[ÚSTAV LÉKARSKÉ BIOCHEMIE 1. LF UK]
Organic Chemistry I
Practical Lesson on Medical Chemistry & Biochemistry
Selected Reactions of Organic Chemistry
Libuše Arnoštová
(edited by Jan Pláteník)
2010
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Content
Theoretical introduction ….. 3
Reagents …..7
Exercise 1 – Reduction of Fehling’s solution ..... 9
Exercise 2 – Reduction of Tollens’ reagent ..... 9
Exercise 3 – Detection of acetone by Lestradet’s test ..... 10
Exercise 4 – Formation of calcium oxalate ..... 10
Exercise 5 – Deacetylation of acetylsalicylic acid ..... 10
Exercise 6 – Reaction of free –OH group with FeCl3 ..... 11
Exercise 7 – Sodium salt of salicylic acid ..... 12
Exercise 8 – Acetylation of salicylic acid ..... 12
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Selected reactions in organic chemistry
Introduction
Organic chemistry is a base for the study of biochemistry and relative areas. Understanding
of the principle of organic reactions leads directly to understanding of the principle of
biological processes.
In this practical lesson we will focus just on some of the most important functional groups
and their reactions. In the first part we will look at the redox reactions of aldehyde group
with Fehling’s and Tollens´ reagent and the reaction of keto group with the Lestradet´s
reagent. We will also perform simple acid-base reaction that leads to insoluble salt of oxalic
acid.
In the second part of the seminar we will try to realize two types of reactions that are very
important for biochemistry as well – esterification and hydrolysis of ester. For this purpose
we have chosen acetylsalicylic acid, known as Aspirin. In the first step, acetyl group will be
removed and in the second step we will esterify this product to obtain the acetylsalicylic
acid again.
The proof will be done with the use of color reaction the principle of which is the formation
of red complex of ferric chloride with the compounds that contain hydroxylic group.
Theoretical part
Oxidation reactions
Aldehydes are easily oxidized and the result of such oxidation is carboxylic acid.
Although this equation provides basic information about the change that occurred in the
organic molecule, we do not have the complete redox pair. So, better way how to describe
this oxidation is reaction like this:
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It contains the whole redox pair and stoichiometric coefficients. In this reaction acetic
aldehyde is oxidized and yields acetic acid while potassium manganate (VII) – permanganate
is reduced to two different compounds with different oxidation states, one having VI and
the other one IV. Reaction occurs in alkaline conditions, so we have one more acid basereaction – acetic acid forms its salt and water.
Similar situation occurs during the reactions with Fehling´s and Tollens reagents.
Fehling´s reagent is prepared by the reaction of copper (II) sulfate with tartrate of
potassium and sodium. The main reagent in this reaction is in fact copper (II) tartrate. This
salt reacts with the aldehyde in alkaline conditions and forms the acid, cuprous oxide and
sodium tartrate. So aldehyde is oxidized to corresponding acid while copper is reduced to
cuprous cation. If an excess of hydroxide is added then copper is reduced to its oxidation
state 0 and so called „mirror“ results.
Tollens´reagent contains the diamminesilver(I) ion, [Ag(NH3)2]+ and oxidizes aldehydic
group as well. By-products are silver, ammonium nitrate and ammonium. It reacts with the
formic acid as well. The reason is obvious when we draw the corresponding formula :
Formic acid is oxidized to carbon dioxide and Ag+ is reduced to elemental silver.
Lestradet’s proof is based on the formation of violet complex of keto group with sodium
nitroprusside Na2[Fe(CN)5NO]. This is the base for the detection of compounds with keto
group (acetone) in urine:
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Calcium oxalate is an almost insoluble salt of oxalic acid. It can be found in urine of some
patients and is usually detected by the use of microscope. In urine it is often present as a
dihydrate or, rarely, monohydrate. It also occurs in some plants, e.g. in Dieffenbachia (in
high concentration) or in vegetables – spinach, rhubarb – in low concentrations. We will
prepare it by exchange of the cation of weak base by the ion of strong base, according to
the equation :
Hydrolysis of acetylsalicylic acid
Acetylsalicylic acid, known under the commercial name Aspirin® ( Bayer) is widely used
drug. It relieves pain, inflammation and flu-like syndroms.
