(a) Give chemical tests to distinguish between the follwing pairs of compounds :
(i) Ethanal and Propanal
(ii) Phenol and Benzoic acid
(b) How will you bring about the following conversions ?
(i) Benzoic acid to benzaldehyde
(ii) Ethanal to but-2-enal
(iii) Propanone to propene
Give complete reaction in each case.

Explanation

(a) (i)Ethanal and propanal can distinguished by iodoform test. Warm each compound with iodine and sodium hydroxide on a water. Ethanal gives yellow crystals of iodoform but propanal does not respond to iodoform test
`underset(Ethanal)(CH_(3)CHO)+3I_(2)+4NaOH overset("Heat")to underset("Idoform"" (yellow ppt)") (HCOONa)+ CHI_(3)+ 3NaI+3H_(2)O`
`CH_(3)CH_(2)CHO overset("I_(2)//NaOH")to No" " yellow" " ppt`
`underset("propanal")(CH_(3)CH_(2)CHO) overset("I_(2)//NaOH")to No" " yellow" " ppt`
(ii) Phenol and Benzoic acid can be distinguished by `FeCl_(3)` test. Add a few drops of neutral `FeCl_(3)` solution.Phenol gives violet coloured ppt but Benoic acid does not produce any ppt.
(b) (i)` (##SB_CHM_XII_DB_2010_E01_035_S01.png" width="80%">
(ii) `2CH_(3)CHOoverset("OH")to CH_(3)overset(OH)overset(|)(CH)-CH-CH_(2)-CHOoverset(Delta)underset(-H_(2)O)(rarr)CH_(3)-CH=CH-CHO` Ethanal " " But-2-enal (iii)
(iii) `CH_(3)-overset(O)overset(||)(C)-CH_(3)overset(LiAIH_(4))to CH_(3)-overset(OH)overset(|)CH-CH_(3)overset(Conc.H_(2)SO_(4))toCH_(3)-CH=CH_(2)`