Chapter 13: Alkenes
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What does it mean for a molecule to be unsaturated?
• It contains at least one carbon-carbon multiple bond
What general formula do alkenes have, and in which situations does it apply?
• CnH2n • It applies for straight chain alkenes that contain only 1 double bond
How is a carbon atom in a double bond in an alkene bonded to the other atoms around it?
• It has 3 sigma bonds (including 1 with the other carbon atom in the double bond) • It has 1 π-bond with the other carbon atom
How is a π-bond formed?
• There is a sideways overlap of p orbitals (as the electrons involved in the bond are in p orbitals) • Each of the 2 carbon atoms contribute 1 electron to the bond • The electron density is concentrated above and below the line joining the nuclei of the bonded atoms
What impact do π-bonds have on the geometry of alkenes?
• They lock the carbon atoms into place (as electron density is above/below the line joining the carbon atoms together) • They therefore can’t rotate around the bond
What are stereoisomers? Required.
• Compounds with the same structural formula but with a different arrangement in space
What is E/Z isomerism? Required.
• A type of stereoisomerism in which different groups attached to each carbon of a C=C double bond may be arranged differently in space because of the restricted rotation of the C=C bond
How can you tell that an alkene doesn’t have E/Z isomerism?
• It has the same 2 groups on (at least) one carbon atom involved in the double bond
What is cis-trans isomerism? Required.
• A type of E/Z isomerism where there are 2 non-hydrogen groups and 2 hydrogen atoms around the carbon-carbon double bond
What is a cis isomer?
• It has hydrogen atoms on the same side of each carbon atom • It is the Z isomer
What is a trans isomer?
• It has hydrogen atoms on the different sides of each carbon atom • It is the E isomer
What are the CIP priority rules?
• The Cahn-Ingold-Prelog rules determine which isomer is the E isomer and which isomer is the Z isomer when molecules do not fit the criteria for cis-trans isomerism
How do you decide which group is higher priority?
• Following on from the bonds off of the carbon involved in the double bond, you compare the first atoms that are different when you compare the 2 groups • The atom with the higher atomic NUMBER makes its group the higher priority group • (Hydrogen always has the lowest atomic number, which is why cis-trans isomerism works)
What is a Z-isomer?
• A compound where the higher priority groups are on the same side of the double bond
What is an E-isomer?
• A compound where the higher priority groups are on the different sides of the double bond