Transition states back-side attack

The. S n reactions between HF and protonated methyl, ethyl, isopropyl, and /-butyl fluorides in the gas phase have been examined at the MP2/6-31+- -G(d,p) level of theory.112 113 The reaction of CH3FH+ clearly occurs via back-side attack as the transition state for this process is of lower energy than the transition state for frontside attack. The EtFH+ can react via a more stable back-side. S N2 reaction or an. S n 1 reaction via front-side attack since the. S N 1 pathway is 4.4 kJmol-1 lower in energy. No. S n2 path could be found for i-PrFH+ and the front- and back-side pathways had equal activation energies for /-BuFH+, which effectively reacts by an. S N1 mechanism. The conclusion is that the preference for back-side attack is reduced as the size of... 

The fact that SN2 reactions always occur with inversion of configuration enables us to form a better picture of the transition state. The nucleophile must approach the carbon from the side opposite the leaving group (back-side attack). The structure of the transition state, with partial bonds to the entering hydroxide and the leaving chloride, is shown in the following structure. Figure 8.3 uses orbitals to show how this process occurs. 

The reaction occurs by back-side attack (Sn2), but positive charge is stabilized by a tertiary carbocation-like transition state (SnI). 

Higher temperatures and polar solvents are considered to switch the reaction mechanism of transmetalation from a four-centered transition state (Se2 (cyclic)) to a back-side attack of the palladium(II) complex (Se2 (open)) (Fig.1). 

It was to account for inversion of configuration that back-side attack was first proposed for substitution of the Sn2 kind. As —OH becomes attached to carbon, three bonds are forced apart until they reach the planar spoke arrangement of the transition state then, as bromide is expelled, they move on to a tetrahedral arrangement opposite to the original one. This process has often been likened to the turning-inside-out of an umbrella in a gale. 

Acid-induced ring opening of 1,2-epoxy-l-methykyclohexane with HBr. There is a high degree of SNi-like carbocation character in the transition state, which leads to back-side attack of the nucleophile at the tertiary center and to formation of the isomer of 2-bromo-2-methylcyclohexano1 that has -Br and OH groups trans. 

Evidently, the transition state for acid-catalyzed epoxide opening L, an SN2-like geometry but also has a lai e amount of Si jl-like carbocaticB character. Since the positive charge in the protonated epoxide is shared the more highly substituted carbon atom, back-side attack of Br occurs ( the more highly substituted site. 

The leaving group is forced out by the nucleophile, as introduced in Section 4.2.7. The transition state is a five-coordinate carbon. The tetrahedral configuration at the carbon atom is inverted. The transition state has a carbon p orbital partially bonded to the nucleophile on one side and the leaving group on the other (back side attack, see Fig. 7.5). The 2 in the name Sn2 indicates that there are two reactant molecules involved in the rate-determining step (the nucleophile and the molecule attacked). 

The C-X bond of alkyl halides and sulfonate esters is polarized such that the carbon has a positive dipole. Halides and sulfonate anions are good leaving groups. Nucleophiles attack primary and secondary alkyl halides, displacing the leaving group in what is known as aliphatic, bimolecular nucleophilic substitution, the Sn2 reaction. The 8 2 reaction follows second-order kinetics, has a transition state rather than an intermediate, and proceeds via back-side attack of the nucleophile on the halide and inversion of configuration. 

Observe what has happened at carbon. Because the motion of the hydrogen atoms is away from the incoming nucleophile, the configuration of the carbon atom they are attached to is inverted in the product. Because iodomethane does not have a stereogenic center, the inversion cannot be detected, but this transition state model of inversion nicely explains the earlier inversion of configuration of 7 to 8. Indeed, a pentacoordinate transition state such as 11 is taken as the transition state for all nucleophilic aliphatic substitution reactions, and they all proceed with 100% inversion of configuration (consistent with back-side attack of the nucleophile). 

SO that there is some formal resemblance to S 2. The above pathway must, however, differ in that attack by Y cannot take place from the back of the carbon atom carrying the leaving group (cf. Sf 2, p. 78), but must occur from the side it is thus often referred to as Sff2(aromatic). Further, on the basis of the above rate law the reaction could be concerted (like Sf 2)—in which case (81) is a transition state—or it could proceed by a stepwise pathway with either step (1) or step (2) as the slow, rate-limiting one—in which case (81) is an intermediate. 

There are two simple ways in which the SN2 reaction of methyl chloride could occur with hydroxide ion. These differ in the direction of approach of the reagents (Figure 8-1). The hydroxide ion could attack chloromethane at the front side of the carbon where the chlorine is attached or, alternatively, the hydroxide ion could approach the carbon on the side opposite from the chlorine in what is called the back-side approach. In either case, the making of the C-O bond is essentially simultaneous with the breaking of the C-Cl bond. The difference is that for the back-side mechanism the carbon and the attached hydrogens become planar in the transition state. 

This one-to-one correlation of inversion with displacement must mean that the incoming iodide enters the molecule from the side of the substitution site opposite to the departing iodide every single time. It initially attacks the back lobe of the sp3 orbital used for bonding with the iodide. The transition state proposed by Hughes and co-workers is shown in 1. Carbon has rehybridized and is... 

When the steric effects in the transition states for retention and inversion are of equal energy, attack of a migrating group on a hot carbocation occurs preferentially from the back side. For example, It")-1 -amino-2-methylbutanol-1 29, is deaminated in aqueous HC104 to afford 2-methylbutanal, 14, (16 percent) with 30 percent inversion of configuration at Ca.42... 

The S 2 transition states and the encounter complexes for back- and front-side attack in the gas-phase reactions between X - and CF3X have been calculated at the... 

The inversion of the configuration at the carbon atom of the epoxide ring where cleaved in the course of the ring opening during polymerisation indicates that the monomer complexed with the metal atom is attacked from the back side by the nucleophilic substituent X [scheme (1)]. If front side nucleophilic attack of this substituent occurred on the coordinated monomer, i.e. via the four-membered transition state as in scheme (2), no inversion but rather the retention of the configuration at the epoxide ring carbon atom where cleaved should be observed however, this is not the case. 

This orange area is on one side of the carbonyl group and in the usual place at the back of the C-13r bond. Each group has become more electrophilic because of the presence of the other— the C=0 group makes the C-Br bond more reactive and the Br makes the C=0 group more reactive. Another way to put this is that the carbonyl group stabilizes the transition state by overlap of its 7t orbital with the full p orbital of the carbon atom under attack. The nucleophile may well attack the carbonyl group but this will be reversible whereas displacement of bromide is irreversible,... 

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