Question

Selective oxidation of $ {{R}_{1}}C\equiv C{{\left( C{{H}_{2}} \right)}_{n}}CH=CH{{R}_{2}} $ with $ Cr{{O}_{3}} $ in acetic acid gives:
(A) $ {{R}_{1}}C\equiv C{{(C{{H}_{2}})}_{n}}C{{O}_{2}}H+{{R}_{2}}CO $
(B) $ {{R}_{1}}C{{O}_{2}}H+H{{O}_{2}}C{{(C{{H}_{2}})}_{n}}CH=CH{{R}_{2}} $
(C) $ {{R}_{1}}CO-CO{{(C{{H}_{2}})}_{n}}CH=CH{{R}_{2}} $
(D) $ {{R}_{1}}C\equiv C{{(C{{H}_{2}})}_{n}}CO-CH{{R}_{2}} $

Answer 1

Hint :We know that the Chromic acid can also be known as Tetraoxo Chromic acid. It is a dark solid which is purple in colour. It is bright orange when wet and it dissolves in water with hydrolysis. It can be used as an oxidizing agent.

Complete Step By Step Answer:
 $ Cr{{O}_{3}} $ is an inorganic compound which is called Chromium trioxide. It is also known as chromium(VI)oxide or chromic anhydride. The oxidation number of chromium in $ Cr{{O}_{3}} $ .
It is marketed under the name of chromic acid because it is the acidic anhydride of chromic acid. This compound has two colors: when it is in the anhydrous form it is a dark-purple solid and in the wet form it is bright orange solid. The color is due to charge transfer even though the chromium has no electrons in d-orbital. It is carcinogenic (it causes cancer) and is a powerful oxidizer, hence it is highly toxic and corrosive. It will ignite organic compounds such as alcohol on contact because it is a powerful oxidizer.
 $ \left[ {{R}_{1}}C\equiv C{{\left( C{{H}_{2}} \right)}_{n}}CH=CH{{R}_{2}} \right]+\left[ Cr{{O}_{3}} \right]\to \left[ {{R}_{1}}CO-CO{{(C{{H}_{2}})}_{n}}CH=CH{{R}_{2}} \right] $
It is also found that a solution of chromic acid and sulphuric acid is a powerful oxidizing agent, and can be also used to clean various laboratory glassware. The dilute solutions of chromic acid are used to oxidise secondary or primary alcohols to aldehydes and ketones. It is also used for qualitative analytical tests for the presence of primary or secondary alcohols.

Note :
Remember that in organic chemistry, it is used for the synthesis of acetic acid or acetone. Since it is a powerful oxidizing agent it is used directly to convert alcohols to carboxylic acid or ketones. It is also known as John’s reagent.