CHEM231 Lab Manual 2013/14 1 Organic Chemistry -1 (Practical) CHEM 231 ________________________________________________________________________ Laboratory Manual 2013/2014 Pepared by Dr. Khaid Shadid & Prof. Dr. Abdulfatah Haikal Islamic University in Madinah
44
Embed
Organic Chemistry -1 (Practical) CHEM · PDF fileOrganic Chemistry -1 (Practical) CHEM 231 ... The capillary melting point tubes should be filled by crushing the
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Determination of Boiling Point (bp) of Pure Acetone
Arrange a simple distillation apparatus. Introduce about 20 mL of acetone and a few boiling
stones in a 50 mL round-bottomed flask. Heat gently so that the distillate collects in the receiver
drop by drop. Make sure that there is a drop of liquid hanging from the bulb of the thermometer
to ensure that the thermometer is reading the correct bp. Absence of this drop indicates
superheating. Wait until 1-2 mL of the distillate have been collected before recording the
temperature. Continue the distillation until about 2 mL of residue are left in the distillation flask,
and record the temperature again. Keep the acetone for the following part.
Separation of a Mixture
(Simple Distillation)
Make a mixture of the two liquids (acetone-water) 20 mL each and pour it into a 100 mL round
bottomed flask. Carry out a simple distillation as before and collect five fractions in the following
boiling ranges: 50-62, 62-72, 72-82, 82-95 plus the fifth fraction which is the residue. Measure the
volume of each fraction and record the results.
CHEM231 Lab Manual 2013/14
19
(Fractional Distillation)
Combine the five fractions and pour into a 100 ml round-bottomed flask, attach the fractionating
column and proceed as for simple distillation. Measure the volume of each fraction as before and
record your results.
CHEM231 Lab Manual 2013/14
20
Boiling Point and Distillation
Report Sheet
Simple Distillation of Pure Acetone
Boiling point of pure acetone: ---------
Separation of a Mixture of Acetone and Water
1.
Fraction Boiling Range, °C Volume of Distillate, mL
Simple Fractional
Composition
I 50-62
II 62-72
III 72-82
IV 82-95
V residue
2. Plot the boiling point versus the volume of distillate for the acetone-water mixtures using
simple and fractional distillation.
Name:----------------------
CHEM231 Lab Manual 2013/14
21
Chromatography
A Separation and Purification Technique
Introduction
Chromatography is a technique that may be used:
1. to separate the components of a mixture as well as 2. to identify organic substances and 3. examine their purity.
Chromatography encompasses several techniques such as column, thin-layer, paper, gas
liquid, etc.
Two principles are basically involved in Chromatography: adsorption (as in thin-
layer Chromatography) and partition (as in paper Chromatography), and certain terms are
common to both types of Chromatography.
In adsorption Chromatography, separation depends on the selective desorption of the
components of a mixture by the eluent (mobile phase) from the surface of a solid
adsorbent (stationary phase). The adsorbent may be packed in a column (Column
Chromatography) or spread as a thin layer on a glass plate as in thin-layer
Chromatography.
In partition Chromatography, separation depends on partition of the components of a
mixture between the stationary and mobile phases. The mobile phase may be a
liquid (liquid-liquid partition Chromatography) or a gas (gas-liquid partition
Chromatography).
CHEM231 Lab Manual 2013/14
22
ANALYSIS OF CHROMATOGRAMS:
In thin layer and paper Chromatography, substances are characterized by their Rf values
(retardation factor). The Rf-value is a number (less than one) which is characteristic of a
compound for a given adsorbent and developing solvent.
The retention factor, or Rf, is defined as the distance traveled by the compound divided by
the distance traveled by the solvent.
For example, if a compound travels 2.1 cm and the solvent front travels 2.8 cm, the Rf is 0.75:
The Rf for a compound is a constant from one experiment to the next only if the
chromatography conditions below are also constant:
solvent system
adsorbent
thickness of the adsorbent
amount of material spotted
temperature
Since these factors are difficult to keep constant from experiment to experiment, relative Rf
values are generally considered. “Relative Rf” means that the values are reported relative to a
standard, or it means that you compare the Rf values of compounds run on the same plate at the
same time.
