Questions tagged [elimination]
Questions whose main focus is on E1, E2, E1cb, or any other type of elimination reaction.
60
questions
0
votes
0answers
29 views
How to rationalise the difference in reactivity between alcoholic and aqueous KOH with alkyl bromides? [duplicate]
Why is it that when we use alcoholic $\ce{KOH}$ with say $\ce{R-Br}$, E2 elimination occurs, whereas with aqueous $\ce{KOH}$, SN reaction takes place, as both have the same nucleophile, ie $\ce{OH^-}$?...
0
votes
0answers
53 views
Why does SN1 not occur as a minor product when a strong nucleophile/strong base attacks a tertiary substrate?
The following is part of a table that appears in my Organic Chem Text (Klein)
Nucleophile Only (e.g. Strong Nucleophile/ Weak base)
$1^\circ - \text{ SN2}\\
2^\circ - \text{ SN1 + SN2}\\ 3^\circ - \...
4
votes
1answer
179 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). ...
0
votes
0answers
18 views
Kinetics for E1cb reactions
We were taught that the kinetics of a reaction is determined by the reactions ‘rate determining step’ which is also the slowest step of the reaction. For E1 as well as E2 reactions the slowest steps ...
3
votes
0answers
46 views
Chugaev and Selenoxide Elimination giving different products
In class we are going over intramolecular elimination. In this question I will provide two similar reactants that provide a different mixture of products.
The chugaev elimination gives a 50/50 ...
2
votes
1answer
734 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 ...
2
votes
2answers
313 views
Reactivity of primary and tertiary alkyl halides
From USNCO 2004, Q58:
The reaction between which pair of reactants occurs the fastest for $[\ce{OH-}] = \pu{0.010 M}$?
(A) $\ce{CH3CH2CH2CH2Cl + OH-}$
(B) $\ce{(CH3)3CCl + OH-}$
(C) $...
3
votes
1answer
62 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)
$\ce{CH_2CH_2}$
...
1
vote
0answers
51 views
Number of products on dehydrobromination
Question:
The total number of alkene products possible from the dehydrobromination of 3-bromo-3-cyclopentylhexane using alcohol KOH is :
This question was asked here before but I am getting more ...
1
vote
1answer
96 views
Can't decide between E1 and E2 mechanisms for this reaction
I can see that due to the substrate being bulky, an elimination reaction will take place and not substitution. The reagent is a strong base. E1 reactions are independent of the base chosen, and E2 ...
0
votes
0answers
133 views
Dehydrogenation/Dehydration of alcohol with Cu
When the vapours of a 1°,2°,3° alcohol are passed over red hot Cu powder at 573K, 1°,2° alcohol undergo dehydrogenation to form an aldehyde or ketone respectively and 3° alcohol undergoes ...
-1
votes
1answer
9 views
Where will the positive charge shift in the E1 mechanism of dehydration of an alcohol? [closed]
My Attempt: according to me the double bond should be in position no. 2 but the correct answer has a double bond in position no.1. Why is it so?
Thanks in advance.
1
vote
1answer
133 views
Elimination reactions of vicinal dibromide
When a vicinal dibromide undergoes elimination reactions in vigorous basic conditions, say in ammonia with $\ce {NaNH2}$, is the product an alkyne or is it a conjugated diene? I would think that the ...
1
vote
0answers
42 views
Please explain the reason why E2 mechanism occurs
I known as that when the weak bases are reacting with 2° alcohol, E1 mechanism occurs. But, in the picture, E2 mechanism is occurred. Why E2 mechanism occurred?
In step [3]
Weak base : pyridine
Did ...
1
vote
1answer
77 views
Do bulky bases participate in substitution reactions?
I see how bulky bases influence the regioselectivity of E2 and E1 reactions. I do not see very much content on the involvement of bulky bases in substitution reactions.
I know that bulky bases like t-...
2
votes
1answer
133 views
Why do *tert*-butyl ethers in diluted sulfuric acid undergo elimation instead of solvolysis?
My organic textbook states the pathway for propyl tert-butyl ether in dilute sulfuric acid will follow E1 with a tertiary carbocation intermediate. It mentions that Sn1 will only occur with good ...
2
votes
2answers
212 views
Can vicinal dihalide produce conjugated alkene in strong base?
In Klein's Organic Chemistry (3rd edition), p. 410, we learn that vicinal dihalide can produce alkyne in strong base. And on Reaxys I see many reactions of this sort. For example:
However, why isn't ...
