Questions tagged [regioselectivity]

For questions involving selectivity as to where a functional group reacts.

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32
votes
1answer
31k views

Is the ammonium substituent (-NH3+) really meta-directing in electrophilic substitution?

If we make a resonance structure for the anilinium ion, with positive charge at either the ortho or the para position, we get a pentavalent nitrogen, which is not possible. So, how is the $\ce{-NH3+}$ ...
26
votes
1answer
36k views

Addition of hydrogen bromide to 1,3-butadiene: thermodynamic and kinetic control

The reaction of one equivalent of hydrogen bromide with 1,3-butadiene gives different products at under different conditions:1 The addition of hydrogen chloride also gives rise to similar products.2 ...
51
votes
2answers
16k views

Regioselectivity of acid-catalyzed ring-opening of epoxides

Not to be confused with what is the mechanism of acid-catalyzed ring opening of epoxides. What is the correct order of regioselectivity of acid-catalyzed ring-opening of epoxides: $3^\circ$ > $2^\...
28
votes
2answers
11k views

Does a Grignard reagent react with enones to give the 1,2- or 1,4- product?

Cyclohexenone, an α,β-unsaturated ketone, is electrophilic at both the carbonyl carbon, as well as the β-carbon. So, it can either undergo 1,2-addition to give the allylic alcohol 1, or 1,4-addition ...
13
votes
1answer
12k views

Why does LiAlH4 reduce the double bond of cinnamaldehyde, whereas NaBH4 does not?

I read an answer here about why $\ce{LiAlH_4}$ does not reduce conjugated double bonds, and that $\ce{NaBH_4}$ does. But in the case of cinnamaldehyde, $\ce{LiAlH_4}$ reduces the conjugated double ...
25
votes
2answers
4k views

Electrophilic Aromatic Substitution: Are protonated OH-groups ortho-/para- or meta-directing?

In Electrophilic Aromatic Substitution ($\text{S}_{\text{E}}\text{Ar}$) reactions groups that can donate an electron pair into the aromatic ring, e.g. $\ce{-OH}$ or $\ce{-NH2}$, are ortho-/para-...
20
votes
1answer
42k views

Why does NaBH4 reduce double bonds conjugated to carbonyl groups, while LiAlH4 does not?

I have been going through reduction of aldehydes using $\ce{LiAlH4}$ and $\ce{NaBH4}$. If there is a double bond conjugated with the carbonyl group, $\ce{LiAlH4}$ doesn't reduce it, leading to an ...
15
votes
2answers
4k views

Does aniline react with diazonium ions at C or N?

In the azo coupling of aniline with benzenediazonium cation, I thought of two possible products 1 and 2 that could be formed. However, I can't work out which will be preferred. What would be the major ...
1
vote
1answer
6k views

Why does radical chlorination and bromination of propane occur at different positions?

When propane reacts with chlorine, we will mostly (theoretically) get an isomer with a chlorine on the end. ($\ce{CH_3-CH_2-CH_2Cl}$). However, when propane reacts with bromine, we will mostly get an ...
11
votes
1answer
1k views

Why does boron add to the less substituted carbon in the hydroboration of an alkene?

In the hydroboration of a typical alkene, the boron fragment ends up attached to the less substituted carbon: What are the electronic or steric factors that lead to this regioselectivity?
12
votes
2answers
4k views

Regioselective enolate formation

Our professor only told us that A is favored, and B is not getting formed. I thought about the resonance structures and saw a contradiction to that what the prof told us. To be honest I think my ...
16
votes
1answer
12k views

Regioselectivity in electrophilic substitution of pyrrole

In the electrophilic aromatic substitution (EAS) of pyrrole, why is the alpha position preferred over the beta position so much?
13
votes
1answer
3k views

Why does treatment of a geminal dihalide with sodium amide lead to an alkyne and not an allene?

I've come across the following problem in Klein's Organic Chemistry, 2nd edition: However, I'm a little bit confused about why the terminal alkyne is produced. I believe that an allene should be ...
6
votes
1answer
1k views

Is there any selectivity between the C-2 and C-4 positions of pyridine in SNAr reactions?

