Ethyl halides protonated

MarkownikofT s rule The rule states that in the addition of hydrogen halides to an ethyl-enic double bond, the halogen attaches itself to the carbon atom united to the smaller number of hydrogen atoms. The rule may generally be relied on to predict the major product of such an addition and may be easily understood by considering the relative stabilities of the alternative carbenium ions produced by protonation of the alkene in some cases some of the alternative compound is formed. The rule usually breaks down for hydrogen bromide addition reactions if traces of peroxides are present (anti-MarkownikofT addition). 

The anion produced by proton abstraction from ethyl acetoacetate is nucleophilic. Adding an alkyl halide to a solution of the sodium salt of ethyl acetoacetate leads to alkylation of the a carbon. 

Acidic ether cleavages are typical nucleophilic substitution reactions, either SN1 or Sn2 depending on the structure of the substrate. Ethers with only primary and secondary alkyl groups react by an S 2 mechanism, in which or Br attacks the protonated ether at the less hindered site. This usually results in a selective cleavage into a single alcohol and a single alkyl halide. For example, ethyl isopropyl ether yields exclusively isopropyl alcohol and iodoethane on cleavage by HI because nucleophilic attack by iodide ion occurs at the less hindered primary site rather than at the more hindered secondary site. 

Carbene complexes have also been prepared by transmetallation reactions. Lithiated azoles react with gold chloride compounds and after protonation or alkylation the corresponding dihydro-azol-ylidene compounds, e.g., (381) or (382), are obtained.22 9-2264 Silver salts of benz-imidazol have also been used to obtain carbene derivatives.2265 Mononuclear gold(I) carbene complexes also form when trimeric gold(I) imidazolyl reacts with ethyl chlorocarbonate or ethyl idodate.2266,2267 The treatment of gold halide complexes with 2-lithiated pyridine followed by protonation or alkylation also yields carbene complexes such as (383).2268 Some of these carbene complexes show luminescent properties.2269-2271... 

Proton nmr halide anion titrations reveal that the ethyl- [79], propyl-[80] and butyl- [81] linked derivatives (Fig. 43) form complexes of 1 1 stoichiometry in acetonitrile solution. Stability constant determinations suggest that the ethyl derivative [79] exhibits selectivity for the chloride anion in preference to bromide or iodide. As the chain length increases, so the selectivity for chloride decreases and also the magnitude of the stability constant which is evidence for an anionic chelate effect with the chloride anion. Receptors containing larger aryl [81], [83], [84] and alkylamino spacers [85] (Fig. 43) form complexes of 2 1 halide anion receptor stoichiometry. 

Looking back on the history of ketone dianion chemistry, one soon notices that dianion species, derived from / -keto esters, have been in continuous steady use in organic synthesis3,4, as shown in Scheme 2. Thus, ethyl acetoacetate can be converted to the corresponding ketone o a -chainon via consecutive proton abstraction reactions. The resulting dienolate anion reacts with a variety of alkyl halides to give products, resulting from exclusive attack at the terminal enolate anions. 

The solvent isotope effect for the acid catalyzed hydrolysis of ethyl diazoacetate (without halide ions) is much smaller than 1 (Table 19, p. 63) as expected for a pre-equilibrium proton transfer mechanism. Furthermore, according to the findings of Roberts et al. [205] the products of ethanolysis of ethyl diazoacetate in C2HsOD solution are C2HS OCHDCOOEt as well as C2 H5 OCD2 COOEt which indicates that H exchange is faster than ethanolysis. 

When a carbocation is formed by loss of a nucleophile, as in the SN1 reaction, elimination of a proton will always compete with substitution, provided that there is a (3-C-H bond in the cation. These eliminations are described as unimolecular eliminations (El)because, as with the competing SN1 reaction, the rate-determining step is the unimolecular reaction of the original halide to give the carbocation. For example, /-butyl chloride in ethanol gives the /-butyl cation, which can either react with a solvent molecule to give the ethyl ether 36 (SN1, solvolysis, reaction 5.25) or a proton can be removed by a solvent molecule to give 2-methylpropene 37 (El, reaction 5.26). If more than one type of J3-C-H... 

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