lecture 3 Flashcards
gemdiols dehydrate to give what
gemdiols dehydrate to give ketones
R R OH OH –> R R =O
covalent component depends mainly on what
the atom size and electronegativity
strong covalent bonds and orbital explanation
small atoms can approach each other closely providing good orbital overlap
why are covalent bonds weaker with NOF
bc the atom size is so small due to high electronegativity,, their lone pairs repel which reduces the amount of efficient orbital overlap
how electronegativity, size, overlap and bond strength change going down the group
down a group = lower electronegativity
= larger atomic size
= weaker overlap of orbitals
= weaker bond
electronegativity and bond energy..
at the top of the group // going to the rhs
larger electronegativity (not NOF) = smaller atom size = efficient orbital overlap = stronger bond
bond energies downa group
decrease
less energy
as atom gets bigger
when is pi bonding preferrred
with small atoms with small radii as it allows for orbitals to get closer and for effective overlap.
when is pi bonding particularly strong
non metals of the second period
BCNOF
when is pi bonding seen less
with atoms further down a group
with heavier atoms
bc theyre larger and more diffuse = large interatomic separation = poor overlap
gemdiols dehydrate to give what
dehydrate to give ketones
RR OH OH —-> RR =O
sometimes sigma bonds are preferred due to what + give an example
sigma bonds are preferred due to ionic interactions
Si-Si (220 kjmol)
Si - O (450kjmol)
gem silan diols dehydrate to give
Si - O - Si bridges
SOS but Si bc silan
when is pi bonding weaker between elements
pi bonding is weaker between 2nd and 3rd row elements due to their orbitals’ sizes + energy being differen.
for ionic systems,, what is a key factor
lattice energy stabilisation is a key factor as lattice energies can be much higher than that of covalent bonds.
entropy and which systems we need to avoid
for great stability we need to avoid systems where small molecules can be formed. WHEN IT IS WARMED!!!
do open chains or rings have more degrees of freedom
open chains have a higher degree of freedom (bc more units in the open chain: 3N)
what will entropy favour at elevated temps
entropy will favour ring opening
- but there diff is a balance between TS and H needed to break the bonds of the ring when u ring open it
what factors indicate the stability of a compound
thermodynamic factors
but we can also consider kinetics
why should we consider kinetics
bc there are many compounds with are intrinsically unstable with respect to their elements but the Ea needed for decomposition to occur is too high.
eg: decomposition is inhibited.
why can a silicone ketone not occur
bc there isnt efficient overlap between the Si and the O
the pi bond would be too weak.
what makes a ketone a possibility when u go from gemdiol to ketone by releasing water
bc the C and the O are similar sizes so the pi bond has efficient overlap
bc theyre both at the top of the group.
larger r groups likeeee
smaller rings
name some small groups which can be released when a reaction is warmed
N2
CO2
H2O
HCl
NO
why can decomposition be inhibited even if the compound is intrinsically unstable
bc sometimes the Ea needed for the reaction to take place is too high.
the decomposition product may be more stable,, but the reactant may have stronger bonds
how can decomposition occur to main group rings and chains
via electrophilic // nucleophilic attacks
increasing stability: what must we avoid in order to prevent coordination with a reactive donor,, a nucleophile
we must avoid low energy acceptor orbitals
improving stability: what do we do if the compound has a reactive acceptor
bulky substituents can prevent an attack by a nucleophile
improving stability: what can we use to reduce the ability of the reactive centre to act as a lewis acid // base
we can use EDG and EWG and their properties - these are already bonded to the central atom and affect its reactivity towards attackers
lewis acid issss
accepts e- pairs from a lewis base to form an adduct
lewis base
donates e- pairs to a lewis acid in order to form an adduct
adduct
basically just the product of 2 things bonding together to form a new molecule. this has all the atoms from each reactant species.
what do EWG and EDG do to the reactive centre atom
they affect its reactivity - either making it a poorer donor or a weaker acceptor
where are multiple bonds between elements containing a valence 2p orbital common
top of the 2p block
C N O etccc
when are multiple bonds between elements containing valence 2p orbitals not common
with heavier atoms
further down the 2p block,, double bonds are seen less here compared to the upper p block elements like c n o
why are multiple bonds (triple and double) not seen as much further down the p block (valence 2p orbitals)
larger atom size
more diffuse orbitals
less efficient overlap
weaker pi bondssss
why are multiple bonds (double and triple) seen more in the atoms closer to the top of the 2p block - ones with valence 2p orbitals
smaller atom
smaller radii
less diffuse orbitals
more efficient overlap
stronger pi bondsss
as heavier atoms with a valence 2p orbital dont form multiple bonds as much due to their size and weak overlap,, what do they form?
oligomers
they max out their single bonds
what is an oligomer
a polymer with only a few repeating units
sigma bonds are maximised
what overlap do the larger 2p orbitals lack,, preventing them from forming pi bonds
they lack spatial overlap
what is spatial overlap girll
the pi bond side overlap 🫶
C=C shape
planar shape
Sn = Sn shape
trans bent
aka lhs R groups tilting down (one wedge and one dashed)
aka rhs R groups tilting up (one dashed and one wedge)
is the Sn=Sn stronger than C=C
nope
C=C is stronger
more efficient spatial overlap bc the atomic radi is smaller
why does Sn = Sn adopt a trans bent structure
to try and form the most stable sigma bond
unhybridised p orbitals overlap to give what
unhybrid p orbitals overlap to give pi bonds
hybridised p orbitals overlap to give what
sigma bond
how can we rationalise bent structures
break them through the middle and see how they associate
breaking the C=C planar shape down thr middle will give what : explain it and where the e- would be
2 Cs with 3 hybridised orbitals and 1 unhybridised p orbital
C will have both aligned up,, one in the unhybridised orbital,, and one in the hybridised orbital (ONE IN THE HYBRID THAT FORMS THE SIGMA,, ONE IN THE ORBITAL THAT FORMS THE PI)
the other C will be the same in terms of e- but the e- will be aligned down
C has 4 e- so when we break the pi bond in the centre,, where will all the e- be
1 in unhybridised orbital
1 in hybridised orbital
1 in the R group bond
1 in the other R group bond
explain how we break the Sn =Sn bond and how they both associate
basically the same thing as the C,, it will have 3 hybridised orbitals which are bonding to 2 R groups and one will have 2 e-,, a pair if u will.
the 2e- in the hybrid orbital will donate these to the unhybridised + vacant p orbital of the other Sn
it gives a dative bond type vibe
describe what the triplet state looks like + if C or Sn prefer this
Carbon is more stable in the triplet state
this is where one C has them aligned up and the other C has them aligned down. but the e- are unpaired and 1 is in the hybrid and 1 is in the unhybrid p orbital.
spin = 1 bc theyre both looking up,, and theyre 1/2 each.
describe the singlet state,, what it looks like and if C or Sn prefers it when we break the double bond
singlet state is when the e- pair is in one of the hybrid orbitals
with R groups on the other hybrids
and with the vacant p orbital
