N. M. Vitkovskaya

Quantum-chemical investigation of mechanisms of reactions of nucleophilic addition to acetylene. 7. Evaluation of possible interactions in C2H2/MOH/DMSO system

Within the framework of the ab initio method SCF MO LCAO, using a model potential, the influence of alkali metal cations on the reactivity of the acetylene molecule has been investigated. Activation of acetylene with respect to a nucleophile is favored by a lowering of energy of the vacant π* orbital upon coordination, a change in its form as a consequence of sufficiently free motion of the cation along the triple bond, and stabilization of the transdistorted form of acetylene in the complex. In the acetylene/alkali/DMSO system, the activating capability of cations is related to the ease of replacement of a solvent molecule in the solvate complex of the acetylene molecule, this capability increasing in the series Li < Na< K.

Quantum-chemical investigation of the mechanisms of nucleophilic addition reactions to acetylene

The interactions between an acetylene molecule and lithium hydrosulfide in the dissociated and undissociated states have been calculated in the framework of the semiempirical MNDO method. It has been established that acetylene can easily form molecular complexes with lithium hydrosulfide, which are stable toward all the possible types of nonradical dissociation. A comparison with the previously considered reaction between C2H2 and LiOH shows that in the case of LiSH, the activation barriers to the isomerization of the complex C2H2·LiSH to vinyl mercaptans are considerably lower.

Quantum-chemical investigation of the mechanisms of nucleophilic addition reactions to acetylene: 6. Cyclotrimerization of acetylene in the presence of bases

A stepwise mechanism for the formation of benzene as a secondary product of a nucleophilic addition reaction to acetylene in media with increased basicity has been proposed. The cyclotrimerization of acetylene takes place as a result of the successive addition of acetylene molecules to an ethynide ion followed by cyclization of the adduct formed. The activation barriers and heats of reaction have been calculated for each of the steps in the framework of the MNDO method. The maximum barrier belongs to the step leading to the formation of a C6H6− ring from the open-chain isomer and has a height of ∼79 kJ/mole.

Quantum-chemical investigation of the mechanisms of reactions involving nucleophilic addition to acetylene 1. Addition of a hydroxide ion

Conclusions1.Various paths for the reaction of acetylene with the hydroxide ion have been evaluated with the aid of quantum-chemical calculations, and it has been shown that the formation of the acetylide ion (HC≡C−) is the preferable direction.2.The formation of anions of the type [HC-CHOH]− (cis and trans) is hampered as a consequence of significant energy barriers and can occur only under severe conditions, and the formation of the [H2C-COH]− and [H2C-CHO]− anions according to the mechanism of the direct addition of an hydroxide ion to acetylene has a low probability.

Quantum-chemical investigation of the mechanisms of reactions involving nucleophilic addition to acetylene. 4. Study of reactions in the acetylene-LIOH system

Conclusions1.The predominant direction of the reaction of acetylene with a molecule of lithium hydroxide in both the dissociated and undissociated states is the formation of the molecular π complex C2H2· LiOH.2.The formation of the complex C2H2· LiOH is accompanied by significant lowering of the energy levels of the vacant π orbitals of acetylene, which creates favorable conditions for nucleophilic attack on the triple bond.

Quantum-chemical investigation of the mechanisms of reactions involving nucleophilic addition to acetylene. 3. Mechanism of the formation of vinylthio anions

Conclusions1.According to the data from the quantum-chemical calculation, the vinylthio anion CH2=CHS− can be observed as a result of the direct addition of an hydrosulfide ion to acetylene with a barrier having a height ≤123 kJ/mole.2.One of the intermediate products in the reaction of acetylene with hydrogen sulfide in the presence of OH− ions is HC≡C−, which adds H2S to form the complex HC≡C−·H2S without a barrier. The CH2=CSH− anion forms from this complex with practically no barrier, and the formation of the vinylthio anion takes place with a barrier not exceeding 42 kJ/mole.

Quantum-chemical investigation of the mechanisms of reactions involving nucleophilic addition to acetylene. 2. Interconversions of products of the reaction of acetylene with the hydroxide ion

Conclusions1.According to the data form quantum-chemical calculations, the formation of vinyloxy anions from acetylene molecules and hydroxide ions occurs with the intermediate participation of [HC=CHOH]− anions (cis or trans), whose further conversion is possible only under fairly severe conditions.2.The formation of the [H2C=COH]− anion according to the mechanisms considered is unlikely.

Nonempirical calculation of surface area of potential energy of ammonium ion

1. The surface areas of the potential energy for the reaction of forming ammonium ion were calculated employing the SCF MO LCAO method on an expanded STO-4GTO basis. 2. The optimum value of the length of the N−H bond in the ammonium ion was determined (1.965 at. units), and is in good agreement with experiment. 3. A value of 211.8 kcal/mole was obtained for the proton affinity of the ammonia molecule, which practically coincides with the experimental value.