Do Now What is the definition of a structural isomer? What are the three structural isomers you know? Give an example for each

Isomers Alkenes with more than 3C can form different types of isomer All end in –ene Chain isomers Position isomers Geometrical isomers

1. Chain Isomers (Branching Isomers) The hydrocarbon chain is arranged differently: pent-1-ene and 2-methylbut-1-ene EXT 1: Draw hex-1-ene and 3-methylpent-1-ene EXT2: Draw and name your own alkene chain isomers

2. Position Isomers Isomers with the double bond in a different position: but-2-ene and but-1-ene EXT 1: Draw hex-3-ene and hex-2-ene EXT2: Draw and name your own alkene position isomers The longer the chain, the more possibilities there will be, so greater number of isomers

p-orbitals of unbonded C atoms containing one electron each. C 1s2 2s2 2p2 Bonding in Ethene ↑↓ ↑↑ Pi bond prevents free rotation about the C-C bond 2C and 4H all in same plane sigma (σ) bonds p-orbitals overlap, above and below the CH plane to form Pi (∏) bond C-C sigma bond + C-C Pi bond = C=C double bond

3. Geometric (E-Z) Isomerism E-Z isomerism tells us about the position of substituents at either side of a carbon-carbon double bond. Two substituents may either be on the same side of the bond (Z) or on opposite sides (E). Substituents groups joined by a single bond can rotate around the single bond, so there are no isomers but there is no rotation around a double bond. Z- and E- isomers are separate compounds and are not easily converted from one to the other.

Z-but-2-ene and E- but-2-ene Geometrical Isomers Form of stereoisomerism: Same structural formula but the bonds are arranged differently in space Occurs ONLY around the C=C double bond Z-but-2-ene and E- but-2-ene EXT 1: Draw Z-hex-3-ene and E-hex-3-ene EXT2: Draw and name your own alkene geometrical isomers Often called E-Z isoomerism Cis and trans in old language!

Why don’t we use cis and trans? Cis (Z): cis-but-2-ene and Z-but-2-ene Trans (E): trans-but-2-ene and E-but-2-ene Cis-trans DOES NOT WORK when more than 2 different substituents around a double bond ... E-Z notation based on atomic numbers: Look at the atoms attached to each of the carbons in the double bond When to 2 atoms of each pair of HIGHER atomic number are on the SAME side of the C=C ... Z isomer When to 2 atoms of each pair of HIGHER atomic number are on the SAME side of the C=C ... E isomer Give examples p 200 of textbook

Examples of Geometric Isomerism

More examples of Geometric Isomerism

Are these geometric isomers?

1-bromo-2-chloro-1-fluoroethene

Task Draw the following compounds and decide whether they show E-Z isomerism: methylpropene 1-chloropropene 2-methylpent-2-ene 2-ethylpent-1-ene 2-bromo-3-chlorobut-2-ene

NO methylpropene YES E Z 1-chloropropene

NO 2-methylpent-2-ene NO 2-ethylpent-1-ene

YES Z E 2-bromo-3-chlorobut-2-ene