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Questions tagged [regioselectivity]

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

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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 ...
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What is the product when 1,4,6-trifluoroazulene is treated with sodium methoxide?

My logic During substitution via Path 1 and Path 2 aromatic intermediates are formed. Via path 3 anti aromatic intermediate is formed. Therefore reaction takes place via path 1 and path 2 only. ...
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470 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 ...
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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? ...
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112 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 ...
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1answer
110 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 ...
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401 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) $\ce{LiAlH4}$, ...
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217 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 $-\ce{OR}$ ...
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171 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 ...
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316 views

Reactivity and selectivity in free radical halogenation of alkanes

I read about the free radical halogenation of alkanes with $\ce{X2}$ and I have the following questions: Given that the first step is the homolytic breaking of the $\ce{X-X}$ bond, and the homolytic ...
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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 ...
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1answer
2k 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 ...
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201 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 ...
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1answer
259 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 ...
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How can organocuprates generated with catalytic Cu(I) be competitive in 1,4-addition to an enone?

Organocoppper 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 $\alpha,\beta$-...
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446 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 ...
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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 ...
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2answers
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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 ...
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Regioselectivity in the aldolcondensation towards (Z)-Jasmon

In the synthesis of (Z)-Jasmone via aldol-condensation as shown below, how can the observed regioselectivity be explained? After all, it is also possible to form the side product shown below. I ...
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Deciding which α-Η gets abstracted in aldol condensation

Consider the base catalysed aldol condensation of methyl ethyl ketone. The first step involves the abstraction of a proton by the base. However I am having trouble deciding whether the proton will be ...
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Carbocation formation in electrophilic addition of hydrogen halides to alkenes

Question: My attempt: I made the carbocation on the carbon labelled $1$ because I think that the positive charge would be slightly more stable on $1$ compared to $2$ due to the inductive effect of $\...
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168 views

Nonsubstituted Cycloalkene HBr Reaction

If I have a cyclohexene with no substituents and add HBr, how do I know what side of the former double bond to place the Br anion? I think I understand Markovnikov's rule, but in my chemistry book ...
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303 views

Why is isopropylbromide formed as the major product but not n-propylbromide from 1-propene?

When 1-propane reacts with HBr, the major product formed is isopropyl bromide and minor product is n-propyl bromide. I know that this is said to happen because the reaction mechanism favours the ...
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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 [...
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1answer
60 views

Separation of alkylated 1,2,4-triazole in solution

I was looking into heterocyclic compounds and their alkylation (i.e. the regioselectivity of the alkylation of the compound). People are able to give pretty exact numbers, for an experiment done in ...
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1answer
427 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 ...
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1answer
153 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 ...
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1answer
215 views

Selectivity in acetonide formation from a polyol

In my total synthesis lecture notes the above example is given in which a polyol is reacted under acetonide forming conditions. The notes point out that this product is the only product, no other ...
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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 ...
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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:...
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Regioselectivity in Friedel–Crafts acylation of thiophene

In the above reaction, I understand that $\ce{AlCl3}$ is acting as a Lewis acid to generate an acylium ion, which is a strong electrophile. Would this electrophile attack at carbon 2 or 3? I think ...
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498 views

Regioselectivity in addition of hydrogen iodide to vinyl chloride

I encountered a question which was stated as: The reaction of vinyl chloride with hydrogen iodide to give 1-chloro-1-iodoethane is an example of anti-Markovnikov addition. Is this true or false? ...
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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 ...
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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 ...
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201 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 ...
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573 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?
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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)...
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1answer
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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 ...
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1answer
439 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 ...
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191 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 ...
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2answers
643 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 ...
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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 ...
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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 ...
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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?
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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, ...
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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 ...
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1answer
592 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 ...
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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 ...
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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 ...
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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 ...