Triple bonds bond enthalpies

The nature of the bonding in this molecule has been the cause of considerable discussion. Its short length (112.8 pm) and its great strength (bond dissociation enthalpy 1072 kJ mol- ) are consistent with the usual triple-bond Lewis structure... 

An aromatic ring and a double or triple bond in the a-position relative to the C—H bond weaken this bond by virtue of the delocalization of the unpaired electron in its interaction with the iT-bond. The weakening of the C—H bond is very considerable for example, D(C—H) is 422 kJ mol-1 in ethane [27], 368 kJ mol-1 in the methyl group of propene [27] (AD = 54 kJ mol-1), and 375 kJ mol-1 in the methyl group of toluene [27] (AD = 47 kJ mol-1). Such decrease in the strength of the C—H bond diminishes the enthalpy of the radical abstraction reaction and, hence, its activation energy. This effect is illustrated below for the reactions of the ethylperoxyl radical with hydrocarbons ... 

As expected, the formal C—C triple bond in benzyne is significantly weaker than in unstrained alkynes, the C=C stretching vibrations of which usually fall in the region 2150 cm. Nevertheless, o-benzyne is better described as a strained alkyne rather than a biradical, which is evident from the large singlet-triplet splitting of 37.5 0.3 kcal/mol as well as the alkyne-like reactivity (e.g., in Diels-Alder reactions). The enthalpy of formation of 4 was determined to be 106.6 3.0 kcal/mol by Wenthold and Squires. For the C=C bond length a value of 124 2 pm was found experimentally, " which comes closer to a typical C C triple bond (120.3 pm in acetylene) rather than a C C double bond (133.9 pm in ethylene). 

Bond length/bond enthalpy. Note that 1 A equals 0.1 nm. 6 Bond lengths are given for bonds in which none of the partner atoms is involved in a double or triple bond. In such cases bond lengths are somewhat shorter.c In carbon disulfide.d In aldehydes. e In ketones, f In carbon dioxide. 

Alkynes are highly reactive building blocks in synthesis which, despite the fact that their positive enthalpy of formation (acetylene Hp = +229.4 kJ/mol) [1] makes them metastable at room temperature, react only at elevated temperature, under increased pressure, and in the presence of suitable catalysts. Under these conditions they are able to take part in a large number of reactions, which are subdivided below into two main groups reactions with retention of or with transformation of the triple bond. For clarity there is further division, in accordance with conventional practice, into the basic reactions of vinylation, ethynylation, carbonylation, and cyclization, although these do not reflect the variety of reaction paths and mechanisms. The cyclization reactions are excluded from the following review since they are dealt with in detail in Section 3.3.8. 

The enthalpy of hydrogenation per triple bond is higher than that of acetylene due to strain induced by inclusion of the triple bonds in a Cg ring. Ahwas measured in isooctane. The datum is not corrected for either Ayap// or Asoynn. 

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