week 5

Question 1

comparing hybridisation and MO theory models

Answer 1

see w5 vid24-25 for comparisons of the models

Question 2

is metallic bonding directional or non-directional

Answer 2

non-directional

Question 3

in metallic bonding, what are bands

how many electrons fit in a given band

Answer 3

• when valence orbitals interact to form molecular orbitals that are spread across the whole molecule
• a band is made from a degenerate set of orbitals with very small gaps between them
• bands extend across the whole structure

eg 2s orbital forms a band, 2p orbitals are degenerate so form a larger band

number of electrons:
- a band made of N number of orbitals can hold 2N number of electrons

Question 4

what is the fermi level

Answer 4

the top of where the band is filled

Question 5

describe using bands why metals conduct electricity in 2 ways

Answer 5

1. there are very small energy gaps between MOs in bands so when voltage is applied, electrons can move from filled to empty MOs without much energy needed
2. if the energy gap between 2 energy levels (eg 2s and 2p) is small enough, there can be overlap and electrons can conduct electricity over the bands

Question 6

how do bands behave in an insulator

Answer 6

if a band is full and there’s a large band gap to the next band, electrons won’t be able to conduct electricity

Question 7

how can bands behave in a semi-conductor

Answer 7

the band gap will be smaller than that of an insulator

conduction is possible if electrons are excited enough

Question 8

reaction of alkene with bromine

what is step 1 in terms of orbital interaction

Answer 8

interaction between alkene π-orbital and bromine between:
- π-bonding MO in alkene is the HOMO
- σu* MO in Br2 is the LUMO
electrons are donated from the HOMO to the LUMO

Question 9

reaction of alkene with bromine

describe the intermediate step in terms of orbitals and bonding

Answer 9

bromonium ion intermediate is formed

now the π-bond is broken, each C has an unhybridised pz orbital and these two have an in phase and out of phase combination

in-phase combination = adjacent lobes are same phase
- this combination interacts with the 4pz orbital of Br

out of phase combination = adjacent lobes are opposite phase
- this interacts with the 4px orbital of Br

energy gap between the two pz orbitals and the Br 4px orbital is greater because the overlap is better

6 electrons in total

• 1 from each C
• 4 from Br

Question 10

reaction of alkene with bromine

what is step 2 in terms of orbital interactions

Answer 10

the bromide ion Br- is added to the molecule

bromide ion has a full set of valence orbitals

• donates electron density to the LUMO of the bromonium cation
• LUMO = anti-bonding MO from overlap between 4pz and carbon pz orbitals (see w5 vid27 for diagram)
• bromide attacks LUMO on the carbon side of the intermediate
• reduces bond order of bromonium ion
• C-Br bonds are formed