12
votes
1answer
332 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 ...
2
votes
0answers
169 views
Multiple leaving groups and SN2/E2
In the first stage of the reaction i am confused in which place will the SN2 happen (we have aprotic polar solvent and 4 leaving groups). In my opinion SN2 will happen either to the carbon with F- ...
3
votes
1answer
228 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 ...
5
votes
0answers
303 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 ...
5
votes
1answer
233 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 ...
-2
votes
1answer
380 views
difference between elimination reactions and oxidation reactions [closed]
I know that oxidation reactions involve the loss of hydrogen. But is the mechanism the same
2
votes
1answer
2k 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
258 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
191 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 ...
1
vote
0answers
33 views
Confused about enolate chemistry [closed]
I'm not so sure how this mechanism is supposed to go. Could anyone provide some pointers?
The answer is supposed to be:
In particular, I'm confused about how to break up the two esters.
-1
votes
2answers
2k views
What is the major product of bromobutane and sodium ethoxide?
I know that the it is SN2 reaction and $\ce{Br}$ is the leaving group. Second Carbon(from right side) is the beta carbon. As the $\ce{Br}$ group leaves, $\ce{OH}$ will create a bond with one of the ...
4
votes
1answer
134 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 $\...
5
votes
2answers
894 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 ...
3
votes
2answers
385 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 ...
8
votes
2answers
359 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 ...
4
votes
1answer
600 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 ...
5
votes
2answers
2k 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 -> ...
0
votes
1answer
53 views
Why does this elimination reaction occur? [closed]
I was confused by why the following elimination reaction hits only one of the hydrogens instead of both isomers that are possible. Please help me out!
7
votes
1answer
211 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 ...
5
votes
3answers
855 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 ...
1
vote
1answer
421 views
Which hydrogen does the nucleophile attack in E2 reaction?
This reaction will be both an E2 and SN2, where the E2 will be major and the SN2 minor (If I have done it correctly). I was drawing out the mechanism as I noticed that I was making my nucleophile ...
1
vote
1answer
389 views
How to determine if a reaction proceeds via SN1, SN2, E1 or E2 mechanism?
Give the main product and reaction type: SN1, SN2, E1 , E2.
As the 1-bromohexane is primary and the nucleophile a strong unhindered base, the reaction should be a SN2 reaction. However, the solvent $...
2
votes
0answers
51 views
Conjugated diene t-buOK reaction [duplicate]
Why does the t-buOK abstract the following hydrogen. Doesn't it promote the Hoffman product so it should take the hydrogen from a secondary carbon instead of a tertiary one? Thank you
1
vote
0answers
29 views
Stereochemistry of eliminations similar to E2' elimination
In the E2 reaction, the proton and the leaving group are anti-periplanar typically, so as to attain best overlap of the C-H bonding orbital and C-LG anti-bonding orbital.
In a similar reaction, with ...
3
votes
1answer
405 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) ...
3
votes
0answers
188 views
Order of reaction greater than molecularity?
We were taught that the order of a reaction is either less than or equal to it's molecularity. But then I came across a mechanism called E1CB, where the fact was totally reversed. Here molecularity is ...
0
votes
1answer
34 views
Why does the carbocation in this scheme react under elimination conditions rather than being attacked by water?
In the final step of the mechanism, elimination occurs over substitution, why is this?
1
vote
3answers
2k views
Saytzeff and beta elimination [closed]
I am not able to understand the difference between Saytzeff's rule and beta elimination reaction. Also I am writing two reactions below:
\begin{align}
\ce{CH3-CH2-CH(Br)-CH3 + \underset{\text{(...
3
votes
1answer
233 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 ...
13
votes
1answer
244 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 ...
3
votes
0answers
44 views
What constitutes something being designated as big, bulky in terms of SN2 vs E2
When deciding between $\mathrm{S_N2}$ and $\mathrm{E2}$ for primary and secondary alkyl halides, is there a particular method or molecular size or any other threshold, which allows us to designate ...
5
votes
2answers
222 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:
...
2
votes
1answer
2k views
1,2-dibromopropane reaction with OH and NaNH2 [duplicate]
We have the reaction of the alkyl halide $\ce{CH3-CHBr-CH2Br}$ with 1) $\ce{OH-}$ 2) $\ce{NaNH2}$ The answer is propyne.
I am unable to understand why we have performed an elimination reaction here. ...
