The document discusses bonding in hydrocarbons, specifically alkanes, alkenes, and alkynes, highlighting the hybridization of carbon atoms and the types of bonds formed. It explains how ethylene (C2H4) features a carbon-to-carbon double bond with sp2 hybridization, resulting in a planar structure, while acetylene (C2H2) has a linear arrangement due to its triple bond. The strength and nature of sigma (σ) and pi (π) bonds are also compared, noting that σ bonds are stronger due to more extensive overlap.

1. Bonding in Alkanes, Alkenes and Alkynes
Dr. K. Shahzad Baig
Memorial University of Newfoundland (MUN)
Canada
Petrucci, et al. 2011. General Chemistry: Principles and Modern Applications. Pearson Canada Inc., Toronto, Ontario.
Tro, N.J. 2010. Principles of Chemistry. : a molecular approach. Pearson Education, Inc.

2. Multiple Covalent Bonds
Ethylene has a carbon-to-carbon double bond in its Lewis structure
Ethylene is a planar molecule with 120o H – C – H, H – C – C and bond
angles
The hybridization scheme that produces a set of hybrid orbitals with a trigonal-planar
orientation is sp2.
The (sp2 + p) orbital set

3. The purple
orbitals are sp2
hybrid orbitals;
the red and blue
orbitals are 2p ,
with the colors
indicating their
phase.
The sp2 hybrid orbitals overlap along the line joining the bonded atoms a σ-bond.
The orbitals overlap in a side-to-side fashion and form a π-bond.
Notice that the phase of the p-orbitals is retained.

4. Bond type # σ
bonds
# π
bonds
Single (C-H) 1 0
Double (C=C) 1 1
Triple (C≡C) 1 2

5. Bonding
the shape of a molecule is determined only by the σ-orbitals forming bonds.
the terminal H-atoms can be easily twist or rotate about the s-bonds that join them to
a C atom.
To twist one –CH2 group out of the plane of the other, however, would reduce the
amount of overlap of the p orbitals and weaken the bond. The double bond is rigid,
and the molecule is planar.
Additionally, in carbon-to-carbon multiple bonds, the
σ-bond involves more extensive overlap than does the
π=bond. As a result, a carbon-to-carbon double bond
(σ + π) is stronger than a single bond (π) but not twice
as strong.
C - C 347 kJ/mol
C = C 611 kJ/mol
C Ξ C 837 kJ/mol

6. Bonding in C2H2
Bonding in acetylene, C2H2, is somewhat similar to that C2H4,
The Lewis structure of features a triple covalent bond, H – C Ξ C – H.
The molecule is linear,
In the triple bond in C2H2 one of the carbon-to-carbon bonds is a σ-bond and two are π-
bonds