SN1 substitution

I Tertiary7 alkyl halides E2 elimination occurs when a base is used, but SN1 substitution and El elimination occur together under neutral conditions, such as in pure ethanol or water. ElcB elimination takes place if the leaving group is two carbons away from a carbonyl group. 

The essential feature of these data is that the rate is independent of the nature of the entering ligand. This behavior is characteristic of an SN1 substitution mechanism. 

FIGURE 20.5 Attack on a trigonal bipyramid transition state during SN1 substitution. 

Inmost instances, a haloorganic subject to either SN2 displacement reaction or SN1 substitution is heated at reflux with the trivalent phosphorus ester with the concomitant formation of the valence-expanded organophosphorus compound and haloorganic by-product, as illustrated with an example in Equation 3.1. 

FIGURE 2.10 Differentiation of SN1 (substitution nucleophilic unimolecular, first order) and SN2 (substitution nucleophilic bimolecular, second order) reactions. 

The work cited in sections 2.4 and 2.5 is representative of the SN1 substitution reactions of metal carbonyls. However, a much more extensive and detailed account has recently been published covering similar reactions of vanadium, chromium, molybdenum, tungsten, rhenium, iron and nickel carbonyls in addition to those of manganese and cobalt2 9a. 

In order to compare SN1 substitution rates in a range of alkyl halides, experimental conditions are chosen in which competing substitution by the SN2 route is very slow. One such set of conditions is solvolysis in aqueous formic acid (HC02H) ... 

The interaction of ge/w-dihalocycloproparenes at C(ll with nucleophiles is consistent with ionization to a cycloproparenyl cation, cf. 5, and subsequent capture of the nucleophile to give products of SN1 substitution at the sp3 centre as illustrated by equation 23 (Section IV. A). There are no known examples of nucleophilic aromatic substitution among the cycloproparenes because of an absence of appropriately functionalized compounds. 

Strong acids promote SN1 substitution reactions by converting an electron-rich ( basic ) atom on the substrate into a good leaving group, e.g., for substitution reactions of tert-butyl derivatives. 

Dibromobenzocyclohexene undergoes SN1 substitution in methanol to give a single methyl ether. 

Adamantyl tosylate undergoes SN1 substitution 1,000 times more slowly than tert-butyl tosylate. 

Water-acetone mixtures offer a sufficiently polar medium that certain alkyl halides can dissociate into a halide anion and a. carbocation The latter then reacts with water to give an SN1 substitution product. 

One of the tosylates shown below undergoes SN1 substitution 100,000 times faster than the other. Intermediate carbocations are involved. 

The substrate is a tertiary alkyl bromide and can undergo SN1 substitution and El elimination under these reaction conditions. Elimination in either of two directions to give regioisomeric alkenes can also occur. 

Primary alcohols favor S 2 substitutions while SN1 substitutions occur mainly with tertiary alcohols. 

Tertiary alkyl halides are essentially unreactive to strong nucleophiles in polar, aprotic solvents, i.e. the conditions for the Sn2 reaction. Tertiary alkyl halides can undergo E2 reactions when treated with a strong base in protic solvent and will do so in good yield since the SN2 reaction is so highly disfavoured. Under non-basic conditions in a protic solvent, El elimination and SN1 substitution both occur. 

Fig. 2.21. Acid catalysed SN1 substitutions of trityl ethers to trityl alcohols, using deprotection procedures from nucleotide synthesis as an example. The table in the center indicates the time (t) it takes to completely cleave the respective trityl groups.

When one follows the reaction arrows in Figure 9.12 from the bottom upward, the following important information can be noted In an acidic water-containing solution 0,0-acetals are hydrolyzed to give carbonyl compounds and alcohols. Such a hydrolysis consists of seven elementary reactions. First, the hemiacetal A (Nu = OR3) and one equivalent of alcohol are produced from the 0,0-acetal and water in an exact reversal of the latter s formation reaction, i.e., through a proton-catalyzed SN1 substitution (in four steps). What follows is a three-step decomposition of this hemiacetal to the carbonyl compound and a second equivalent of the alcohol. 

Most carbocations are quite unstable and have only a fleeting existence as intermediates in reactions such as the SN1 substitution. However, some, such as the triphenylmethyl carbocation, are stable enough that they can exist in significant concentrations in solution or even can be isolated as salts. 

Normally, carbocations are encountered as transient intermediates along the pathway of reactions such as the SN1 substitution. However, under conditions in which no nucleophiles or bases are available to react with them, carbocations can have significant lifetimes. Because superacids are very weak nucleophiles (see Section 4.10) and are quite polar, they provide an environment in which carbocations have lifetimes long enough to allow them to be studied by a variety of instrumental techniques. 

It is important to note that the use of polar solvents is only a necessary but not a sufficient condition for the feasibility of SN1 substitutions or for the preference of the SN1 over the SN2 mechanism. 

Die ambidenten Anioncn dcr Thiophosphorsaure-0,0-diester reagieren unter Bcdingun-gen der SN2-Substitution mit Alkyl-halogeniden am S-Atom zu Thiophosphorsaure-0,0,S-triestern (s. S. 584) unter Bedingungen der SN1-Substitution (z.B. mit Tosylaten) wird am O-Atom alkyliert149 ... 

Reactivity toward SN1 substitution mechanisms follows the stability of carbocations ... 

Bromocyclohexene is a secondary halide, and benzyl bromide is a primary halide. Both halides undergo SN1 substitution about as fast as most tertiary halides. Use resonance structures to explain this enhanced reactivity. 

Under certain conditions, when (R)-2-bromobutane is heated with water, the SN1 substitution proceeds twice as fast as the SN2. Calculate the e.e. and the specific rotation expected for the product. The specific rotation of (R)-butan-2-ol is —13.5°. Assume that the SN1 gives equal amounts of the two enantiomers. 

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