It is often easier to work backwards from the product.
Ozonolysis
Ozonolysis of alkenes gives aldehydes as products.
∴ "CH"_3"CHO" implies "CH"_3"CH=CHCH"_3CH3CHO⇒CH3CH=CHCH3
Heating with alc. "KOH"KOH
Alkyl halides are dehydrohalogenated to alkenes by alc. "KOH"KOH.
∴ "CH"_3"CH=CHCH"_3 implies "CH"_3"CH"_2"CHXCH"_3CH3CH=CHCH3⇒CH3CH2CHXCH3
where "X = Br or Cl"X = Br or Cl
Now, we put them together and write the forward reactions.
underbrace("CH"_3"CH"_2"CHXCH"_3)_color(red)(bb("A"),color(white)(l) "2-halobutane") stackrelcolor(blue)("Alc. KOH" color(white)(mm))(→) underbrace("CH"_3"CH=CHCH"_3)_color(red)(bb("B"),color(white)(l) "but-2-ene") stackrelcolor(blue)("O"_3 color(white)(mm))(→) underbrace("CH"_3"CHO")_color(red)("ethanal")
