Questions tagged [regioselectivity]

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

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48
votes
2answers
15k 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^\...
29
votes
3answers
28k 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 ...
25
votes
1answer
27k 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 ...
24
votes
1answer
22k 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 $\ce{-NH3+}$ ...
22
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2answers
5k 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 ...
22
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
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3answers
8k 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?
20
votes
1answer
596 views

Regioselectivity of epoxide hydrogenation using Pt versus Ni catalyst

What conditions are applicable for the transformations X and Y, respectively? (a) $\ce{LiAlH4}$, and $\ce{NaBH4}$ (b) $\ce{NaBH4}$, and $\ce{BCl3}$ followed by $\ce{NaBH4}$ (c) $\ce{LiAlH4}$, ...
14
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1answer
10k 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?
14
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1answer
9k 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 ...
13
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2answers
2k 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 ...
13
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1answer
2k 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 ...
13
votes
1answer
221 views

Ring opening of aziridinylmethyl and oxiranylmethyl radicals

When a radical is alpha to a three-membered ring, it is common for the ring to open via β-scission (to relieve ring strain): When the ring contains a heteroatom there are a couple of different ...
12
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2answers
3k 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 ...
11
votes
1answer
35k 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 ...
11
votes
1answer
619 views

Why do Grignard reagents add to propargyl alcohols?

In Phil Baran's lectures on heterocyclic chemistry at Scripps, one of the lecture problems is concerned with this synthesis of Vioxx: The first step supposedly involves an anti addition of the aryl ...
11
votes
1answer
389 views

Most activated position on para-terphenyl for EAS

Para-terphenyl: it doesn't look pretty with all those math-y numbers, but those are going to come helpful in answering my question! A question asked me to tell the expected product when this reacts ...
11
votes
2answers
690 views

Alkylation of 1-naphthol in trifluoroethanol versus in DMSO

I am supposed to work out which product is formed in which solvent when naphthalen-1-olate is treated with benzyl iodide. DMSO is a polar aprotic solvent, as it does not possess an acidic hydrogen. ...
10
votes
2answers
2k views

Why is SN2 favored greatly over SN2' in this reaction?

Secondary allylic halides can be produced in high yield with the following Mitsunobu-type reaction:[1] The yield being $94~\%$. The last step in the mechanism is: Why is the $\mathrm{S_N2}$ reaction ...
10
votes
1answer
265 views

Why is nitrosobenzene electrophilic on O in organocatalytic oxygenations?

In 2003 Zhong1 and MacMillan2 reported the use of nitrosobenzene, PhNO, in the organocatalytic asymmetric α-oxygenation of carbonyl compounds: which is all nice, except for the slightly ...
10
votes
1answer
249 views

Regioselectivity of an amination reaction of 2,6-dichloro-3-nitropyridine

In the formation of ethyl 4-(6-chloro-3-nitropyridin-2-yl)piperazine-1-carboxylate G, why is the substitution favoured at the 2-position? I would have said that the 6-position substitution is ...
9
votes
3answers
21k 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 ...
9
votes
2answers
841 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 ...
8
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2answers
540 views

How does Hofmann elimination work in this reaction?

In my Chemistry class, we were given this question to solve, and I'm not sure I fully understood the solution. I understand that since the substrate is bulky, Saytzeff elimination can't happen, so it ...
8
votes
1answer
903 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?
8
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1answer
909 views

Regioselectivity of electrophilic aromatic substitution in imidazo[1,2-a]pyrazine

I am supposed to rationalise the regioselectivity of bromination of 8-chloroimidazo[1,2-a]pyrazine, as depicted in the scheme. The left ring kind of looks like a pyridazine. If it's anything like a ...
8
votes
1answer
5k 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)?
8
votes
1answer
268 views

Selectivity in aldol condensation of 7-oxo(6,6-²H₂)octanal

I am supposed to find the product of the following reaction: I know that six-membered rings are more stable than five-membered rings, but the C-D bonds are stronger, so it confuses me where will the ...
8
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0answers
314 views

Sulfonyl azides: diazo or azide transfer?

