Hydrocarbon Molecular & Electron Geometry

Alkanes

• Alkanes only have $single bonds$ in the molecule
  • the carbons are $all fully bonded to 4 other atoms$
• All the carbons have $tetrahedral molecular and electron geometry$ and are $sp3 hybridized$.
• These atoms have $freedom of rotation$ since their bonds are all sigma bonds.

Example: Butane

• All the carbons are bonded to 4 other atoms as you can see above.
• The $bond angle$ between each atom is $109.5 degrees$.

Cyclohexane

• Cyclohexane is special because it has different conformations with varying stability.
• The $chair conformation$ is the $most stable$ of the many conformations it has.
• The ones shown below are the chair and boat.

Alkenes

• Alkenes have a $double bond somewhere$ in their parent chain.
• The carbons not involved in the double bond would be similar to carbons in an alkane, so we won’t focus on those.
• Let’s just focus on the double bond using ethene as an example.

• There are $only 3 electron domains$ so the $electron geometry is trigonal planar$.
• There are $no lone pairs$ so the $molecular geometry$ is also $trigonal planar$.
• These carbons are $sp2 hybridized$ and have a $bond angle of 120 degrees$ between the 2 hydrogens.

Benzene

• Benzene is also a $planar hexagonal$ molecule.
• All the carbons are $sp2 hybridized$.
• The carbon atoms’ $molecular and electron geometry is trigonal planar$.

Alkynes

• The carbons not in the triple bond would have the same characteristics as alkanes.
• If there is a double bond along with a triple bond in the molecule, the carbons in the double bond would have the same characteristics as those in an alkene.
• Look at ethyne, the most basic alkyne:

• There are only $2 electron domains$, so the $electron geometry of the carbons is linear$.
• There’s only one atom bonded to each carbon, so the $molecular geometry is also linear$.
• The bond angle between carbon and hydrogen is $180 degrees$.
• The carbons are all $sp hybridized$.