Summary of the subject (Midterm)
BS10057: Organic Chemistry
by
Ms. Anantanuch Intajak
ID : 5710056
Ms. Kornkanok Ketbumrung
ID : 5710071
Ms. Anchidta Tangsuk
ID : 571022
Module 1
Crystallization
Crystallization is a technique which chemists use to purify solid
compounds. It is one of the fundamental procedures each chemist must
master to become proficient in the laboratory. Crystallization is based on the
principles of solubility; compounds tend to be more soluble in hot liquids
then they are in cold liquids. If a saturated hot solution is allowed to cool, the
solute is no longer soluble in the solvent and forms crystals of pure
compound. Impurities are excluded from the growing crystals and the pure
solid crystals can be separated from the dissolved impurities by filtration.
What Happens During a crystallization
To crystallization an impure, solid compound, add just enough hot
solvent is added to it to completely dissolve it. The flask then contains a hot
solution, in which solute molecules – both the desired com pound and
impurities – move freely among the hot solvent molecules, and they begin to
leave the solution and form solid crystals. During this cooling, each solute
molecule in turn approaches a growing crystal and rests on the crystal
surface. If geometry of the molecule fits that of the crystal, it will be more
likely to remain on the crystal than it is to go back into the solution.
Therefore, each growing crystal consists of only one type of molecule, the
solute. After the solution has come to room temperature, it is carefully set in
an ice bath to complete the crystallization process. The chilled solution is
then filtered to isolate the pure crystals and the crystals are rinsed with the
chilled solvent.
This first series of diagrams shows what happens if you let a
crystallization proceed slowly: first by setting the flask at room temperature
undisturbed until crystals form, and then carefully on ice. The red bar to the
right of each image is a thermometer, to indicate the temperature. The yellow
triangles are an impurity in the hot solution of orange hexagons. If the
solution is allowed to cool slowly, the impurities may attach briefly to the
growing crystal lattice, but they soon leave as a compound with a more
suitable geometry comes in to take their place. Suitable hexagons stay more
readily in the growing lattice, and eventually pure crystals of orange
hexagons are formed.
This second series of diagrams shows what happens if you cool the
solution too quickly. The yellow triangle impurities are trapped inside the
crystals being formed by the orange hexagons, thus, the crystals isolated are
impure. Note that slow crystallization gives larger crystals than fast
crystallization. Small crystals have a large surface area to volume ratio and
impurities are located on the surface of the crystals as well as trapped inside
the matrix.
Module 2
Melting point determination
Pure, crystalline solid have a characteristic melting point, the
temperature at which the solid melt to become a liquid. The transition
between the solid and the liquid is so sharp for small samples of a pure
substance that melting points can be measured to 0.1℃. The melting point
of solid oxygen, for example, is -218.4℃.
Liquids have a characteristic temperature at which they turn into
solids, known as their freezing point. In theory, the melting point of a solid
should be the same as the freezing point of the liquid.
It is difficult, if not impossible, to heat a solid above its melting point
because the heat that enters the solid at its melting point is used to convert
the solid into a liquid. It is impossible, however, to cool some liquids to
temperatures below their freezing point without forming a solid. When this is
done, the liquid is said to be supercooled.
An example of a supercooled liquid can be made by heating solid
sodium acetate trihydrate (𝑁𝑎𝐶𝐻3 𝐶𝑂2 3𝐻2 𝑂). When this solid
melts, the sodium acetate dissolves in the water that was trapped in the
crystal to form a solution. When the solution cools to room temperature, it
should solidify. But it often doesn’t. if a small crystal of sodium acetate
trihydrate is added to the liquid, however, the contents of the flask solidify
within second.
A liquid can become supercooled because the particles in a solid are
packed in a regular structure that is characteristic of that particular substance.
Some of these solids form easily; others do not. Some need a particle of dust,
or a seed crystal, to act as a site on which the crystal can grow. In order to
form crystals of sodium acetate trihydrate, 𝑁𝑎+ 𝑖𝑜𝑛𝑠, 𝐶𝐻3 𝐶𝑂2−
ions, and water molecules must come together in the proper orientation. It is
difficult to organize themselves, but a seed crystal can provide the
framework on which the proper arrangement of ion and water molecules can
grow.
