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I was taught at an institution I study in that using $\ce{K2Cr2O7}$ in the presence of $\operatorname{dil}.\ce{H2SO4}$ will oxidise $1^{\circ}$ alcohols to aldehydes, and $2^{\circ}$ alcohols to ketones. This is supported in here too.

At the same time, I came across Jones reagent ($\ce{Na2Cr2O7}$ in the presence of $\operatorname{dil}.\ce{H2SO4}$) on Khan Academy. But this says that it is an extremely strong oxidising agent and can oxidise Aldehydes to Carboxylic Acids too (thus giving acids on the oxidation of $1^{\circ}$ alcohols).

I do not understand how the above two reagents have different oxidising abilities. Is my premise itself wrong or is there any reason for it to have different oxidising abilities?

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  • $\begingroup$ Jones Reagent usually oxidises primary alcohols to acids via the aldehyde. Only aldehydes which do not form hydrates (eg benzylic or allylic) are stable to Jones conditions and are not oxidised further. The reagents you refer to are all the same regardless of how they ate prepared. $\endgroup$
    – Waylander
    Jan 6, 2018 at 17:49
  • $\begingroup$ @Waylander So the sources that say in case of Potassium dichromate with Sulfuric acid oxidises primary alcohols to aldehydes are partially correct, and those aldehydes will be oxidised to acids? Please add your comment as an answer. $\endgroup$
    – Siddharth Venu
    Jan 6, 2018 at 18:17

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Sodium or potassium dichromate or chromium trioxide in dilute aq. sulfuric acid all give rise to Jones reagent. This is used, though less than it used to be due to toxicity and cost issues, to oxidize secondary alcohols to ketones and primary alcohols to carboxylic acids via the aldehyde hydrate. In the case where the aldehyde does NOT form a hydrate with water (e.g. benzylic and allylic aldehydes), then that aldehyde will be stable to the conditions and be the major reaction product. Aliphatic aldehydes will generally give carboxylic acids.

Read more here.

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