1. Alkenes AS Chemistry

2. Understand that alkenes and cycloalkenes are unsaturated hydrocarbons. Explain the overlap of adjacent p-orbitals to form a π-bond. Explain the trigonal planar shape around each carbon making up the C=C bond of alkenes. Week 15 © Pearson Education Ltd 2008 This document may have been altered from the original

3. Basic concepts Alkenes are unsaturated hydrocarbons with at least one C=C double bond. Aliphatic alkenes with one double bond have the general formula: C n H 2n Alkenes are more reactive than alkanes Take part in Addition reactions Alkenes can form E/Z including cis-trans isomers

4. Naming of Alkenes Select the longest chain of C atoms containing the double bond – End in ENE Number the chain starting from the end nearer the double bond – Use a number to indicate the first carbon in the double bond Prefix with substituents, side chain positions are based on the number allocated to the first carbon in the double bond

5. Ethene Week 15 © Pearson Education Ltd 2008 This document may have been altered from the original

6. Covalent bonds are formed by overlap of orbitals. An sp 2 orbital from each carbon overlaps to form a single C-C bond. The resulting bond is called a SIGMA (δ) bond. THE STRUCTURE OF ALKENES

7. The two 2p orbitals also overlap to form a second bond. This is known as a PI (π) bond. For maximum overlap and hence the strongest bond, the 2p orbitals are in line. This gives rise to the planar arrangement around C=C bonds. THE STRUCTURE OF ALKENES

8. two sp 2 orbitals overlap to form a sigma bond between the two carbon atoms ORBITAL OVERLAP IN ETHENE - REVIEW two 2p orbitals overlap to form a pi bond between the two carbon atoms s orbitals in hydrogen overlap with the sp 2 orbitals in carbon to form C-H bonds the resulting shape is planar with bond angles of 120º

9. Nature of the double bond The π-bond fixes the carbon atoms in position – Preventing any rotation of the bond. In alkenes, each carbon involved in the double bond – Uses three of its electrons in the formation of 3 σ- bonds – And uses one of its electrons in the formation of a π-bond

10. Shape of an ethene molecule Three regions of electron density surround each carbon atom in the double bond Pairs of electrons repel each other as far apart as possible The electron pairs repel each other to give a trigonal shape Ethene is a planar molecule, all atoms are in the same plane. Week 15 © Pearson Education Ltd 2008 This document may have been altered from the original

11. Cyclohexene Cylic alkenes have closed rings of carbon atoms containing one or more double bonds Most common is cyclohexene Do not follow the same general formula as aliphatic alkenes Week 15 © Pearson Education Ltd 2008 This document may have been altered from the original Sometimes used in exam questions Treat them in the same way as aliphatic alkenes because they react the same way

12. Shapes of alkenes Key words to use – Trigonal – Planar – 120° bond angle – Electron pair replusion

13. Carotene, C 40 H 56 Week 15 © Pearson Education Ltd 2008 This document may have been altered from the original How many double bonds? Aliphatic or cyclic?

14. Class work Complete questions on page 127 Complete Naming Alkenes Worksheet

15. Answers to Questions 1.C 4 H 8 2.A) 2. B) 3. E Z

16. Answers to Questions 4. a) 2-methylbut-2-ene b) hexa-2,4-diene Worksheet