Bonding in Alcohols

Names and Lewis Structures

We name alcohols as alkanols. Find the longest carbon chain that includes the OH group. The numbering of carbon atoms on the chain should put the OH group at the lowest number possible.

Number of Carbons Alkane Alcohol
1 methane methanol
2 ethane ethanol
3 propane propanol
4 butane butanol
5 pentane pentanol
6 hexane hexanol

In the line drawings above, the end of every line is a carbon atom unless otherwise assigned. Every carbon atom makes for bonds. Any bond that is not specified is to a hydrogen atom.

Electron Bookkeeping

The total electron count is the sum of the number of non-bonding electrons and bonding electron pairs around every atom. The bonding pairs are counted twice, once for each atom in a bond.

Each atom can have a maximum that is the same as the nearest noble gas. For hydrogen, the maximum electron count is 2. For carbon and oxygen, it is 8.

To calculate the formal charge, take the sum of the non-bonding electrons and 1/2 of the bonding electrons. Compare this number with the valence electron count around the atom.

For carbon, the valence electron count is 4, so 5 electrons around a carbon gives it a charge of -1 and 4 electrons around a carbon gives it a charge of +1. All atoms in this molecule are uncharged.

For oxidation state, break the bonds and give all bonding electrons to the more electronegative of the pair. Compare the sum of the electrons around an atom with its valence number. Use Roman numerals for oxidation state to differentiate from formal charge.

Molecular Orbital Diagrams

We'll use hybrid molecular orbital diagrams for these molecules because they have more than 2 atoms.

The first step is to decide on the hybridization of each atom. In this molecule, every carbon is 2sp3 hybridized, oxygen is also 2sp3 hybridized, and hydrogen uses it 1s orbital.There are a total of 12 + 20 + 4 = 36 atomic and hybrid orbitals.

Now use the atomic and hybrid orbitals to make 36 molecular orbitals with the same average energy as the orbitals they were made from:
  1. The Lewis structure shows that there are 4 C-C bonds so the MO diagram will have 4 sigma bonding and 4 sigma antibonding orbitals. (8 molecular orbitals)

  2. The 1 C-O bond leads to 1 sigma bonding MO and 1 sigma antibonding MO.(2 molecular orbitals)

  3. The 11 C-H bonds in the Lewis structure leads to 11 sigma bonding MOs and 11 sigma antibonding MOs. (22 molecular orbitals)

  4. The O-H bond gives 1 sigma bonding MO and 1 sigma antibonding MO. (2 molecular orbitals)

  5. Finally, there are 2 2sp3 hybrid orbitals on oxygen that are not used for bonding. These become non-bonding molecular orbitals. (2 molecular orbitals)

The total number of electrons for the diagram is the sum of the valence electrons of C, H, and O: 5(4) + 12(1) + (6) = 38 electrons

Professor Patricia Shapley, University of Illinois, 2011