In order for you to play Portal 2 on macOS with minimal graphics settings, your computer must meet the following requirements
Operating System:
MAC OS X 10.6.7 or higher
Processor (CPU):
Intel Core Duo Processor (2GHz or better)
Memory (RAM):
2 GB RAM
Graphics Card:
ATI Radeon 2400 or higher / NVIDIA 8600M or higher / Intel HD Graphics 3000
Storage:
8 GB available space