Esters and Thioesters

Question 1

As the number of charges are increased do resonance forms become more or less significant?

Answer 1

Less significant

Question 2

When are negative charges most stable?

Answer 2

When placed on the least basic atom

Question 3

Which resonance form is best for positive charges?

Answer 3

One with all the octets filled

Question 4

What are cyclic esters called?

Answer 4

Lactones

Question 5

What are lactones synthesised from?

Answer 5

Hydroxy carboxylic acids

Question 6

What is the geometry of an ester?

Answer 6

Trigonal planar

Question 7

What are the C-C-O and O-C-C bond angles of esters?

Answer 7

120°

Question 8

Are esters more or less polar than alcohols?

Answer 8

Less

Question 9

Can esters act as H bond acceptors?

Answer 9

Yes

Question 10

Can esters act as H bond donors?

Answer 10

No

Question 11

What is a thioester?

Answer 11

Functional group characterised by a sulfur atom flanked by one carbonyl and one carbon of any hybridisation

Question 12

Are thioesters reactive towards nucleophiles?

Answer 12

Yes

Question 13

Give an example of a key molecule which contains a thioester

Answer 13

Acetyl CoA

Question 14

What is the major route of thioester synthesis?

Answer 14

Condensation of thiols and carboxylic acids in the prescence of dehydrating agents

Question 15

What is formed when a thioester reacts with an amine?

Answer 15

An amide

Question 16

What are the 2 alternative approaches for synthesising thioesters?

Answer 16

1. Reaction of thiol with alkene
2. Displacement reaction of K salt of thioacetic acid in acetone

Question 17

How are thioesters named?

Answer 17

Like esters but ‘thio’ is inserted into 2^nd part of the name e.g. methyl thioethanoate

Question 18

Compare the magnitude of the δ+ charge on carbon in esters and thioesters

Answer 18

It is large in esters and moderate in thioesters

Question 19

Why is there a large δ+ charge on the carbon in esters?

Answer 19

Due to the electronegative oxygen

Question 20

Why is there a moderate δ+ charge on the carbon in thioesters?

Answer 20

Due to the prescence of less electronegative Sulfur (compared with oxygen)

Question 21

Why do thioesters have poor resonance?

Answer 21

C and S are not in the same row on the periodic table

Question 22

Why do esters have good resonance?

Answer 22

C and O are in the same row on the periodic table so have good double bond character

Question 23

Do esters or thioesters contain the better leaving group?

Answer 23

Thioesters

Question 24

Why is SR’ a good leaving group from thioesters?

Answer 24

Has a larger atomic size so easier to break the bond

Question 25

Why is OR' a moderate leaving group?

Answer 25

Due to the highly electronegative O

Question 26

What do good leaving groups tend to be?

Answer 26

Weak bases

Question 27

Which orbital are lone pairs on a thioester in?

Answer 27

3p

Question 28

Why do thioesters have less conjugation than ordinary esters?

Answer 28

* lone pairs are in 3p rather than 2p orbitals * these orbitals are too large to overlap efficiently with 2p orbital of the carbon atom

Question 29

What does conjugation mean in term of molecules?

Answer 29

The extent to which a molecule contains connected p orbitals with delocalised electrons

Question 30

Are thioesters or esters more easily attacked by nucleophiles?

Answer 30

Thioesters

Question 31

Are thioesters more or less reactive than esters?

Answer 31

Much more reactive

Question 32

Why are thioesters more energy rich than esters?

Answer 32

Esters have 2 resonance forms and so are stabilised, whilst thioesters do not

Question 33

What is the only difference between the biological and chemical synthesis of Coenzyme A thioesters?

Answer 33

Biologically ATP is the dehydration agent

Question 34

Why does nature use thioesters rather than esters?

Answer 34

* reactive * high energy systems * easy to add things onto and take things off * acyl carbon more susceptible to nucleophilic attack * produces a better leaving group

Question 35

Are thioesters more or less easily converted into enolate ions than esters?

Answer 35

More easily converted

Question 36

Are thioesters good alkylating agents?

Answer 36

Yes

Question 37

Why are thioesters good alkyating agents?

Answer 37

* stabilisation of the resulting carbanion by resonance makes the hydrogen attached to the alpha carbon more acidic * the enolate form is likely to predominate which overall makes the thioester more nucleophilic and able to act as an akylating agent

Question 38

Do esters and thioesters have the capacity to act as H bond acceptors?

Answer 38

Yes

Question 39

What is required for intramolecular esterification?

Answer 39

H+

Question 40

What is the product of intramolecular esterification?

Answer 40

Lactone and H₃O⁺