You gain the following benefits while you do not have a weapon in your offhand unless the weapon is a light weapon and you have a feature that allows duel wielding under such conditions. Such as the dualwielder feat or crossbow expert feat.

• When you take the Attack action and attack with only a whip, you can use a bonus action to make a disarming attack, the target must make a strength or dexterity save, DC is 8 + proficiency modifier + ability modifier. This attack uses the same ability modifier as the primary attack. The weapon’s damage die for this attack is a d4, and it deals slashing damage.
• While you are wielding a whip, other creatures provoke an opportunity attack from you when they enter the reach you have with that weapon.
• While you are wielding a whip you deal an additional d4 damage as long as you are wielding a shield or a light weapon in the other hand.