Write a note on the preparation of aldehydes and ketones from the oxidation of alcohols.

(N/A) Preparation of aldehydes and ketones by oxidation of alcohols:
Aldehydes and ketones are generally prepared by the oxidation of primary and secondary alcohols,respectively.
These reactions involve the cleavage and formation of bonds. They are also known as dehydrogenation reactions because they involve the loss of dihydrogen from an alcohol molecule.
Depending on the oxidising agent used,a primary alcohol is oxidised to an aldehyde,which can be further oxidised to a carboxylic acid.
$RCH_2OH \xrightarrow{(K_2Cr_2O_7 + H_2SO_4), CrO_3, (O), -H_2O} RCHO$
$1^\circ$-Alcohol $\rightarrow$ Aldehyde
$CH_3CH_2OH \xrightarrow{(K_2Cr_2O_7 + H_2SO_4), CrO_3, (O), -H_2O} CH_3CHO$
$(b)$ $A$ better reagent for the oxidation of primary alcohols to aldehydes in good yield is pyridinium chlorochromate $(PCC)$,a complex of chromium trioxide with pyridine and $HCl$.
$(c)$ Secondary alcohols are oxidised to ketones by chromic anhydride $(CrO_3)$ in anhydrous medium.
$R-CH(OH)-R' \xrightarrow{CrO_3, (O), -H_2O} R-CO-R'$
Examples:
$(i) CH_3CH(OH)CH_3 \xrightarrow{CrO_3, (O), -H_2O} CH_3COCH_3$
$(ii) CH_3CH(OH)CH_2CH_3 \xrightarrow{CrO_3, (O), -H_2O} CH_3COCH_2CH_3$
$(iii) C_6H_5CH(OH)CH_3 \xrightarrow{CrO_3, (O), -H_2O} C_6H_5COCH_3$
Under strong reaction conditions such as strong oxidising agents $(KMnO_4)$ and elevated temperatures,cleavage of various $C-C$ bonds takes place,and a mixture of carboxylic acids containing a lesser number of carbon atoms is formed.