To be certain, you should:
* Consult your owner's manual: This is the best resource for the exact location on your specific Impala.
* Look for a diagram under the hood: Many cars have diagrams printed under the hood pointing out key components, including AC service ports.
* Use a shop manual specific to your year and model: This will provide detailed diagrams and explanations.
Without knowing the year of your Impala, I can't give a more precise location. Be cautious when working with refrigerant; it's dangerous and requires proper handling and equipment. If you're not comfortable working on your AC system yourself, it's best to take your vehicle to a qualified mechanic.