The larger an Rf of a compound, the larger the distance it travels on the TLC plate. When
comparing two different compounds run under identical chromatography conditions, the
CHEM231 Lab Manual 2013/14
23
compound with the larger Rf is less polar because it interacts less strongly with the polar
adsorbent on the TLC plate. Conversely, if you know the structures of the compounds in a
mixture, you can predict that a compound of low polarity will have a larger Rf value than a
polar compound run on the same plate.
The Rf can provide corroborative evidence as to the identity of a compound. If the identity of a
compound is suspected but not yet proven, an authentic sample of the compound, or standard,
is spotted and run on a TLC plate side by side (or on top of each other) with the compound in
question. If two substances have the same Rf value, they are likely (but not necessarily) the same
compound. If they have different Rf values, they are definitely different compounds. Note that
this identity check must be performed on a single plate, because it is difficult to duplicate all the
factors which influence Rf exactly from experiment to experiment.
In gas-liquid chromatography, compounds are characterized by their retention times.
THIN-LAYER CHROMATOGRAPHY (TLC):
This is one application of adsorption chromatography in which an adsorbent, usually silica gel
or alumina, is spread out as a thin layer on an inert surface, such as a glass plate or microscope
slide. The mixture is applied at one end of the coated plate and, as the mobile phase (a liquid)
moves up the solid adsorbent by capillary action, the adsorbed components of the mixture get
desorbed and carried along at different rates by the moving solvent. Adsorption of the
components of the mixture, on the surface of the adsorbent, occurs to differing extents
depending on their structural features and polarity. The more strongly adsorbed a given
compound is, the slower it is transported by the mobile phase, and conversely, the more weakly
adsorbed the compound is, the faster it is transported up the stationary phase. The result is that
the components of the mixture are separated into different zones or spots.
Separation by thin-layer chromatography
CHEM231 Lab Manual 2013/14
24
Separation by thin-layer chromatography depends on:
1. the kind and activity of the adsorbent (stationary phase). 2. the polarity of the eluent (mobile phase). 3. the chemical nature of the components of the mixture.
The most common adsorbents employed in TLC are silica (SiO2.xH2O) and alumina (Al2O3.
xH2O), and the activity of these adsorbents is largely determined by their water content. For a
given adsorbent and compound, the greater the polarity of the eluent, the greater is its ability to
dislodge a compound from the surface of the adsorbent, and therefore the higher the Rf-value.
Preparation of TLC Plates. Large glass plates (20x20 cm) are commonly used for quantitative separations, while microscope slides are usually used for qualitative purposes. A homogeneous slurry of the adsorbent in a volatile organic solvent (chloroform or dichloromethane) is poured over the glass plates and allowed to air-dry at room temperature. Microscope slides can be coated, two at a time, by dipping them into the slurry for sometime then holding them vertically to air-dry. The jar of adsorbent must be shaken thoroughly before each use to homogenize the slurry.
Spotting. The mixture to be analyzed is dissolved in a suitable solvent (1% solution). With a drawn capillary tube, a small amount of this solution is spotted on the TLC plate about 1 cm from the bottom. The spots should have a diameter not larger than 1-2 mm, since larger spots result in "tailing" and overlapping of close spots. Once the solvent evaporates from the spots, the plate is ready for developing.
Development of the Chromatogram. The eluent, also called developing solvent, is chosen on the basis of the nature and polarity of the compounds being studied. It is best to choose the solvent that will give a satisfactory separation within the range of 0.2-0.8 Rf values. The plate is placed in a developing chamber (e.g. a covered beaker) containing the solvent and lined with filter paper soaked in the solvent to help saturate the atmosphere with solvent vapors. When the solvent front reaches the finish line, the plate is removed from the beaker and placed on the bench top to air-dry.
