Oxonium ions, secondary stability

When 1-hexyne is treated with a catalytic amount of sulfuric acid in an aqueous solvent, initial reaction with the acid gives the expected secondary vinyl carbocation 103, and the most readily available nucleophile in this reaction is water (from the aqueous solvent). Nucleophilic addition of water to 103 leads to the vinyl oxonium ion 104. Loss of a proton in an acid-base reaction (the water solvent is the base) generates a product (105) where the OH unit is attached to the C=C unit, an enol. Enols are unstable and an internal proton transfer converts enols to a carbonyl derivative, an aldehyde, or a ketone. This process is called keto-enol tautomerization and, in this case, the keto form of 105 is the ketone 2-hexanone (106). (Enols are discussed in more detail in Chapter 18, Section 18.5.) Note that the oxygen of the OH resides on the secondary carbon due to preferential formation of the more stable secondary carbocation followed by reaction with water, and tautomerization places the carbonyl oxygen on that same carbon, so the product is a ketone. When a disubstituted alkyne reacts with water and an acid catalyst, the intermediate secondary vinyl cations are of equal stability and a mixture of isomeric enols is expected each will tautomerize, so a mixture of isomeric ketones will form. 

The intensity of the molecular ion from primary and secondary alcohols is normally quite low, and there usually is no molecular ion detectable for tertiary alcohols. One of the most common fragmentation patterns for alcohols is loss of a molecule of water to give a peak corresponding to the molecular ion minus 18 (M - 18). Another common pattern is loss of an allyl group from the carbon bearing the —OH group to form a delocalization stabilized oxonium ion and an allyl radical. The oxonium ion is particularly stable because of delocalization of charge. 

In the dehydration of alcohols, we recall that a methyl group can migrate to an adjacent electron-deficient center if a more stable carbocation results (Section 9.19). In this case, the rearrangement of a methyl group yields a carbocation that is formally a secondary ion. However, it is a stabilized hydroxy carbocation, or oxonium ion. The nonbonded electron pair on the oxygen atom is delocalized to the positive carbon atom. 

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