Tert-Butyloxonium ion

Step 2 Dissociation of tert butyloxonium ion to give a carbocation... 

The tert butyloxonium ion formed m this step is an intermediate It was not one of the initial reactants nor is it formed as one of the final products Rather it is formed... 

Only one species tert butyloxonium ion undergoes a chemical change m this step Therefore the step is ummolecular... 

Like tert butyloxonium ion tert butyl cation is an intermediate along the reaction pathway It is however a relatively unstable species and its formation by dissociation of the alkyloxonium ion is endothermic Step 2 is the slowest step m the mechanism and has the highest activation energy Figure 4 8 shows a potential energy diagram for this step... 

The transition state is closer m energy to the carbocation (tert butyl cation) so Its structure more closely resembles the carbocation than it resembles tert butyloxonium ion The transition state has considerable carbocation character meaning that a significant degree of positive charge has developed at carbon... 

FIGURE 4 8 Potential en ergy diagram for dissocia tion of tert butyloxonium ion to tert butyl cation... 

The major difference between the two mechanisms is the second step The second step m the reaction of tert butyl alcohol with hydrogen chloride is the ummolecular dis sociation of tert butyloxonium ion to tert butyl cation and water Heptyloxonium ion however instead of dissociating to an unstable primary carbocation reacts differently It IS attacked by bromide ion which acts as a nucleophile We can represent the transition state for this step as... 

Like the reaction of tert butyl alcohol with hydrogen chloride step 2 m which tert butyloxonium ion dissociates to (CH3)3C and water is rate determining Because the rate determining step is ummolecular the overall dehydration process is referred to as a ummolecular elimination and given the symbol El... 

Step 3 Deprotonation of tert butyloxonium ion Water acts as a Brpnsted base... 

The bond between the carbon and oxygen of the tert-butyloxonium ion breaks heterolytically, leading to the formation of a carbocation and a molecule of water. 

The product is a tert-butyloxonium ion (or protonated tert-butyl alcohol). 

In the second step the intermediate tert-butyl cation reacts rapidly with water to produce a tert-butyloxonium ion, (CH3)3COH2, which in the third step, rapidly transfers a proton to a molecule of water producing tert-butyl alcohol. 

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