Proton carbocation

Nonbonded electron pair donors (w-donors) are expectedly readily protonated (or coordinated) with superacids. Remarkably, this includes even xenon, long considered an inert gas. The protonation of some 7T-, (T- and -bases and their subsequent ionization to carbocations or onium ions is depicted as follows ... 

Treating 5.5 g of 2-amino-4,5-dimethylthiazole HCl with 0.66 g of solid sodium hydroxide 15 min at 220°C yields 53% of 4.4. 5.5 -tetramethyT 2,2 -dithiazolylamine, whose structure w as proved by identification with the produa obtained from the reaction between dithiobiuret and 3-bromo-2-butanone (467). This result is comparable to the reaction between 2-aminopyridine and its hydrochloride to yield bis(pyridyl-2)amine (468). Gronowitz applied this reaction to 2-aminothiazole, refluxing it with its hydrochloride 4 hr in benzene and obtained the dimeric 2-aminothiazole (236). He proposed a mechanism (Scheme 143) that involves the addition of a proton to the 5-position of the ring to give 234. The carbocation formed then reacts on the 5-position of a second... 

Step 3 IS new to us It is an acid-base reachon m which the carbocation acts as a Br0n sted acid transferrmg a proton to a Brpnsted base (water) This is the property of carbo cations that is of the most significance to elimination reactions Carbocations are strong acids they are the conjugate acids of alkenes and readily lose a proton to form alkenes Even weak bases such as water are sufficiently basic to abstract a proton from a carbocation... 

Water may remove a proton from either Cl or C 3 of this carbocation Loss of a proton from C 1 yields the minor product 2 3 dimethyl 1 butene (This alkene has a disubstituted double bond )... 

In Chapter 4 you learned that carbocations could be captured by halide anions to give alkyl halides In the present chapter a second type of carbocation reaction has been introduced—a carbocation can lose a proton to form an alkene In the next section a third aspect of carbocation behavior will be described the rearrangement of one carbo cation to another... 

Step (2) Ethanol acts as a base to remove a proton from the carbocation to give the alkene products (Deprotonation step)... 

Dehydrohalogenation of alkyl halides (Sections 5 14-5 16) Strong bases cause a proton and a halide to be lost from adjacent carbons of an alkyl halide to yield an alkene Regioselectivity is in accord with the Zaitsev rule The order of halide reactivity is I > Br > Cl > F A concerted E2 reaction pathway is followed carbocations are not involved and rearrangements do not occur An anti coplanar arrangement of the proton being removed and the halide being lost characterizes the transition state... 

Primary alcohols do not dehydrate as readily as secondary or tertiary alcohols and their dehydration does not involve a primary carbocation A proton is lost from the (3 carbon m the same step m which carbon-oxygen bond cleavage occurs The mechanism is E2... 

Section 5 15 Dehydrohalogenation of alkyl halides by alkoxide bases is not compli cated by rearrangements because carbocations are not intermediates The mechanism is E2 It is a concerted process m which the base abstracts a proton from the p carbon while the bond between the halogen and the a carbon undergoes heterolytic cleavage... 

We can gam a general understanding of the mechanism of hydrogen halide addi tion to alkenes by extending some of the principles of reaction mechanisms introduced earlier In Section 5 12 we pointed out that carbocations are the conjugate acids of alkenes Therefore strong acids such as HCI HBr and HI can protonate the double bond of an alkene to form a carbocation... 

Addition begins m the usual way by protonation of the double bond to give m this case a secondary carbocation This carbocation can be captured by chloride to give 2 chloro 3 methylbutane (40%) or it can rearrange by way of a hydride shift to give a tertiary carbocation The tertiary carbocation reacts with chloride ion to give 2 chloro 2 methylbutane (60%)... 

Step 1 Protonation of the carbon-carbon double bond in the direction that leads to the more stable carbocation... 

The notion that carbocation formation is rate determining follows from our previous experience and by observing how the reaction rate is affected by the shucture of the aUcene Table 6 2 gives some data showing that alkenes that yield relatively stable carbocations react faster than those that yield less stable carbocations Protonation of ethylene the least reactive aUcene m the table yields a primary carbocation protonation of 2 methylpropene the most reactive m the table yields a tertiary carbocation As we have seen on other occa sions the more stable the carbocation the faster is its rate of formation... 

