Molecular Reactions Cyclic Transition States

The great majority of reactions involve intermediates, as illustrated in the previous three chapters. A smaller number of uni- or bimolecular reactions appear to proceed from reagents to products without detectable intermediates. These reactions take place in the gas phase or in solution in non-polar solvents. The absence of substantial solvent effects (Chapter 3) indicates little build up of charge in the transition state. 

Butadiene dimerizes to 4-vinylcyclohexene (4-ethenylcyclohexene, 1) (reaction 7.1). The absence of intermediates suggests a cyclic movement of three electron pairs, (which could equally well have been written in the opposite direction, or as single electron movements). The transition state would involve partial bond-making and breaking in the six-membered transition state as shown. Reactions involving such cyclic transition states are known as pericyclie reactions. However, ethene does not dimerize to cyclobutane (reaction 7.2) under thermal conditions, even though a cyclic movement of two pairs of electrons could have been invoked. 

This chapter explores the reasons why some molecular reactions take place whereas others do not, and introduces the concepts of frontier orbitals and transition state aromaticity. 

Having worked through this chapter, you should  

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Experimental activation energies presented in Table XI indeed are consistent with this observation. Furthermore, we can postulate that molecular eliminations that proceed with the formation of five-membered or larger cyclic transition states should exhibit activation energies that are much closer to the heats of reaction (O Neil and Benson, 1967). 

Catalytic hydrogenation transfers the elements of molecular hydrogen through a series of complexes and intermediates. Diimide, HN=NH, an unstable hydrogen donor that can only be generated in situ, finds some specialized application in the reduction of carbon-carbon double bonds. Simple alkenes are reduced efficiently by diimide, but other easily reduced functional groups, such as nitro and cyano, are unaffected. The mechanism of the reaction is pictured as a transfer of hydrogen via a nonpolar cyclic transition state. 

Termination of the autoxidation chain process occurs as peroxyl radicals couple to yield non-radical products. This reaction takes place through an unstable tetroxide intermediate. Primary and secondary tetroxides decompose rapidly by the Russell termination mechanism to yield three non-radical products via a six-membered cyclic transition state (Fig. 95). The decomposition yields the corresponding alcohol, carbonyl compound, and molecular oxygen (often in the higher energy singlet oxygen state) three... 

The kinetics of the gas-phase elimination of 3-hydroxy-3-methylbutan-2-one have been investigated in a static system, seasoned with allyl bromide, and in the presence of the free chain radical inhibitor toluene.14 The reaction was found to be homogeneous, unimolecular and to follow a first-order rate law. The products of elimination are acetone and acetaldehyde. Theoretical estimations suggest a molecular mechanism involving a concerted non-synchronous four-membered cyclic transition state process. 

The pyrolysis of 2-bromo-2-butene in a static system, with seasoned vessels, and even in the presence of a free radical inhibitor, was autocatalyzed by the HBr product109. However, under maximum catalysis with HBr gas, the reaction is molecular in nature and follows first-order kinetics. The overall rate coefficient was given by the following Arrhenius equation log kx (s-1) = (13.57 0.56)-(200.4 6.8) kJmol-1 (2.303R7)-1. The mechanism was suggested to involve a six-membered cyclic transition state as described in equation 22. 

Rates of loss of water from the molecular ions of hexanol-3, 3-d2 and hexanol-4, 4-d2 have been measured over the time range from tens of picoseconds to 10 ps [229]. Two distinct reactions were identified, one involving a 5-membered and the other a 6-membered cyclic transition state. The former appeared to have the higher frequency factor, i.e. the looser transition state or higher densities of states in the transition state. It is important to bear in mind that what are being compared here are a five-membered ring with a propyl side chain and a six-membered ring with an ethyl side chain. That the former is looser ... 

This chapter is an introduction to qualitative molecular orbital theory and pericyclic reactions. Pericyclic reactions have cyclic transition states and electron flow paths that appear to go around in a loop. The regiochemistry and stereochemistry of these reactions are usually predictable by HOMO-LUMO interactions, so to understand them we need to understand molecular orbital theory, at least on a qualitative basis. 

Molecular Reactions with Non-cyclic Transition States... 

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