Jamieson, Fausset and Brown on Exodus 5:2

2. And Pharaoh said, Who is the Lord--rather "Jehovah." Lord was a common name applied to objects of worship; but Jehovah was a name he had never heard of. Pharaoh estimated the character and power of this God by the abject and miserable condition of the worshippers and concluded that He held as low a rank among the gods as His people did in the nation. To demonstrate the supremacy of the true God over all the gods of Egypt, was the design of the plagues.

I know not the Lord, neither will I let Israel go--As his honor and interest were both involved he determined to crush this attempt, and in a tone of insolence, or perhaps profanity, rejected the request for the release of the Hebrew slaves.