Some OChem Questions

[redkbro]

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Hey everyone, this is my first post on here. I've been studying for about a month using the Kaplan blue book exclusively with my textbooks to back up confusing topics. Got my exam in nine days and there are a few orgo concepts that i'm still struggling to grasp.

1. Sn2, Sn1, E1, E2

I know all the basic stuff that the book tells you about Nu, LG, solvent and substrate, but anytime I encounter a problem about this stuff, I blank on the nucleophiles and telling which is best out of the given options. Can anyone give me a failproof method for determining this, as well as which of the 4 rxns will occur?


2. Oxidation of alkenes/alkynes. More specifically, hot KMnO4 and O3. I understand that KMnO4 will give either carboxylic acids (internal) or CO2 and c.a. (terminal), and I'm pretty sure that applies to both alkenes and alkynes. Right? But the problem is O3..with reducing conditions (Zn, S, or H2O), it will give alkenes or alkynes --> carbonyl products, whether internal or terminal, yes? and then with alkyne oxidizing conditions (H2O2), internal --> carb. acids and terminal --> c.a. + CO2?? and reducing of either --> c.a.?? Sense my confusion?

3. Grignard vs. Gilman. I understand Grignard is MgX and Gilman is CuLi, but these to reagents are used in all the same circumstances, right? i.e. reduction of carbonyls?


Looking to put in 8+ hour days for the next week to nail all this stuff home before the big day. I'm a little worried that Kaplan leaves out some crucial specific details on bio and ochem, but I guess we'll see on test day.

Also, does anyone know of a good place for free gen chem practice? Even if i think i know the material, I have no place to apply it to see if i'm on the right track, ya know?

Wish me luck ya'll

 

[needzmoar]

2. oxidative cleavage of an alkyne will result in an acid.
3. They're not exactly the same. The Gilman is a much weaker Nu than the Grignard. Here are some of the differences:

Gilman:
a) 1,4 addition of a beta unsaturated carbonyl (Michael donor)
b) Will form ketones in the presence of acyl halides

Grignard:
a) extremely strong base - - - 1,2 addition is observed (direct addition) to beta unsaturated carbonyls
b) Will form a tertiary alcohol in the presence of acyl halides.

 

[Antonia]

Let me see if I can help with question 1. I was extremely confused with it too until I watched Chad's videos (very helpful!).

so to distinguish, you first look at the Nucleophile of the reaction. If it is a strong nucleophile (it has a negative charge: NaOCH2CH3), then the reaction will tend to go E2 or Sn2. I think of it as, the Nucleophile can't wait for the LG to come off first, because it is too strong, like a bully, it just wants to "kick out" the LG, can't wait for the LG to leave by itself.

next, to distinguish b/w E2 or Sn2, you look at the carbon attached to your LG. if it is secondary or teriary, then the LG is sort of "blocked," so the reaction will favor E2. if primary or methyl, then Sn2.

if the Nu is weak (HOCH2CH3, it will usually do Sn1 or E1. I don't really know how to distinguish those two (i think it could go either way), but be careful of rearrangements, because you form a carbocation. Also, in order for the reaction to do Sn1 or E1, it must have a protic solvent, because the protic solvent will dissociate into components with + and - charge and further stablize the carbocation.

For E2/Sn2 however, you could have either aprotic or protic solvent, but aprotic is preferred.

 

[needzmoar]

Let me see if I can help with question 1. I was extremely confused with it too until I watched Chad's videos (very helpful!).

so to distinguish, you first look at the Nucleophile of the reaction. If it is a strong nucleophile (it has a negative charge: NaOCH2CH3), then the reaction will tend to go E2 or Sn2. I think of it as, the Nucleophile can't wait for the LG to come off first, because it is too strong, like a bully, it just wants to "kick out" the LG, can't wait for the LG to leave by itself.

next, to distinguish b/w E2 or Sn2, you look at the carbon attached to your LG. if it is secondary or teriary, then the LG is sort of "blocked," so the reaction will favor E2. if primary or methyl, then Sn2.

if the Nu is weak (HOCH2CH3, it will usually do Sn1 or E1. I don't really know how to distinguish those two (i think it could go either way), but be careful of rearrangements, because you form a carbocation. Also, in order for the reaction to do Sn1 or E1, it must have a protic solvent, because the protic solvent will dissociate into components with + and - charge and further stablize the carbocation.

For E2/Sn2 however, you could have either aprotic or protic solvent, but aprotic is preferred.

nice. i was too lazy to type all that out 😛

For SN1 vs E1, they tend to accompany each other (competing reactions) but because the transition state for the substituted product is slightly lower in energy, SN1 tends to dominate (slightly).

 

[redkbro]

@needzmoar do you think that complexity is above the scope of the DAT?

@Antonia- great explanation, that cleared it up for me, thanks. but i'm still wondering, to distinguish strong/weak base (which means how well it accepts H or electrons), would the atom attached to the O have to be in an ionic relationship? like in the examples you gave, the difference between Na+-O-R and H-O-R?

 

[mh0000]

@needzmoar do you think that complexity is above the scope of the DAT?

@Antonia- great explanation, that cleared it up for me, thanks. but i'm still wondering, to distinguish strong/weak base (which means how well it accepts H or electrons), would the atom attached to the O have to be in an ionic relationship? like in the examples you gave, the difference between Na+-O-R and H-O-R?

I haven't taken the exam yet, but I think it's fair game considering these reaction types are central to organic chemistry. A lot of people seem to fail to recognize that many of the reactions they memorize are really just manifestations of these basic reaction types. As to your question, alkoxy anions would be a mildly strong base (not nearly as strong as hydrides, NH2-, grignard, etc), while a neutral alcohol is not.

 

[needzmoar]

@needzmoar do you think that complexity is above the scope of the DAT?

Not sure, I haven't taken it yet. Word on the street is that it's comparable to the o. chem section of destroyer.

 

[needzmoar]

I haven't taken the exam yet, but I think it's fair game considering these reaction types are central to organic chemistry. A lot of people seem to fail to recognize that many of the reactions they memorize are really just manifestations of these basic reaction types. As to your question, alkoxy anions would be a mildly strong base (not nearly as strong as hydrides, NH2-, grignard, etc), while a neutral alcohol is not.

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