Because it is difficult to heat solid to temperatures above their melting
point, and because pure solid tend to melt over a very small temperature
range, melting points are often used to help identify compounds. We can
distinguish between the three sugars known as ( MP = 150℃), fructose (MP
= 103-105℃), and sucrose (MP = 185-186℃), for example, by determining
the melting point of a small sample.
Measurements of the melting point of a solid can also provide
information about the purity of the substance. Pure, crystalline solid melt
over a very narrow range of temperatures, whereas mixtures melt over a
broad temperature range. Mixture also tend to melt at temperatures below
the melting points of the pure solids.
Module 3
Distillation
Distillation is the process of heating a liquid until it boils, capturing
and cooling the resultant hot vapors, and collecting the condensed vapors. In
the modern organic chemistry laboratory, distillation is a powerful tool, both
for the identification and the purification of organic compounds. The boiling
point of a compound-determined by distillation- is well-defined and thus is
one of the physical properties of a compound by which it is identified.
Distillation is used to purify a compound by separating in from a non-volatile
or less-volatile material. When different compounds in mixture have
different boiling points, they separate into individual components when the
mixture is carefully distilled.
Distillation for Boiling Point Determination
Boiling points are usually measured by recording the boiling point (or
boiling range) on a thermometer while performing a distillation. This method
is used whenever there is enough of the compound to perform a distillation.
The distillation method of boiling point determination measures the
temperature of the vapors above the liquid. Since these vapors are in
equilibrium with the boiling liquid, they are the same temperature as the
boiling liquid. The vapor temperature rather than the pot temperature is
measured because if you put a thermometer actually in the boiling liquid
mixture, the temperature reading would likely be higher than that of the
vapors. This is because the liquid can be superheated or contaminated with
other substances, and therefore its temperature is not an accurate
measurement of the boiling temperature.
If you are using the boiling point to identify a solid compound which
you have isolated in the lab, you will need to compare its boiling point with
that of the true compound. Boiling points are listed in various sources of
scientific data, as referenced on the Chemical Information page on this
website.
If you look up the boiling point of a compound in more than one
source, you may find that the values reported differ slightly. The literature
boiling point depends on the method and ability of the technician taking the
boiling point, and also on the purity of the compound. While theoretically all
boiling points should be constant from source to source, in reality the
reported boiling points sometimes vary. Therefore, always reference the
source of the physical data which you write in your lab report.
Module 4
Extraction and Chromatography
1.Extraction of liquids from solids and from liquids.
Solvent extraction is one method used to separate natural and synthetic
organic compounds. The principle used here is the varying solubilities that
different solvents have; solvent extraction can be classified into two
categories:
1. Solid – liquid extraction: use of a solvent to remove liquid
from a solid material.
2. Liquid – liquid extraction: use of a solvent to remove a liquid
from liquid.
In a liquid – liquid extraction we normally use organic solvent to
extract a substance from an aqueous solution. This type of extraction uses a
separatory funnel.
You must be careful when using the separatory funnel. Before using
the funnel be sure that you grease the stopcock (if the stopcock is made of
Teflon, then you don’t have to grease it)
If you are right handed, hold the funnel in your right hand and use your
index finger to push down on the glass stopper to hold it in place. Use your
left hand to hole the bottom of the funnel so that the thumb and index finger
can easily open and close the stopcock. Hold the funnel so that the top is
lower than the bottom. When using the funnel, hold it as describe above and
shake as quickly as possible so that the added solvent will mix with the
solution to be extracted as much as possible. While shaking, there will be an
increase in pressure inside the funnel; this can be release by opening and
closing the stopcock. When the extraction is finished, put the funnel in a ring
and clamp it upright to the ring stand. The solution in the funnel will separate
into two layers.
Remove the glass stopper before opening the stopcock to remove the
bottom layer. The remaining liquid can be removed from the funnel by
pouring it out of the hole on top.
Remove the glass stopper and stopcock and put a small piece of paper
between them and the funnel. This will prevent the glass stopper and glass
stopcock from becoming stuck to the funnel. The main idea between this type
of extraction is that adding a small amount of solvent many times will extract
more material than using large amount of solvent for one extraction.