Sulfonyl azides, $\ce{RSO2N3}$, are capable of acting as sources of electrophilic azide ("$\ce{N+3}$"), but are also capable of effecting diazo transfer, as shown in the scheme below, taken from ref 1:...
7
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1answer
109 views

Regioselectivity in the formation of an imidazo[1,2-a]pyrimidine

In the heterocycle synthesis below (taken from Org. Process Res. Dev. 2006, 10 (3), 398–402), a 5,6-fused heterocycle (an imidazo[1,2-a]pyrimidine) is formed by treating 2-amino-4-(trifluoromethyl)...
7
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2answers
1k views

Hydroboration/oxidation vs acid-catalyzed hydration of 1-phenylpropene

I am trying to predict the products of both the hydroboration/oxidation and acid-catalyzed hydration of 1-phenylpropene: Both C1 and C2 of the double bond are secondary carbons. However, I have the ...
7
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0answers
72 views

How can organocuprates generated with catalytic Cu(I) be competitive in 1,4-addition to an enone?

Organocopper compounds (e.g. $\ce{LiCuR_2}$) are often prepared from an organolithium or Grignard reagent when a soften nucleophile is needed, such as for a conjugate addition to an $\alpha,\beta$-...
6
votes
2answers
658 views

Claisen rearrangement in substituted ring

This is an example of Claisen rearrangement, property of allyl vinyl/phenyl ethers. I believe that the allylic migration should take place to the position ortho to both $\ce{-CH3}$ and $-\ce{OR}$ ...
6
votes
1answer
7k 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 ...
6
votes
1answer
750 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 ...
6
votes
1answer
972 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 ...
6
votes
1answer
2k views

Reaction of propylene oxide with ammonia or hydrazoic acid

I am supposed to identify the products A and B in the following scheme: The three options I have been given are: I think that since $\ce{HN3}$ is a stronger nucleophile than $\ce{NH3}$, B should ...
6
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0answers
585 views

Why does phenol react on oxygen in the Gattermann–Koch reaction?

When discussing the Gattermann–Koch reaction on phenol substrates, it has been said that O-acylation is a problematic competing reaction as in a phenol, there's high electron density on the O atom [...
5
votes
2answers
2k views

Product of intramolecular aldol condensation of 3-methyloctane-2,7-dione

What is the major product formed when 3-methyloctane-2,7-dione is treated with sodium hydroxide? My main issue is where the base deprotonates. Two enolates are possible (I neglected the other ...
5
votes
1answer
15k views

Which is the major product formed on nitration of benzyl methyl ether?

Which is the major and minor product formed from the reaction of benzyl methyl ether with dinitrogen pentoxide? I know that $\ce{NO2+}$ is behaving as an electrophile here and attacking the aromatic ...
5
votes
1answer
90 views

Regioselectivity in opening of cationic three-membered rings

The mechanism of electrophilic addition on alkenes often involves an electrophile attacking the double bond and causing an electromeric shift of electrons. The electrophile then forms a bridged cation ...
5
votes
1answer
251 views

Why is a 7-endo radical cyclisation favoured over a 6-exo in this synthesis?

Normally, according to Baldwin's rules, 3-exo to 7-exo cyclisations are preferred over endo cyclisations. But in the following example, a 7-endo-trig reaction is favoured over a 6-exo-trig and I am ...
5
votes
0answers
632 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 ...
5
votes
0answers
232 views

Stability of extended enolates

I'd expect an extended enolate to be more stable than a normal enolate, when both can form, but my textbook suggests otherwise. What is the stability of an extended enolate compared with a normal ...
5
votes
0answers
48 views

Increasing C-alkoxycarbonylation of enolates

I am working with N-tosyl-2-piperidone and attempts to use the lithium enolate of this compound to perform an nucleophilic acyl substitution reaction on an several chloroformate esters in THF has lead ...
5
votes
0answers
149 views

Regioselectivity in Ziegler–Natta polymerisation of α-olefins

Why does the Ziegler–Natta polymerization display the regioselectivity shown in the image? Internal nucleophiles (coordinated to the metal) should attack the alkene on the less substituted end, like ...
4
votes
2answers
269 views

Regioselectivity of deprotonation of 1,4-dicarbonyl

In the intramolecular aldol condensation of hexane-2,4-dione (above), why does the base abstract one of the $\ce{-CH3}$ protons instead of one of the $\ce{-CH2}$ protons? Both sets of protons are α to ...
4
votes
2answers
136 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? ...
4
votes
1answer
175 views

Stereochemical outcomes in opening of vinyl epoxides

Explain the outcomes of the following reactions. In each case the major product is shown. Firstly, I don't understand why the chloride always attacks on the right hand side of the epoxide. Also,...
4
votes
1answer
153 views

Can someone explain my professor's answers for these alkene additions? [closed]

(a) Why would cyclohexane expand into cycloheptane? Cyclohexane is stable. (b) Why would cyclopentane not ring expand into cyclohexane? In the process a tertiary carbocation would be formed just like ...