Esters and Thioesters Flashcards

1
Q

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

A

Less significant

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2
Q

When are negative charges most stable?

A

When placed on the least basic atom

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3
Q

Which resonance form is best for positive charges?

A

One with all the octets filled

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4
Q

What are cyclic esters called?

A

Lactones

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5
Q

What are lactones synthesised from?

A

Hydroxy carboxylic acids

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6
Q

What is the geometry of an ester?

A

Trigonal planar

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7
Q

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

A

120°

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8
Q

Are esters more or less polar than alcohols?

A

Less

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9
Q

Can esters act as H bond acceptors?

A

Yes

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10
Q

Can esters act as H bond donors?

A

No

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11
Q

What is a thioester?

A

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

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12
Q

Are thioesters reactive towards nucleophiles?

A

Yes

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13
Q

Give an example of a key molecule which contains a thioester

A

Acetyl CoA

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14
Q

What is the major route of thioester synthesis?

A

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

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15
Q

What is formed when a thioester reacts with an amine?

A

An amide

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16
Q

What are the 2 alternative approaches for synthesising thioesters?

A
  1. Reaction of thiol with alkene
  2. Displacement reaction of K salt of thioacetic acid in acetone
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17
Q

How are thioesters named?

A

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

18
Q

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

A

It is large in esters and moderate in thioesters

19
Q

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

A

Due to the electronegative oxygen

20
Q

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

A

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

21
Q

Why do thioesters have poor resonance?

A

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

22
Q

Why do esters have good resonance?

A

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

23
Q

Do esters or thioesters contain the better leaving group?

A

Thioesters

24
Q

Why is SR’ a good leaving group from thioesters?

A

Has a larger atomic size so easier to break the bond

25
Why is OR' a moderate leaving group?
Due to the highly electronegative O
26
What do good leaving groups tend to be?
Weak bases
27
Which orbital are lone pairs on a thioester in?
3p
28
Why do thioesters have less conjugation than ordinary esters?
* lone pairs are in 3p rather than 2p orbitals * these orbitals are too large to overlap efficiently with 2p orbital of the carbon atom
29
What does conjugation mean in term of molecules?
The extent to which a molecule contains connected p orbitals with delocalised electrons
30
Are thioesters or esters more easily attacked by nucleophiles?
Thioesters
31
Are thioesters more or less reactive than esters?
Much more reactive
32
Why are thioesters more energy rich than esters?
Esters have 2 resonance forms and so are stabilised, whilst thioesters do not
33
What is the only difference between the biological and chemical synthesis of Coenzyme A thioesters?
Biologically ATP is the dehydration agent
34
Why does nature use thioesters rather than esters?
* 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
35
Are thioesters more or less easily converted into enolate ions than esters?
More easily converted
36
Are thioesters good alkylating agents?
Yes
37
Why are thioesters good alkyating agents?
* 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
38
Do esters and thioesters have the capacity to act as H bond acceptors?
Yes
39
What is required for intramolecular esterification?
H+
40
What is the product of intramolecular esterification?
Lactone and H3O+