Hydrolysis of acetylsalicylic acid yields salicylic acid. Reaction represents a typical alkaline
hydrolysis of an ester and so salt of the acid and alcohol are the results of this reaction.
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Reverse reaction, acetylation of salicylic acid by acetic anhydride, provides acetylsalicylic
acid, according to the equation:
The proof that we have really prepared acetylsalicylic acid can be done with the use of
Uffelmann´s reagent, which is Fe(III) chloride. All the hydroxyacids provide color reaction
with this reagent.
Conversion of the free acid to its sodium salt leads to the increase of polarity and therefore
to improvement of its solubility in water. Sodium salt is obtained by reaction of free acid
with sodium hydroxide.
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Reagents
1.
Fehling I: Copper sulfate, CuSO4 solution
2.
Fehling II: Na-K-tartrate (Seignett’s salt) solution in sodium hydroxide
3.
Formaldehyde, HCOH
4.
Silver nitrate, AgNO3 100 g/l solution
5.
Sodium hydroxide, NaOH 2 mol/L
6.
Aqueous ammonia (ammonium hydroxide), NH4OH
7.
Formic acid, HCOOH
C (CORROSIVE !)
8.
Acetone, H3C-CO-CH3
F
9.
Lestradet’s reagent (powder):
10.
Oxalic acid, (COOH)2
11.
Calcium chloride, CaCl2 10 g/L
Xi
12.
Hydrochloric acid, HCl diluted
C
13.
Acetylsalicylic acid, C9H8O4
Xi
N
(TOXIC, IRRITANT, MUTAGENIC,
CANCEROGENIC)
C
C
N
Xi
ammonium sulfate
sodium carbonate anhydrous
sodium nitroprusside
Xn
Xn
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20 g
20 g
0.2 – 1 g
14.
Sodium hydroxide, NaOH 50%
C (CORROSIVE !)
15.
Sulfuric acid, H2SO4 diluted
16.
Uffelmann reagent: ferric chloride, FeCl3 solution
17.
Acetanhydride, (CH3CO)2O
18.
Phosphoric acid, H3PO4
C
C (CORROSIVE !)
C (CORROSIVE !)
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Exercise 1 – Reduction of Fehling’s solution
Principle:
Fehling solution is obtained by mixing of Fehling I and Fehling II solutions 1:1. In the
presence of an aldehyde, reddish cuprous oxide appears.
Procedure:
Preparation of Fehling´s reagent : mix Fehling I and Fehling II solutions in 1:1 (volume ratio,
so 1 portion of solution I and 1 portion of solution II are mixed).
Put few drops of formaldehyde into the test tube and add about 3 ml of Fehling’s solution.
After brief warming, a precipitate of reddish Cu2O appears. Make also a blind test (without
formaldehyde).
Exercise 2 – Reduction of Tollens´ reagent
Principle:
Formic acid reduces Tollens´reagent and silver of the oxidation state 0 is formed. Formic
acid is oxidized to carbon dioxide.
Procedure:
Aqueous solutions of AgNO3 (100 g/l) and NaOH (2 mol/l) are mixed in 1:1 ratio.
Precipitation of Ag2O occurs.
Add ammonia as long as precipitated Ag2O just dissolves. The excess of ammonia is not
desirable as it lowers the sensitivity of the reagent.
Mix 0.5 ml HCOOH with 1 ml of water and 2 ml of Tollens´ reagent. Warm carefully.
Elemental silver appears as the dark precipitate or the shiny mirror on the test tube.
Blind test: Tollens´ reagent itself does not form any precipitate, even after warming.
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Exercise 3 – Detection of acetone by Lestradet´s test
Principle:
Compounds containing a keto group form colored compound with sodium nitroprusside.
This reaction can be used as an evidence of occurrence of acetone in urine.
