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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 ...
orthocresol's user avatar
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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 ...
ringo's user avatar
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19 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 ...
orthocresol's user avatar
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18 votes
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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 ...
Waylander's user avatar
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16 votes
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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 ...
user55119's user avatar
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15 votes
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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 ...
NotEvans.'s user avatar
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15 votes
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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 ...
orthocresol's user avatar
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14 votes
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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 ...
ringo's user avatar
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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 ...
Zhe's user avatar
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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 ...
DavePhD's user avatar
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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 ...
Jan's user avatar
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12 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 ...
orthocresol's user avatar
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11 votes
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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 ...
orthocresol's user avatar
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10 votes
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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 ...
Jan's user avatar
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9 votes
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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 ...
NotEvans.'s user avatar
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9 votes
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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 ...
ron's user avatar
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9 votes
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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 $\...
Aniruddha Deb's user avatar
8 votes
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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 ...
ron's user avatar
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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-...
Dennis Cao's user avatar
8 votes
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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 ...
orthocresol's user avatar
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8 votes
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Product of oxymercuration-demercuration reaction

Well, the answer is correct except instead of -OH it must be -OMe because the reagent used is Hg(OAc)2/ MeOH and not Hg(OAc)2/H2O. Here's an illustration of the rection using methanol
daf's user avatar
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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 ...
Zhe's user avatar
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7 votes
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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" ...
orthocresol's user avatar
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6 votes
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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 ...
Zhe's user avatar
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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 +...
Jan's user avatar
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6 votes
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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. ...
Jan's user avatar
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6 votes
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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 ...
orthocresol's user avatar
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6 votes
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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 ...
NotEvans.'s user avatar
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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 ...
orthocresol's user avatar
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6 votes
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Why does LiAlH4 reduce the double bond of cinnamaldehyde, whereas it does not reduce the double bond in crotonaldehyde?

α,β-Unsaturated carbonyl compounds, for the most stable intermediate with respect to $\ce{H-}$ ion prefer 1,4-addition. However, $\ce{LiAlH4}$ prefers 1,2-addition over 1,4-addition. This can be ...
napstablook's user avatar
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