In addition to it being wrong to hit your cat, punishment simply doesn't work and is likely to make your situation worse. Clever though Kitty is about many things, she won't understand that you're punishing her for scratching the couch. She will only compute that sometimes when you catch her she is treated badly. This may make her insecure and stimulate her to scratch more or develop other undesirable behavior problems. Eventually you will break the trust and security that is the basis for your cat's relationship with you, and you will find it very difficult to catch her for any reason at all.
Cats have excellent memories and hold serious grudges.
Understanding your cat's need to scratch is more than just an act of charitabilty on your part. It's the key to channeling Kitty's efforts to more acceptable areas. Marking their territory Scratching is a territorial instinct by which cats place their mark and establish their turf. Through scratching, cats mark their domains with more than just visible signs of claw marks. Cat's paws also have scent glands that leave their own special scent on their territory.