Transition states partial bonds

In the transition state, a bond between oxygen and carbon is partially formed and the bond between carbon and chlorine is partially broken. The configuration of the carbon begins to invert. 

Broadly, then, if the substitution site is primary, and therefore access to it is not hindered sterically, the nucleophile approaches it and, by donation of its electron pair, forms a partial bond to. carbon while the leaving-group—carbon bond begins to break (Equation 4.7). At the transition state, both bonds partially... 

The Sn2 reaction of methyl iodide (iodomethane) with hydroxide ion is a concerted reaction, taking place in a single step with bonds breaking and forming at the same time. The middle structure is a transition state, a point of maximum energy, rather than an intermediate. In this transition state, the bond to the nucleophile (hydroxide) is partially formed, and the bond to the leaving group (iodide) is partially broken. Remember that a transition state is not a discrete molecule that can be isolated it exists for only an instant. 

The origin of the alpha effect is currently a subject for much speculation. Edwards and Pearson (1962) have suggested that, since the nucleophile partially loses two electrons in the transition state (by bonding... 

A reaction coordinate diagram. The dashed lines in the transition state indicate bonds that are partially formed or partially broken. 

In order to explain the observed substituent and solvent effects on the rate of the Ciaisen rearrangement, Coates et al. [28] suggested a dipolar character for the transition state "... partial delocalization of a non-bonded electron pair from the donor substituent generates a significant degree of enolate-oxonium ion-pair character that stabilizes the TS more than the ground state,... (Scheme 11.22). 

Calculated transition structures may be very sensitive Lo the level of theory employed. Semi-empirical methods, since they are parametrized for energy miriimnm structures, may be less appropriate for transition state searching than ab initio methods are. Transition structures are norm ally characterized by weak partial" bonds, that is, being broken or formed. In these cases UHF calculations arc necessary, and sometimes even the inclusion of electron correlation effects. 

Dashed lines in transition state structures represent partial bonds that is bonds in the process of being made or broken... 

The transition state for this step involves partial bond formation between tert butyl cation and chloride ion... 

The regioselectivity of elimination is accommodated m the E2 mechanism by noting that a partial double bond develops at the transition state Because alkyl groups... 

Carbon is partially bonded to both the incoming nucleophile and the departing halide at the transition state Progress is made toward the transition state as the nucleophile begins to share a pair of its electrons with carbon and the halide ion leaves taking with it the pair of electrons m its bond to carbon... 

Any discussion based on reactivity ratios is kinetic in origin and therefore reflects the mechanism or, more specifically, the transition state of a reaction The transition state for the addition of a vinyl monomer to a growing radical involves the formation of a partial bond between the two species, with a corre sponding reduction of the double-bond character of the vinyl group in the monomer ... 

If substituent X is an electron donor and Y an electron acceptor, then the partial bond in the transition state is stabilized by a resonsance form [IV] which attributes a certain polarity to the emerging bond ... 

A stronger bond between the nucleophilic atom and carbon is reflected in a more stable transition state and therefore a reduced activation energy. Since the 8 2 process is concerted, the strength of the partially formed new bond is reflected in the energy of the transition state. 

The transition state involves six partially delocalized electrons being transformed from one 1,5-diene system to another. The transition state could range in character from a 1,4-diradical to two nearly independent allyl radicals, depending on whether bond making or bond breaking is more advanced. The general framework for understanding the substituent effects is that the reactions are concerted with a relatively late transition state with well-developed C(l)—C(6) bonds. 

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