in the C=O bond, give the hybridisation for each atom
C
- sp2 hybridised
- 3 sp2 orbitals, one used for σ(C-O)
- one p-orbital left for π-system
O
- sp2 hybridsed
- one sp2 orbital used for σ(C-O), left over 2 are n.b. and contain two lone pairs
- one p-orbital for π-system
in the C=O bond, describe the pi-system and its polarisation
p-orbital overlap between C and O lead to a pi-bond
difference in electronegativity between C and O = polarisation of bond = pi-bond is skewed towards O
π* orbital is skewed towards C
in the C=O bond, where is the LUMO
LUMO is the π* orbital
which is electrophilic at carbon, nucleophile likely to attack the carbon
nucleophilic attack on C=O
- where does the nucleophile attack?
- which two orbitals are interacting?
- describe transfer of electrons
- nucleophile attacks the LUMO of the carbonyl group, the π* orbital that is empty and skewed towards carbon
- the HOMO of the nucleophile interacts with LUMO of carbonyl
- electrons are being transferred from HOMO to LUMO
nucleophilic attack on C=O
- how does the nucleophilic attack change the bonding and hybridisation of the carbonyl atoms?
- nucleophilic attack leads to a tetrahedral intermediate
- electrons filling π* orbital causes pi-bond to break and sigma bond is formed between the carbon and the nucleophile
- carbon is now sp3 hybridised
- pair of electrons that were once in the pi-bond are now a negative charge on the oxygen
nucleophilic attack on C=O
what are two outcomes for the tetrahedral intermediate
- nucleophilic addition: the O- can gain a proton and give an alcohol product
- nucleophilic substitution: the O- can collapse and form a carbonyl group again and leaving group is removed
define leaving group
a molecule fragment that leaves a molecule with a pair of electrons
nucleophilic substitution on C=O
what determines the stability of the product when the C=O reforms?
the stability of the product depends on the stability of the leaving group anion
most stable anion = best leaving group
more stable anion = more electronegative
how is leaving group ability determined
how does reactivity change with leaving group ability
pKa can show leaving group ability
if X- is the leaving group, the lower the pKa value of the acid HX, the better the leaving group
stronger acids make better conjugate base leaving groups
as you increase pKa (worse leaving groups) reactivity decreases
describe what happens to the MOs of the carbonyl group when an amide (NH2 group) is present
in an amide, the nitrogen lone pair is in a p-orbital and will overlap with the empty π* orbital of the carbonyl, this is delocalisation
this creates a new bonding and anti-bonding MO
- bonding MO is lower in energy and stabilised
- anti-bonding MO becomes too high in energy, making it less ready to react with nuclephiles
what are 3 conditions for nucleophilic substitution of C=O
need a good enough nucleophile
need an electrophilic enough carbonyl
need a good leaving group
