Arrange the following in the increasing order of their property indicated (any 2):
(A) Benzoic acid, Phenol, Picric acid, Salicylic acid (pka values).
(B) Acetaldehyde, Acetone, Methyl tert butyl ketone (reactivity towards NH2OH).
(C) ethanol, ethanoic acid, benzoic acid (boiling point)
(A) Picric acid < salicylic acid < benzoic acid <phenol
The pKa value of an acid is inversely dependent on its acidity and/or its stability .
The acidity of a compound is directly dependent on the -I effect of the groups attached to the benzene.
- Since picric acid has 3 -NO 2 groups and an -OH group, the -I effect on benzene is highest. Hence, it has the highest acidity and lowest pKa value.
- In salicylic acid , there is one -COOH group and an -OH group.
- In benzoic acid , there is only one -COOH group. Therefore, its pKa value is higher than that of salicylic acid.
- Hence phenol has the lowest acidity and highest pKa value due to its single -OH group.
(B) Methyl tert butyl ketone < acetone< Acetaldehyde
-NH 2 OH would get attached to the carbon that needs an electron cloud to share most.
- The N in -NH 2 OH will share its electrons with the C atom.
- This means, the chain with the most electronegative C atom will have highest reactivity towards -NH 2 OH
Due the the +I effect of -CH 3 towards the C atom with the ketone group,
- Methylterbutylketone, due to its 3 -CH 3 groups , will be least reactive .
- Acetaldehyde, due to the presence of only one -CH 3 group will be most reactive towards -NH 2 OH.
(C) Ethanol <ethanoic acid < benzoic acid (boiling point of carboxylic acids is higher than alcohols due to extensive hydrogen bonding , boiling point increases with increase in molar mass)
Boiling point of a substance is the temperature at which force of attraction and bonds between molecules break and allow free movement of molecules.
- The presence of hydrogen bonds , or higher molecular mass leads to higher boiling points.
- Since Ethanoic Acid and Benzoic Acid have -COOH groups, they have Hydrogen bonding making them stable.
- Moreover, the phenyl ring in benzoic acid provides more stability to the compound.
Hence, Benzoic acid will have the highest boiling point, followed by Ethanoic Acid and then Ethanol.