26 votes

Why do BH3 and NaBH4 have different selectivities?

Your assertions are correct, for the most part. $\ce{BH3}$ is a Lewis acid, and it does not reduce carbonyl groups by directly donating a hydride group like $\ce{NaBH4}$ does. The $\ce{NaBH4}$ ...
ringo's user avatar
  • 24k
12 votes
Accepted

Monoepoxidation of 1,2-dimethylcyclohexa-1,4-diene using m-CPBA

The interaction of the alkene with m-CPBA is primarily nucleophilic/electrophilic in nature, i.e. the π MO of the alkene attacks the O–O σ* orbital. Therefore, the more electron-rich alkene (i.e. more ...
orthocresol's user avatar
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8 votes

m-CPBA Selectivity between Baeyer Villiger Oxidation and Epoxidation?

The answer appears to be "it depends" (perhaps this should not surprise an experienced organic chemist). According to the abstract of the paper by Mendelovici and Glotter [1] in steroid chemistry: ...
Waylander's user avatar
  • 22k
8 votes

Why does the Rosenmund reduction stop at the aldehyde stage?

In general, the Rosenmund reduction isn't actually used very often, mainly since the catalyst is also very good at reducing other functional groups which may be present in the molecule (the catalyst ...
NotEvans.'s user avatar
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7 votes

Reduction of a ketone in the presence of an aldehyde

An easy and convenient method to achieve this is the use of the Luche reduction.[1,2] The ‘organic chemist’s bible’, Strategic Applications of Named Reactions in Organic Synthesis by L. Kürti and B. ...
Jan's user avatar
  • 67.7k
7 votes
Accepted

Reduction of a ketone in the presence of an aldehyde

The best strategy is probably to chemoselectively protect the aldehyde before the borohydride reduction. Protecting groups are derivatives of certain moieties in a molecule that are far less reactive ...
Curt F.'s user avatar
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6 votes
Accepted

Will a Grignard reagent react with an epoxide or episulfide first?

There are quite a few reasons why the episulfide might open before the epoxide: Sulphur is less electronegative than oxygen. Therefore, by Bent's rule , more atomic s-character will be directed by ...
Yusuf Hasan's user avatar
  • 3,186
6 votes
Accepted

Why is a Grignard reagent not protonated by chloramine?

The $\ce{N-H}$ protons are relatively weakly acidic and this makes deprotonation kinetically less favored if an alternative nucleophilic attack mechanism is available. With $\ce{NH3}$ no such ...
Oscar Lanzi's user avatar
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6 votes
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Selective transfer of least/most stable anion from organometallic reagents

It’s not really a dichotomy, it’s just two very different reaction mechanisms. For the cuprate addition, which also applies to general Grignard-type additions, in principle the reaction is reversible ...
Jan's user avatar
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5 votes

Chemoselectivity in reduction of 3′-nitroacetophenone

The key question is how the reduction by the two reagents proceeds. For tetraborohydride the reduction proceeds as a standard nucleophilic addition of hydride ion to carbonyl group. For tin/HCl the ...
permeakra's user avatar
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5 votes
Accepted

Reaction of hydrazine and hydroxylamines with carboxylic acids

For simplicity, I will consider the reaction of acetic acid with ammonia, because the same principles apply. The two alternatives are: $$ \underbrace{\ce{CH3C(O)NH2}}_{\mathrm{amide}} + \ce{H2O} \...
TAR86's user avatar
  • 6,881
5 votes

m-CPBA Selectivity between Baeyer Villiger Oxidation and Epoxidation?

The selective Baeyer-Villiger Oxidation of unsaturated ketones is definitely depends on the conditions used. I do not want to eloborate this fact any further because it was well documented in ...
Mathew Mahindaratne's user avatar
4 votes

Will a Grignard reagent react with an epoxide or episulfide first?

I'm not sure I buy the strain argument either. Is there enough difference between the reactivities that you would get selectivity? In the real world I think you would get a mixture, possible complex ...
Waylander's user avatar
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4 votes
Accepted

Single electron reduction of ketone by Na or Zn

Let the carbonyl compound under consideration be acetone. Case I: Hydrogen Abstraction The species formed are NaH and an allyl-type radical. The resonating system includes an electronegative oxygen ...
Eashaan Godbole's user avatar
3 votes

Why does magnesium prefer to insert into C–Br bonds over C–Cl bonds?

As noted earlier, the determining factor is the greater strength of the carbon-chlorine bond versus carbon-bromine, which leads to a higher activation energy for the magnesium to react with the former....
Oscar Lanzi's user avatar
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3 votes
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Are alkenes or alkynes reduced first with diimide?

Your given answer to the textbook question is seemingly correct. I said seemingly, because this reaction is not well understood to date, even though its first introduction was given at time as far as ...
Mathew Mahindaratne's user avatar
3 votes
Accepted

Can an ether be selectively hydrolysed in the presence of an ester?

There are certain ethers which are somewhat easier to remove than others. If you stipulate that the ether must have a CH2 group immediately adjacent to oxygen, then a couple of possibilities, off the ...
orthocresol's user avatar
  • 70.9k
3 votes

Why does bromine add to a double bond instead of a triple bond if both are present?

I'm going to take "more reactive" as in the alkene undergoes bromination at a faster rate than the alkyne, which seems to imply that the activation energy of the transition state is greater. A ...
dlq's user avatar
  • 226
2 votes

How can an aromatic ketone be reduced in the presence of a nitro group?

A discussion about alternative reduction methods was here Clemmensen reduction vs. Wolff-Kishner reduction. I've not seen nitro groups reduced by the acid-triethylsilane or tetramethyldisiloxane ...
Beerhunter's user avatar
  • 2,201
2 votes

nBuLi and tBuLi can take part in halogen metal exchange OR deprotonate. Is there any way to predict which it favours?

According to Bordwell $\mathrm{p}K_\mathrm{a}$ Table, $\mathrm{p}K_\mathrm{a}$ of $\ce{PhCH2COSPh}$ is $16.9$, which is compatible with alcoholic $\ce{OH}$. We all know that Grignard reagents and ...
Mathew Mahindaratne's user avatar
2 votes

Performing a reaction on one side of piperazine ring

Making your own BOC piperazine is not hard, the general principle is to do the reaction with the monohydrochloride - there is a good procedure here from piperazine, piperazine di-HCl, t-BuOH and ...
Waylander's user avatar
  • 22k
1 vote

Can organocuprates do SN2 reactions with tosylates?

Cuprates (also known as Gilman reagents) do not generally react with isolated ketones. If they did the widely used reaction of cuprates with alpha/beta unsaturated ketone would not work to give the ...
Waylander's user avatar
  • 22k
1 vote
Accepted

Why is the nitrile group not hydrolyzed as well during hydrolysis of imine in Thorpe reaction?

It has to do with hybridization. The nitrogen in a nitrile is sp hybridized; imine sp2 and ammonia sp3. The more s-character in hybrid orbitals, the less basic the electron pair (protonated nitrile ...
user55119's user avatar
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