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8/13/2019 Classification of Organic Compounds by Solubility
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Chapter 5
Classification of Organic Compounds by Solubility
Deductions based upon interpretation of simple solubility tests
can be extremely useful in organic structure determination.
Both solubility and spectrometric analyses often lead to the
same kinds of structural deduction.
Solubility involves
the formation of one layer, if the compounds are miscible, or formation of two layers, if the compounds are immiscible.
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The solubility of organic compounds can be divided into two
major categories:
Solubility in which a chemical reaction is the driving force
Solubility in which simple miscibility is the only mechanism
involved, such as dissolving ethyl ether (CH3CH2OCH2CH3) in
carbon tetrachloride (CCl4)
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Solubili ty Tests for the Identif ication of Functional Groups
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Test the solubility of a substance in
Water
5% NaOH
5% NaHCO3
5% HCl
Cold concentrated H2SO4
This can provide three kinds of information:
(i) Presence of a functional group.
For instance, because hydrocarbons are insoluble in water,
observing that an unknown is partially soluble in waterindicates that a polar functional group is present.
A substance is said to besoluble if it dissolves to the
extent of 3.3 g/100 mL of solvent
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(ii) Solubility in certain solvents often leads to more specificinformation about functional group.
(iii) Certain deductions about molecular size and compositionmay sometimes be made from solubility tests.
Example:
i.e., solubility in 5%
NaOH of a water-
insoluble unknown
is a strong indicationof an acidic
functional group.
in many homologous series of monofunctionalcompounds, the members with fewer than about 5 carbon
atoms are water soluble, whereas the higher homologs areinsoluble.
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Procedure for Determination of Solubility:
Solid:Solid: R.T. warm for water solubility cool to R.T.
Liquids:Liquids: ~25 mg (0.05 ml one drop) in 0.75 mL (15 drops)
Testing with litmus paper
Heat should not be applied for ether solubility, and also whensolubility in acid or alkali is being determined, because itmight cause hydrolysis to occur.
If the mixture is shaken thoroughly, the time required for theunknown to dissolve should not be more than 1 to 2 min.
Solubility in Concentrated AcidSolubility in Concentrated Acid
Place 0.6 mL of conc. H2SO4 in a test tube, and add 0.05 mL(~1 drop) or 25 mg of the unknown.
Unknowns that react with H2SO4 to produce heat and/or color
changes should be classified as soluble even if the sampledoes not appear to dissolve.
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The dielectric constant is the
measure of the ability of the
solvent to separate ionic
charges.
The dielectric constant is
related to the polarity of the
solvent.
To dissolve ionic compounds,
a high dielectric constant is
required, but it is not the only
characteristic of an effective
ion solvent.Example:Example: HCN is a very poor
solvent for salts such as NaCl
The high dielectric constant
and hydrogen-bonding abilityof water (and other hydroxylic
solvents) which combine to
make it a good solvent for
salts, also make it a poorsolvent for nonpolar
substances.
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Most organic molecules have both a polar and a nonpolar
entity, it can be deduced that solubility would depend on thebalance of the two parts.
As the percentage of the hydrocarbon portion increases,
water solubility decreases and ether solubility increases.
Like dissolves like
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Generally, solubility of a substance is inversely related to its
melting point: high melting point low solubility.
Among cis-trans isomers, the cis form generally is more
soluble.
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In general, an increase in molecular weight leads to an
increase in intermolecular forces in a solid and decreased
solubility.
Similarly, glucose vs. starch or cellulose, and amino acids vs. proteins
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Compounds having a branched chain is more soluble than
the corresponding straight-chain compound.
Branching lowers intermolecular forces and decreasesintermolecular attraction.
The position of the functional group in the carbon chain alsoaffects solubility.
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Solubility in WaterSolubil ity in Water
Water is a poor solvent for hydrocarbons.
Presence of double bonds, triple bonds or aromatic ringsdoes not affect the polarity greatly similar to alkanes intheir solubility.
Substituting halogen for a hydrogen decreases the watersolubility.
Salts are extremely polar and are usually water soluble.
Acids and amines are more soluble than nonpolarcompounds (due to H-bonding).
Solubility of amines decreases as the basicity decreases.
Many tertiary amines are more soluble in cold than in hotwater (at lower temperatures, the solubility of the hydrates is
involved).
S l bilit i 5% H d hl i A id S l ti
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Solubility in 5% Hydrochloric Acid SolutionSolubility in 5% Hydrochloric Acid Solution
1o, 2o, and 3o aliphatic amines form polar ionic salts with HCl.
Aliphatic amines are readily soluble in 5% HCl.
The presence of conjugated aryl groups decreases thebasicity of the nitrogen atom.
Disubstituted amides (RCONR2) of sufficiently high molecularweight to be water insoluble are soluble in 5% HCl solution.
soluble in 5% HCl insoluble in 5% HCl insoluble in 5% HCl
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Solubility in 5%Solubility in 5% NaOHNaOH and 5% NaHCOand 5% NaHCO33
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Ka 1x10-10
insoluble in NaHCO3 soluble in NaOH
soluble in NaHCO3
soluble in NaHCO3
Ald h d d k t ffi i tl idi t t ith
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Aldehydes and ketones are sufficiently acidic to react with
aqueous alkali, but they are too weakly acidic to dissolve to
any measurable extent in 5% NaOH solution.
When two carbonyl groups are attached to the same carbon,
the acidity increases sharply.
Esters with five or six carbon atoms that are almost
completely soluble in water may be hydrolyzed by continued
shaking with 5% NaOH solution. The alkali should not be
heated and the solubility or insolubility should be recordedafter 1-2 minutes.
S l bilit i O i S l tS l bilit i O i S l t
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Solubility in Organic SolventsSolubility in Organic Solvents
The solubility of organic compounds in organic solvents should
be determined in order to plan for a variety of laboratoryoperations.
These include choosing solvents fororganic reactions,
dissolving substrates for spectral analyses,
cleaning glassware,extraction,
Thin layer and other chromatography,
Crystallization.
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Solubil ity in Cold Concentrated Sulfuric AcidSolubility in Cold Concentrated Sulfuric Acid
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Solubil ity in Cold, Concentrated Sulfuric AcidSolubility in Cold, Concentrated Sulfuric Acid
Cold, concentrated H2SO4 is used with neutral, water-
insoluble compounds containing no elements other thancarbon, hydrogen and oxygen.
If the compound is unsaturated, is readily sulfonated, or
posses a functional group containing oxygen, it will dissolvein cold, concentrated H2SO4
Alkanes, cycloalkanes, and their halogen derivatives areinsoluble in H2SO4
Si l ti h d b d th i h l d i ti
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Simple aromatic hydrocarbons, and their halogen derivatives
do not undergo sulfonation and are insoluble in H2SO4
However, the presence of two or more alkyl groups on the
aromatic ring permits the compound to be sulfonated.
Dissolve readily in H2SO4
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THE ENDTHE END