Question 1 - Chemistry CBSE Class 12 Sample Paper for 2022 Boards [Term 2] - Solutions to CBSE Sample Paper - Chemistry Class 12

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)

Answer

(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, 

Hence,

• 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.