carbonyl groups (structure/bonding/reactivity)

imine

oxime

hydrazone

acetal

bonding in carbonyl compounds

• C-C MO

• C-O MO

• describe

• O orbitals lower in energy (more electronegative)

• the C-O MOs are lower in energy - carbonyls are not nucleophilic as nucleophiles have high-lying HOMOs

• the HOMOs in C-O are the oxygen lone pairs

• the LUMOs are quite low-lying making carbonyls electrophilic

• the π orbital is polarised towards O as O is more electronegative. this means the π MO is closer in energy to O than C and the electrons in this bond lie closer to O.

• the π* orbital lies closer in energy to C

bond strength order for single + double C-C + C-O

C-C < C=C < C-O < C=O

C=O more than twice as strong as C-O

why are carbonyls reactive towards nucleophiles

where does the nucleophile attack

• the C=O bond is strong but polarised

• nucleophilic species are attracted to the partially positive carbon where they can attack the large lobe of the low-lying LUMO (C=O π*)

• the bonding C=O orbital has greater orbital density towards the more electronegative O atom

• the antibonding C=O orbital correspondingly has a greater coefficient on the C atom

resonance forms of C=O

water as a nucleophile

for aldehydes + ketones

can add itself to carbonyls to generate hydrates or 1,1-diols

the reactions are reversible and generally the equilibrium lies to the side of the carbonyl compound as a strong C=O bond must be broken - the hydrates are “disfavoured enthalpically and entropically”

draw the mechanism for ketone + water

trend in this (hydrate equilibrium)

origins of trends in hydrate equilibrium

• steric component - larger substituents than H disfavour going to sp³

• electronic component - stabilisation of carbonyl by hyperconjugation/conjugation lost on going to sp³

• these effects mirror those seen in reactivity

  • aldehydes are generally more reactive than ketones

  • alkyl substituted carbonyls more reactive than aryl

  • large substituents

oxidation of secondary alcohols

oxidation of primary alcohols

oxidation does not stop at the aldehyde but instead form acids via the hydrate

how to get only aldehyde from primary alcohol

• need to avoid presence of water so that hydrate and hence carboxylic acid cannot form

• do this by using pyridinium dichromate, a form of the dichromate anion which is soluble in organic solvents

sodium borohydride as a reducing agent

• the boron is not nucleophilic - all the electrons are in B-H bonds

• the reacting HOMO is a B-H bond

• reactions with sodium borohydride are carried out in protic solvents, often alcoholic eg MeOH, EtOH, giving the alcohol directly

• all 4 hydrides can be delivered from ^-BH₄

reducing agent in cells/in nature

NADH - the reduced form of NAD⁺ (nicotinamide adenine dinucleotide)

alternative reducing agent to sodium borohydride

lithium aluminium hydride, LiAlH₄

much more reactive and must be used in aprotic solvents as it has a very violent reaction with water/alcohols

reduction with LiAlH₄

why are both reducing agents used

lithium aluminium hydride reduces many more functional groups than sodium borohydride so the reaction outcomes are different

carboxyl derviative

C/H substituent of ketone/aldehyde replaced by a heteroatom group

addition of heteroatom Nu to carboxyl derivative

the heteroatom on the carboxyl is a potential leaving group so when the tetrahedral intermediate is formed it can either reform the starting materials or form a different product (substitution)

mechanism of Nu substitution name and rules

known as addition/elimination

always has 2 steps with tetrahedral intermediate being formed - never draw as S_N2

ester formation from acids (Fischer esterification)

• form by condensation of carboxylic acids with alcohols to give esters and wwater

• reaction is slow on its own but catalysed by acid

Fischer esterification mechanism for butanoic acid and methanol

don’t forget sulfuric acid catalyst

equilibrium for Fischer esterification

• reaction is an equilibrium and roughly thermoneutral due to similar structure and bonding of carboxylic acids and esters

• get a mixture unless equilibrium shifted by

  • using the alcohol in large excess (suitable for volatile alcohols like MeOH, EtOH)

  • or driving off the water eg by azeotropic distillation

polyesters

condensation polymerisation of carb acs and alcs can be used to make polyesters

the high temperatures of the industrial process drive off the water by-product as steam, shifting the equilibrium