38
votes
Accepted
Addition of hydrogen bromide to 1,3-butadiene: thermodynamic and kinetic control
Thermodynamic and kinetic control
This is a classic example of the concept of thermodynamic versus kinetic control of a reaction. Take a look at this energy profile diagram.1 The horizontal axis is a ...
- 70.8k
20
votes
Why can't Pd/C and H2 reduce both the alkene and carbonyl portions of α,β-unsaturated carbonyls?
You hit it right on the nose. The real key piece of information is that given enough time, all the unsaturated bonds will be reduced. This tells you that though the reduction is thermodynamically ...
- 23.9k
17
votes
Accepted
Does aniline react with diazonium ions at C or N?
This is a really interesting question and the answer is that the reaction of benzenediazonium chloride with aniline is a bit different to most of the reactions of benzenediazonium salts in that the ...
- 21.1k
17
votes
Does aniline react with diazonium ions at C or N?
In general, for anilines reaction at nitrogen is kinetically faster than reaction at carbon. However, the C-substituted product is usually more stable than the N-substituted product, so can prevail ...
- 70.8k
15
votes
Accepted
Why do SN2 reactions of alkyl halides proceed differently with KCN and AgCN?
The cyanide anion is an example of an ambident nucleophile, these are nucleophiles which, usually due to delocalisation, are able to attack an electrophile from two or more atoms. In the case of the ...
- 17k
15
votes
Accepted
Why does LiAlH4 reduce the double bond of cinnamaldehyde, whereas NaBH4 does not?
Cinnamaldehyde (1) does not undergo 1,4-reduction but rather 1,2-reduction with LiAlH4. The product of the reduction is 3-phenylpropan-1-ol (2). If conjugate addition were to occur, enolate 3 would be ...
- 16k
15
votes
Accepted
Regioselectivity of epoxide hydrogenation using Pt versus Ni catalyst
First of all, I should clarify that hydrogenolysis of epoxides does not always proceed with such predictable regioselectivity simply based on a switch in catalyst. Often it depends on the substituents ...
- 70.8k
14
votes
Accepted
Why does boron add to the less substituted carbon in the hydroboration of an alkene?
For the hydroboration of isobutene
there is a steric aspect:
And though sterics may play some part, it doesn't explain near 100% regioselectivity. More important in determining regiochemistry, I ...
- 23.9k
14
votes
How does Hofmann elimination work in this reaction?
Wow, this is an amazing question. The expected reactivity is strongly dependent on the exact structure.
For starters, trimethylammonium is about the size of a tert-butyl group. So, expect an A value ...
- 17.2k
12
votes
Why can't Pd/C and H2 reduce both the alkene and carbonyl portions of α,β-unsaturated carbonyls?
This issue is discussed in the following references:
Hydrogenation of chalcones using hydrogen permeating through a Pd and palladized Pd electrodes Electrochimica Acta vol. 55, pages 5831–5839.
and
...
- 40.1k
12
votes
Why do SN2 reactions of alkyl halides proceed differently with KCN and AgCN?
The different reactivities of $\ce{KCN}$ and $\ce{AgCN}$ with alkyl halide, giving nitriles and isonitriles, respectively, has often been cited as a prime example of Pearson’s hard and soft acids and ...
- 66.9k
11
votes
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 ...
- 70.8k
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 ...
- 70.8k
10
votes
Accepted
Why can toluene be chlorinated at either the benzylic position or the ring depending on the conditions?
The halogenation of toluene is possible by two mechanisms. In German, they are referred to by the shorthands KKK and SSS, where KKK stands for Katalysator, Kälte, Kern (catalyst, cold and core) while ...
- 66.9k
9
votes
Accepted
Why is nitrosobenzene electrophilic on O in organocatalytic oxygenations?
Using proline as the catalyst provides enantioselectivity to the process, but also allows N vs O regioselectivity to be controlled (though not always perfectly).
Houk (and subsequently others) applied ...
