36 votes
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Why does alcoholic KOH prefer elimination whereas aqueous KOH prefers substitution?

In alcoholic solution, the $\ce{KOH}$ is basic enough ($\mathrm{p}K_{\mathrm{a}} =15.74$) to deprotonate a small amount of the alcohol molecules ($\mathrm{p}K_{\mathrm{a}}= 16–17$), thus forming ...
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16 votes

Use of aqueous KOH and alcoholic KOH in dehydrohalogenation reactions

Aqueous $\ce{KOH}$ is alkaline in nature i.e. it dissociates to produce a hydroxide ion. These hydroxide ions act as a strong nucleophile and replace the halogen atom in an alkyl halide. $$\ce{RCl + ...
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14 votes

How does Hofmann elimination work in this reaction?

Wow, this is an amazing question. The expected reactivity is strongly dependent on the exact structure. For starters, trimethylammonium is about the size of a tert-butyl group. So, expect an A value ...
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12 votes
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Rearrangement with sodium iodide (ring opening)

I think the final product will also contain some Bromine atoms attached to it. and the no. of double bonds will be one less than that of the compound given in the picture. I may be wrong but with all ...
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10 votes
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Reaction of conjugated alkene with KNH2

Here is a flowchart of what the commenters have stated. An α-elimination occurs stepwise 1 --> 2 --> 3. There is a phenyl migration (bridged?) to afford tolane 4. Look here for this chemistry.
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10 votes

Determining if the product would be cis or trans after an anti elimination on fischer projections

As you have mentioned, the basic Fischer operations are: Vertical positions are below the plane of paper and horizontal positions are Above the plane of paper, thus you have already remember that when ...
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9 votes
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Why does d/l-2,3-dibromobutane on reaction with NaI/acetone give cis-2-butene?

The process of elimination of two bromides which you are thinking is not the correct way how the reaction occurs in this case. First there will be a nucleophilic substitution (preferably SN2) by $\ce{...
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9 votes
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Elimination of bromine with sodium iodide and electrocyclization

Pöytä: Your conclusion that D contains four bromine atoms symmetrically distributed is sound. The treatment of D with NaI serves to substitute one, if not two bromine atoms with iodine to ...
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9 votes
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Determining if the product would be cis or trans after an anti elimination on fischer projections

There are two possible eliminations that can happen (see the image below) As you can see, we then have two final products depending upon which $\ce{H}$ is eliminated. From the image 1a is cis and 2a ...
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8 votes
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Why does treatment of a geminal dihalide with sodium amide lead to an alkyne and not an allene?

Sodamide is bloody dangerous. If your bottle is too old, it likely has begun to oxidize some and you have potential azides or other polynitrogen compounds that may lead to an explosion. Even with ...
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  • 6,488
8 votes

Why does alcoholic KOH prefer elimination whereas aqueous KOH prefers substitution?

$\ce{OH-}$ acts as a nucleophile. Reactions carried out in alcohol tend to be elimination reactions, and reactions carried out in water (aqueous) tend to be substitution reactions. If water were ...
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  • 476
8 votes
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Why is this β hydrogen in trans position to the leaving group?

Your assessment of which groups are trans to each other is correct but you have not taken into account that the molecule can adopt other conformers. The classic confomation for E2 elimination is when ...
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  • 15k
8 votes

Dehydration of primary alcohols without a β-hydrogen

In this case, $\mathrm{E2}$ elimination is impossible regardless of condition used, because of lack of $\beta$-hydrogens. However, in acidic conditions, it is possible to have elimination reaction. ...
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7 votes
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How to tell/control whether sodium ethoxide will react by a substitution or elimination pathway?

The above reactions belong to Williamson Synthesis of Ethers, probably the best of the alternatives for prepartion of ethers! The Williamson Ether synthesis reactions follow SN2 mechanism. Since the ...
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  • 1,933
7 votes

Why does this elimination reaction occur?

Conjugated dienes are significantly favorable to non-conjugated dienes (because of delocalization and hybridization, etc.), so this will be the major product. It is not to say that the other reaction ...
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7 votes

Why does alcoholic KOH prefer elimination whereas aqueous KOH prefers substitution?

