how will u convert : Ethanal to but-2-enal ? PLZ EXPLAIN

you can use aldol condensation...in prescence of dil. NaOH ethanal (2molecules ) will combine and then will lose water to give you but-2-enal..

2ch3-cho ----- ch3-ch(oh)-ch2-cho

this will lose water to give but 2 enal....thumbs up if it helps