9c Flashcards
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!!
why is the gauche conformation preffered when u have a heteroatoms
bc the molecule is ready for cyclisation!!!
there is an increased rate of cyclisation
theres a good interaction between the filled CH sigma orbital and the C-R antibonding orbital ,, allowing cyclisation to occur faster!!
whats the main bit of the thorpe infold effect // gem dimethyl // gem dialkyl effect
states that intramolecular cyclisation occurs at a faster rate if there are substituents on the backbond of the molecule!!!
aka the more random substituents we add,, the faster cyclisation occurs
why does the thorpe ingold effect work
bc by adding more substituents the bond angle between the main groups,, aka the ones involved in the intramolecular cyclisation becomes smaller,, therefore it enforces the cyclisation ready geometry!!!
thorpe ingold effect: angle betwee special R groups when there are no substituents,, apart from H obvs
larger angle,, the H’s dont repel eachother and therefore dont push together the special R groups.
thorpe ingold effect: angle between special R groups when u have Me as a substituent instead of H’s
u have a smaller angle between the special R groups!!
the methyls repel eachother and kinda push the special R groups closer to one another
when u think of thorpe ingold effect,, what should we think of ,, iteam wise to help us understand it
u would think of scissors!!
and how when random substituents are closer together due to less repulsion,, the other ends,, the special R groups,, get further away!! therefore making the molecule lessss geometrically ready for cyclisation!!
if the substituents are H’sss,, is the anti or gauche conformer preferred
if the H’s are the substituents and u just have the 2 special R groups,,, the anticonformation is preferred!!
the gauche will be the higher energy conformer.
if the substituents are methyl groups,, will the gauche or anti conformation be preferred
if the substituents are methyl groups,, both conformations will be similar in energy
this is bc there will be gauche and anti interactions in both conformation and there will be the same of each in each conformation!!
aka anti and gauche conformers will both have 1 anti and 2 guache interactions,, maybe not of the same type,, but they still have them, which is why their energies might be similar.
describe the graph of the anti and gauche confomers when u have H and Me as substituents
okay so there are 2 peaks and 3 troughs for each!!
the peaks are the energies needed to go from anti to gauche (from more to less favourable) and the energy needed for the cyclisation process (largest hump)
for the Me substituent conformer,, the gauche and anticonformer have simialr energies meaning they have a smaller activation energy to go between the 2,, this means if u have Me as substituents,, u need less energy for the molecule to get in the cyclisation geometry and for cyclisation to occur!!
if u have H’s as substituents,, the anti conformation has less energy than the gauche,, this means energy is needed to for the correct geometry to occur,, and then even more energy is needed for cyclisation to occur.
so basically for the conformer with Me as substituents,, less energy is needed in total for cyclisation to occur.
what is the last step of cyclisation
the bond formation process
bond formation in cyclisation is usually what
its normally sn2
bc it involves reversing the configuration
it also includes attacking from the back: allowing the homo and lumo to overlap and have constructive interference.
what angle do nucs need to attack from
theburgi dunitz trajectory!!
why do nucs need to attack via the burgi dunitz trajectory
to allow bonding interactions via goo orbital overlap
and to prevent the bonding and antibonding interactions from cancelling out!!
it allows sigma bonds to form and pi bonds to break.
why might certain cyclisations reactions not work,, even tho they form a 5 membered ring with amine as a nuc and a carbonyl as the electrophile
bc the bonding and antibonding interactions might cancel eachother out
the non bonding orbital aka the lone pair may be too far from the antibonding orbital for anything to acc happen
whats special about triple bonds and them being the electophile in cyclisation reactions
the fact that they have 2 sets of empty pi * orbitals
that form a cylindrical shape // symmetry meaning its easier for the pi* orbitals to overlap with the non bonding ones.
intermolecular cyclisation example
when cyclisation occured between deuterated and non deuterated molecules!!
there wasnt one fully hydrogen or one fully deuterium,, it was a mix!!
baldwins ring closure rules: what does the n stand for
what is
tet
trig
dig used for
whats exo and endo used for
n = the number of members in the cyclic product ring!!
tet = single bond
trig = double bond
dig = triple bond
endo = u attack the furthest thing,, aka the end of the ring
u form a bridge type thing
exo = u attack the 2nd last thing in the ring!!
order of the
n
exo / endo
tet, trig, dig thing
n - exo/endo - tet/trig/dig etc
