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Biological Chemistry 1A > Esters and Thioesters > Flashcards

Esters and Thioesters Flashcards

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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
Q

Why is OR’ a moderate leaving group?

A

Due to the highly electronegative O

26
Q

What do good leaving groups tend to be?

A

Weak bases

27
Q

Which orbital are lone pairs on a thioester in?

A

3p

28
Q

Why do thioesters have less conjugation than ordinary esters?

A
  • 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
Q

What does conjugation mean in term of molecules?

A

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

30
Q

Are thioesters or esters more easily attacked by nucleophiles?

A

Thioesters

31
Q

Are thioesters more or less reactive than esters?

A

Much more reactive

32
Q

Why are thioesters more energy rich than esters?

A

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

33
Q

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

A

Biologically ATP is the dehydration agent

34
Q

Why does nature use thioesters rather than esters?

A
  • 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
Q

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

A

More easily converted

36
Q

Are thioesters good alkylating agents?

A

Yes

37
Q

Why are thioesters good alkyating agents?

A
  • 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
Q

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

A

Yes

39
Q

What is required for intramolecular esterification?

A

H+

40
Q

What is the product of intramolecular esterification?

A

Lactone and H3O+

Biological Chemistry 1A (10 decks)

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