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
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26 votes
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Does Br2/H2O oxidize all aldehydes including carbohydrates?

Aldehydes, including aldoses, are oxidized to their respective carboxylic acids in the presence of $\ce{Br2}$ in $\ce{H2O}$. The reason this reaction is often discussed with carbohydrates is that it ...
ringo's user avatar
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22 votes
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Can hydrogen on platinum reduce carboxylic acids and esters?

This is based off my memory, as far as I can remember what my teacher taught me. Note, instead of $\ce{Pt}$, you can go for $\ce{Ni}$ (or Raney Nickel) or even $\ce{Pd}$ as a catalyst. Functional ...
Berry Holmes's user avatar
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22 votes
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In what way could benzoin give Tollen's test?

Of your three "thoughts", you are correct that an aldol reaction is not an option. Not only is the product "crowded", but the reaction is reversible. [BTW, an aldol condensation occurs when water is ...
user55119's user avatar
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22 votes
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Reasons for negative iodoform test

As @Waylander pointed out, it appears this reaction has not been performed and/or recorded in any literature, so it is quite dangerous to speculate. But keeping that aside, A 3D perspective reveals ...
William R. Ebenezer's user avatar
21 votes
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Can an amide nitrogen be a hydrogen bond acceptor?

If I understood you correctly, you are talking about the peptide bond nitrogens ($\ce{R-C(=O)-\color{red}{N}H-R}$). This is, reduced to its significant chemical functional group an amide, more ...
Jan's user avatar
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20 votes

Does folic acid contain a benzyl or a phenyl group?

Does folic acid contain a benzyl or a phenyl group? This is the question asked in the title. At the first glance to the structure, one would say folic acid consists of phenyl function but not benzyl ...
Mathew Mahindaratne's user avatar
19 votes
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Does acetic acid give a positive result with the iodoform test?

The iodoform reaction proceeds by the mechanism shown below. (The final proton transfer need not occur between the two partners, any other hydroxide could abstract the acid’s proton and any water ...
Jan's user avatar
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19 votes
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Why does LiAlH4 reduce an amide to an amine, but only reduce a ketone/aldehyde to an alcohol?

You've got it correct in your comment with the $\ce{N}$ lone pair assisting. The reason why the $\ce{N}$ lone pair kicks oxygen out, and not the other way round, is because oxygen-based groups are ...
orthocresol's user avatar
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19 votes
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Mechanism of a dephenylation reaction

First, the methoxy- group can be demethylated by $\ce{AlCl3}$, beginning with $\ce{AlCl3}$ coordination to the ether oxygen, followed by an $\mathrm{S_N2}$ reaction: $\hspace{3.6cm}$ $\hspace{0cm}$ ...
ringo's user avatar
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19 votes
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How can benzaldehyde have a pKa of 14.9?

Significant amount of geminal diol of benzaldehyde exists in an aqueous solution of benzaldehyde at 25 °C because $\mathrm{p}K_{\text{hyd}} = 2$ (Ref. 1) The $\mathrm{p}K_{\mathrm a}$ of benzyl ...
Mathew Mahindaratne's user avatar
18 votes
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Mechanism for basic hydrolysis of α-chloronitrile to ketone?

Your analysis is slightly incorrect, actually. If you do a $\mathrm{S_N2}$ on the chloride, hydrolyse the nitrile, and decarboxylate, you end up with the wrong oxidation state: Instead, if you ...
orthocresol's user avatar
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18 votes
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ICHO Synthesis Problem 2018

You have asked quite a number of questions in your post so let's tackle it one by one. Why does the oxidation take place at that particular nitrogen atom? To understand this, we first need to ...
Tan Yong Boon's user avatar
15 votes
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What is the significance of the 2 and 4 positions in 2,4-DNP?

