General Flashcards
What does chemical shift tell you?
It tell you what the protons are attached to
What is the chemical shift for aromatic compounds and why is it this?
6.5-8.5ppm shows aromatic protons
This tells you about shielding and deshielding
The chemical shifts is higher as aromatics can have more deshielded protons due to the double bonds
What does integration tell you?
Integration tells you how many protons a signal is worth and tells you about the symmetry of the molecules
What is multiplicity?
Multiplicity is the shape of the signal
E.g singlet, doublet, triplet and quartet
Multiplicity is due to 3 bond coupling
Equivalent protons do not couple to each other
Why do aromatic protons occur at such a downfield shift?
This due to ring current due to the delocalisation of electrons in the pi system
What happens when you apply a magnetic field to an aromatic?
This sets up a ring current in pi system (above&below)
Ring current induces a local magnetic field
Local magnetic field opposes Bo at centre of ring
Local magnetic field reinforces Bo outside of the ring
This means protons outside the ring and more strongly deshielded
What are aromatic compounds?
These can have many different structures
All are cyclic and have delocalised pi electrons which can delocalised all the way round the ring
Describe the bonding in benzene?
The 6 carbons are linked in a planar hexagonal- unsaturated
Each carbon is sp2 hybridised that link to form a planar ring which forms sigma framework
Bonds equal 120
Each carbon has an empty unhybridised p orbital which overlap to produce two continuous ring of pi electrons above and below the ring
Why does benzene not match Keluke?
- all bonds in benzene are the same length
- only reacts in substitution so stability of ring is maintained
- benzene is more stable as electrons are delocalised rather than being localised in 3 pi bonds
Why is there no similarity in reactions of benzene and alkenes?
Alkenes undergo electrophilic addition of bromine but benzene does not
This is because pi systems is more stable
Aromatic compounds want to keep their pi systems whereas alkenes want to lose theirs
How can the stability of benzene be measured?
Hydrogenation reaction and measure the enthalpy changes
Benzene is far more stable than k
Resonance energy of stabilisation is 152KJmol-1
The theoretical structure gives out more heat which shows the reaction is more exothermic and so the reactant is more reactive
Outline the MO diagram for pi bonding in an alkene
Each carbon contributes one p orbital and one electron = 2 AOs
This means there are 2 molecular orbitals
Both electrons are held in the pi bonding MO which is lower in energy
Outline the MO diagram for benzene
Benzene contributes 6 p orbitals and 6 electrons = 6AO and so 6MOs
3 pi bonding MO
3 pi antibonding MO
All bonding are filled - why benzene is so stable
What is Huckle’s rule?
Determines aromatic structures
1) must be cyclic
3) uninterrupted ring of pi electrons/ ring of adjacent p orbitals
3) planar
4) 4n+2 pi electrons
What are some addition points to huckle’s rule?
If ring has nitrogen, nitrogen lone pair is not needed
Lone pairs from heteroatoms are involved in aromaticity
Small rings and charges are allowed
Metals are allowed
