CH5008 Assessment Details and Practical Procedures

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.

Procedure

Mount a 100 cm3 round-bottom flask, equipped with a magnetic stirrer, above a stirrer-hotplate (DO NOT SWITCH ON THE HOTPLATE!). Fit the flask with an adaptor carrying

a dropping funnel and a condenser.  Place a solution of 6 cm3 of cyclohexene in 6 cm3 of diglyme in the flask and add a solution of 1.4 g of sodium borohydride in
10 cm3 of diglyme.  Cool the flask in an ice bath, begin stirring and slowly add from the dropping funnel a solution of 2 cm3 of boron trifluoride etherate (care –

avoid skin contact) over a period of about 12-15 minutes. Stir the reaction for an additional 15 min. and carefully add 10 cm3 of water to destroy any excess NaBH4.

When no further hydrogen is evolved make the solution alkaline (pH 11-12 using indicator paper) by the addition of 6 cm3 of dilute NaOH solution, followed by 6 cm3 of

30 % hydrogen peroxide solution (added in 0.5 cm3 portions using a teat pipette).

Pour the reaction mixture into a separating funnel containing 20 cm3 of ice-water and extract the cyclohexanol with two 20cm3 diethyl ether extractions. Wash the

combined ether extracts with saturated sodium chloride solution, separate the ether layer and dry this over anhydrous magnesium sulphate (1-2 spatula loads).  Decant

off the dried ether layer into a beaker, and evaporate on a steam bath to leave the crude cyclohexanol.  (Cyclohexanol is a liquid with  a high boiling point, it will

not evaporate however long you leave the flask on the steambath!) Purify the cyclohexanol by distillation.

Record the yield, boiling point and IR spectrum of the product.

NB If the amount of product you produce is quite small, pool your material with other colleagues and do a bulk distillation.

EXPERIMENT 3: DIELS-ALDER REACTION OF MALEIC ANHYDRIDE WITH CYCLOPENTADIENE

Note: This experiment involves a distillation step. Distillation techniques are demonstrated in a video which you should view before this experiment. The link to this

video is
www.rsc.org/learn-chemistry/resource/res00001070/distillation

Introduction

This experiment illustrates the Diels-Alder cycloaddition reaction, covered in lectures in Semester 2. It is one of a number of reactions whose outcome is governed by

the role of orbital symmetry. In the first step of this procedure cyclopenta-1,3-diene is prepared by thermal ‘cracking’ of the dimer (dicyclopentadiene). In the

second step cyclopentadiene is reacted with maleic anhydride (a strong dienophile) to form a Diels-Alder adduct.

Procedure

Place dicyclopentadiene (30 cm3) and a few anti-bumping granules in a 100 cm3 round-bottom flask and set up the apparatus for distillation, with the receiving flask in

an ice-bath. Raise the temperature of the heating mantle slowly until cyclopentadiene (monomer) collects slowly (dropwise) in the receiving flask. The distillation

temperature should not exceed 65 oC. Collect 3-5 cm3 of the distillate and keep it in the ice-bath until use. Allow the apparatus to cool and, when safe to handle,

return the unused dicyclopentadiene to the bottle provided. Rinse the apparatus with acetone and return it to the box – it will be needed later on in the practical by

another group.

Dissolve maleic anhydride (1.0 g) in toluene (15 cm3). Cool the solution in an ice bath and, when cold, add 1.0 g of cyclopentadiene (monomer). Swirl the flask and

leave to stand in the ice-bath; record the time taken for crystals to appear. Filter the cyclopentadiene-maleic anhydride using a Buchner filtration apparatus, record

the yield and obtain an IR spectrum. An NMR spectrum of the product will be posted on the weblearn site: interpret this for inclusion in your portfolio for assessment

in Semester 2.

Return the unused cyclopentadiene (monomer) to the bottle used to collect dicyclopentadiene (dry the outside of the round bottom flask before pouring it back in – it

is important that the recovered dicyclopentadiene/cyclopentadiene is not contaminated with water).

REPORT
S FOR SYNTHESIS PRACTICALS
The report will need the following:
?
The date the practical was carried out.
?
An Introduction
–
describe the reaction and explain the mechanism (the process by
which the reaction occurs).
?
The Method
–
in the past tense, passive voice with a description of what
YOU
did and
any observations you carried out.
?
Results
–
give a record of the observed yield and calculate the %yield in the standard
fashion
–
calculate the theoretical yield and (ob
served/calculated)x100 give %yield. List
the melting range (normally 2
–
3 degree range rather than a specific temperature unless
you have done it very, very slowly) if asked. Look up the melting point of the pure
compound and list this (plus the source of y
our data) for comparison. Include the IR
spectrum plus any assignment of the bands. Check the functional groups in the product
to see what you need to assign.
?
Discussion
–
i
s the yield good or bad
–
where do you think you might have lost material
–
can yo
u find a literature reference to a yield?
–
is the M.Pt. lower/higher than
expected, if so why? List the detail of any equipment used to record any analytical data
(i.e. the make and model of the IR machine).
As part of the discussion also answer the
quest
ions from the pro
–
forma.

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