Transition states bromide reaction

Calculate activation barriers for bromide addition t( methyl bromide, ethyl bromide, 2-propyl bromide an( 2-methyl-2-propyl bromide using energies for Sn transition states bromide+methyI bromide, bromide- ethyl bromide, bromide+2-propyl bromide and bromides 2-methyl-2-propyl bromide) and Br (at left). Whicl reaction is fastest Slowest ... 

HI is also used in organic chemistry to convert primary alcohols into alkyl halides. This reaction is an Sn2 substitution, in which the iodide ion replaces the "activated" hydroxyl group (water). HI is preferred over other hydrogen halides in polar protic solvents because the iodide ion is a much better nucleophile than bromide or chloride, so the reaction can take place at a reasonable rate without much heating. The large iodide anion is less solvated and more reactive in polar protic solvents and thus causes the reaction to proceed faster because of stronger partial bonds in the transition state. This reaction also occurs for secondary and tertiary alcohols, but substitution occurs via the SnI pathway. 

The major difference between the two mechanisms is the second step The second step m the reaction of tert butyl alcohol with hydrogen chloride is the ummolecular dis sociation of tert butyloxonium ion to tert butyl cation and water Heptyloxonium ion however instead of dissociating to an unstable primary carbocation reacts differently It IS attacked by bromide ion which acts as a nucleophile We can represent the transition state for this step as... 

Sketch a potential energy diagram for the reaction of 1 heptanol with hydrogen bromide paying careful attention to the positioning and structures of the intermediates and transition states... 

The bond highlighted m yellow is the peptide bond ) Pencyclic reaction (Section 10 12) A reaction that proceeds through a cyclic transition state Period (Section 1 1) A honzontal row of the penodic table Peroxide (Section 6 8) A compound of the type ROOR Peroxide effect (Section 6 8) Reversal of regioselectivity oh served m the addition of hydrogen bromide to alkenes brought about by the presence of peroxides m the reaction mixture... 

Enby 6 is an example of a stereospecific elimination reaction of an alkyl halide in which the transition state requires die proton and bromide ion that are lost to be in an anti orientation with respect to each odier. The diastereomeric threo- and e/ytAra-l-bromo-1,2-diphenyl-propanes undergo )3-elimination to produce stereoisomeric products. Enby 7 is an example of a pyrolytic elimination requiring a syn orientation of die proton that is removed and the nitrogen atom of the amine oxide group. The elimination proceeds through a cyclic transition state in which the proton is transferred to die oxygen of die amine oxide group. 

Use of the Hammond Postulate requires that the reverse reactions both be fast. Obtain energies for the transition states leading to 1-propyl and 2-propyl radicals ipropane+Br end and propane+Br center), and draw a reaction energy diagram for each (place the diagrams on the same axes). Is use of the Hammond Postulate justified Compare the partial CH and HBr bond distances in each transition state to the corresponding distances in propane and hydrogen bromide, respectively. Does the Hammond Postulate correctly predict which bond distances will be most similar Explain. 

Sn2 reactions proceed through transition states in whicl the central carbon has five neighbors instead of the usua four, e.g., for reaction of bromide and methyl bromide. 

Alteration of the relative reactivity of the ring-positions of quinoline is expected and observed when cyclic transition states can intervene. Quinoline plus phenylmagnesium bromide (Et20,150°, 3 hr) produces the 2-phenyl derivative (66% yield) phenyllithium gives predominantly the same product along with a little of the 4-phenylation product. Reaction of butyllithium (Et 0, —35°, 15 min) forms 2-butylquinoline directly in 94% yield. 2-Aryl- or 6-methoxy-quinolines give addition at the 2-position with aryllithium re-agents, and reaction there is so favored that appreciable substitution (35%) takes place at the 2-position even in the 4-chloroquinoline 414. Hydride reduction at the 2-position of quinoline predominates. Reaction of amide ion at the 2-position via a cyclic... 

We call the carbocation, which exists only transiently during the course of the multistep reaction, a reaction intermediate. As soon as the intermediate is formed in the first step by reaction of ethylene with H+, it reacts further with Br in a second step to give the final product, bromoethane. This second step has its own activation energy (AG ), its own transition state, and its own energy change (AG°). We can picture the second transition state as an activated complex between the electrophilic carbocation intermediate and the nucleophilic bromide anion, in which Br- donates a pair of electrons to the positively charged carbon atom as the new C-Br bond starts to form. 

The addition reactions of alkyllithium-lithium bromide complexes to a-trimethylsilyl vinyl sulfones that have as a chiral auxiliary a y-mono-thioacetal moiety derived from ( + )-camphor are highly diastereoselective. A transition state that involves chelation of the organolithium reagent to the oxygen of the thioacetal moiety has been invoked. The adducts are readily converted via hydrolysis, to chiral a-substituted aldehydes22. 

The fast reaction clearly involves the transition state (XLVIII) as confirmed by the observation of Reutov etal.123 that all of the label in the mercuric bromide... 

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