Chem 4563
Organic Qualitative Analysis
Solubility Tests
Solubility tests should be performed on every unknown. They are extremely useful in
determining the nature of themajor functional groups present in the unknown compound. The tests
are very simple and require only a small amount of sample. In addition, the solubility tests will
determine whether the compound is a strong base (amine), a weak acid (phenol), a strong acid
(carboxylic acid) or a neutral compound (aldehyde, ketone, alcohol ester). The common solvents
used to determine solubility are: water, ether, 5% NaOH, 5% NaHCO3 , 5% HCl, 96% H 2 SO4 , and
85% H3 PO4. A summary scheme of solubility classification is given on p 92 and 93
of Shriner.
1.
Solubility in Water.
Compounds with four or less carbons, and which contain polar groups of oxygen, nitrogen or
sulfur, will often be soluble in water. Almost any functional group containing these elements will
lead to water solubility for low molecular weight (C4) compounds. Compounds with five or six
carbons with those elements will often have borderline solubility or be insoluble. Branching of the
alkyl chain of the molecule lowers intermolecular forces between its molecules. Yhis is usually
manifested in a lower melting or boiling point, and a greater solubility in water for the branched
compound thann for a corresponding straight chain compound. This occurs simply because the
molecules of the pranched compound are more easily separated from one another. Thus t-butyl
alcohol is more soluble in water than is n-butyl alcohol.
When the ratio of O,N or S atoms in a molecule to the carbon atoms is increased, the solubility
of that compound in water will likewise increase. This is due to an increase in the number of polar
functional groups. Thus 1,5-pentanediol is more soluble in water than is 1-pentanol .
As the size of the alkyl chain of a compound is increased beyond 4 carbons, the influence of
polar functional groups is diminished and water solubility begins to decrease (this is formulated as the
5-cCarbon Rule). A few examples of these generalizations are given below.
SOLUBLE
BORDERLINE
INSOLUBLE
H3C O
CH3 O
O
H2
H3C C C OH
CH
C
H3C
C
C
H3C
C
OH
C
C
OH
CH3
H2
H2 H2
CH3
OH
OH
H3C
OH
H3C
CH3
The π-electrons of an aromatic ring are more polarizable
thus a benzene ring is more soluble than a typical
hydrocarbon
2.
Solubility in 5% HCl.
The possibility of an amine should be considered immediatly if a compound is soluble in dilute
HCl. Aliphatic amines (RNH2 , R 2 NH, R3 N) are basic compounds which readily dissolve in acid
because they form hydrochloride salts that are soluble in aqueous medium.
H
R
NH2 + Cl
R NH2 + HCl
The substitution of an aromatic ring (Ar) for an alkyl group (R) reduces the basicity of the
amine somewhat, but the amine will still protonate, and it will wtill be generally soluble in dilute
aqueous acid. This reduction in basicity is due to resonance delocalization of the unshared electrons
on the amino nitrogen of the free-base. The delocalizaton is lost on protonation, a problem which
does not exist for aliphatic amines. The substitution of two or three aromatic ringson an amine
nitrogen reduces the basicity even further. Di- and tri-aryl amines will not dissolve in HCl since they
do not protonate easily. Some very high molecular weight amines like tribromoaniline (MW 330)
may also be insoluble in dilute acid.
N
N
N
N
Delocalization lowers basicity of triaryl amines
3.
Solubility in 5% NaOH and 5% NaHCO 3
Compounds which dissolve in sodium bicarbonate, a weak base, are stron acids. Compounds
which dissolve in sodium hydroxide, a stron base, may be either strong or weak acids. Thus one can
distinguish between weak and strong acids by determining their solubility in both strong (NaOH) and
weak (NaHCO3 ) base.
For the purpose of this experiment, carboxylic acids (strong acid pKa~5) are generally
indicated when the unknown compound is soluble in both bases, while phenols (weak acid pKa~10)
are indicated when a compound is soluble only in NaOH.
Compounds dissolve in aqueous base solutions because they form sodium salts that are
soluble in aqueous medium. However, the salts of some high molecular weight compounds are not
soluble and will precipitate out. The salts of long chain fatty acids such as myristic (C14), palmitic
(C16) and stearic(C18) acids are in this category, these salts form soaps.
Both phenols and carboxylic acids produce resonance stabilized conjugate bases.
In phenols,
substitution of an electron withdrawing group (nitro or cyano) at the ortho and para positions of the
ring increases the acidity. Phenols with two or three nitro groups at the ortho and para positions are
strong acids that will dissolve in both strong and weak bases.
STRONG ACIDS
WEAK ACIDS
Carboxylic acids
Sulfonic Acids
Ortho and para substituted
di- and trinitrophenols
4.
Phenols
Nitroalkanes
β-diketones
Solubility in Conc. Sulfuric Acid and Phosphoric Acid.
Many compounds are soluble in cold, concentrated sulfuric acid. Of the compounds
commonly encountered, alcohols, ketones aldehydes and esters fall in this category. Other
compounds which also dissolve include alkenes, alkynes, nitroaromatics and amides. Since several
different types of compounds are soluble in sulfuric acid, further chemicalt tests are required.
Compounds which are insoluble in sulfuric acid are extremely weak bases. It is the extreme
acidity of concentrated sulfuric acid along with its powerful ionizing ability that allows these
compounds to be soluble in sulfuric acid. Thus sulfuric acid is able to protonate alcohols, ketones
aldehydes,ester an unsaturated hydrocarbons. The resulting ions produced are soluble in the
medium..
R OH
+ H2SO4
R OH2 + HSO4
O
R C R' + H2SO4
O H
R C R' + HSO4
O
R C OR" + H2SO4
R
H
R
+ H2SO4
R
O H
R C OR" + HSO4
R
R
R + HSO4
R R
Ionizing Power of Conc. H2SO4
Phosphoric acid, on the other hand is not nearly as strong an ionizing solvent as is sulfuric
acid. It is capable of ionizing most of the oxygen containing compounds such as alcohols,aldehydes,
and esters, but is not able to ionize unsaturated hydrocarbons such as alkenes,alkynes or aromatics.
Thus these compounds will be insoluble in this medium.
5.
Inert Compounds
Compounds not soluble in conc. Sulfuric Acid or any of the other solvents are said to be inert.
These compounds include the alkanes, most simple aromatics, and the alkyl halides.
Read:
"The Systematic Identification of Organic Compounds", Chapter 5
Section 1 pp 90-112. Know how to use chart on p 93 to determine
solubility classes.