Questions tagged [elimination]

Questions whose main focus is on E1, E2, E1cb, or any other type of elimination reaction.

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37
votes
5answers
41k views

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

What is the basic difference between aqueous and alcoholic $\ce{KOH}$? Why does alcoholic $\ce{KOH}$ prefer elimination whereas aqueous $\ce{KOH}$ prefers substitution?
19
votes
3answers
792 views

Why are two three-membered rings preferred to a single five-membered ring in the reaction of a dihalide with alcoholic KOH?

The given answer is 1,1-dicyclopropylmethanone: Step 1 is abstraction of acidic H by the strong base. Then, there's surely going to be anchimeric assistance. But there can be two places where the ...
14
votes
2answers
2k views

What will be the product when 3,4-dibromocyclohexene is reacted with sodium followed by heating with selenium?

I'm tutoring few Class 12 students for their chemistry exams. I'm stuck with few problems. I don't know, if the key is wrong or I am wrong. According to me product (A) must be the cyclic diene, ...
13
votes
1answer
1k views

Rearrangement with sodium iodide (ring opening)

I'm not able to figure out the mechanism for this reaction. I don't see how sodium iodide in acetone could lead to rearrangement, and surprisingly ring opening! What is this reaction called, if it ...
13
votes
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 ...
13
votes
1answer
428 views

Does (2-fluoroethyl)benzene undergo elimination via E1cb or E2?

I have seen in two different sources claiming that $\ce{PhCH2CH2F}$ gives styrene $\ce{PhCH=CH2}$ when treated with alcoholic $\ce{KOH}$, but one source suggests an E1cb mechanism, while the other ...
11
votes
2answers
10k views

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

I am not able to decide which among 1,3-butadiene and 2-butyne is the major product of this reaction. Will there be even a major product out of the two or will there be a mixture? According to me, ...
10
votes
1answer
646 views

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 ...
9
votes
2answers
679 views

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

I find these kind of problems really confusing for some reason, but I saw a way on the web and tried doing it that way: I considered the basic Fischer operations (Vertical: Below the plane of paper, ...
8
votes
1answer
142k views

Use of aqueous KOH and alcoholic KOH in dehydrohalogenation reactions [duplicate]

What is the difference between aqueous KOH and alcoholic KOH? How do they react differently in dehydrohalogenation?
8
votes
1answer
2k views

Reaction of conjugated alkene with KNH2

I could not think of this reaction fitting into any of the reactions I know except the aryne mechanism. And hence I came up with the following mechanism after seeing the answer (which I've written in ...
8
votes
1answer
202 views

Mechanism of reaction of ketones with isoalkanes under conc. sulfuric acid and heat

Taken from the book of GOC by Dr. O.P Tandon, Himanshu Pandey, Dr. A.K. Virmani, Can anyone elaborate on the mechanism?
7
votes
1answer
359 views

Elimination of bromine with sodium iodide and electrocyclization

I've been trying to solve the following reaction scheme for a while now and I am having trouble especially with the products E and F. For product D I got the following, but I'm unsure about ...
7
votes
1answer
166 views

When is Zaitsev product formed in pyrolytic elimination reaction?

I recently came across a problem of choosing which product to be made in case of pyrolytic elimination reactions. I was taught that usually Hoffman products are formed in such reactions. For e.g.: In ...
7
votes
1answer
283 views

How to decide what pathway a reaction will follow, elimination, addition to a carbonyl or nucleophilic substitution?

There is a substrate such as this, and it is treated with alcoholic KOH. I know that a hydrogen in $\alpha$-position to the carbonyl group will be abstracted, leaving a carbanion. My query lies in ...
6
votes
2answers
2k views

Origin of alkenes from Wurtz Reaction

According to Wikipedia: Also, since the reaction involves free radical species, a side reaction occurs to produce an alkene. This side-reaction becomes more significant when the alkyl halides are ...
6
votes
1answer
747 views

Major product in dehydration of alcohol

Consider the dehydration of the following molecule with conc. $\ce{H2SO4}$ at $443 \text K$: The possible products are: The first one is the Zaitsev product and the second one Hofmann product. ...
6
votes
1answer
1k views

Is E1cB possible in the reaction of cyclopentadiene with acetone in presence of ethanolate?

