Some organic compounds dissolve in water and some are not. There are some reasons which will decide solubility of organic compounds in water. In this lesson, we identify those soluble compounds and why they dissolve in water with reasons.
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.
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.
If organic compounds molecular mass is small, they have a higher possibility to dissolve in water.
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.
| 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 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.
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.
Benzene is not soluble in water because benzene cannot make strong intermolecular forces such as hydrogen bonds or dipole dipole interactions.
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.
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 can make hydrogen bonds due to -OH bond. But due to large phenyl group, phenol does not dissolve in water.
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 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.
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 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 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 is insoluble in water due to large phenyl group. But aniline can form hydrogen bonds.
Amides can form hydrogen bonds due to -NH2 group. Amides which have smaller alkyl group have high solubility in water.
Now, we are going to list very common organic compounds in the laboratory which dissolve in water very well.
Alkanes can be prepared by the reaction of grignard reagent and water.
All orgaic compounds of alkanes, alkenes, alkynes, alkyl halides habe the lowest solubility in water.
Non-polar organic compound dissolve in non-polar organic solvents such as hexane, benzene and more.
Yes. Sometimes you can identify organic compounds by looking their behavior with water.
pKa of propyne is 25. So then what is the Ka value of propyne?
pKa = -logKa
pKa = -log(25)
Questions
Alcohols, carboxylic acids, carboxylic acid chlorides, amines, esters are usually soluble in water. But, when those compound's molecular mass increases, solubility in water is decreased.
As examples, ethanoic acid is soluble in water. But, benzoic acid is not soluble in water. both of compounds are carboxylic acids.
Yes. Some organic compounds are soluble only in organic solvents. They do not dissolve in water. If they do not dissolve in water, those organic compounds are non-polar compounds. As an example, methane is soluble in ethanol, diethyl ether, benzene, toluene, methanol, acetone. But insoluble in water (22.7 mg of CH4/ 1 litre of water)
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