How easy is it to scratch or cut my esophagus if i swallowed robitussin-dm capsules but cracked them first in my mouth, also how long does stomach acid take to dissolve the capsule.

Take it easy: Those capsules will get digested by the stomach acid, as they are not made of steel. You should be alright.

Doubtful: Doubt that the medication could harm your esophagus to do any damage. Most medications would dissolve within minutes even without the presence of acid. Just drop a tablet of capsule in tap water and see how fast it disintegrates.