When cyclopentadiene 1 and acetone 2 are reacted with sodium ethoxide in ethanol, what product is obtained? I was told that the correct product 3 is formed by nucleophilic addition of cyclopentadienyl ...
6
votes
2answers
4k views

Elimination reaction with 1,2-dibromo-4-methylcyclohexane

I was doing a question from a practice exercise on the topic of elimination reactions and alkene synthesis. It is shown below: I predicted that the product would be 1,2-diiodo-4-methylcyclohexane ...
6
votes
1answer
1k views

Elimination of HCl from isomeric chlorocyclohexadienes

Which of these is most easily dehydrohalogenated? a. 5-Chlorocyclohexa-1,3-diene (1) b. 3-Chlorocyclohexa-1,4-diene (2) c. 1-Chlorocyclohexa-1,4-diene (3) d. Both 1 and 3 I encountered this question ...
5
votes
3answers
618 views

Dehydration of primary alcohols without a β-hydrogen

What happens in dehydrations of primary alcohols (in presence of acid catalyst at high temperatures) which don't have a β-hydrogen? For example, in the case of neopentyl alcohol, will it dehydrate as ...
5
votes
2answers
313 views

Predicting product of amine elimination

From section 'Nitrogen Containing Compounds' I spent quite a lot of time here but could not figure out the answer. It's not mentioned in my school text book either. The solution given in my book is: ...
5
votes
3answers
2k views

Hofmann rule and deviation from E2 mechanism

I've learnt that The best reaction conditions for synthesis of alkene by dehydrohalogenation are those that promote an E2 mechanism. (From "Organic Chemistry" by TW Graham Solomons) But, in the ...
5
votes
1answer
248 views

Mechanism for the conversion of dimethyl cyclohexa-1,4-diene-1,2-dicarboxylate to dimethyl benzene-1,3-dicarboxylate using bromine

Question Convert the first compound into the second using only $\ce{Br2}$ I tried solving it by making a 3-ring with $\ce{Br+}$, and then making one of the oxygens break that ring. Than I used $\ce{...
4
votes
2answers
368 views

What are the organic reactions that are taking place in this reaction?

I know that A is the product of a Knoevenagel condensation between the aldehyde and the diethyl malonate, due to the acidic protons of the latter, but I don't know what the reaction between that ...
4
votes
1answer
9k views

Why does d/l-2,3-dibromobutane on reaction with NaI/acetone give cis-2-butene?

This is what I have read/know about the dehalogenation of vicinal dihalides by NaI/acetone: The two halides which are leaving must be on opposite sides (from the mechanism). In case they are present ...
4
votes
1answer
374 views

Reaction of alkyl bromide in presence of piperidine

What are the products of this reaction? I believe there'll be E2 elimination to give the products. Not sure. Help would be greatly appreciated. The above is the product which I thought might be ...
4
votes
1answer
7k views

Reaction of dihalides with excess of NaNH2

Gem-dihalides react with $\ce{NaNH2}$ to give terminal alkynes. Why is a terminal alkyne prefered? For example, I had thought that 2-pentyne would be preferred over 1-pentyne as it would be more ...
4
votes
1answer
676 views

Cyclic elimination reaction

Which of the following will give cyclic elimination by heating? phenol propanoic acid trimethyl isopropyl ammonium hydroxide cyclohexyl propanoate Cyclic elimination by heating is ...
4
votes
1answer
4k views

Why do polar aprotic solvents favour SN2 over E2?

I was reading about Substitution and Elimination Reactions and I came across the following on MasterOrganicChemistry: Polar protic solvents tend to favor elimination (E2) over substitution (SN2). ...
4
votes
1answer
603 views

When does a pinacol undergo dehydration or rearrangement?

What happens when the above pinacol is treated with - (a) conc. sulphuric acid (b) dil. sulphuric acid I'm confused between pinacol rearrangement and dehydration (E1 or E2?), as both possibilities ...
4
votes
2answers
862 views

Why does the E1cb mechanism proceed in the specific way it does?

We have been learning the E1cB mechanism in organic chemistry, but we have not been delving into the motivation as to why this mechanism occurs instead of others. Consider the following mechanism: I ...
4
votes
0answers
38 views

E1CB mechanism with replacement of hydrogen step?

In this question by Jay, one of the sub parts of the total question has the following reaction identified as $\ce{E1CB}$ pathway: $$\ce{Ph-CH2-CH2Br} \text{ on treatment with } \ce{C2H5OD/C2H5O-} \\ \...
4
votes
0answers
48 views

When does reaction of alcohol with alumina give ethers and when an alkene?

