11
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Regioselectivity of deprotonation of 1,4-dicarbonyl
Here, both the α-$\ce{CH2}$ and α-$\ce{CH3}$ groups have very similar acidities since they are both α to only one carbonyl group. [This is different compared to, for example, a 1,3-dicarbonyl such as ...
- 69.6k
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-...
- 16.9k
8
votes
Accepted
What determines whether an aldol reaction will occur again or eliminate?
Partially yes. The big problem with aldol reactions in general is that there are a lot of species that can react together to form stable compounds resulting in low yield. There are numerous ways to ...
- 6,083
8
votes
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Product of intramolecular aldol condensation of 3-methyloctane-2,7-dione
For the enolates that can be formed, the others have already given comprehensive answers: All four enolates are possible and since we are under thermodynamic control (weak base), we expect o observe ...
- 66k
8
votes
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What is the correct reaction mechanism?
I see what went wrong. Such questions are often confused with aldol or cannizarro as they have a carbonyl group and a base is given.
Actually this is a completely different name reaction called ...
- 445
7
votes
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Why cyclohexane-1,4-dicarbaldehyde does not give aldol condensation?
Your claim is worded very poorly, and taken in the broadest sense, is not true. However, it's more complex than an outright yes or no.
If we're interested in fundamental reactivity, it suffices to ...
- 69.6k
7
votes
Aldol Condensation with Cyclic Molecule
It certainly could undergo the acid-base equilibrium you described, if there's enough energy in the system. Your problem is omitting an important piece of information, which is the temperature at ...
- 949
6
votes
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Will 2-methylbutanal undergo aldol condensation
There is no reason why the aldehyde cannot undergo an aldol reaction with itself. Enolates are not rendered unstable by the presence of additional alkyl substituents; in fact they are considered to be ...
- 69.6k
6
votes
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Self aldol condensation of butanal under basic conditions
Conceptually, we can think of this reaction as occurring in three stages. I've provided diagrams for the self-condensation of acetaldehyde, however you should be able to draw an analogous mechanism ...
- 16.9k
6
votes
Base-promoted rearrangement of β-ketoester
This problem has probably been on everyone's mechanistic chemistry exam at some time or another. Does the alkyl (allyl) just "jump" from one side of the carbonyl group to the other? In most examples, ...
- 14.8k
6
votes
Will cinnamaldehyde undergo self-aldol condensation?
Not every α-hydrogen automatically means that a ketone is enoliseable. It would need to be an α-hydrogen capable of aligning with the π system of the $\ce{C=O}$ bond such that ...
- 66k
6
votes
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Regioselectivity in Claisen condensation and aldol reaction
Claisen and aldol condensations are thermodynamically controlled,* so it is not a question of which enolate forms, it is a question of which product forms.
In your first example, product A can be ...
- 69.6k
6
votes
What does the intramolecular aldol condensation of 6-oxoheptanal form?
Intramolecular-Aldol reactions involves heat, as you have mentioned, and the base used (NaOH) isn't much affected by steric factors. Therefore, the preferred product should be thermodynamically ...
- 423
5
votes
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Reduction using hydrazine and glycol in alkaline medium
If you look carefully, the reagent for the first step is $\ce{O3/H2O}$, which is used for Oxidative ozonolysis. If the mechanism of ozonolysis is drawn, you will realise that a molecule of Hydrogen ...
- 5,525
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 ...
- 16.9k
5
votes
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What does the intramolecular aldol condensation of 6-oxoheptanal form?
B.Anshuman has given a short answer, but I felt more explanation is needed to understand this situation. This reaction can be done in kineticlly controlled conditions (e.g., using $\ce{LDA/THF}$ as a ...
- 38.1k
4
votes
Acidity of aldehydes and ketones and enolate formation
I would like to first comment on the reasoning given as to why aldehydes are more acidic than ketones. The usually provided reasoning is, as stated here:
H atoms are regarded as having no electronic ...
- 9,744
4
votes
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Which Alpha-H will be extracted in intramolecular condensation rxn between ester and ketone
Your immediate concern seems to be which hydrogen is abstracted, 1 or 2, in structure 1 (see below). Examination of pKa’s is useful. pKa’s: EtOH, 16; alkyl ketone, ~20; aliphatic ester, ~25; and β...
- 14.8k
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 (...
- 69.6k
3
votes
How does this ring expansion on a steroid system occur?
It is clear that the first step is ozonolysis with a slightly reductive workup, yielding two ketones. You seem to have already established that. The second step features a base and an alcohol as a ...
- 9,744
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 ...
- 16.9k
3
votes
Aldehyde and ketone reactions
Step P is std base-driven alpha alkylation of ketone. Step Q Wacker details here takes the terminal alkene to methyl ketone then step R is base-catalysed internal aldol mechanism here gives the ...
- 20.2k
3
votes
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Does 4-methylbenzaldehyde give haloform and aldol reactions?
First, Cannizzaro reaction is not given by ketones (your statement says ketones give cannizzaro reaction).
Counter to your analysis, the following reaction will not occur since $\ce{H}$ bonded to ...
- 4,252
3
votes
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Decide major product for first order elimination reaction
The product that you have drawn is very unlikely to be the major product of a mixed aldol condensation. The actual major product in said reaction would be (assuming I get the substrates right) would ...
- 6,710
3
votes
Effect of +I on the alpha hydrogen in aldol reaction
Is the alpha hydrogen acidic enough to react with base?
The answer is yes. As you said +I effect of the methyl group stabilizes the carbocation. It is weakly acidic but not weak enough to not be ...
- 445
3
votes
Accepted
Acidity of alpha hydrogens in enolate formation
I'm interested in the OP's curiosity. I'd agree with Waylander about disfavoring to make 4-membered rings. Yet, it is possible to have two six-membered ring products. The first $\alpha$-proton ...
- 38.1k
2
votes
Regioselectivity of deprotonation of 1,4-dicarbonyl
You can actually take either. The other two $\alpha$ hydrogens are more acidic thermodynamically, but if you used a hindered (kinetic) base at a low temperature, you can predominatly de-protonate at ...
- 17.1k
2
votes
Which Alpha-H will be extracted in intramolecular condensation rxn between ester and ketone
TL;DR
The hydrogen near the ketone will be plucked.
Comparing positive charge characters
Ketones and esters both have carbonyls. The oxygen of the carbonyl pulls electrons away from the carbonyl ...
- 1,508
2
votes
Deciding which α-Η gets abstracted in aldol condensation
There have been tons of articles, review articles and even books written on the subject. The tl;dr version is that it depends on the conditions you use.
In general, the less-hindered enolisation (in ...
- 66k
2
votes
How does dehydration of an alcohol occur in alkaline medium?
Usually aldol products are dehydrated under acidic conditions or at elevated temperatures . But it can also be done employing strong bases like alkoxide or hydroxide ion , and it follows E1CB ...
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