The ALDEHYDE family has a =O at the BEGINNING of the carbon chain.

The names end in al.
The "al" is from the word "aldehyde".

There are four carbons, hence butan.
The =O is present at the beginning of the chain.
Therefore no number is required.
These compounds are called aldehydes and their names end in al.

Hence butanal

A chlorine is at carbon 2.

Hence the name is:

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