stonecold's chem questions :)

[simonhu81292]

They are not symmetry!! so we can't assume that the CH3 and CH2's will have the same chemical shift !!
so be careful!~~~

[Blakhitman]

    H H O    H  H
    |  |  ||     |  |
H-C-C-C-O-C-C-H
    |  |          |  |
    H H         H H

All the hydrogen environments are not equivalent because of the asymmetrical nature of the compound (wow that sounds smart ). For example.

[stonecold]

Sorry, I still don't get it.

Why is CH₃-CH₂-COO different to COO-CH₂-CH₃ ?

I thought, (and know I'm wrong) that it was just adjacent protons that affected the nuclear spin...

[Blakhitman]

yea the spin coupling is only affected by the adjacent protons, however the number of environments depends on symmetry, the last environment (relative to my drawing) is different to the first because of its place in the molecule, example, it's 1 C away from the O while the first is 2 Cs away.

Very bad explanation, but this is how I see it.
                          O
                          ||
If there was no  C-O  then yes they'd be the same.

[kyzoo]

Sorry, I still don't get it.

Why is CH₃-CH₂-COO different to COO-CH₂-CH₃ ?

I thought, (and know I'm wrong) that it was just adjacent protons that affected the nuclear spin...

The closer protons are to electronegative atom groups (such as O, Fl or Cl), the less nuclear shielding and hence higher chemical shift they experience.

[Mao]

Sorry, I still don't get it.

Why is CH₃-CH₂-COO different to COO-CH₂-CH₃ ?

I thought, (and know I'm wrong) that it was just adjacent protons that affected the nuclear spin...

Nope, any atom can affect it, and they can affect it a few groups away. Another O, F, Cl, P, S, etc... In general, unless they are in exactly the same environment (such as the cases of symmetrical molecules), they are different.

[Blakhitman]

Didn't like chemical engineering Mao?

[Mao]

Didn't like chemical engineering Mao?

Nah, chem eng is good fun, but I just realised I liked science so much more.

[stonecold]

This is in my SAC tomorrow, so if anyone could explain it would be a great help.

I tried googling it, but how does HCl hydrolyse starch?

[m@tty]

I don't know how correct this is but to me it sounds as though it could be the same as real hydrolosis(with water) but with -Cl^- substituted for -OH^-...

ie. HCOOCH₃(l) + HCl (aq) --> HCOCl (l) + HOCH₃ (l)

I have never heard of it before though, so don't rely on only my speculation, especially for a SAC. Just putting an idea out there...

[stonecold]

I see what you mean.  I think your sort of right, as apparently it is some technique used decades ago, where only 50% glucose is produced.

It's just that starch has ether links and not ester links.

Maybe it is this:

-COC-(l) + HCl (aq) --> -C(C)(C)(H)(OH) (l) + -C(C)(C)(H)(Cl) (l)

I think it balances, and the carbons have 4 bonds.  Take a look at a glucose molecule.

[stonecold]

This is what you get

[kenhung123]

That seems more like a spontaneous clorination and hydrogenation reaction

[m@tty]

Well, it is. HCl is aqueous so it is dissociated into H⁺ and Cl^-.

[Mao]

Most hydrolysis uses acid catalysis, the Cl- is just a spectator ion.

The mechanism would go something like:

1. C-O-C + H+ ---> C-O⁺H-C
the O in glycosidic bond accepts a H+, becomes an O+ (electrophilic attack)

2. C-O⁺H-C ---> C-OH + C⁺
the O+ withdraws electrons from C-O bond, becomes neutral hydroxyl, the glycosidic bond is severed

3. C⁺ + H2O --> C-O⁺H₂
the C+ attracts a water from solution, O in water donates electron to C, becomes O+ (nucleophilic addition)

4. C-O⁺₂ --> C-OH + H+
the O+ withdraws electrons from OH bond, H becomes H+ and leaves the molecule, catalyst is regenerated

but then this is way too advanced for VCE level