Esters and Thioesters Flashcards
As the number of charges are increased do resonance forms become more or less significant?
Less significant
When are negative charges most stable?
When placed on the least basic atom
Which resonance form is best for positive charges?
One with all the octets filled
What are cyclic esters called?
Lactones
What are lactones synthesised from?
Hydroxy carboxylic acids
What is the geometry of an ester?
Trigonal planar
What are the C-C-O and O-C-C bond angles of esters?
120°
Are esters more or less polar than alcohols?
Less
Can esters act as H bond acceptors?
Yes
Can esters act as H bond donors?
No
What is a thioester?
Functional group characterised by a sulfur atom flanked by one carbonyl and one carbon of any hybridisation
Are thioesters reactive towards nucleophiles?
Yes
Give an example of a key molecule which contains a thioester
Acetyl CoA
What is the major route of thioester synthesis?
Condensation of thiols and carboxylic acids in the prescence of dehydrating agents
What is formed when a thioester reacts with an amine?
An amide
What are the 2 alternative approaches for synthesising thioesters?
- Reaction of thiol with alkene
- Displacement reaction of K salt of thioacetic acid in acetone
How are thioesters named?
Like esters but ‘thio’ is inserted into 2nd part of the name e.g. methyl thioethanoate
Compare the magnitude of the δ+ charge on carbon in esters and thioesters
It is large in esters and moderate in thioesters
Why is there a large δ+ charge on the carbon in esters?
Due to the electronegative oxygen
Why is there a moderate δ+ charge on the carbon in thioesters?
Due to the prescence of less electronegative Sulfur (compared with oxygen)
Why do thioesters have poor resonance?
C and S are not in the same row on the periodic table
Why do esters have good resonance?
C and O are in the same row on the periodic table so have good double bond character
Do esters or thioesters contain the better leaving group?
Thioesters
Why is SR’ a good leaving group from thioesters?
Has a larger atomic size so easier to break the bond