26
votes
Accepted
How is exact racemization possible?
Racemization isn't "exact," but rather very very close to equality. It is just simple probability.
Think of flipping a coin, p=probability for heads, and q=probability of tails. Now for a fair flip ...
- 22.3k
23
votes
Why do we get slightly more inversion product than retention product in SN1 reaction?
It is well known that SN1 reactions often give incomplete racemisation:
Although many first-order substitutions do give complete racemization, many others do not. Typically there is 5–20% inversion, ...
- 17.1k
19
votes
Why do we get slightly more inversion product than retention product in SN1 reaction?
As we know, $\ce{S_N1}$ reaction is stereochemically non specific. This means, that it does not force the formation of one stereoisomer, as $\ce{S_N2}$ does. In $\ce{S_N1}$ reaction, presence of ...
- 5,647
14
votes
Accepted
Stereoselectivity of dihydroxylation with osmium tetroxide
Dihydroxylation responds to steric effects and proceeds at less hindered positions. What will be the major product in the following reaction if the reagent used is osmium tetroxide followed by sodium ...
- 17.1k
12
votes
Accepted
Why does d/l-2,3-dibromobutane on reaction with NaI/acetone give cis-2-butene?
The process of elimination of two bromides which you are thinking is not the correct way how the reaction occurs in this case. First there will be a nucleophilic substitution (preferably SN2) by $\ce{...
- 5,593
11
votes
Accepted
Why do Grignard reagents add to propargyl alcohols?
The addition of Grignards to propargyl alcohols occurs via the alkoxide salt, after deprotonation:[1]
Here:
the reactivity may be explained by the stabilising influence of complexation;
the ...
- 71.1k
10
votes
Accepted
Why do stabilised ylids lead to trans alkenes in the Wittig reaction?
TL;DR: With stabilised ylids, the trans oxaphosphetane is formed, under kinetic control. This is due to minimisation of dipole-dipole repulsions in the transition state for oxaphosphetane formation:
...
- 71.1k
10
votes
Accepted
Models for diastereoselectivity in nitronate additions
Dave Evans taught me that you should generally analyze these types of problems by drawing out allyl conformations.
I've drawn the nitronate to minimize 1,3-allylic strain. Now, the top face of the ...
- 17.3k
9
votes
Can a stereospecific / stereoselective reaction produce a racemic mixture of enantiomers?
You haven't mixed up your terms: a stereoselective reaction is defined as one which preferentially forms one stereoisomer over another. Likewise, all stereospecific reactions are also stereoselective, ...
- 71.1k
9
votes
Accepted
Why is the endo product the major product in a Diels-Alder reaction?
What is the endo-selectivity of Diels-Alder reactions?
If the diene used in the Diels-Alder reaction has asymmetric substituents at the end carbons, and if the dienophile is unsymmetric, then two ...
- 5,647
8
votes
Why does acetate aldol with Evans auxiliary give almost no diasteroselectivity?
1. The acetate aldol reaction
Before looking at selective variants, its worth pointing out what an acetate aldol is.
An acetate aldol (below), is the simplest kind of aldol in which the β-...
- 17.1k
8
votes
Stereoelectronics of the Houk model
1. Origins of the Houk model
The initial idea behind the Houk model was infact provided by Kishi, who proposed that alkenes with a chiral centre on the adjacent carbon adopted a 'reactive conformation'...
- 17.1k
8
votes
How is exact racemization possible?
It's just theory vs. real life. When mixing components, you always have limitations with the purity of chemicals and the accuracy of the balance available.
When you look at chemical reactions which ...
- 633
7
votes
Accepted
Explaining stereochemistry in a sigmatropic 1,3-alkyl migration
Problem #1 is that if you interchange the methyl and hydrogen groups, you get diastereomers, not enantiomers. But this is a relatively minor point. I assume your question is about the ...
- 71.1k
6
votes
Accepted
Stereoelectronic effects in torquoselectivity of 4π electrocyclic opening
The torquoselectivity of the above reactions is dictated by stereoelectronic effects, which (in this case) are significant enough to take precedence over steric effects. In general, electron-donating ...
