Organic Structures - Organic Chemistry Concepts: An EFL Approach - page 7

The sigma (σ) and pi (π) types of covalent bonds come from the relative direc-
tion of the axes of the overlapping bonding atomic orbitals. A σ bond has direct
overlap along the orbital axis. This gives a bonding orbital that is cylindrically
symmetrical. A π bond results from the less efficient sideways overlap of orbitals
that are in the same plane.
We can estimate the strength of the π bond as about 273 kJ/mol by using the
bond energies of the C-C and C ] C as given in Table 1.2 . Therefore it is much
weaker than σ bond (347 kJ/mol). This fact is important because it explains the
higher reactivity of multiple bonds.
To find the hybrid state of any carbon atom, simply count the number of differ-
ent atoms bonded directly to it. An sp 3 carbon bonds to four other atoms with
single σ bonds. An sp 2 carbon bonds to three other atoms with two single and
one double bond. An sp carbon bonds to only two other atoms with one single
and one triple bond or two double bonds.
1.5 POLAR BONDS AND ELECTRONEGATIVITY
The polarity of a chemical bond shows how the bonding electrons are shared
between the bonded atoms. Figure 1.6 shows the range from the extremes of
ionic, between anions and cations, and perfect covalent, in which identical
atoms or groups share the bonding electrons equally. All situations between
these are examples of polar covalent bonding.
FIGURE 1.6
The bonding range from ionic to covalent. The symbol δ is often used to show a partial/small amount of
charge.
In polar bonds one nucleus attracts the bonding electrons more than the
other. Electronegativity measures the attraction which a bonded atom has for
the bonding electrons. As the electronegativity difference between the bonded
atoms increases, the polar character of the bond between them increases. Fur-
ther details and values are listed in Appendix 1.
In organic chemistry, we talk about the polarity of a bond in terms of the induc-
tive effect (I) . This shows the ability and direction with which an atom or group
of atoms polarizes a covalent bond by donating or withdrawing electron density.