CHEM231 Lab Manual 2013/14
25
Visualization of Spots. Compounds on the plate are located according to their characteristics:
a). If the spots are colored, they can be observed in ordinary light.
b). If the compounds are colorless, they can be seen under UV-light where they appear as dark
spots on a white background.
c). Colorless spots may also be located with an indicator. Most organic compounds form
complexes with iodine giving dark brown spots when the plate is exposed to iodine vapor.
Sulfuric acid may also used to make colorless spots visible. Most organic compounds turn black
when sprayed with sulfuric acid.
COLUMN CHROMATOGRAPHY: تم إضافة هذا الجزء المهم باألسفل الرتباطه الوثيق بما سبق لكنة غير مطلوب
In column chromatography, the stationary phase, a solid adsorbent, is placed in a vertical
glass (usually) column and the mobile phase, a liquid, is added to the top and flows down
through the column (by either gravity or external pressure). Column chromatography is
generally used as a purification technique: it isolates desired compounds from a mixture.
The mixture to be analyzed by column chromatography is applied to the top of the column.
The liquid solvent (the eluent) is passed through the column by gravity or by the application of
air pressure. An equilibrium is established between the solute adsorbed on the adsorbent and
the eluting solvent flowing down through the column. Because the different components in the
mixture have different interactions with the stationary and mobile phases, they will be carried
along with the mobile phase to varying degrees and a separation will be achieved. The
individual components, or elutants, are collected as the solvent drips from the bottom of the
column.
Column chromatography is separated into two categories, depending on how the solvent
flows down the column. If the solvent is allowed to flow down the column by gravity, or
percolation, it is called gravity column chromatography. If the solvent is forced down the
column by positive air pressure, it is called flash chromatography, a "state of the art" method
currently used in organic chemistry research laboratories. The term "flash chromatography" was
coined by Professor W. Clark Still because it can be done in a “flash."
CHEM231 Lab Manual 2013/14
26
Columns Packed Column Column Chromatography
The Adsorbent
Silica gel (SiO2) and alumina (Al2O3) are two adsorbents commonly used by the organic chemist
for column chromatography. These adsorbents are sold in different mesh sizes, as indicated by
a number on the bottle label: “silica gel 60” or “silica gel 230-400” are a couple examples. This
number refers to the mesh of the sieve used to size the silica, specifically, the number of holes in
the mesh or sieve through which the crude silica particle mixture is passed in the
manufacturing process. If there are more holes per unit area, those holes are smaller, thus
allowing only smaller silica particles go through the sieve. The relationship is: the larger the
mesh size, the smaller the adsorbent particles.
Adsorbent particle size affects how the solvent flows through the column. Smaller particles
(higher mesh values) are used for flash chromatography, larger particles (lower mesh values)
are used for gravity chromatography. For example, 70–230 silica gel is used for gravity columns
and 230–400 mesh for flash columns.
Alumina is used more frequently in column chromatography than it is in TLC. Alumina is quite
sensitive to the amount of water which is bound to it: the higher its water content, the less polar
sites it has to bind organic compounds, and thus the less “sticky” it is. This stickiness or activity
is designated as I, II, or III, with I being the most active. Alumina is usually purchased as
activity I and deactivated with water before use according to specific procedures. Alumina
comes in three forms: acidic, neutral, and basic. The neutral form of activity II or III, 150 mesh,
is most commonly employed.
CHEM231 Lab Manual 2013/14
27
The Solvent
The polarity of the solvent which is passed through the column affects the relative rates at
which compounds move through the column. Polar solvents can more effectively compete with
the polar molecules of a mixture for the polar sites on the adsorbent surface and will also better
solvate the polar constituents. Consequently, a highly polar solvent will move even highly polar
molecules rapidly through the column. If a solvent is too polar, movement becomes too rapid,
and little or no separation of the components of a mixture will result. If a solvent is not polar
enough, no compounds will elute from the column. Proper choice of an eluting solvent is thus
crucial to the successful application of column chromatography as a separation technique. TLC
is generally used to determine the system for a column chromatography separation. Often a
series of increasingly polar solvent systems are used to elute a column. A non-polar solvent is
first used to elute a less-polar compound. Once the less-polar compound is off the column, a
more-polar solvent is added to the column to elute the more-polar compound.
