[hellohelp!]

im doing ocr f324, wanted to know if you need to know the actual mechanism of how an ester is formed, or do you only need to know that a c.acid react with alcohol to form an ester. Thanks

[TiTo20]

(Original post by hellohelp!)
im doing ocr f324, wanted to know if you need to know the actual mechanism of how an ester is formed, or do you only need to know that a c.acid react with alcohol to form an ester. Thanks

No you don't, the actual mechanism is very complicated =]


This was posted from The Student Room's iPhone/iPad App

[thegodofgod]

(Original post by TiTo20)
No you don't, the actual mechanism is very complicated =]


This was posted from The Student Room's iPhone/iPad App

Really? I would have thought it's simply nucleophilic addition-elimination:

- lone pair on O of ROH attacks d+ C of RCOOH
- C=O bond of RCOOH breaks
- C=O bond reforms
- OH from RCOOH gets kicked off
- OH^- that's kicked off attacks one of the protons on ROH₂⁺ on the ester
- RCOOR' + H₂O formed

[illusionz]

(Original post by TiTo20)
No you don't, the actual mechanism is very complicated =]


This was posted from The Student Room's iPhone/iPad App

It's not very complicated. You've seen things very similar to it.

(Original post by thegodofgod)
Really? I would have thought it's simply nucleophilic addition-elimination:

- lone pair on O of ROH attacks d+ C of RCOOH
- C=O bond of RCOOH breaks
- C=O bond reforms
- OH from RCOOH gets kicked off
- OH^- that's kicked off attacks one of the protons on ROH₂⁺ on the ester
- RCOOR' + H₂O formed

That's correct, but you'll almost always have a catalyst in there as the reaction you described is extremely slow!!

[thegodofgod]

(Original post by illusionz)
That's correct, but you'll almost always have a catalyst in there as the reaction you described is extremely slow!!

Concentrated sulfuric acid

[illusionz]

(Original post by thegodofgod)
Concentrated sulfuric acid

Yup. Can you see how it will catalyse the reaction, from your mechanism?

[thegodofgod]

(Original post by illusionz)
Yup. Can you see how it will catalyse the reaction, from your mechanism?

- d- O in H-O-SO₃H attacks the d+ C of RCOOH
- C=O bond of RCOOH breaks
- C=O bond reforms
- OH from RCOOH gets kicked off
- OH^- that's kicked off attacks the proton on ROH(SO₃H)⁺ on the ester
- RCOOSO₃H + H₂O formed


- lone pair on O of ROH attacks d+ C of RCOOSO₃H
- C=O bond of RCOOSO₃H breaks
- C=O bond reforms
- OSO₃H from RCOOSO₃H gets kicked off
- OSO₃H^- that's kicked off attacks one of the protons on ROH₂⁺ on the ester
- RCOOR' formed and H₂SO₄ re-formed, so it is a catalyst

[umair.khan]

You dont need to learn the mechanism for it, just the conditions required for the reaction. Reflux, conc sulfuric acid

[illusionz]

(Original post by thegodofgod)
- d- O in H-O-SO₃H attacks the d+ C of RCOOH
- C=O bond of RCOOH breaks
- C=O bond reforms
- OH from RCOOH gets kicked off
- OH^- that's kicked off attacks the proton on ROH(SO₃H)⁺ on the ester
- RCOOSO₃H + H₂O formed


- lone pair on O of ROH attacks d+ C of RCOOSO₃H
- C=O bond of RCOOSO₃H breaks
- C=O bond reforms
- OSO₃H from RCOOSO₃H gets kicked off
- OSO₃H^- that's kicked off attacks one of the protons on ROH₂⁺ on the ester
- RCOOR' formed and H₂SO₄ re-formed, so it is a catalyst

Nope, it's a lot simpler and more logical than that!!

The reason COOH isnt usually electrophilic is because when deprotonated it's negatively charged.

