S1-L12: Organic Structure and Bonding Flashcards
Describe S orbital (refer to figure 1)
- spherical
- lower in energy than other orbitals in same shell
Outline what the P orbital is (refer to figure 2)
- dumb bell shape
- nodal plane passes through nucleus
- higher in energy than S orbital
- 3 p orbitals in same shell
How does bonding in organic molecules occur?
- through linear combination of atomic orbitals (LCAO)
- orbitals combine to form hybrid atomic orbitals (sp, sp2, sp3) AND molecular orbitals (sigma, sigma, pi, pi)
Define the following type of orbitals:
1-Hybrid atomic orbital
2-Molecualr orbital
1-combination of atomic orbitals from same atom
2-combination of atomic OR hybrid atomic orbitals from different atoms
-second row of elements (like C/O/N) hybridise using their 2s AND 2p orbitals to form sp, sp2, sp3 HAO’s
What is hybridisation and it’s use? (refer to figure 3)
- C electronic configuration: 1s2, 2s2, 2p2
- ->2 unpaired e-‘s yet C forms 4 bonds
- one of 2s e-‘s can be promoted into vacant p orbital
- ->requires energy input
- ->favourable as decreased e- -e- repulsion
- 2s AND 2p orbital now mix to form various hybrid atomic orbitals
Explain sp^3 hybridisation
- all 4 atomic orbitals combine to give same n. of new hybrid atomic orbitals
- all 4 sp3 orbitals equivalent with same energy (degenerate)
- orbitals point towards corners of tetrahedron
- overlap produces sigma bonds
- used for bonding in saturated compounds
What is sp^2 hybridisation?
- S and two P orbitals combine to give 3 new hybrid atomic orbitals
- 3rd orbital unchanged
How is the positioning of the different parts of this hybridisation significant?
- sp2 orbital point towards corners of triangle
- ->used for sigma bonding
- p orbital perpendicular to them
- ->used for pi bonding
- sp2 hybridisation used for bonding in alkenes/ carbonyls AND aromatic rings
Outline sp hybridisation (figure 6)
- s and one p orbital combine to give two new hybrid atomic orbitals
- other two orbitals unchanged
How are the sp orbitals structured and what is the significance of this? (figure 7)
- sp orbitals at 180 degrees to each other (linear molecule)
- ->used for sigma bonding
- p orbitals perpendicular to them
- ->used for pi bonding
- sp hybridisation used for bonding in alkynes AND nitriles
Outline the properties of hybrid orbitals
- hold e-‘s closer to nucleus
- bond becomes shorter AND so stronger
- more directional SO have more effective bonding interactions
- more s-character in orbital- the lower the energy
- ->increased stability of e- in that orbital
Explain the formation of Sigma bonds and their structure (refer to figure 8)
- sigma bonds formed by direct overlap
- e- density is along line of bond- directly between atoms
- symmetric wrt 180 degrees rotation about bond
- bonds may be formed between s-s/ p-p/ s-p OR hybridised orbitals
- bonding molecular orbital lower in energy than original atomic orbitals
Outline the sigma bonding in methane (figure 9)
- 4 sp3 hybrid orbitals form tetrahedron
- ->add 4 H atoms
- ->combine sp3 AND 1s
- ->each MO orbital same AND has sigma symmetry
Describe sigma bonding in ethane
- each C uses sp3 orbitals to bond to H’s
- remaining sp3 orbital on each C overlaps to form C-C bond
How do pi bonds form?
- formed by “parallel” overlap of p orbitals
- e- density above AND below plane of bond
- ->no e- density directly between bonded atoms