Interactions of the Compound and the Adsorbent
Compounds interact with the silica or alumina largely due to polar interactions.
Analysis of Column Eluents
If the compounds separated in a column chromatography procedure are colored, the progress
of the separation can simply be monitored visually. More commonly, the compounds to be
isolated from column chromatography are colorless. In this case, small fractions of the eluent
are collected sequentially in labeled tubes and the composition of each fraction is analyzed by
thin layer chromatography. (Other methods of analysis are available; this is the most common
method and the one used in the organic chemistry teaching labs.).
This Figures shows the use of column chromatography
1 2 3 4 5 …..
Column Chromatography
CHEM231 Lab Manual 2013/14
28
Alcohols
Classification and tests
Objective:
1. Determine chemical and physical properties of alcohols.
2. Classify an alcohol as primary, secondary, or tertiary.
3. Perform a chemical test to distinguish between the classes of alcohols.
4. Write the formulas of the oxidation products of alcohols
Structures of Alcohols
Alcohols are organic compounds that contain the hydroxyl group (–OH). The simplest alcohol
is methanol. Ethanol is found in alcoholic beverages and preservatives, and is used as a solvent.
2-Propanol (isopropanol), also known as rubbing alcohol, is found in astringents and perfumes.
Classification of Alcohols
In a primary (1°) alcohol, the carbon atom attached to the –OH group is bonded to one other
carbon atom. In a secondary (2°) alcohol, it is attached to two carbon atoms and in a tertiary (3°)
alcohol to three carbon atoms.
CHEM231 Lab Manual 2013/14
29
1. Properties of Alcohols Materials: 4 test tubes, pH paper, stirring rod, methanol, ethanol, 2-propanol, t-butyl
alcohol (2-methyl-2-propanol).
Odor: Place 5 drops of each of the alcohols, phenol to four separate test tubes. Carefully
detect the odor of each.
Solubility in water Add about 2 mL of water (40 drops) to each test tube. Shake and
determine whether each alcohol is soluble or not. If the substance is soluble in water, you
will see a clear solution with no separate layers. If it is insoluble, a cloudy mixture or
separate layer will form. Record your observations.
Acidity Obtain a container of pH paper. Place a stirring rod in one of the alcohols and touch
a drop to the pH paper. Compare the color of the paper with the chart on the container to
determine the pH of the solution. Record your observations. Note: DISPOSE OF ORGANIC SUBSTANCES IN DESIGNATED WASTE CONTAINERS!
2. Oxidation of Alcohols
The Bordwell-Wellman test solution contains potassium dichromate dissolved in sulfuric
acid. It is an orange-yellow solution. The orange-yellow color is due to the Cr2O7 2- ion. The
oxidation number of chromium is +6. This reagent will oxidize primary and secondary
alcohols and, in turn, becomes reduced. The chromium in dichromate is reduced to chromic
ion, Cr+++.
A greenish colored solution results. This color change from orange-yellow to green serves
as an indicator for the presence of a primary or secondary alcohol. A primary alcohol is
oxidized first to an aldehyde, which will be further oxidized to an acid.
Materials: 4 test tubes, methanol, ethanol, 2-propanol, t-butyl alcohol (2-methyl-2-
6. Reaction with calcium chloride CaCl2 General Carboxylic acid + CaCl2 + NH4OH precipitate 1. Oxalic acid: white ppt will dissolve in dil HCl, will not dissolve in acetic acid 2. Tartaric acid: white ppt with test tube scratching. The ppt will dissolve in acetic
acid 3. Citric acid: white ppt after heating, will not dissolve in acetic acid