Download Structure and Bonding in Ethene: The Pi Bond

11-2 Structure and Bonding in Ethene: The Pi Bond
The double bond consists of sigma and pi components.
Ethene is planar. It contains two trigonal carbon atoms having bond angles close
to 120o.
The hybridization of the carbon atoms is best described as sp2.
The three sp2 orbital on each carbon form  bonds to two hydrogen atoms and to
the other carbon atom.
The remaining unhybridized p orbital on each carbon overlap to form a  bond.
The electron density of the  bond is equally distributed above and below the
plane of the molecule.
The relative energies of the bonding and antibonding  and  orbitals can be
summarized:
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  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  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  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  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.