Carbonyl chemistry- O/S Nucleophiles with aldehydes and ketones Flashcards
What are the two main ways a nucleophile attacks a carbonyl
- Nucleophilic addition
2. Nucleophilic substitution
What is another method of addition to carbonyls
- Addition using an oxygen nucleophile
- Results in the loss of carbonyl oxygen
- Specific to aldehydes and ketones
What happens if you add OHCH2CH2OH to an aldehyde with h+
- Forms an acetyl + H2O
- The carbonyl oxygen is lost and forms the O in H2O
- Oxygens in acetyl come from the OHCH2CH2OH
How is it known that the carbonyl oxygen forms the H2O
- Used oxygen 18 on the carbonyl and saw that it was O18 in the H2O product and normal O in the acetyl
What is formed when you add water to an aldehyde/ketone
- A hydrate
- Two alcohol groups attached to the R groups
- Reversible reaction
Why is it difficult to isolate hemi-acetal
- Add alcohol to aldehyde/ketone produces a hemi-actetla
- Most work-ups require water but adding water shifts the equilibrium back towards the aldehyde/ketone
- Entropy- negative- 2 starting materials= 1 product
- So equilibrium is on the left as DG>0
How are acetals formed
- Adding alcohol to aldehyde/ketone first forms hemiacetal and then adding more forms acetal
- Acetal can be isolated
- But still has entropy issues.
What is a hemi-acetal
- RC(OH)OR
How can a hemiacetal be formed
- Form very slowly in presence of alcohol- not a great nucleophile
- Rate of formation is increased by either acid or base
Why does adding an acid speed up the rate of hemiacetal formation
- The aldehyde/ketone forms an oxonium ion
2. This is much more electrophilic as carbonyl carbon has more electron density pulled away from it
Why does adding a base speed up the rate of hemiacetal formation
- OH- deprotonates the alcohol so it becomes a much better nucleophile
Why do ring structures form from a aldehyde/ketone with alcohol group on other end
- DH is slightly -ve
- DS is neutral
- DG <0, so equilibrium lies to right
How can an acetal be formed
- Formed very slowly in presence of alcohol
- Rate of formation increased by acid only
- Add heat and pTSA (acid)
What is pTSA
- Common organic acid
2. Nice crystalline powder to work with as dry
What is a problem with acetal formation and how is it solved
- Water is produced which can reverse the reaction
- Excess of ethanol pushes equilibrium to the right and swamps out water produced
- Remove the H2O from reaction by MgSO4, using a molecular sieve, using Dean Stark reagent
What is the more stable alcohol reactant
- Diol more stable than monomeric alcohol
2. As cyclic acetyl are more stable than non-cyclic
How can acetals be hydrolysed
- Reverse of formation
2. Acid and add water back in
What can acetals be used for
- They are protecting groups
2. Protect carbonyl carbon from attack- especially for aldehydes and ketones
What can sulfur nucleophiles be used for
- Useful in synthesis
- Can protect carbonyl groups- diathiane formation
- Form using acid, Toluene and Dean Stark- same as acetals formation
How can you remove the sulfur protecting group to reform the carbonyl
- Need Acid + Water + HgCl2 = carbonyl back
2. Or add raney-Ni + H2 = reduced so no Oxygen after
What is the Baeyer Viliger reaction
- Converts a ketone to an ester
How can you convert a ketone to an ester
- Using peracids
2. Forms an oxonium ion intermediate and then forms an ester
Describe properties of peracids
- Nucleophilic atom- OH
2. Carrying good leaving group o-c(=o)-R
