chemistry, F. G. Mann and B. C. Saunders, 4th Ed., Longmans, 1960, details of ester hydrolysis, p.354.

2. Vogel’s Textbook of Practical Organic Chemistry, revised by B. S. Furniss et al, 4th Ed., Longman, 1978; details of solvent extraction, p.127; details of ester hydrolysis, p.1120.

3. Handbook of Chemistry and Physics, R. C. Weast, Ed., 64th Ed., CRC Press, 1983/84.

Background:

Ester hydrolysis may be catalysed by either acid or base. Both reactions are reversible. However, in the case of base catalysis, any carboxylic acid produced reacts further with the hydroxide to give the acid anion, RCOO-. Thus as the acid is formed by the first reaction it is removed by the second, so that eventually all the ester is converted into its hydrolysis products. This makes the base-catalysed reaction more efficient than the acid catalysed reaction.

Procedure

Place 2.0 g of potassium hydroxide pellets, 7 cm3 of diethylene glycol and 2 cm3 of water in a 100cm3 round-bottomed flask, add a few antibumping granules, and assemble the apparatus for reflux. Using an electric heating mantle (“isomantle”), heat to reflux and maintain reflux until all the potassium hydroxide has dissolved. Allow the solution to cool to room temperature. Remove the reflux condenser and add 3.0 g of phenyl benzoate. Swirl the contents of the flask, reassemble the apparatus and reflux for 15 minutes. Cool again, and transfer the flask and contents to an ice/water bath for additional chilling. Add, dropwise, with swirling, sulphuric acid (3 mol dm-3, available as a bench reagent) until a faint but permanent precipitate (of benzoic acid) is observed. Continue adding the acid until the solution of is acid to Universal indicator paper. Break up any large solid deposits that may occur with a glass rod.

Add aq. sodium carbonate solution (available as a bench reagent) with vigorous swirling until the precipitate redissolves and the solution is definitely alkaline to indicator paper. (CAUTION:Add carbonate slowly to avoid excessive frothing and risk of spillage). Pour the resultant solution into a separating funnel and extract twice with diethyl ether to remove the phenol. Dry the combined ether layers and evaporate the solvent in a pre-weighed beaker on a steam bath. Calculate the yield of phenol.

Collect the aqueous extracts in a conical flask and, dropwise, add dilute hydrochloric acid (4 mol dm-3, available as a bench reagent) until the benzoic acid precipitates and the solution is just acid to Universal indicator paper. Filter off the benzoic acid, using the Büchner method. Wash the product with ice-cold water and then recrystallise it if time permits. Calculate the yield of benzoic acid.

EXPERIMENT 2: PREPARATION OF CYCLOHEXANOL FROM CYCLOHEXENE

Introduction

This experiment illustrates electrophilc addition to a double bond, covered in lectures during Semester 2. The alkene involved is cyclic – lectures in semester 2 will discuss the effect that ring-conformation has on this and related reactions. The first step of this reaction is usually referred to as a ‘hydroboration’. It involves an ‘anti-Markovnikov’ addition to a C=C double bond.

 

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