Chapter 13 - Alkenes

Describe an alkene

• Alkenes are unsaturated hydrocarbons

• Contain at least one carbon carbon double bond

• General formula: CnH2n

• Alkenes can be branched have more than one double bond or be cyclic

• Branches alkenes fit the general formula, alkenes with more than one double bond and cyclic alkenes do not fit the general formula

How is a sigma bond formed

This is formed by the head-on overlap of atomic orbitals

How is a pi bond formed

A pi bond is formed by the sideways overlap of two p-orbitals, one from each carbon atom of the double bond

Describe the double bond in an alkene

• Each carbon atom of the double bond uses 3/4 electrons for 3 sigma bonds

• This means one electron is left on each carbon atom not involved in any sigma bonds

• This electron is in a p-orbital

• Each carbon atom contributes one electron to the electron pair in the pi bond

• Pi electron density is concentrated above and below the line joining the nuclei of the bonding atoms

• The pi bond locks the two carbon atoms in position and prevents them from rotating around the double bond (unlike alkanes)

Describe the shape around a double bond

Shape is trigonal planar, bond angle of 120

What is a structural isomer

Same molecular formula

Different structural formula

What is a stereoisomer

Same structural formula

Different arrangement of atoms in space

What are the conditions for E/Z isomerism

• A C=C double bond

• Different groups attached to each carbon of the double bond

How do you determine which is the E and which is the Z isomer

E - Opposite sides

Z - Same side

What is Cis-trans isomerism

Cis-trans isomers need the same conditions as E/Z isomerism as well as:

one of the attached groups on each carbon atom must be the same

Cis isomer - Z - same

Trans isomer - E - opposite

Describe Cahn-Ingold-Prelog rules

• Atoms attached to each carbon in the double bond are given priority based on their atomic number

• Higher atomic number = Higher priority

• If the groups of higher priority are on the same side of the double bond, this is the Z isomer

• If the groups of higher priority are on opposite sides of the double bond, this is the E isomer