[pfnm]

Several questions:

1.

Addition of X2 to unsymmetrical alkenes: eg addition of Br2 to 2-methylbut-2-ene - lecturer says the reaction forms enantiomers (3S)-2,3-dibromo-2-methylbutane + (3R)-2,3-dibromo-2-methylbutane.

What ratio of products areformed? Or are both enantiomers formed in a racemic mixture?

2.

Addition of Br2 to (Z)-2-pentene. The nucleophile Br- adds to the sterically unhindered carbon. The notes say that there is a major product formed, but if the initial bromonium ion can form on both the top face or the bottom face of the alkene, wouldn't (2R,3R)-2,3-dibromopentane be as likely to form as (2S,3S)?
Is the bromonium ion equally likely to form at the top face, and the bottom?

3. Nucleophilic ring opening -
Starting with 2,2-dimethyloxirane, solvent CH3OH in acid, the product according to the notes is 2-methoxy-2-methyl-propan-1-ol.


The lecture notes show first the attack of the oxirane oxygen on the proton in solution, then indicates that the C-O bond and the attack of the nucleophile CH3OH is by 'loose SN2 mechanism':


Looks in the notes like this:



Why, if this rxn occurs in a protic solvent, and the CH3OH is attacking a tertiary carbocation (or a structure that looks stable enoguh to be a tertiary carbocation), is this going by SN2 and not SN1? Don't polar protic solvents destabilise SN2 transition states?

4. A product formed by oxidative addition (dihydroxylation) looks like this:



In the notes, the name is:

(4R,5S,6R)-4-chloro-6-methoxy-5-methyl-oct-1-en-5-ol.

But this name adds to a lower total:

(3R,4S,5R)-5-chloro-3-methoxy-4-methyloct-7-en-4-ol.

Which would be correct?

5. Lastly with reaction mechanisms: I have seen different methods. Is it best to include electron pairs and dipoles on atoms where appropriate, or can a clearer mechanism be drawn without these?

Thank you, if any clarification is needed on any question let me know. Any help is appreciated.

(If I ought to split these into 5 separate topics I can do so)

[pfnm]

for Question 2:


This is taken from the notes:



The text with the image reads:

"UNSYMMETRICAL ALKENES
STERIC EFFECTS
− If both carbons of the alkene produce the same
type of carbocation (eg both secondary) then steric
effects become important
− The nucleophile will attack the least sterically
hindered position to produce the major product
(note that a minor amount of the other
regioisomer may also be produced)"

Why do the notes suggest a major product, wouldn't the enantiomers be formed in equal proportion, so a racemic mix?
Because initially the bromonium ion must form on the top face or the bottom face of the alkene, and it is equally likely to form on the top face as the bottom face.

Any help is appreciated.

[orgopete]

Yes, a racemic mixture would result with both isomers equally. The bromide would open at top and bottom statistically equally. I would expect little difference in the two secondary carbons as well, though with a dibromide product, the difference cannot be discerned. If the bromohydrin were being prepared, you could determine the ratio more accurately.

[Smrt guy]

1.  Any reaction that begins without optically active reactants and does not use optically active reagents/catalysts will result in products without optical activity (i.e. racemic or achiral products).

2.  The R,R and S,S products will be formed in a racemic mixture but the reaction is anti- stereoselective.  This means that the product will have the Br atoms oriented anti to each other where applicable.  In this case, it means that R,R and S,S will be formed preferentially relative to R,S and S,R.

3.  The transition state does not look like a traditional S_N1 or S_N2 reaction, but somewhere in between.  A weak C-O bond to the tertiary carbon is displaced by the incoming nucleophile.  Tertiary centers cannot typically undergo S_N2 substitution but the reaction takes place because the lengthened C-O bond in the transition state allows the approach of the incoming nucleophile.

4.  I quit trying to deal with nomenclature after undergrad.  No one uses it except in patents really.

5.  With reaction mechanisms it is important to explicitly include all lone pairs that are being donated in the reaction.  Any that are not involved can be omitted but it is not wrong to include them.  I don't know what you mean about dipoles but typically you should draw all compounds in their most stable resonance form (although drawing them in a less stable one to clarify their reactivity is not wrong).

[pfnm]

Thank you both tremendously.

Smrt guy, out of interests sake, how do chemists in any industry refer to molecules without specific nomenclature? Do they use images, or abbreviated names or something else altogether?

Quite an emphasis seems to be put on it for first and second year organic chemistry in my case.

Thanks again

[orgopete]

Re: nomenclature

Smrt guy's comments are nearly correct. If we look at a bit of history, Chemical Abstracts provided two ways to find compounds, the MF and nomenclature. That has been superseded by structure searching. While nomenclature was absolutely essential, now it is not so essential. Depending on what you might be looking for, you may encounter incorrect structures being assigned in the literature. Either the structure was incorrectly created from the name or the product was wrong, but this is not common. In those cases, it can be helpful to know different nomenclatures.

If SG's experience is anything like mine, my first impression with IUPAC nomenclature was, "Who uses that?" I thought CAS nomenclature was more standard. However, subsequently I have thought nomenclature was part of systematic thinking. Systematic thinking could be thought of as the virtual essence of organic chemistry. Systematic thinking allows a chemist to understand what methyl ethyl chickenwire is, than to come up with an IUPAC consistent name for it.

Now, if we jump back to the original question about the names, both options allow a chemist to draw the same compound. That is, the name is readily interpretable. For the purpose of searching for a compound from the name, then the name should be unique. That should allow one to reliably search for a name without considering all of the possible alternatives.

Since the original question asked about the rules, the name in the notes (while understandable) is not correct, but not for the reason stated. The lowest locants apply to choose between two options. This name follows a priority rule. The highest ranked substituent takes highest priority and receives the lowest locant. In this case, it is an alcohol. It determines the numbering of the chain. The name then simply follows alphabetically and with the according locants.

[Smrt guy]

Strictly speaking almost all names are common names.  It can be quite a hassle when you are looking for a compound in say the aldrich catalog and you have to figure out what it is called (although often if you know the IUPAC name it will be shown on the appropriate page along with the correct search term).  The internet has certainly changed things but I think that a lot of the problem comes from the persistence of archaic common names.  In many cases, IUPAC names are inconvenient for everyday purposes anyway.  No one wants to say a name that is five lines long.