1. HYBRIDIZATION SIGMA AND PI BONDS
L.O. understand the formation of sigma and pi bonds and know their occurrence and the implication for molecular shapes
The concept of mixing atomic orbitals into new hybrid orbitals (with different energies and shapes

2. Methane (CH4)

3. New Shape for New Orbitals
109.5°
sp3 hydrid orbital

4. Ethane (C2H6)
109.5°

5. Check Point (b) How many  bonds are there in a molecule of
(i) propane?
(ii) butane?
(iii) methylpropane?
Answer
(b) (i) 10  bonds
(ii) 13  bonds
(iii) 13  bonds

6. Alkenes Contain Carbon-Carbon Double bonds
Ethene
1  bond

7. sp2 Hybridization
Unhybridized p

9. Example
The bond enthalpy of the carbon-carbon double bond in ethene is +612 kJ mol–1 whilst the bond enthalpy of the carbon-carbon single bond in ethane is +348 kJ mol–1. Explain briefly why the bond enthalpy of the carbon-carbon double bond is less than twice that of the carbon-carbon single bond.
Solution:
A carbon-carbon double bond is a  bond and a  bond. The  bond is formed by the head-on overlap of orbitals; distributed along the internuclear axis. The  bond is a strong bond and has a high bond enthalpy.
On the other hand, a  bond is formed by the side-way overlap of orbitals, it is less effective than the head-on overlap of orbitals, a  bond is weaker than a  bond.
Hence, the bond enthalpy is less than twice that of the carbon-carbon single bond (i.e. a  bond)
Answer

10. Check Point
(a) State the difference between the  bond and the  bond in the carbon-carbon double bond.
Answers

11. Restricted to rotation
Differences between the  bond and the  bond in the carbon-carbon double bond:
 bond
 bond
Head-on overlap of the sp2 hydridized orbitals of two carbon atoms
Side-way overlap of the vacant p orbitals of two carbon atoms
The bonding electrons in are localized symmetrically along the internuclear axis
The electrons in the  bond appear as two lobes, one above and one below the internuclear axis
Stronger
Weaker
Free to rotate
Restricted to rotation

12. Check Point (b) How many  and  bonds are present in a molecule of
(i) propene?
(ii) but-1-ene?
(iii) but-2-ene?
Answer
(b) (i) 8  bonds and 1  bond
(ii) 11  bonds and 1  bond
(iii) 11  bonds and 1  bond

13. Alkynes Contain Carbon-Carbon Triple Bonds
Ethyne
1  bond
1  bond

14. 2 Unhybridized p for pi bonds
In 
sp Hybridization
2 Unhybridized p for pi bonds

15. Check Point (a) How many  and  bonds are present in a molecule of
(i) propyne?
(ii) but-1-yne?
(iii) but-2-yne?
Answer
(b) (i) 6  bonds and 2  bonds
(ii) 9  bonds and 2  bonds
(iii) 9  bonds and 2  bonds

16. Check Point (c) State the bond angles indicated in the compound below:
Answer
x: 109.5°
y: 120°
z: 180°

17. Check Point (c) State the bond angles indicated in the compound below:
Answer
(c) x: 109.5°
y: 120°
z: 180°