The two dimers of (CH3)2C=CH2 are formed by the mechanism shown m Figure 6 16 In step 1 protonation of the double bond generates a small amount of tert butyl cation m equilibrium with the alkene The carbocation is an electrophile and attacks a second molecule of 2 methylpropene m step 2 forming a new carbon-carbon bond and generating a carbocation This new carbocation loses a proton m step 3 to form a mixture of 2 4 4 tnmethyl 1 pentene and 2 4 4 tnmethyl 2 pentene... 

Step 3 Loss of a proton from this carbocation can produce either 2 4 4 trimethyl 1 pentene or 2 4 4 trimethyl 2 pentene... 

Protonation of the double bond occurs in the direction that gives the more stable of two possible carbocations... 

The first step protonation of the double bond of the enol is analogous to the pro tonation of the double bond of an alkene It takes place more readily however because the carbocation formed m this step is stabilized by resonance involving delocalization of a lone pair of oxygen... 

Of the two resonance forms A and B A has only six electrons around its positively charged carbon B satisfies the octet rule for both carbon and oxygen It is more stable than A and more stable than a carbocation formed by protonation of a typical alkene... 

Step 2 The carbocation transfers a proton from oxygen to a water molecule yielding a ketone... 

Protonation at end of diene unit gives a carbocation that IS both secondary and allylic most stable carbocation product is formed from this carbocation... 

Protonation at C 2 gives a carbocation that IS secondary but not allylic less stable carbocation not formed as rapidly... 

Section 10 10 Protonation at the terminal carbon of a conjugated diene system gives an allylic carbocation that can be captured by the halide nucleophile at either of the two sites that share the positive charge Nucleophilic attack at the carbon adjacent to the one that is protonated gives the product of direct addition (1 2 addition) Capture at the other site gives the product of conjugate addition (1 4 addition)... 

Only the benzylic chloride is formed because protonation of the double bond occurs m the direction that gives a carbocation that is both secondary and benzylic... 

Protonation in the opposite direction also gives a secondary carbocation but it is not benzylic... 

Because electrophilic attack on benzene is simply another reaction available to a carbocation other carbocation precursors can be used m place of alkyl halides For exam pie alkenes which are converted to carbocations by protonation can be used to alkyl ate benzene... 

Step 4 The oxygen protonated carbmolamme loses water to give a nitrogen stabilized carbocation CeHs H... 

Step 5 The nitrogen stabilized carbocation is the conjugate acid of the imine Proton transfer to water gives the imine... 

FIGURE 22 5 The diazo mum ion generated by treatment of a primary al kylamine with nitrous acid loses nitrogen to give a car bocation The isolated prod ucts are derived from the carbocation and include in this example alkenes (by loss of a proton) and an al cohol (nucleophilic capture by water)... 

The tertiary carbocation formed m this step can react according to any of the various reaction pathways available to carbocations One of these is loss of a proton to give a double bond... 

Loss of a proton from the tertiary carbocation formed m this step gives limonene an abundant natural product found m many citrus fruits Capture of the carbocation by water gives a terpmeol also a known natural product... 

The tert butyl group is cleaved as the corresponding carbocation Loss of a proton from tert butyl cation converts it to 2 methylpropene Because of the ease with which a tert butyl group is cleaved as a carbocation other acidic reagents such as trifluoroacetic acid may also be used... 

Elimination unimolecular (El) mechanism (Section 5 17) Mechanism for elimination characterized by the slow for mation of a carbocation intermediate followed by rapid loss of a proton from the carbocation to form the alkene Enamine (Section 17 11) Product of the reaction of a second ary amine and an aldehyde or a ketone Enamines are char actenzed by the general structure... 

Cocatalysts of two types occur (/) proton-donor substances, such as hydroxy compounds and proton acids, and (2) cation-forming substances (other than proton), including alkyl and acyl haUdes which form carbocations and other donor substances leading to oxonium, sulfonium, halonium, etc, complexes. 

Electrophile Addition Reactions. The addition of electrophilic (acidic) reagents HZ to propylene involves two steps. The first is the slow transfer of the hydrogen ion (proton) from one base to another, ie, from Z to the propylene double bond, to form a carbocation. The second is a rapid combination of the carbocation with the base, Z . The electrophile is not necessarily limited to a Lowry-Briiinsted acid, which has a proton to transfer, but can be any electron-deficient molecule (Lewis acid). 

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