The distribution law says that the compound will distribute itself
between the two layers at a constant rate called the distribution or partition
coefficient (K):
𝐾=
𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑 𝑥 𝑖𝑛 𝑠𝑜𝑙𝑣𝑒𝑛𝑡 𝐴
𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑 𝑥 𝑖𝑛 𝑠𝑜𝑙𝑣𝑒𝑛𝑡 𝐵
At a constant temperature
From this you can see that if X is extracted from solvent A by using
solvent B, the volume of X in solvent A will decrease. Also, if we extract X
by using small amount of solvent B and repeat it many times, we will get an
almost constant flow of X out of solvent A.
The material obtained from this extraction will be in a crude form. We
can purify it by crystallization, distillation, and chromatography.
2.Chromatography
Chromatography coms form the Greek word “chromatos” which
means color. Chromatography was first used to separate the component
colors from the pigments of plants. More recently chromatography has come
to mean the separation of substances from mixtures, in general, and usually
does not involve colored substance.
Chromatography can be used for both qualitative analysis
(identification of the components in a mixture) and for quantitative analysis
(determining how much of a substance is in a mixture).
Chromatography is a separation process that is achieved by distributing
the substance to be separated between two phase,
- A moving phase
- A stationary phase
Those substances distributed preferentially in the moving phase will
pass through the chromatography system faster than those that are distributed
preferentially in the stationary phase. As a consequence, the substances will
be eluted from the chromatographic system in the inverse order of the
magnitude of their distribution coefficients with respect to the stationary
phase.
In general, a moving phase will be either gas or liquid which give rise
to the basic types of chromatography;
1. Gas chromatography (GC) where the moving phase is a gas
2. Liquid chromatography (LC) where the moving phase is
liquid.
The stationary phase will normally be either a liquid or a solid which
give rise to four sub groups of chromatography,
1. Gas-liquid chromatography (GLC)
2. Gas-solid chromatography (GSC)
3. Liquid-liquid chromatography (LLC)
4. Liquid-solid chromatography (LSC)
Module 5
Solubility of organic Compounds
When one substance is soluble in another, it will mix homogeneously
with another compound, but not react with it. Usually this will happen when
a substance is dissolved in water or an aqueous solution. The solubility of
organic compounds depends on the size, shape, polarity, acidity, or basicity
of the molecules, substances with the same functional group will have the
same solubility. Substances that do not dissolve in any solvent are called
inert substances.
A. General Rules for the Physical solubility Properties of Substances
1. The alcohol group will dissolve in water; n-hexane doesn’t dissolve in
water but partially dissolves in methanol. It is more soluble in absolute
ethanol and is completely soluble in n-Butanol or other higher
molecular weight alcohol, Esters will dissolve in alcohols and ethers.
2. Branched chain molecules dissolve more readily than straight chain
molecules.
3. For many types of organic molecules, there are series of molecules
which differ from each other only by the number of carbon atoms in
the chain. For these compounds the general rule is the higher number
of carbon atoms, the higher their solubility in non-polar solvents and
the lower their solubility in polar solvents.
4. High molecular weight substances are relatively insoluble in nonreactive solvents. For these substances, part of the solubility will be in
form of colloidal suspensions. For example, glucose and methyl
acetate readily dissolve in water. But when in polymer form or a
methyl acetate chain, they are insoluble in water.
5. Solids with low melting points dissolve more readily than those with
high melting points because thy have fewer attractive forces between
the molecules.
6. If solvent molecules can easily surround the substance molecules, the
substance will dissolve readily. Molecules surrounded by solvent
molecules are said to be solvated.
B. General Rules for the Chemical Solubility Properties
1. Hydrocarbon alkanes, are not acid nor are basic.
2. Alkyl halides and Aryl halides normally are neutral and do not dissolve
in dilute acid or base.
3. Alcohols and phenol are highly polar compounds. Under normal
conditions, alcohol is neutral but phenol is a weak acid and can
dissolve in strong bases such as NaOH.
4. Ether; are less polarized than alcohols, and are neutral so that are
insoluble in acid or bases.
5. Aldehydes and Ketones are more polar than ester but less polar than
alcohol. Under normal conditions, they are neutral, not soluble in
dilute acid or base except when they contain an alpha-hydrogen.
6. Carboxylic acids have very high polarity and stronger acidity than
phenols but weaker than organic acids. In general, carboxylic have
p𝐾𝑎 = 4 − 5. they can dissolve in strong bases such as NaOH.