This problem given in my book confused me a little bit: when an excess of ethyl alcohol vapour is passed over heated alumina ($250^\circ$C), the main product is? I read that this reaction gives an ...
4
votes
0answers
210 views

Hofmann and Zaitsev products

It is known that, with regard to the elimination reaction, the Zaitsev product is the thermodynamically more stable product, which is also often the more substituted alkene while the Hofmann product ...
3
votes
1answer
522 views

Why is this β hydrogen in trans position to the leaving group?

Problem: One of the most widely used methods for the formation of $\ce{C=C}$ is β - elimination reaction of the following type: The spatial arrangement for a bimolecular elimination (E2) of the type ...
3
votes
2answers
302 views

Is the elimination reaction between 1 1 dibromo cyclopropane and phenyl lithium feasible in alkaline medium (and some heat)

I found this question in one of my question papers for the JEE , My sir gave me an explanation that due to resonance between lone pairs of the bromide and π bond formed. This reasoning doesn't seem ...
3
votes
2answers
6k views

How to tell/control whether sodium ethoxide will react by a substitution or elimination pathway?

Why is the behavior of sodium ethoxide inconsistent? In a reaction with 1-bromobutane, the anionic part of sodium ethoxide substitutes for $\ce{Br}$: $$ \ce{CH3CH2CH2CH2Br + C2H5ONa -> ...
3
votes
2answers
435 views

Recognising a rate-determining step in E1

If there is a hydride/methyl shift possible in E1, then is the carbocation formation the rate-determining step (RDS), or the shift, or is it considered a cumulative step and happens all at once?
3
votes
1answer
113 views

Quaternary Ammonium salts and peroxy acid

I came across this question in my test: This compound is treated with $\ce{CH_3CH_2I}$ and then with $\ce{H_2SO_5}$ and heated. The product is: A) B) $\ce{CH_2CH_2}$ ...
3
votes
1answer
1k views

Why does this E2 reaction occur on the less substituted carbon?

In compound C it eliminates the beta hydrogen attached to the less substituted carbon, rather than attacking the beta hydrogen attached to the tertiary carbon. Why is this the case?
3
votes
1answer
282 views

Stability of carbocations with -M groups in beta position

First part The original question was to arrange the following compounds in the order of their rate of dehydration with conc. $\ce{H2SO4}$:- (A) $\ce{CH3-CH(OH)-CH2-CH=O}$ (B) $\ce{CH3-CH(OH)-CH2-C(=O)...
3
votes
1answer
83 views

Mechanism of acid-catalyzed ring opening of a cyclopropane ring

I have a question regarding following synthesis which apparently only needs a sour medium to occur: What exactly happens as a mechanism? What I thought was: The alcohol group gets protonated and ...
3
votes
1answer
587 views

Does SN2 favor strong nucleophile and SN1 favor weak nucleophile? Does E2 favor a strong base, and E1 favor a weak base?

I understand if you have a protic solvent, it will stabilize the strong base (to form weak acid) or the strong nucleophile. The protons would react with them. To have an SN2 or an E2, so you need an ...
3
votes
1answer
514 views

Elimination mechanism in alkenes

Do E2 elimination reactions occur in alkenes? If not, what mechanism is used? If they do, are they limited by any stereochemical rules as alkanes do (antiperiplanar)? My question is pernitent to the ...
3
votes
1answer
7k views

What conditions would make SN1 products the major products over E1 products, and vice versa?

It seems that if a carbocation intermediate is formed through solvolysis, then the product(s) are going to always be a pretty even mix of SN1 and E1 products. Are there any conditions that greatly ...
3
votes
1answer
159 views

What would be the product of following elimination reaction?

Question Doubt The answer is given B. But why cant it be C after all formation of cyclopropane would increase angle strain?
3
votes
1answer
333 views

Comparing between SN2 and E2

Question I am confused between the questions B. And C. The earlier is SN2 and later is E2. But what makes the difference?
3
votes
1answer
115 views

Does elimination occur when bromonapthalene is reacted with sodium amide in potassium hydroxide?

What should be the product of this reaction? Some books show that an elimination reaction would happen since $\ce{NH2-}$ is a strong enough base to take away a hydrogen on a benzene ring and create a ...
3
votes
0answers
29 views

Why can SN2 reactions occur when the reagents are a primary substrate and a weak nucleophile/base?

The reagents are ethyl bromide and water. In this Khan Academy video at 5:20, the narrator says because ethyl bromide is a primary substrate, the reaction is neither SN1 or E1. This makes sense ...