- 17k
9
votes
Accepted
Most activated position on para-terphenyl for EAS
So, how do I find the most activated position in this compound?
Start by drawing resonance structures of the various possible intermediates (sigma complexes) formed when $\ce{Br^{+}}$ attacks the ...
- 84k
9
votes
Accepted
Regioselectivity in coupling reactions of α-naphthol
The product at the para-position is more stable as it allows the compound to exist in the quinoid form, rather than only in the benzoid form:
This is important as in your case, if the substituent $\...
- 6,208
8
votes
Accepted
Regioselectivity in electrophilic aromatic substitution of benzofuran and indole
In electrophilic substitution of aromatic systems sigma complexes are typically formed as reaction intermediates. Let's look at the sigma complexes formed in the reaction of benzofuran and indole ...
- 84k
8
votes
Accepted
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 ...
- 66.9k
8
votes
Accepted
Why does treatment of a geminal dihalide with sodium amide lead to an alkyne and not an allene?
Sodamide is bloody dangerous. If your bottle is too old, it likely has begun to oxidize some and you have potential azides or other polynitrogen compounds that may lead to an explosion.
Even with ...
- 6,586
8
votes
Claisen rearrangement in substituted ring
Another great example of how it's important to look at actual evidence. There have been various studies on Claisen rearrangement selectivity, here's a combined experimental/theoretical one on meta-...
- 969
8
votes
Accepted
Regioselectivity in iodolactonisation of γ,δ-unsaturated carboxylic acid
There are several arguments. Firstly, five-membered ring formation is generally kinetically more favourable than 6-membered ring formation. As ring size increases, the entropy of activation also ...
- 70.8k
7
votes
Accepted
Which is the major product formed on nitration of benzyl methyl ether?
1) $\ce{NO+2}$ electrophile.
Yes
2) Benzene ring is attached to an activating group, so meta positions
are enriched and electrophile has chances of attacking ortho/para
positions.
The effect ...
- 84k
7
votes
Carbocation formation in electrophilic addition of hydrogen halides to alkenes
The rate determining step is carbocation formation. After which, depending on conditions, the trapping of the cation may or may not be slow enough for [1,2]-shifts.
The [1,2]-shifts are probably ...
- 17.2k
7
votes
Accepted
Why does nucleophilic aromatic substitution occur at C-2 and C-4 of pyridine?
Nucleophilic aromatic substitution on pyridines (or related heterocycles such as quinolines or pyrimidines) regioselectively occurs at the 2- and 4- positions. These can be thought of as being "ortho" ...
- 70.8k
6
votes
Accepted
Why does sulfonation of aniline occur para rather than ortho?
The sulfonyl group is kind of bulky due to the size of the sulfur, so steric hindrance is probably making it energetically disfavored for the group to be placed ortho. It's unlikely you can see this ...
- 17.2k
6
votes
What determines the high degree of selectivity in free radical brominations?
First, let’s take a look at the radical chain reaction that leads to the substitution in question:
$$\begin{align}\ce{Br^. + H-R &-> HBr + R^.}\tag{1}\\[0.4em]
\ce{R^. + Br-Br &-> R-Br +...
- 66.9k
6
votes
Accepted
Regioselectivity in the formation of an imidazo[1,2-a]pyrimidine
I propose the route in the following scheme. I don’t see any unfavourable intermediates. In short, the first step would be formation of a Schiff base with the extracyclic amino group and the aldehyde. ...
- 66.9k
6
votes
Accepted
How is the acetoxy group ortho/para-directing in electrophilic aromatic substitution?
Phenoxide is more reactive than phenol towards electrophilic substitution. However, the number of lone pairs isn't the factor that controls it, because it is not possible for both lone pairs to be ...
- 70.8k
6
votes
Accepted
Separation of alkylated 1,2,4-triazole in solution
Joule and Mills Heterocyclic Chemistry [1] confirms that 1,2,4-triazoles alkylate with ~9:1 selectivity for N1 over N3, citing a paper in Tetrahedron Letters from an industrial group at Merck as the ...
- 17k
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