9c Flashcards
(42 cards)
stereoelectronic effect brief definition
how the orientation of an orbital effects the molecules properties
if the basicity of 2 things is the same,, what can make one more reactive than the other
the orientation of their lone pair.
is one of them is more exposed and in a more open region ,, it will be more nucleophilic and therefore reactive
the more positive the electrochemical potential,, the what
the easier it is to oxidise
aka the more electron rich it issss
what must overlap with an aromatic ring to make it. more e- rich
if theres a lone pair in an orbital that overlaps with the p orbitals of the aromatic!!
whats the anomeric effect
electronegative groups like being axial rather than equitorial!!!
aka in the cyclohexane chairs,, they will prefer to be in the axial position when theyre close to an oxygen etc
what causes the anomeric effect to occur
stabilisation of the groups due to the neighbouring non bonding e- !!
aka the lone pair of a group near it ,, stabilises it.
big groups in cyclohexane chairs normally like being where
they normally like being equitoral
to avoid 1-3 diaxial interactions
if oxygen is one of the vertices on cyclohexane,, and theres a big electronegative group on the C next to it,, where would the group like to be positioned
it would like to be positioned axially
as the molecule will be stabilised by the e- in the oxygens lone pair (in the non bonding orbital) overlap with the C-R antibonding orbital!! this stabilises the molecule
its called the anomeric effect // gauche interaction
does the gauche interaction // anomeric effect only occur in cyclic structures
nopeee
they occcur in aliphatic structures too!!!
when u draw the newman projection u want the 2 groups to be gauche ,, this allows the lone pairs of the oxygen to overlap with the C - R antibonding orbital
allowing stabilisation
this is why some molecules can be a bit bent,, to allow that constructive overlap to occur!!!
when we say theres good overlap,, what do we mean by that,, aka in what way can we see if orbitals have constructive overlap or not
if the orbitals are parallel to one another!!!!!
not perpencdicular but parallelll!!!
whats an ether
carbonyl - o - c
is s- trans or s- cis ether preferred
the s-trans ether is preffered ,, this is where it looks like its a zigzag just going straight across!!
aka how u normally draw an aliphatic molecule.
why is the s-trans ether preferred
bc it allows for constructive overlap between the ocygens lone pair in the non bonding orbital and the C-O sigma * antibonding orbital ,, stabilisign the structure
hyperconjugation can occur due to e- from lone pairs orrrr
e- from CH sigma bonds
to corresponding antibonding orbitals
okay so lets say we have 1,2 - di fluoro ethane ,, what does the gauche configuration look like and why is this one favoured
u would draw the newman projection and have the F’s be gauche,, 90* apart bc theyre the bigger groups and the groups we focus on
then when these are in the gauche interactio nwe can draw out the molecule with that little twist in it.
then look at the CF sigma** orbital and the CH sigma orbital and see if theyre parallel.
they usually are and this tells us that hyperconjugation is used to stabilise the moelcule as e- from the CH go to the CF sigma**.
stabilisation occurs due to the gauche effect!!
what molecules normally have ring strain
the small membered rings
bc the bond angles in the ring are much smaller than their normal hybridisation bond angles
so theyre strained by having to stay in that shape.
this makes them more unstable and reactive. think of them about to burst open
how does ring strain affect the formation of a ring that size
the larger the ring strain ,, the more energy is needed to form a ring that size
the smaller the relative rate value,, the what in terms of formation time
the smaller the relative rate of formation,, the slower the ring takes to form.
meaning its formation is probably not favoured.
for saturated rings,, the 5 membered ring has a larger relative rate of formation value,, what does this mean
for saturated moelcules,, 5 membered rings form the fastest.
this contradicts the rings strain argument as 5 membered rings have a larger ring strain value that 6 membered ones.
thermodynamics and ring formation: what is the delta H and delta S values of 3-4 and 7-8 membered rings
3-4 have a large delta H bc their formation is an endothermic process bc its not favoured due to ring strain.
their delta S is negative,, bc theyre a small and not so floppy molecule meaning only a bunch of conformers can be made.
7-8
they have a small delta H bc they have less ring strain and therefore its easier to form them,, their formation requires less energy input.
their negative delta S is large!!! aka mor negative aka less disordered. bc theyre more floppy meaning its harder to get the exact conformer.
why are 5 membered saturated rings easier to form that 6 membered rings
bc 5 membered rings do have ring strain!! however their change in S, entropy, when theyre made is smaller than the entropy penalty u pay making a 6 membered ring.
normally,, is gauche or the anticonformation preferred
normally ,, when u have normal hydrocarbon molecules,, the anticonformation is preffered and it reduces torsial and steric strain
whats ban about the antconformation
its bad for the cyclisation process,, bc the orbitals are too far from eachother.
when u have heteroatoms in a chain,, is the gauche or anti conformation preferred
if there are heteroatoms,, the gauche conformation is preferred!!
