Question

Why carbon at left called alpha carbon instead of carbon at right as rule.

Answer 1

When you start learning organic chemistry, it’s common to use Greek letters to denote this. So the carbon adjacent to a carbonyl is called an “α (alpha) carbon”, two carbons away is called a “β carbon”, and so on.
If we have a double bond between the α carbon and the β carbon it’s common to call it “α,β-unsaturated”. So we can have α,β unsaturated ketones, aldehydes, esters, and so on.
It can keep going beyond gamma, of course, but it’s rare to see it progress beyond ε (epsilon).
Another thing: aldehydes, esters, carboxylic acids, and so on, can only have one “alpha” carbon each, wheras ketones can have two. Sometimes you’ll see one set of Greek symbols marked with ‘ (prime) symbols to distinguish them. The location of the prime is completely arbitrary.
For esters, the OR group is not denoted “alpha”. It’s usually just called the “alkoxy” group.