Procedure:
Moisten round slice of filtration paper and place it on the glass mat. Sprinkle the filtration
paper with acetone and add a few crystals of the reagent. A violet color will appear after a
while.
Exercise 4 – Formation of calcium oxalate
Principle:
Oxalic acid forms soluble salt with two ammonia cations. These cations can be substituted
by cations from stronger bases (sodium, calcium etc.). In the solution of strong mineral
acids, those salts dissolve readily, in water they are almost insoluble.
Procedure:
Dissolve a small amount of oxalic acid in water. Add 10 drops of ammonia – after the
addition of ammonia the resulting mixture should have roughly neutral pH (check with pH
paper). Then pour approximately 1 ml of aqueous CaCl2 (10g /l). White precipitate appears.
Dissolve the precipitate by addition of 1 ml of aqueous solution of HCl. Notice the pH in
which the precipitate dissolves completely with the use of indicator paper.
Exercise 5 – Deacetylation of acetylsalicylic acid
Principle:
As a result of alkaline hydrolysis of ester, from one mole of the ester one mole of sodium
salt of the acid and one mole of alcohol result. Sodium salt of an acid is soluble in water
while free acid, obtained by acidification of this salt has limited solubility. This is the reason
why free acid crystallizes easily from the solution after cooling.
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Warning: Attention! During this exercise you will be working with strong acids and bases,
wear protection glasses and gloves. Neutralization should be done in a way that acid is
added to the COLD reaction mixture CAREFULLY and SLOWLY.
Be aware of “hidden boil“ and add small pieces of porous material or stirrer into the flask.
Procedure:
Put 1 g of acetylsalicylic acid to 125 ml Erlenmayer´s flask, add 5 ml of 50% NaOH and warm
to boiling point in hot water bath (add stirrer in order to avoid problems with ‘hidden
boiling’). Boil the reaction mixture for 10 minutes and then let it cool down – you can use
cold water to speed up cooling.
To the cold reaction mixture carefully add diluted sulfuric acid until white crystals of salicylic
acid start to appear. At this moment pH should be slightly acidic.
Filter out crystals with the use of paper filter and dry them with filtration paper.
Exercise 6 – Reaction of free –OH group with FeCl3
Principle:
FeCl3 reacts with free hydroxyl group of organic acids and colored compound is formed.
Using a glass rod, put a small portion of the hydrolysis product to a test tube, add water and
mix. Add a drop of FeCl3 solution and notice the change of the color.
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Exercise 7 – Sodium salt of acetylsalicylic acid
Principle:
Acetylsalicylic acid is poorly soluble in water. By conversion to its sodium salt the solubility is
enhanced.
Procedure:
Put a small amount of salicylic acid obtained in the previous experiment to the test tube and
add 1 ml of water. Then add 2 ml of NaOH (the same that was used for hydrolysis) and mix.
Exercise 8 – Acetylation of salicylic acid
Principle:
We can use anhydrides for preparation of esters of carboxylic acids. Anhydride reacts with
alcohol and forms ester and acid.
Safety: be careful, acetic anhydride is irritating agent, avoid contact with skin and do not
inhale. Phosphoric acid is corrosive agent, use protective equipment - glasses and gloves.
Procedure:
Pour 100 ml of water into 400 ml beaker and warm to boiling. Put product of the previous
reaction (Exercise 5) to the 125 ml Erlenmayer flask with the use of plastic spoon.
Add 1.5 ml of acetic anhydride and 2 drops of phosphoric acid.
Place the flask into the water bath and warm 15 minutes. Let it cool to lab temperature; you
can use cold water to speed up the process. You can also add 30 ml of cold water and 2 ice
cubes directly into the reaction mixture.
During the cooling stir the mixture with a glass rod and rub the walls and the bottom of the
flask. Crystals of acetylsalicylic acid will appear after a while. Separate crystals by filtration
and dry them with the filtration paper.
Literature:
Exercises 1-4 were adopted from Kraml J. a kol.: Návody k praktickým cvicením z lékařské
chemie a biochemie, Praha 1991
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