No Crimean War is a big one. Napoleon III style himself an defender of the Faith to win over Catholic Support at the same time of supporting Italian unification. And from that, Napoleon III and Nicholas I rivalry of who was the 'protector' of Christian peoples within the Ottoman Empire.
Without the Second Empire, you won't have the Crimean War, or something very different. You still have the British and them not wanting the Russian Empire to take Constantinople, but they be limited to naval actions. You still have Russia and it deep dream of a revived Byzantium ran deep within the Russian Empire, and would find a reason to war with the Turks.
TTL Crimean War may be an Russo-Ottoman War with British naval support.