In strong acid you can protonate the carbonyl oxygen, making it much more electrophilic than before, and hence it is attacked by the relatively poor nucleophilie ROH.

[thegodofgod]

(Original post by illusionz)
Nope, it's a lot simpler and more logical than that!!

The reason COOH isnt usually electrophilic is because when deprotonated it's negatively charged.

In strong acid you can protonate the carbonyl oxygen, making it much more electrophilic than before, and hence it is attacked by the relatively poor nucleophilie ROH.

Ah - didn't think of that

Although I thought that the 'alternative route' provided by H₂SO₄ was longer, so wasn't too convinced, but I thought that the RCOOSO₃H may be very unstable, so the second stage of the mechanism may be very quick

Edit: Is OH^- a relatively weak nucleophile as O is electronegative, so the lone pair isn't as available to attack the electrophile?

Cheers!

[illusionz]

(Original post by thegodofgod)
Ah - didn't think of that

Although I thought that the 'alternative route' provided by H₂SO₄ was longer, so wasn't too convinced, but I thought that the RCOOSO₃H may be very unstable, so the second stage of the mechanism may be very quick

Edit: Is OH^- a relatively weak nucleophile as O is electronegative, so the lone pair isn't as available to attack the electrophile?

Cheers!

No. OH^- is a good nucleophile. ROH is a poor nucleophile. The simple reason for this difference is the charge on the two species.

[thegodofgod]

(Original post by illusionz)
No. OH^- is a good nucleophile. ROH is a poor nucleophile. The simple reason for this difference is the charge on the two species.

Ah, that makes sense.

Okay, so which one of NH₂^- or OH^- would be a better nucleophile?

I'm thinking that they both have the single negative charge, but it would be OH^-, as oxygen is more electronegative than nitrogen, so the O region of OH^- would have a greater negative charge density than the N of NH₂^-, meaning that OH^- would be more attracted to positive charge than NH₂.

Cheers

[illusionz]

(Original post by thegodofgod)
Ah, that makes sense.

Okay, so which one of NH₂^- or OH^- would be a better nucleophile?

I'm thinking that they both have the single negative charge, but it would be OH^-, as oxygen is more electronegative than nitrogen, so the O region of OH^- would have a greater negative charge density than the N of NH₂^-, meaning that OH^- would be more attracted to positive charge than NH₂.

Cheers

There are two main factors which determine the nucleophilicity of a species. The charge, and the energy of the lone pair (higher = more nucleophilic)

The charge on the species is one factor, so ROH < RO^- < O^2-

(also relevant for the topic of this thread - protonation of the carbonyl C=O makes it positively charged and hence more electrophilic)

This gives us the difference between species wheere the electron pair comes from an identical atom.

However, when comparing N vs O, we need to consider the energy of the lone pairs. As Zeff is higher for O than N, the lone pair is lower in energy for O, and hence O is a worse nucleophile than N.

So, for species of equal charge, we expect the nitrogen analogue to be more nucleophilic than the oxygen one.

[thegodofgod]

(Original post by illusionz)
There are two main factors which determine the nucleophilicity of a species. The charge, and the energy of the lone pair (higher = more nucleophilic)

The charge on the species is one factor, so ROH < RO^- < O^2-

(also relevant for the topic of this thread - protonation of the carbonyl C=O makes it positively charged and hence more electrophilic)

This gives us the difference between species wheere the electron pair comes from an identical atom.

However, when comparing N vs O, we need to consider the energy of the lone pairs. As Zeff is higher for O than N, the lone pair is lower in energy for O, and hence O is a worse nucleophile than N.

So, for species of equal charge, we expect the nitrogen analogue to be more nucleophilic than the oxygen one.

Okay - is Zeff sort of a scale, like the Pauling scale for electronegativity? Haven't come across it before

Would I be correct in assuming that F has a higher 'Zeff' than O, so it would have a lower energy lone pair than O, and thus would be a weaker nucleophile? (Assuming the charge is constant)