The reason is quite straightforward- $\ce{OH^-}$ is a weak base and a stronger nucleophile specially under polar protic conditions. Hence substitution occurs. $\ce{RO^-}$ is a strong base owing to ...
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  • 63
7 votes

Origin of alkenes from Wurtz Reaction

The best explanation I was able to find is given by Avery A. Morton , John B. Davidson , Barton L. Hakan, J. Am. Chem. Soc., 1942, 64 (10), 2242–2247. If I understand it correctly, this boils down ...
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7 votes
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Predicting product of amine elimination

In chemistry, you don't solve things like that; you just recall the right class of reactions (Hofmann elimination in this case) and apply that knowledge. You can hardly expect to rediscover that from ...
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7 votes
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Is E1cB possible in the reaction of cyclopentadiene with acetone in presence of ethanolate?

I believe you were right to conclude E1cB would occur. Both of your considerations are exactly on point. The amount of ring strain is minimal -- if we think that it's OK to deprotonate to form a sp2-...
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7 votes
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Rate of dehydration of alcohols having keto group

A carbonyl group normally destabilizes carbocations but stabilizes carbanions. In this particular case, the rate of dehydration isn't guarded by carbocation stability, but by CH- acidity. In acidic ...
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7 votes

How to determine if a base is bulky?

This question about the effect of the bulkiness of bases on the mode of E2 elimination has been studied and was published in 1956 by Brown, et al.[1] The elimination of 2-bromo-2-methylbutane was ...
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7 votes
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Major product in dehydration of alcohol

The major product is the Zaitsev one i.e 1-methylcyclohexene. The OH needs to be protonated first in order for it to leave and moreover it is a very fast leaving group (also we have the case that $\ce{...
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  • 192
7 votes
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Mechanism of reaction of ketones with isoalkanes under conc. sulfuric acid and heat

This was going to be a comment but it got too long. TL;DR: - This is not an answer, rather a justification for why the question is (probably) wrong. This paper (linked by @Rishi) gives us ...
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  • 1,454
6 votes

What is the major product in the reaction of 2,3-dichlorobutane with sodium amide in liquid ammonia?

These are both reasonable mechanisms, and the question outlines well the factors favoring each. In favor of mechanism I: Low temperature suggests kinetic deprotonation Statistically more terminal ...
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6 votes

Reaction of alkyl bromide in presence of piperidine

@User6376297 raised a question: "Could the cyclopropane ring behave like a double bond, making this the analogue of an allyl bromide, and yielding $\ce{R2NCH2CH2CH=CH-tBu}$ by attack on a ...
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6 votes
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How were the Hoffman's and the Zaitsev's rule formulated even before the discovery of the electron?

This excerpt is from an article in J Chem Ed (J. Chem. Educ., 1961, 38 (6), p 297 DOI: 10.1021/ed038p297) describing the contents of Markovnikov's 1870 paper in Liebig’s Annalen (translated into ...
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6 votes
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Recognising a rate-determining step in E1

Anindya Prithvi has said everything you needed to hear in two sentences. It is true that rate determining step for E1 mechanism is the carbocation formation. Thus, I present here the detail energy ...
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6 votes
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Reaction of cyclohexene oxide with LDA

Because of the steric hindrance of the two iPr groups, LDA is a very poor nucleophile. It is, as you note, a very strong base. The two products observed both arise from deprotonation of cyclohexene ...
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6 votes
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Elimination reaction mechanism decision (order = 1 or 2)

$\ce{C-H}$ bond and $\ce{C-D}$ bonds are not the same in energy. $\ce{C-D}$ bond is stronger. So breaking $\ce{C-D}$ bond is more difficult and slower than breaking $\ce{C-H}$ bond. I hope you know ...
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  • 1,773
5 votes
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Hofmann rule and deviation from E2 mechanism

The difference between the different eliminations like E1, E2 or Hoffmann elimination is the proper choice of the base and proper solvent medium. Let's take your example i.e. 2-bromo-2-methylbutane, ...
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