classification of organic compounds by solubility

8/13/2019 Classification of Organic Compounds by Solubility

1/25

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.


2/25

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)


3/25

Solubili ty Tests for the Identif ication of Functional Groups


4/25


5/25

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


6/25

(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.


7/25

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.


8/25

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.


9/25

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


10/25

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.


11/25

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


12/25

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.


13/25

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


14/25

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


15/25

Solubility in 5%Solubility in 5% NaOHNaOH and 5% NaHCOand 5% NaHCO33


16/25

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


17/25

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


18/25

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.


19/25

Solubil ity in Cold Concentrated Sulfuric AcidSolubility in Cold Concentrated Sulfuric Acid


20/25

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


21/25

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


22/25


23/25


24/25


25/25

THE ENDTHE END

classification of organic compounds by solubility

Documents