RMgX→H2OHCHOX ; ‘X’ cannot be

H-CH|=O+RMgX→H-CR|H|-OMgX→H2OH-CR|H|-OH(1o alcohol)+Mg(X)OHSo what so ever is the size and length of -R group in RMgX, if it is treated with HCHO followed by hydrolysis the ultimate product is RCH2OH(1°)1, 3, 4 are 1° alcohols ;2 is 2° alcohol which cannot be obtained using HCHO and RMgX.