SEM-III, CORE COURSE-7
ORGANIC CHEMISTRY-3
TOPIC: CARBONYL AND RELATED COMPOUNDS
Dr. Kalyan Kumar Mandal Associate Professor St. Paul’s C. M. College Kolkata PPT: 4
• OXIDATION OF ALCOHOLS WITH PDC AND PCC
• PERIODIC ACID AND LEAD TETRAACETATE OXIDATION OF 1,2-GLYCOLS OPPENAUER OXIDATION
Oxidation of primary and secondary alcohols to aldehydes and ketones respectively using aluminium tertiary butoxide in presence of a hydride ion acceptor like acetone, cyclohexanone, benzophenone p-benzoquinone, etc., is called Oppenauer oxidation. Facts • In case of oxidation of primary alcohol, non-enolisable ketones with a relatively low reduction potential such as p-benzoquinone is used. • Tertiary butoxide is used as the reagent, since tertiary butanol produced is not oxidised under these conditions. • This reagent is particularly useful for oxidising unsaturated secondary alcohols because it does not affect the double bond.
• Primary alcohols including the unsaturated alcohols may also be oxidised to aldehydes if acetone is replaced by p-benzoquinone. • In general, quinones and aromatic ketones are better hydrogen acceptors than acetone. • The reaction is completely reversible and therefore, to shift the reaction towards the product-side, addition of large excess of the hydride acceptor is used. Mechanism • Initially the starting alcohol reacts with aluminium t-butoxide to form a new alkoxide.
• This intermediate alkoxide then reacts with acetone or other acceptor molecule and a hydride ion (H-) from the substrate alcohol is transferred to the carbonyl carbon atom of acetone.
• The reaction proceeds through the formation of a six-membered cyclic transition state.
• The specific (H-) ion transfer from the carbon atom of alcohol molecule to carbonyl carbon atom of acetone has been demonstrated by using RCD(OH)R type labelled alcohol which led to the formation of CH3CD(OH)CH3. Mechanism Oxidation of Primary Alcohols FACTS • The outcome of oxidation reactions of alcohols depends on the substituents on the carbinol carbon and in order for each oxidation step to occur, there must be at least one hydrogen atom on the carbinol carbon. • Primary alcohols can be oxidised to aldehydes or further to carboxylic acids. • Oxidation of alcohols is normally carried out with Cr(VI) reagents but
these, like the Jones’ reagent (Na2Cr2O7 in H2SO4), are usually acidic. – In aqueous media, the carboxylic acid is usually the major product. – Some pyridine complexes of Cr(VI) compounds solve this problem by
having the pyridinium ion (pKa 5) as the only acid. – Both PDC and PCC can convert alcohols into aldehydes and ketones, especially in dichloromethane at room temperature and allow the oxidation to be stopped at the intermediate aldehyde. – PDC is less acidic than PCC and is therefore more suitable for the oxidation of acid-sensitive substrates. OVER-OXIDATION OF ALDEHYDES • Aqueous methods like the Jones oxidation are no good for this, since the aldehyde that forms is further oxidized to acid via its hydrate. The oxidizing agent treats the hydrate as an alcohol and oxidizes it to the acid. • The key thing is to avoid water—so PCC in dichloromethane works quite well. The related reagent PDC (pyridinium dichromate) is particularly suitable for oxidation to aldehydes Pyridinium Chlorochromate (PCC) Corey-Suggs Reagent
• Pyridinium chlorochromate is used as an oxidising agent in organic chemistry. • The reagent is useful to oxidise primary and secondary alcohols to the corresponding aldehydes and ketones respectively.
• The oxidation of primary alcohols to aldehydes is advantageous in that sense that over oxidation to the corresponding carboxylic acid does not occur.
• PCC is also important for its high selectivity. For example, when an allyl alcohol is oxidized, unsaturated aldehyde is formed as sole product. This oxidation is known as Babler oxidation. Preparation • Chlorochromic acid can by prepared by the dissolution of chromium trioxide in 6 M aqueous hydrochloric acid. Addition of pyridine gives pyridinium chlorochromate as orange crystals.
• PCC is soluble in many organic solvents, and especially dichloromethane at room temperature has been used in most cases, whereas DMF promotes the over-oxidation of primary alcohols into carboxylic acids. Mechanism Pyridinium Dichromate (PDC) Pyridinium dichromate is the pyridinium salt of dichromate that can be obtained by addition of pyridine to a solution of chromium trioxide in water. FACTS • Initially, PDC was used either as a solution in DMF or as a suspension in dichloromethane. In DMF solution, PDC oxidizes primary and secondary allylic alcohols to the corresponding α,β- unsaturated carbonyl compounds. • Over-oxidation of primary allylic alcohols is not observed and (E,Z) isomerization does not take place. Mechanism with PDC OXIDATION OF 1,2-DIOLS
USING
• PERIODIC ACID (HIO4)
• LEAD TETRAACETATE [Pb(OAc)4] • Cleavage of 1,2-diols to carbonyl compounds can be accomplished by periodic acid (HIO4) or lead tetraacetate (Pb(OAc)4]. The nature of the products depends on the nature of the substituents in the diol used. FACTS • Oxidation of 1,2-glycols occurs more rapidly with syn-diol than with the corresponding anti-isomers and this fact indicates the formation of cyclic intermediates. • The fact that anti-isomers are oxidized suggest that the reaction proceeds through a non-cyclic intermediate for these compounds. This could be the non-cyclic ester formed in the first step. • These oxidations are also subject to steric hindrance, e.g., glycol is oxidised much faster than pinacol by periodic acid. • The rate determining step for glycol oxidation is the fission of the complex, whereas that for pinacol is the formation of the complex. • The alcohol reacts to form a cyclic intermediate. The intermediate then undergoes a rearrangement of the electrons, cleaving the C-C bond and forming two C=O bonds. Mechanism with HIO4 Mechanism for the ANTI-DIOL Mechanism with Pb(OAc)4 QUESTION
Write the order of rates of oxidation of the following 1,2-diols and give reasons:
(i) HOCH2CH2OH (ii) MeCH(OH)CH(OH)Me
(iii) Me2C(OH)C(OH)Me2
(iv) Me2C(OH)CH(OH)Me