organic bonding - electron effects Flashcards
(21 cards)
what are the 3 limitations of kekules model of benzene?
- benzene is less reactive than model predicts
- has equal bond lengths - experimentally measured
- less stable than expected compared to cyclohexene
what does a structure like benzene - with multiple alternating C=C - imply about its physical geometry? + why is this strange?
combining different pairs of p-orbitals puts double bonds in different positions
e.g. could be C-C=C-C=C or C=C-C=C-C
now for a symmetrical molecule like benzene this isn’t a problem as the 2 are equal, but for an asymmetrical derivative (e.g.1-bromo-2-chloro-benzene) these 2 forms would be different but there is still only 1 form of this molecule - this is what is strange
explain why benzene compounds don’t all have 2 stereoisomers with opposite double/single bonds
rather than π electrons being localised to each C=C bond, they are spread out/delocalised across the whole ring
why is the delocalised model of benzene more accurate?
it accounts for the 3 limitations of kekules model + lack of stereoisomers
how can delocalisation of benzene be demonstrated with kekules model?
through resonance structures, the real structure is a hybrid of the 2 resonance ones
conjugation definition
p-orbital overlap creating a π system that allows delocalisation of electrons across a molecule - possible in structures that have alternating single and double bonds, or an uninterrupted chain of p-orbitals/lone pairs - this is a stabilising effect
why is conjugation a stabilising effect?
this is because then the 2 double bond systems interact this way the overall energy is lower than 2 separate alkenes
how can you tell which in/out-of-phase combinations of 2 π systems/bonds are higher or lower in energy?
more nodes between p-orbitals/more out-of-phase interactions = higher in energy
what is the effect of conjugation on the HOMO and LUMO?
conjugation lowers the HOMO-LUMO energy gap (this can be seen on MO diagrams), this means it is much easier to promote an electron from HOMO -> LUMO in a conjugated system
- this effect can be multiplied - the more conjugated a compound is the smaller the HOMO-LUMO energy gap
why does conjugation change a molecules interaction with light?
this is because conjugation lowers the HOMO-LUMO energy gap, which changes the particular wavelength of light needed to promote an electron, which means that the colour seen/wavelengths of light absorbed/transmitted changes
for a conjugated system, how is net bonding calculated?
= p-orbital interactions - no. nodes
when conjugation is possible in a molecule that also has a lone pair, what type of hybridisation will be adopted?
sp/sp2 - not sp3
although lone pairs are usually more stable in sp3 orbitals due to repulsion, conjugation requires a p-orbital, and as it is so stabilising the lone pair will reside in a p-orbital
what does ɸ mean in organic bonding?
represents an MO
resonance definition
the idea that a true structure (resonance hybrid) is an average of 2 or more hypothetic localised structures (resonance forms) - these are weighted according to their contribution /thermodynamic stability
give 4 rules for resonance structures
- minimise the no. of electron deficient atoms with <8 valence electrons - not significant contributors to the true structure
- minimise the no. of formally charged atoms - less significant contributors to the true structure
- if formal charges are unavoidable minimise separation for different charges and maximise separation for like charges
- place any -ve charge on the most electronegative atom and any +ve charge on the most electropositive
what is mesomerism
resonance - they’re the same thing
what are resonance/mesomeric effects in π systems?
electron donation/withdrawal effects through a π-bonded network
inductive effects definition
electron donation/withdrawal effects operating in σ-bonded networks
EDG definition
an electron donating group/+M group/π electron donor which donate electron density into systems
- often reveals -ve character in terminal C/other C atoms that heavily influence reactivity
EWG definition
an electron withdrawing group/-M group/π electron acceptor which withdraws electron density from systems - often reveals +ve character in terminal C/other C atoms that heavily influence reactivity
how do EDGs/EWGs influence reactivity of aromatics?
EDGs increase reactivity and are 2,4-directing (δ- positions)
EWGs decrease reactivity and are 3-directing (δ+ positions)
- directing effects can be proven through resonance structures
