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
