#CH_3CH(OH)CH_2CH_2CH_3# (pentan-2-ol) being secondary alcohol on oxidation first tansformed into ketone.
#CH_3CH(OH)CH_2CH_2CH_3stackrel([O])->
CH_3COCH_2CH_2CH_3#
Further oxidation of ketone is difficult but can be performed in extreme conditions and oxidation product is obtained following Popoff's rule.
Popoff's rule states that during oxidation of unsymmetrical ketones, the > C = O group remains with the smaller of the two alkyl groups.
Pentan-2-one (#CH_3COCH_2CH_2CH_3#) is an unsymmetrical ketone inwhich two different alkyl groups are bonded with #>CO# group. So following Popoff's rule the #>CO# group will remain bonded with smaller alkyl group #-CH_3# to produce caboxylic acid as #CH_3COOH# and the residual part will produce another carboxylic acid of three carbon atoms i.e. Propanoic acid (#CH_3CH_2COOH#).
