Structure and bonding: study guide

Key ideas

Carbon forms four bonds, and the way it mixes its orbitals sets the shape of a molecule. Hybridization, sigma and pi bonds and the degree of unsaturation describe the structure behind a formula.

• sp³ carbon has four sigma bonds and is tetrahedral (109.5°), sp² has a double bond and is trigonal planar (120°), and sp has a triple bond and is linear (180°).
• A single bond is one sigma bond. A double bond is one sigma and one pi bond, and a triple bond is one sigma and two pi bonds.
• The degree of unsaturation, DoU = (2C + 2 + N − H − X) ÷ 2, counts rings plus pi bonds.
• Isomers share a molecular formula but differ in structure. A stereocenter is a carbon with four different groups.

Key equations

• DoU = (2C + 2 + N − H − X) ÷ 2
• single bond = 1σ
• double bond = 1σ + 1π
• triple bond = 1σ + 2π

Worked example

For C₆H₆, DoU = (2 × 6 + 2 − 6) ÷ 2 = 4: one ring and three C=C, as in benzene. For CH₂=CH–C≡CH, count the C–H bonds (4), then the C=C (1σ + 1π), C–C (1σ) and C≡C (1σ + 2π). That gives 7 sigma and 3 pi bonds, with the carbons sp², sp², sp and sp.

Common mistakes

• Counting a double bond as two sigma bonds. It is one sigma and one pi.
• Forgetting to count the C–H bonds when totaling sigma bonds.
• Counting halogens as hydrogens in the DoU formula without the sign being right. Halogens count like hydrogens, oxygen is ignored.

Practice

Test yourself with practice questions on this topic: Structure and bonding