7. Carboxylic derivatives. There are many kinds of Carboxylic
derivatives that are polar and have high acidity, such as acid halide and
acid anhydrides. Some are neutral such as esters and amides. Both
ester and amides do not dissolve in dilute acids or bases.
8. Amines are compounds that have the general formula
𝑅𝑁𝐻2 𝑤ℎ𝑒𝑟𝑒 R is an alkyl or aryl group. These substances are
polar and have basic properties with𝑝𝐾𝑏 = 3 − 9 . These
readily dissolve in acid.
9. Organic salts usually are salt of carboxylic acid or salt of amines which
have high polarity. The compound can ionize to form cations or
anions, so these compounds readily in water. The acidity depends on
the different organic salt and can tested using pH paper.
C. Basic Rules for the properties of Solvents
1. Solubility in water. Water is polar solvent; therefore, compounds
which can dissolve in water are polar compounds. They can be acids,
bases, or neutral and most importantly, the molecular weight is always
low or the carbon chain is less than 5 carbon atoms.
2. Solubility in ether. Ether is a solvent that dissolve many organic
compounds; both high molecular or low molecular weight compounds.
Because ether contains an ethyl group it is polar. But, because the
polar of the compound is not very strong, it can dissolve compound
with low polarity and not so strong.
3. Solubility in base. Organic compounds which normally dissolve in
water but also dissolve in base are mostly compounds with acid
properties. That is, when the base reacts with acid we will get salt
which dissolve well in water. Strong acids will dissolve in both strong
and weak base, but compounds which are weak acid will dissolve only
in strong base.
4. Solubility in acid. Organic compound which normally not dissolve in
water but dissolve in acid are mostly compounds with basic properties.
When a weak acid reacts with base, the product is a salt which dissolve
very well in water. If we concentrated sulfuric acid as a solvent, the
compound does not have to be base because concentrated sulfuric acid
can react with many compounds. Therefore compounds which are not
bases but react with concentrated sulfuric acid will also dissolve in
weak acids.
Because different compounds dissolve in different solvents, we can
identify compounds by what they dissolve in. We can divide substances into
the following categories based upon their solubility.
𝑆1 𝑔𝑟𝑜𝑢𝑝: Dissolve in both water and in ether. This group has moderate
polarity and is composed of chains of less than 5 carbon atoms. Mostly, there
is only one functional group. These compounds could be basic, acidic group
and neutral and this can be determined by pH paper.
𝑆2 𝑔𝑟𝑜𝑢𝑝: Can dissolve in water but not in ether. These compounds have
a polarity and are composed of more than one functional group. The
functional group will consist of many oxygen and nitrogen atoms such as in
sugars, amino acids, carboxylic salt and amine base salt, etc.
𝐴1 𝑔𝑟𝑜𝑢𝑝: Not soluble in water but dissolve in NaOH which is base and
dissolve in 𝑁𝑎𝐻𝐶𝑂3 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 which is weak base. These
compounds are rather strong acids and consist of more than 5 carbon atoms.
𝐴2 𝑔𝑟𝑜𝑢𝑝: Insoluble in both water and 𝑁𝑎𝐻𝐶𝑂3 but will dissolve in
NaOH. These compounds are weak acids and consist of more than 5 carbon
atoms.
B group: Insoluble in water and NaOH but will dissolve in dilute acids.
These compounds are normally in bases such as primary, secondary and
tertiary amines and hydrazines.
M group: Insoluble in water and NaOH and do not dissolve in acids in the
presence of N and S. These compounds are things suach as like tertiary
nitrogen compounds, amides, nitrates, azo compounds, ester of sulfuric.
N group: Not dissolve in water or NaOH and do not dissolve in dilute acids
but dissolve in concentrated sulfuric acid, that is, these compounds will cause
reaction with sulfuric acid. If these compounds do not consist of nitrogen and
sulphur in the molecules, they can be unsaturated carbon compounds, such as
alkenes and alkynes. They also have moderate polarity and consist of more
than 5 carbon atoms.
I group: Not dissolve in any kind of solvent and sulphur. These compounds
are inert hydrocarbon compound such as saturated hydrocarbon compounds,
aromatic, alkyl halides and aryl halides hydrocarbon.
Chart of solvent identifying