- 71.1k
6
votes
Accepted
Stereoselectivity of ring closure in intramolecular iodolactonisation
The results of the iodolactonization example presented in this question were published in 1982 by Bartlett, et al.1 While @Aniruddha Deb has captured the essence of the reaction, there is more than ...
- 16.4k
5
votes
Cram's rule for nucleophilic addition to carbonyl groups
Cram's rule, proposed by Donald Cram in 1952,[1] was the earliest model proposed for rationalising the stereoselectivity of nucleophilic additions to carbonyl groups with α-stereocentres. The model ...
- 71.1k
5
votes
Effect of protecting group on diastereoselectivity of LiAlH4 reduction of α-hydroxyketones
I think Jan did an excellent job of answering your question, however I am writing a slightly longer answer largely with more details concerning the Felkin–Anh rule, just so that someone who is not ...
- 8,465
5
votes
Chelation-controlled aldol: why is the α substituent pseudo-axial?
Having the methyl group psuedo-axial means that when the nucleophile approaches, the transitiation state is chair-like. The energy penalty for having the methyl group initially in the less favourable ...
- 17.1k
5
votes
Accepted
Does syn-dihydroxylation form racemic mixture?
Oxidation of alkene to 1,2-syn-diol by cold basic $\ce{KMnO4}$ is a common classification test for alkene in undergraduate organic chemistry laboratory courses. OP had correctly recognized that this ...
- 39.5k
4
votes
Accepted
Why does only the cis-isomer of potassium dioxalatodiaquochromate(III) form?
I'm not an inorganic chemist, but to my understanding, both cis- and trans-dioxalatodiaquochromate(III) ions exist (Note that some also called it as Diaquabis(Oxalato)Chromate (III)). According to ...
- 39.5k
4
votes
Is this photochemical [2+2] cycloaddition enantioselective?
If there is no source of chirality present in the reaction mixture, then there is no reason for the reaction to be enantioselective. In this case there are two possible sources of chirality: firstly ...
- 71.1k
4
votes
Elimination mechanism in alkenes
No E1elimination. E2 in both cases. The E-isomer has a good overlap; syn elimination. The Z-isomer goes through anti elimination.
- 16.4k
4
votes
Effect of protecting group on diastereoselectivity of LiAlH4 reduction of α-hydroxyketones
The $\ce{TBDPS}$ group (tert-butyldiphenylsilyl) is a very bulky protecting group. A silicon atom is bonded to the oxygen, which already acts as a larger carbon atom. That is then substituted by three ...
- 67.3k
4
votes
Accepted
Origin of diastereoselectivity in Simmons–Smith cyclopropanation of allylic alcohol
Yes. Right after Clayden mentions this stereoselectivity, he proceeds to state that this is due to "coordination between the zinc atom and the hydroxyl group in the transition state" (pp. 1067-8, 1st ...
- 71.1k
4
votes
Stereoselectivity of ring closure in intramolecular iodolactonisation
Without further details, I would say that both products are possible.
The reaction that you have given here is an Iodolactonization reaction. You have correctly identified that the tert-butyl group ...
- 6,218
4
votes
Why is only the (E,E)-isomer obtained in Claisen-Schmidt condensation?
If your Claisen condensation was run with lots of heat (reflux?), as is typical, then you will obtain the thermodynamic product. For the alkenes, the (E)-isomer is more thermodynamically stable (...
- 71.1k
3
votes
Stereo selective reaction products
Enyzmatic reduction (as would occur in yeast) could easily be a stereoselective process - probably via selective delivery of a hydride from NADH.
As the other respondent notes, there is simply no way ...
- 126
3
votes
Why do Zr, Sn, and Ti enolates form syn aldols selectively?
For most aldol reactions, the relative 1,2-diastereoselectivity is a consequence of the enolate geometry.*
Both tin and titanium enolates are easily formed, and have been shown to exist predominantly ...
- 17.1k
3
votes
Accepted
Stereoselectivity of Roush crotylboration
This figure from this reference (p. 170). might help you visualize things.
As you can see, the stereochemistry is given by electronic repulsion between the carbonyl group and the incoming aldehyde.
- 1,539
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