lecture 4 Flashcards
is C more stable in the singlet or triplet state and why
triplet fragment
(energy change from going to singlet to triplet is negative : exothermic)
energy to pair electrons up in the same orbital is higher than the energy needed to promote the e- to a higher energy p orbital (higher bc not hybrid and no s character)
why are C=C bonds planar
they form strong multiple bonds
fragments have triplet ground states or small singlet-triplet energies.
which state does Sn like and why
Sn likes being in the singlet state,, where e- is paired in the hybrid orbital (meaning energy needed to go from S -> T is positive,, bc its unflavoured and will require energy to do so)
energy required to pair e- in the same orbital is lower than the energy needed to promote the e- to the p orbital
bc the energ ygap between sp2 and p is larger! and due to interelectron repulsion.
what is interelectron repulsion
when e- repel eachother in the same orbital // others
and the inert pair effect
if a singlet is preferred,, will the delta E s->t be positive or negative
it would be positive bc ur changing from smt the compound likes and is stable in,, to smt it doesnt like and smt its not stable with.
what is a genuine triple bond made up of
1 sigma bond
2 pi bonds that are degenerate
do lots of compounds have the ability to have a genuine triple bond
nope!! only a few classes of compounds can acc form a genuine triple bond
what classes of compounds can form genuine triple bonds
compounds with a valence 2p orbital
where orbital overlap is efficient
name things triple bonds are seen between
alkynes
cyanide
di nitrogen
nitronium cation
alkyne
c triple c
cyanide
c triple N-
- charge on N
di nitrogen
N triple N
nitronium cation
N triple O +
what structure do heavier elements have when they form triple bonds
heavier atoms with multiple bonds,, aka double or triple bonds,, will adapt a trans bent structure
are heavy main group compounds more likely to form double or triple bonds
theyre more likely to form triple bonds
however forming a triple bond using 3 orbitals means theres only one orbital for a substituent or a lone pair
bc heavy main group elements that form triple bonds only have 1 orbital to either bond to a substituent or a lone pair what does this mean
steric protection of the triple bond usually requires an exceptionally bulky ligand.
to prevent it from attacking other things due to the bulky groups repelling any possible electrophiles.
what is used to keep electrophiles away from multiple bonds aka double or triple bonds
bulky groups are used as a double// triple bond restricts thr amount of substituents bonded to it. which means the multiple bond isnt sterically protected as theres nothing protecting it.
so we use a bulky base as a substituent as this will sterically hinder electrophiles from attacking the multiple bond
why are there fewer compounds with a potential triple bond than a double bond
when we have a triple bond,, theres only one orbital left to bond to a substituent
when we have a triple bond we use an s electrons: sp
as seen in double bonded compounds: describe the availability of s electrons down a group
down a group,, s electrons are less available for bonding due to the inert pair effect: due to their shielding and penetrative properties: also the larger energy difference between s and p orbitals the lower u go. prevents s from being used. can lead to lower than expected OS due to p e- being used instead
s electrons penetrate closer to nuc: more stable,, less chemically reactive
as seen in double bonded compounds: describe a trans bent structure
double bond with the left r groups pointed down and the right handed r groups pointed up.
each sidde has a dashed and a wedged side.
the bond strength between the main group elements is reduced
as seen in double bonds: do we need to sterically protect triple bonds
yessss with a bulky base
to prevent electrophilic addition to the triple bond
how would we draw a rlly scientific and really cool triple bond
two circles to be the main group element
they each have 1 e- which are opposite spins
then theres an e- pair in one of the hybrid orbitals of each main group element and theres an arrow from this to an empty hybrid orbital.
so its like a dative type bond between the 2 main group elements.
and theres a sigma bond between them
inorganic rings can be either
homocyclic
heterocyclic
homocyclic inorganic ring
only one element is used
heterocyclic inorganic ring
more than element is used
two or more elements are used
every inorganic non metal has what
they have a homocyclic ring
C-C is a really strong bond which meansss
they have extensive ring chemistry
cyclic alkanes and aromatics
what is boron the only one known to form
boron is the only main group element known to form an extensive pi delocalised homocyclic rings and cages (boron hydrides)
some E-E bonds are strong but their XXX are stronger leading to XXXX
some E-E bonds are strong but their E-X bonds are much stronger (ionic contribution)
this leads to instability with respect to hydrolysis, hydrogenation or halogenation
Group 3 and 5 elements like to bond with which element
they like to bond with OXYGEN,, over itseld, H or Cl due to O being so electronegative,, it gives ionic contributions due to the electronegative differences.
recall the pi delocalised heterocycles containing N
Borazines
Cyclophosphazenes
Cyclothiazenes
describe borazines
6 membered BN ring
B bonded to H
N bonded to H
conjugated double bonds
describe cyclophosphazenes
6 membered PN ring
P bonded to 2 Cl
N not bonded to anything
conjugated double bonds
describe cyclothiazenes
6 membered N S ring
S bonded to Cl
N bonded to nothing
conjugated double bonds
when u see pi delocalised heterocycles what should u think
cyclic ring
made up of 2 or more different elements
conjugated double bonds
what are the 2 synthetic methods for making inorganic rings
condensation
azide route
how is condensation used to make inorganic rings
driven by entropy
loss of HCl or H2 (from the N-H and E-Cl)
elevated temps are required (120-150) to drive off the HCl formed.
a base can be added to prevent HCl being formed as it is dangerous
which base can be added to condensation to prevent HCl being formed when inorganic rings are being synthesised
pyridine (6 ring with N)
or Nh3
driving force for condensation reactions when making inorganic rings is
entropy
bc TS bc gas is being made
when making inorganic rings via condensation,, what do we do when using milder conditions
we use an alkali metal salt or a trimethysilyl derivative
the salt formation is the driving force of the reaction : thermodynamically driven
NaCl LiCl
what is the other,, less popular route for making inorganic rings
thr azide routeeee
why is the azide route less favoured
bc azides are explosiveeee
esp if large quantities of inorganic rings are needed
what is used to make main group azides at a low tempppp
mild condensation reactions using
LiN3
NaN3
Me3SiN3
low temp
warming these releases N2 to generate reactive monomers which oligomerise
decomposition of the azide depends on the substituents
what is an oligomer
a polymer made up of only a couple of repeating units
its phycial properties are dependant on chain length
Borazanes: what are they
aliphatic BN bondssssss
borazine BN is isoelectronic with what
C2
that means BN can be used to replace CC bonds
Alkyne andddd BN form
Borazyne
B triple N
alkene and BN
borazene
B=N
alkane and BN
borazane
B-N
describe amine-borane adducts and how theyre made
N dative bond to B using lone pair
H3N–>BH3 or R groups
theyre colourless
low mp
crystalline solids
isoelectronic with alkanes
donor acceptor adducts
whats an adduct again
when 2 molecules react and bond together and the product has all the atoms of the separate molecules
lewis structure of an amine-borane adduct
R3N+ —> BR3 -
bc N donates its lone [air to B meaning its more positive relative to before,, when it still had its lone pair and was neutral
partial charges structure of amine-borane
R3N - — BR3 +
but partial charges
bc N is more electronegative than B
Borazines
cyclic!!!!
borazine = limozene = car = cyclic bc wheels.
inorganic benzene
BN cycle with H on each thing
B3H3N3
conjugation swell.
whats an azide
and azide is used in the azide synthesis route to make inorganic, pi delocalised heterocyclic rings.
N=N=N
middle N is (+)
N thats on the end is (-)
