Solubility of Organic Compounds in Water and Reasons

Some organic compounds dissolve in water and some are not. There are some reasons which will decide solubility of organic compounds in water. This lesson we identify those soluble compounds and why they dissolve in water.

In this tutorial, we study which organic compounds are soluble in water and why they dissolve? Sometimes we can see rather than solubility, organic compounds react with water and dissolve or make precipitates.



Requirements to dissolve in water

Hydrogen bonds

When there are hydrogen bonds ( F-H , O-H , N-H , Cl-H bonds can make hydrogen bonds ) between organic compound and water molecules, ability of dissolving in water is high. Organic compounds such as alcohols, phenol, aldehyde, ketone, carboxylic acids, amines and more can make hydrogen bonds.

But remember that, there are organic compounds which are insoluble in water and they can make hydrogen bonds too such as phenol.


Molecular mass

If organic compounds molecular mass is small, they have a higher possibility to dissolve in water.


Size of alkyl group

C-H bond is not enough polarized to make hydrogen bond because difference of electronegativity of between carbon and hydrogen is small. Therefore C-H bond are non-polar. So hydrocarbon part of molecule does not help to dissolve in water.

When hydrocarbon part of molecule becomes simple or small, it also increases the solubility. This means, when a non-polar part is so large than polar part, solubility decreases.


Solubility of different organic compounds types

Organic compound type Solubility in water
Alkanes Insoluble
Alkenes Insoluble
Alkynes Insoluble
Benzene Insoluble
Alkyl halides Insoluble
Alcohols Alcohols upto four carbon atoms (methanol, ethanol, propanol, butanol) are much soluble in water
Phenol Insoluble
Aldehyde Aldehydes with less molecular mass are soluble
Ketone Aldehydes with less molecular mass are soluble
Carboxylic acid
  • Carboxylic acids with small alkyl groups are soluble. (formic, ethanoic/acetic, propanoic acids are soluble)
  • All aromatic carboxylic acids are insoluble.
Carboxylic acid chlorides Reacts with water and form soluble HCl and carboxylic acid.
Amides Amides with less molecular masses (small alkyl groups) are soluble.
Amines Amines with less molecular masses are soluble.

Now, we take each organic compound type to study their solubility in water.



Hydrocarbons

Alkane, alkene and alkyne belong to the hydrocarbon type. In hydrocarbons, there are only carbon - hydrogen bonds in the organic compound. So polarization is weak to make strong intermolecular forces between water molecules. Therefore, alkane , alkene and alkyne are not soluble in water.

  • Methane, ethane, propane other alkanes are not soluble in water.
  • Ethene, propene are also not soluble in water.
  • Acetylene, propyne and so do not dissolve in water.

Benzene

Benzene is not soluble in water because benzene cannot make strong intermolecular forces such as hydrogen bonds or dipole dipole interactions.


Alkyl halides

All alkyl halides are insoluble in water. Alkyl halides can form dipolar - dipolar interactions. But it is not enough to dissolve due to non-polar hydrocarbon group.


Alcohols

Methanol, ethanol dissolve very well in water. Propanol dissolve little bit. Butanol is completely insoluble in water. When alkyl group gets longer or larger, solubility of alcohols decreases.


Phenol

Phenol can make hydrogen bonds due to -OH bond. But due to large phenyl group, phenol does not dissolve in water.


Aldehyde and Ketone

Aldehyde and ketone which have less molecular mass, will dissolve in water. Formaldehyde, Ethanal, butanal are soluble aldehydes. Benzaldehyde is insoluble in water. Acetone (propanone), butanone are soluble ketones.


Carboxylic acids

Carboxylic acids are soluble in water because they can form hydrogen bonds with water. But when alkyl group gets larger, solubility decreases. Formic acid, ethanoic acid, propanoic acids are water soluble. Remember that, carboxylic acids dissolve in water and dissociate to give an acidic solution.

Aromatic carboxylic acids

All aromatic carboxylic acids are insoluble in water. Benzoic acid forms a white precipitate in the water. Benzoic acid can form hydrogen bonds, but due to large phenyl group, (phenyl group is a non-polar part) benzoic acid does not dissolve in water.



Carboxylic acid chlorides

Carboxylic acid chlorides readily dissolve in water and reacts with water to give carboxylic acid and hydrochloric acid (HCl). Benzoyl chloride reacts with water to produce benzoic acid which is a white precipitate.



Amine

Amine can make hydrogen bonds with water. Amine which have less molecular mass are soluble in water. But solubility is low when alkyl group is large. Methanamine (methylamine), ethanamine, propylamine are soluble in water.


Aniline | Benzanamine

Aniline is insoluble in water due to large phenyl group. But aniline can form hydrogen bonds.



Amide

Amides can form hydrogen bonds due to -NH2 group. Amides which have smaller alkyl group have high solubility in water.

List of soluble organic compounds in water

Now, we are going to list very useful organic compounds which dissolve in water very well.

  • CH3COOH
  • CH3CH2OH
  • CH3NH2
  • CH3CH2NH2

List of organic compounds which hydrolysis when add to the water

  • Carboxylic acid chloride - Carboxylic acid and HCl are given as products when carboxylic acid reacts with water.
  • Benzenediazonium chloride - Phenol is given as the product when benzenediazonium chloride reacts with water.

Can we identify organic compounds by checking their solubilities in water?

Yes. Sometimes you can identify organic compounds by looking their behavior with water.

Examples

  • Identify ethanol and aniline - Both are liquids in room temperature. Add ethanol to water. You can see ethanol dissolve very well in water. But aniline separate from water and form a layer. We can easily see those aniline layer and water layer.

propyne pKa

pKa of propyne is 25. So then what is the Ka value of propyne?

pKa = -logKa

pKa = -log(25)



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