deuag6aaaaabcoaagnmperteso 觉 ” 8888188 容 aei8yg5ooaemese 11-3 Physical Properties of Alkenes und EesRateb9ne.aahgeowrmtada2epeoevam6he 3 3 Thermal isomerization allows us to measure the strength of the pi bond. Thermal isomerization involves the interconversion of the cis form and the trans form of a double bond at high temperature. During the isomerization process, the π bond between the two carbon atoms is broken and the p orbitals on the two carbon atoms become perpendicular to each other (transition state). The activation energy for this process is roughly the same as the π contribution to the double-bond energy. The measured activation energy for this process is about 65 kcal mol-1. The total energy of the ethene double bond is 173 kcal mol- 1, which means the σ bond energy must be about 108 kcal mol-1. The alkenyl hydrogens are more tightly held in alkenes than the C-H bonds in the corresponding alkanes. As a result, addition to the weaker π bond characterizes the reactivity of alkenes in radical reactions, rather than hydrogen abstraction. 11-3 Physical Properties of Alkenes The boiling points of alkenes are very similar to the corresponding alkanes. The melting points of alkenes are lower than those of the corresponding alkanes. The presence of a trans double bond lowers the melting point slightly, while the presence of a cis double bond lowers the melting point significantly more. The effect of a double bond on melting point is due to the disruption of packing of molecules in the crystal lattice compared to the packing of saturated molecules. Cis double bonds often exhibit weak dipolar character. The degree of s orbital character in a sp2 carbon is larger than in an sp3 carbon (alkane) which makes the sp2 carbon a weak electron withdrawing group. Trans double bonds, on the other hand, generally have little dipolar nature since the dipoles involved oppose each other. The electron-attracting character of the sp2 carbon also accounts for the increased acidity of the alkenyl hydrogen, compared to its saturated counterpart. Ethene is still a very poor source of protons compared to alcohols or carboxylic acids