Questions tagged [regioselectivity]

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

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5
votes
1answer
117 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 ...
2
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1answer
1k views

How is the acetoxy group ortho/para-directing in electrophilic aromatic substitution?

Why is the $\ce{-OAc}$ group o,p-directing in electrophilic aromatic substitution? How can I compare the activating effect of $\ce{-OAc}$ with $\ce{-OH}$ and $\ce{O-}$? Is it to do with the number of ...
10
votes
1answer
294 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 ...
11
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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?
7
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1answer
123 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)...
6
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1answer
4k 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 ...
11
votes
1answer
830 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 ...
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 ...
4
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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 ...
4
votes
2answers
340 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 ...
26
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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 ...
22
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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?
4
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0answers
374 views

Regioselectivity of the Beckmann rearrangement

It seems to be that the Beckmann rearrangement is a nice way to prepare secondary amines. This can be achieved by removal of oxygen via the use of $\ce{HI}$/$\ce{P}$. However, if ketones are used, ...
5
votes
2answers
27k views

Why can toluene be chlorinated at either the benzylic position or the ring depending on the conditions?

My book mentions a reaction for the preparation of benzaldehyde with toluene in which side chain chlorination of toluene gives (dichloromethyl)benzene, which upon hydrolysis gives benzaldehyde: Why ...
6
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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 ...
2
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2answers
4k views

Why does 1,2-dichloro-4-nitrobenzene undergo SNAr para and not meta to the nitro group?

In the reaction of 1,2-dichloro-4-nitrobenzene with sodium ethoxide, why does ethoxide end up substituting the chlorine para to the nitro group rather than the chlorine meta to the nitro group? I ...
5
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0answers
52 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 ...
2
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0answers
486 views

Selectivity in Suzuki coupling of 2,4-dichloropyrimidine

I've seen many papers (see e.g. Synthesis 2010, 16, 2721–2724) in which the 4-chloro group of 2,4-dichloropyrimidine is selectively reacted under Suzuki coupling conditions. Is there any way where ...
5
votes
2answers
4k 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 ...
13
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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 ...
7
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1answer
17k 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 ...
33
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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 ...
3
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2answers
219 views

Regioselectivity in nitration of a 1,2-disubstituted aromatic compound

For 1,1,4,4-tetramethyl-1,2,3,4-tetrahydronaphthalene: What would be the products following a standard nitration with $\ce{NO2+}$? My reasoning is thus: Since the substituent group is alkyl then it ...
11
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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)?
5
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0answers
158 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 ...
11
votes
2answers
891 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. ...
32
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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 $\ce{-NH3+}$ ...
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 ...
15
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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 ...
16
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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?
11
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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 ...
8
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1answer
1k 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 ...
7
votes
2answers
2k 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 ...
12
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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 ...
3
votes
1answer
757 views

Preference of certain positions during chlorination of an alkene

I am currently doing the questions: Write equations for the reaction of chlorine with propene and 2-butene using structural formulas. So I know that propene is $\ce{H3C-CH=CH2}$ and chlorine is $\ce{...
2
votes
1answer
678 views

Why should epoxides be more reactive at a position where positive charge is stabilised?

In the above reaction, I am told that the nucleophilic chloride anion attacks the trisubstituted carbon because it has the most partial positive character due to its ability to stabilize positive ...
1
vote
1answer
2k views

What is Markovnikov's rule?

What is Markovnikov's rule? How can we use it to predict the products of additions to alkenes? And is it always obeyed, or are there exceptions to this rule?
9
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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 ...
25
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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 ...
3
votes
1answer
1k views

Regioselectivity in Kharasch addition of bromotrichloromethane

When hex-1-ene is treated with bromotrichloromethane in the presence of a peroxide initiator (e.g. dibenzoyl peroxide), what is the regioselectivity of the addition? I know that the radical addition ...
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^\...

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