How will you convert ethanal into the following compounds?
$(i)$ Butane$-1,3-$diol
$(ii)$ But$-2-$enal
$(iii)$ But$-2-$enoic acid

(N/A) $(i)$ Ethanal undergoes aldol condensation with dilute alkali to form $3-$hydroxybutanal,which upon reduction with $NaBH_4$ yields butane$-1,3-$diol.
$CH_3CHO$ $\xrightarrow{dil.NaOH} CH_3-CH(OH)-CH_2-CHO$ $\xrightarrow{NaBH_4} CH_3-CH(OH)-CH_2-CH_2OH$
$(ii)$ Ethanal undergoes aldol condensation with dilute alkali to form $3-$hydroxybutanal,which upon heating undergoes dehydration to yield but$-2-$enal.
$CH_3CHO$ $\xrightarrow{dil.NaOH} CH_3-CH(OH)-CH_2-CHO$ $\xrightarrow{\Delta, -H_2O} CH_3-CH=CH-CHO$
$(iii)$ But$-2-$enal is oxidized using Tollen's reagent to yield but$-2-$enoic acid.
$CH_3-CH=CH-CHO \xrightarrow{[Ag(NH_3)_2]^+OH^-} CH_3-CH=CH-COOH$