Probably any DNP could work if you could get it. But, because of the directing character of substitutents in electrophilic substitution you inherently favor the 2,4 isomer: the hydrazine group (or ...
Oscar Lanzi's user avatar
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15 votes
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Rearrangements of α-aminoketones in Clemmensen reduction

Regarding this question, there are two things that should be stated beforehand. Firstly, the mechanism of the Clemmensen reduction is not well-understood. There are two plausible pathways that have ...
orthocresol's user avatar
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14 votes
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Why are beta-ketoacids better at decarboxylation than gamma or delta?

It has to do with the stability of the electrons that are ejected from the bond that breaks. In B-ketoacids, the electrons can resonate onto the oxygen and form an enol/enolate intermediate, which is ...
TOLA3HPPA's user avatar
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14 votes

Why can enolizable aldehydes undergo Cannizzaro reactions?

There is never an either–or in chemistry; it is typically a both instead. In the case of a hydroxide ion approaching an aldehyde with an α-hydrogen, it can both deprotonate the hydrogen creating ...
Jan's user avatar
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14 votes
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How can this ketone show Cannizzaro reaction?

There are a number of discussions on this site regarding the Cannizzaro reaction. In strong base there are competing reactions (aldol self-condensation) that can occur with ketones that have at least ...
user55119's user avatar
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14 votes

Reasons for negative iodoform test

There have been a few answers for this question, which make sense on steric hindrance preventing the idoform formation. However, one thing I wouldn’t agree with is almost everybody’s claim of not ...
Mathew Mahindaratne's user avatar
13 votes

How can an α,β-unsaturated ketone tautomerise?

If you have an α,β-double bond next to your carbonyl, a γ hydrogen can also take part in tautomerism. Take crotonaldehyde $\ce{H3C-CH=CH-CHO}$: $$\ce{O=CH-CH=CH-CH3 <<=> HO-...
Jan's user avatar
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13 votes

Are groups other than carbonyl reduced by Clemmensen reduction?

I would look at it in the following way: The oxidation state of the carbonyl carbon is +II, while the oxidation state of the nitrogen in the nitro group is +III. So nitrogen is in a higher oxidation ...
logical x 2's user avatar
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13 votes
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Will the bond order of CO be greater in its existence as carbon monoxide or in metal carbonyl?

In figure 1 you can see the molecular orbital scheme of a $\ce{[ML6]}$ complex including σ and π interactions. The class of ligands is not important; however, care should be taken when ...
Jan's user avatar
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13 votes
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Mechanism of homologation of aldehyde to alkyne: Ohira–Bestmann reaction

This reaction is the Ohira–Bestmann modification1,2 of the Seyferth–Gilbert homologation.3,4 In the original Seyferth–Gilbert homologation, a diazomethane phosphonate is used to convert carbonyl ...
orthocresol's user avatar
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13 votes
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Mechanism of radical cyclisation to form hydroazulene core

This reaction is taken from Tetrahedron Lett. 1995, 36 (17), 3015. (DOI: 10.1016/0040-4039(95)00502-4) The heat (provided by refluxing) is required to initiate the reaction by the decomposition ...
orthocresol's user avatar
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13 votes

Is there intramolecular hydrogen bonding in salicylaldehyde?

It can indeed hydrogen bond, this can (to a certain extent) be observed by NMR. Due to the internal hydrogen bonding, the phenolic proton is surprisingly consistent, and shows no dependence on ...
NotEvans.'s user avatar
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13 votes
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Decarboxylation of Salicylic acid

From (1): Salicylic acid decarboxylated to give phenol and carbon dioxide, at a convenient rate for kinetic measurements, at $200-230^\circ$, in a homogeneous solution of benzoic acid and certain ...
andselisk's user avatar
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12 votes
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Why does the carbonyl group in an acid anhydride have two stretching frequencies?

The two observed C=O frequencies are due to the symmetric and asymmetric stretching modes of the anhydride. Source: Introduction to Spectroscopy, Pavia and Lampman You can see that the lower ...
NotEvans.'s user avatar
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