Having problems upgrading i3 with i7 CPU

Hi,

I have a 2019 21.5” iMac i3 3.6 GHz I3-8100 with 8GB

I first upgraded the HDD to SDD which works perfectly

Next I upgraded the RAM from 8GB to 32GB and it too works perfectly.

Finally I bought a new CPU [Intel Core i7-8700 3.2 GHz] but it didn’t work.

I’ve already tried re-installing my old CPU [Intel Core i3-8100 3.6 Ghz] and that works just fine again.

What was the problem? And which CPU do I have to buy to upgrade my iMac.

Thank you for your attention.