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Organische Chemie I
For what do we use thin-layer chromatography (TLC)?
• To monitor/control a reaction.
• To test the purity of a compound.
• To set up conditions to separate a mixture of compounds by
flash column chromatography.
• To compare qualitatively a compound in question with a known
substance.
Organische Chemie I
What is disposed of in the yellow canister?
non-halogenated substances
Organische Chemie I
What is disposed of in the white canister?
substances containing water (acids, ...)
Organische Chemie I
When the mobile phase is liquid and the stationary phase is solid;
What chromatography type is suitable for these conditions?
Liquid-Solid Chromatography (LSC)
Organische Chemie I
What is the Hückel's 4n+2 rule?
This rule is used to calculate which (polycyclic) aromatic hydrocarbon structures are stable and which aren't.
For examlpe when drawing naphthalene (two benzene rings connected to eachother):
n is the amout of rings you have (here: 2). So when you just add two benzene rings (one ring has 6 pi electrons) the total amount of electrons results in 12. But this is wrong.
According to Hückel: 4*2 + 2= 10 electrones
So keep in mind while drawing these structures!
Organische Chemie I
What would happen if an addidion of an nucleophilic alcohol to an electrophilic carbonyl group occurs and the electrophilic carbonyl group is an aldehyde or a ketone?
It would collapse back to the starting materials by eliminating the alcohol
Organische Chemie I
Why are tetrahedral intermediates unstable?
Has to do with the leaving group
Once the nucleophile has been added to the carbonyl compound, the stability of the product (or here tetrahedral intermediate) depends on how good the groups are attached to the new tetrahedral carbon atom and at leaving with the negative charge. For the tetrahedral intermediate to collapse (and therefore be just an intermediate and not the final product) one of the groups has to be able to leave and carry off the negative charge from the alkoxide anion formed in the addition
the most stable anion will be the best leaving group
Organische Chemie I
Why do carboxylic acids not undergo substitution reactions under basic conditions?
For a substitution reaction of any carboxylic acid, the leaving group is OH-. As water is a basic environment, acids are about as same electrophilic as esters (not pretty good electrophiles). As esters react well with NH3 to give amides, carboxylic acids should react the same with NH3 - but they don't.
An acid-base reaction occurs instead of a substitution reaction: As carboxylic acids are pretty strong in OC, the hydrogen from the OH group gets deprotonated and an ammonium salt results.
Organische Chemie I
Why does it take more energy to break a C-C sigma bond than a C-C pi bond?
Orbital overlap greater in C-C sigma bond; results in a stronger bond
where as C-C pi bond orbitals only overlap partially; weaker bond
Organische Chemie I
What does isoelectronic mean?
two atoms, ions or molecules that have the same number of electrones and a similar electroneconfiguration, eventhough their from different elements
Organische Chemie I
What are organometallic reagents?
What are the (dis)advantages when using them?
Molecules that have a carbon-metal bond
-> often Li of Mg
-> solvent often THF or ether (like Et2O); because these do not have any acidic protons (that's why their called "aprotic solvents")
-> addition to aldeyde gives a secondary alcohol
-> addition to ketone gives a tertiary alcohol
advantage:
- highly polarized towards carbon (powerful nucleophiles)
disadvantage:
- very strong bases (reaction has to take place in absence of water; water destroys organometallics very rapidly (protonating them to give alkanes))
- used at low temperature -78°C (because of their high reactivity; sublimation temperature of solid CO2)
- also react with oxygen (reaction has to take place in a dry, inert atmosphere of N2 or Ar)
Organische Chemie I
What are the ideal conditions for a reaction between a Nu and an aldehyde/ ketone?
1) Nu is strongly attracted to the partial positive charge on the carbonyl C
2) Nothing (like other atoms/ substituents) blocks the Nu's path to the C in order to come close enough to it to make bond
3) The substituents on the aldehyde/ ketone are not too big to repel one another in the transition state and the product
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