An elimination reaction is a type of chemical reaction where several atoms either in pairs or groups are removed from a molecule. The removal usually takes place due to the action of acids and bases or action of metals. It can also happen through the process of heating at high temperatures.
Elimination reaction often competes with substitution reactions. In this reaction, a substrate (typically an alkyl halide) eliminates one equivalent (unit) of acid to form an alkene. Two possible mechanisms are available for this elimination reaction – E1 and E2 mechanisms.
|Definition: Elimination reaction is a type of reaction is mainly used to transform saturated compounds (organic compounds which contain single carbon-carbon bonds) to unsaturated compounds (compounds which feature double or triple carbon-carbon bonds).
Besides, it is an important method for the preparation of alkenes.
Elimination Reaction Chemistry Questions with Solutions
Q1. Assuming both the reactions as E1, where will the expected ratio between KH/KD lie between?
a.) nearly I
b.) nearly 3
c.) nearly 5
d.) anything in between 2 and 8
Correct Answer– (a.) nearly I
Q2. Predict the product for the following elimination reaction.
Q3. Which of the following statements is correct?
a.) E2 is a concerted reaction in which bonds break and new bonds form at the same time in a single step
b.) Order of reactivity of alkyl halides towards E2 dehydrohalogenation is found to be 3° > 2° > 1°
c.) In E2 elimination different stereoisomer (diastereomer) converts into different stereo product
d.) All of the above
Correct Answer– (d.) All of the above
Q4. Find the following statement (s) is/are true about the following eliminations-
(1) Hoffmann product is major product in I
(2) Saytzeff product is major product in I
(3) Hoffmann product is major product in II
(4) Saytzeffproduct is major product in II
a.) 1 and 2
b.) 1 and 4
c.) 2 and 3
d.) 3 and 4
Correct Answer– (b.) 1 and 4
Q5. Choose the incorrect statement about alkyl bromide having molecular formula C5H11Br?
a.) One isomeric alkyl bromide undergoes E1 elimination at the fastest rate.
b.) Only one is incapable of reacting by the E2 mechanism.
c.) Only one isomer gives a single alkene on E2 elimination.
d.) 2-Bromopentane gives the most complex mixture of alkenes on E2 elimination.
Correct Answer– (c.) Only one isomer gives a single alkene on E2 elimination.
Q6. Specify the reagent used in the Elimination reaction (𝛃 – elimination) in case of alkyl halides.
Answer. The reagent used in the Elimination reaction (𝛃 – elimination) in case of alkyl halides is alcoholic KOH.
Q7. Fill in the blank.
By elimination reaction alkyl halide is best converted to ____.
Answer. By elimination reaction alkyl halide is best converted to alkene in the form of dehydrohalogenation.
R – CH2 – CH2 – X → R – CH = CH2.
Q8. Define 𝛃 – elimination reaction.
Answer. When hydrogen atom is eliminated from the β-carbon atom (carbon atom next to the carbon to which halogen is attached).
Q9. What is nucleophilic elimination reaction?
Answer. Nucleophilic substitution is a fundamental class of reactions in which an electron-rich nucleophile selectively binds or attacks the positive or partially positive charge of an atom or group of atoms as a substitute for a so-called leaving group.
Q10. Why alcohol is used in elimination reaction?
Answer. Since most alcohols are weaker acids than water, the left side is preferred. The elimination of water from an alcohol is termed dehydration. Given that water is a far better leaving group than the hydroxide ion, acid-catalysis.
Q11. How many distinct alkenes can result from E2 elimination of the compound below? Give their structures and IUPAC names.
Answer. Q12. What is Saytzeff’s rule?
Answer. There are haloalkanes that can undergo elimination in two different ways resulting in two different products. Alkenes with less number of hydrogens on the double-bonded carbon atoms are the preferred product. This process is known as Saytzeff’s rule.
According to Saytzeff rule in dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms.
Q13. What is the difference between substitution and elimination reactions?
The distinction between replacement and elimination reactions is that replacement responses substitute one replacement with another while elimination reactions merely remove the replacement.
Q14. Which diastereomer of 1-bromo-4-t-butylcyclohexane, the cis or the trans, undergoes
elimination more rapidly when treated with sodium ethoxide? Explain your answer.
Answer. The presence of the bulky t-butyl group effectively locks the ring into the most stable conformation, with the bulky group being equatorial.
The cis isomer is the only one of the two that fulfils the anticoplanar arrangement for E2, where the leaving group and adjacent proton must be in the same plane and anti to each other.
15. Arrange the alkyl halides (R – Cl, R –Br, R – F, R – I) according to their reactivity in the elimination reaction.
Answer. The arrangement of alkyl halides according to their reactivity in elimination reaction is as follows-
R – F > R – Cl, R – Br, R – I.
Practise Questions on Elimination Reaction
Q1. Which of the following reactions does not show an addition elimination mechanism?
Q2. Which of the following reacts by the E1 mechanism in ethanol most readily?
Q3. Fill in the blank.
Elimination reaction generally occurs with the formation of ___.
Q4. Write the detailed, step-by-step mechanism for the reaction depicted below.
Q5. The major elimination (E – 1) product P is:
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Dehydration of Alcohols