Video Transcript
In a molecule of ethene, all five
of the 𝜎 bonds exist in the same plane. What region of space does the 𝜋
bond occupy in relation to the central 𝜎 bond in a molecule of ethene? (A) Surrounding the 𝜎 bond in a
tube, (B) above and below the 𝜎 bond, (C) around the 𝜎 bond lengthways, (D) above
the 𝜎 bond, or (E) parallel to the 𝜎 bond on the same plane.
Sigma bonds are covalent bonds that
are formed through the head-on or end-to-end overlap of different types of
hybridized or unhybridized bonding orbitals. The question mentions the presence
of five 𝜎 bonds in ethene. The five 𝜎 bonds present in each
molecule of ethene are shown by the black lines, wedges, and dashes. The definition of 𝜎 bonds mentions
hybridized orbitals. Orbital hybridization is the
concept of mixing atomic orbitals into new and different types of hybrid
orbitals.
The question also mentions 𝜋
bonds. A 𝜋 bond is a type of covalent
bond that is formed through the sideways overlap of two adjacent p-subshell
orbitals. There is a 𝜋 bond between the two
carbon atoms in ethene. We need to figure out which region
of space shown in blue the 𝜋 bond occupies in ethene. Before we do this, let’s look at
the 𝜎 bonds in ethene. To do this, first of all, we need
to clear a little space.
Carbon has four outer shell or
valence electrons. Two of them are in a 2s orbital and
the other two are in two of the three 2p orbitals. One of the 2s electrons can be
promoted into the 2pz orbital. Then, the 2s and 2p orbitals can
hybridize. Orbitals can hybridize in different
ways, but in the case of ethene, the s orbital and two of the p orbitals will
hybridize, forming sp2 hybridized orbitals, but the pz orbital will remain
unhybridized. Each of the two carbon atoms in
ethene will have three sp2 orbitals. Two of these orbitals will overlap,
forming a carbon-carbon 𝜎 bond. The other four sp2 orbitals in
ethene overlap with 1s orbitals of hydrogen, forming four more 𝜎 bonds.
So, we now know how the five 𝜎
bonds in ethene are formed. They are formed from the overlap of
two sp2 orbitals or an sp2 orbital and a 1s orbital. We’ve now considered all of the sp2
orbitals, so we need to consider the 2pz orbitals. All of the 𝜎 bonds in ethene are
in the same plane, and carbon pz orbitals lie above and below this plane. It is the interaction or sideways
overlap of these two adjacent p orbitals that causes the formation of the 𝜋
bond. So, the region of space that the 𝜋
bond occupies is above and below the carbon-carbon 𝜎 bond.
The drawing shown is most similar
to the drawing shown in option (B). So, the answer to the question
“What region of space does the 𝜋 bond occupy in relation to the central 𝜎 bond in
a molecule of ethene?” is (B) above and below the 𝜎 bond.