In nucleophilic aromatic substitution (SNAr) reactions on pyridines, displacement of leaving groups at C-2 and C-4 is favoured because the negative charge in the intermediate can be delocalised onto ...
22
votes
3answers
10k views

Why do SN2 reactions of alkyl halides proceed differently with KCN and AgCN?

Alkyl halides react with $\ce{KCN}$ to form alkyl cyanides as the main product, whereas the use of $\ce{AgCN}$ leads to isocyanides as the chief product. Why does this happen?
2
votes
1answer
211 views

Regioselectivity in coupling reactions of α-naphthol

I have seen in the following reaction of azo coupling where coupling takes place at position 4 of $\alpha$-naphthol. Now the electron density accumulation is similar at both position $4$ as well as $...
6
votes
1answer
1k views

Regioselectivity in Claisen condensation and aldol reaction

I'm having a hard time determining regio-selectivity in those two reactions. At the image above I've drawn one example for each on of them. My question is how do we choose where the deprotonation ...
2
votes
1answer
2k views

Why does sulfonation of aniline occur para rather than ortho?

According to a textbook by K. L. Chugh: Sulfonation [of aniline] is a reversible reaction. The p-isomer, being more stable, does not get desulfonated easily. Hence, p-sulfanilic acid is the major ...
9
votes
3answers
24k views

Nitration of naphthalene and anthracene

In the nitration of compounds such as naphthalene and anthracene, how can you determine which would be the major product? I tried drawing resonance structures for the intermediate formed because ...
4
votes
2answers
983 views

What determines the high degree of selectivity in free radical brominations?

In my organic chemistry class, we were asked what determines the relatively high degree of selectivity in the reaction of a bromine radical with primary, secondary, and tertiary C–H bonds. I said it ...
33
votes
3answers
35k views

Why can't Pd/C and H2 reduce both the alkene and carbonyl portions of α,β-unsaturated carbonyls?

Why is it that the major product of the reduction of chalcones the ketone and not the monoalcohol? In other words, Why isn't the major product a benzyl alcohol? From what I understand, catalytic ...
11
votes
1answer
6k views

Regioselectivity in electrophilic aromatic substitution of benzofuran and indole

Why does benzofuran react preferentially at position 2 when nitrated or formylated, while indole reacts preferentially at position 3 (source: Joule and Mills, Heterocyclic Chemistry 5th ed., p 371)?
15
votes
1answer
12k views

Why is it the middle ring of anthracene which reacts in a Diels–Alder?

I'm wondering why maleic anhydride adds to the middle cycle of anthracene, and not the outer two. I would have expected that a Diels–Alder with the outer ring would be better, because I expected a ...
4
votes
1answer
6k views

Why does nucleophilic aromatic substitution occur at C-2 and C-4 of pyridine?

In the following reaction of 2,3,4-tribromopyridine with sodium methoxide in methanol,[1] why are only the C-2 and C-4 bromines replaced with methoxy groups? Why doesn't the bromine at C-3 also get ...
9
votes
2answers
2k views

Nucleophilic aromatic substitution on pyrimidines: C2 vs C4 selectivity

[There are many similar questions on Chemistry.SE (e.g. 1, 2) and a quick search will reveal all of them, but nothing on this in particular.] The reaction of 2,4-halopyrimidines with nucleophiles ...
5
votes
2answers
192 views

Regioselectivity in iodolactonisation of γ,δ-unsaturated carboxylic acid

In the last step of the iodolactonisation of 3,3-dimethylpent-4-enoic acid,[1,2] why does the carboxylate attack the iodonium to form a 5-membered ring, when a more stable 6-membered ring is possible? ...
3
votes
1answer
603 views

How does base size affect the ratio of kinetic/thermodynamic enolate?

One of the factors that favor kinetic products is bulky bases. So, I'd expect that, for a certain substrate, the use of a more bulky base to favor even more the kinetic product. However, in the table ...
5
votes
0answers
680 views

Why do t-butyl bromide and silver(I) nitrite form the nitroalkane, not the nitrite ester?

We know that reaction of $\ce{AgNO2}$ with alkyl halides is SN1 type reaction and follows HSAB principle. So in the reaction of $\ce{(CH3)3C Br}$ we would expect $\ce{AgBr}$ to form and also a ...