13.1 Properties of the Alkenes & 13.2 Stereoisomerism

Question 1

What are alkenes?

Answer 1

• Unsaturated hydrocarbons

Question 2

What does it mean for a molecule to be unsaturated?

Answer 2

• It contains at least one carbon-carbon multiple bond

Question 3

What general formula do alkenes have, and in which situations does it apply?

Answer 3

• CnH2n
• It applies for straight chain alkenes that contain only 1 double bond

Question 4

How is a carbon atom in a double bond in an alkene bonded to the other atoms around it?

Answer 4

• It has 3 sigma bonds (including 1 with the other carbon atom)
• It has 1 π-bond with the other carbon atom

Question 5

How is a π-bond formed?

Answer 5

• 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

Question 6

What impact do π-bonds have on the geometry of alkenes?

Answer 6

• 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

Question 7

What are stereoisomers? Required.

Answer 7

• Compounds with the same structural formula but with a different arrangement in space

Question 8

What is E/Z isomerism? Required.

Answer 8

• 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

Question 9

How can you tell that an alkene doesn’t have E/Z isomerism?

Answer 9

• It has the same 2 groups on (at least) one carbon atom involved in the double bond

Question 10

What is cis-trans isomerism? Required.

Answer 10

• A type of E/Z isomerism where there are 2 non-hydrogen groups and 2 hydrogen atoms around the carbon-carbon double bond

Question 11

What is a cis isomer?

Answer 11

• It has hydrogen atoms on the same side of each carbon atom
• It is the Z isomer

Question 12

What is a trans isomer?

Answer 12

• It has hydrogen atoms on the different sides of each carbon atom
• It is the E isomer

Question 13

What are the CIP priority rules?

Answer 13

• The Cahn-Ingold-Prelog rules determine which isomer is the E isomer and which isomer is the Z isomer when the molecules do not fit the criteria for cis-trans isomerism

Question 14

What is a Z-isomer?

Answer 14

• A compound where the higher priority groups are on the same side of the double bond

Question 15

What is an E-isomer?

Answer 15

• A compound where the higher priority groups are on the different sides of the double bond

Question 16

How do you decide which group is higher priority?

Answer 16

• 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.)