Transition state crowding

Rate IS governed by stability of car bocation that is formed in loniza tion step Tertiary alkyl halides can react only by the SnI mechanism they never react by the Sn2 mecha nism (Section 8 9) Rate IS governed by steric effects (crowding in transition state) Methyl and primary alkyl halides can react only by the Sn2 mecha nism they never react by the SnI mechanism (Section 8 6)... 

Steiic effects of another kind become important in highly branched substrates, in which ionization is facilitated by relief of steric crowding in going from the tetrahedral groimd state to the transition state for ionization. The ratio of the hydrolysis rates in 8OV0 aqueous acetone of t-butyl /F-nitrobenzoate and 2,3,3-trimethyl-2-butyl p-nitrobenzoate is 1 4.4. 

Extensions of the enamine alkylation to a-tetralones have also been used (245-248), but product yields were lower, presumably due to steric crowding in a transition state where generation of an imonium salt gives rise to a repulsion between a methylene group on nitrogen and a peri aromatic proton. 

Next, examine the Sn2 transition states as space-filling models. Are you able to identify unfavorable nonbondec (steric) interactions that are not present in the reactants If so, which Sn2 reaction is likely to be most affected b] steric interactions Least affected Rationalize you observations. Hint Compare CBr bond distances in thf Sn2 transition states. How do these change with increasec substitution at carbon What effect, if any, does this havf on crowding ... 

What other factors might be responsible for difference in activation energies Compare atomic charges anc electrostatic potential maps for the Sn2 transition states Does the increase in steric crowding lead to enhanced o diminished charge delocalization Explain. How, if at all would this be expected to affect the energy barrier Why ... 

Aldehydes are generally more reactive than ketones in nucleophilic addition reactions for both steric and electronic reasons. Sterically, the presence of only one large substituent bonded to the C=0 carbon in an aldehyde versus two large substituents in a ketone means that a nucleophile is able to approach an aldehyde more readily. Thus, the transition state leadingto the tetrahedral intermediate is less crowded and lower in energy for an aldehyde than for a ketone (Figure 19.3). 

This simple strategy was not used before because (34) is the xo adduct from (35), and it was supposed that the endo adduct would be preferred. However, Stork examined the reaction carefully and realised that the enda transition state would be impossibly crowded (of p 420), Heating (35) to 140°C does... 

The replacement of five waters in [Cr(H20)6]3+ by NH3 in [Cr(NH3)5H20]3+ causes a 20-fold increase in the lability of the single remaining water (which is quite small by comparison with such replacement in [Ni(H20)6]2 +, as seen from Table V) and appears to decrease the importance of the a-activation mode in the approach to the transition state (see Section III,G). Assuming that the free radii (222) of H20 (138 pm) and NH3 (169 pm) reflect their relative electrostricted coordinated radii, it is anticipated that the first coordination sphere of [Cr(H20)6]3+ will be less crowded than that of [Cr(NH3)5H20]3+ and that the extent of a-activation character in the substitution of water in the latter species will decrease. 

Benzyne reacts with 7-dehydrocholesteryl methyl ether to form the ene products (110) and (111, X = H), while with tetrafluorobenzyne we isolated the anticipated cyclo-adduct (112) in addition to an ene product (111, X = F) 152>. We expected that less crowding would obtain in the transition states leading to cyclo-adducts from cholesta-2,4-diene, and obtained from a reaction with benzyne both the a- and 3-adducts (113, X = H) and (114). In a reaction with tetrafluorobenzyne we only obtained the a-adduct (113, X = F). 

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