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Elimination vs Substitution Revision Q&A (Class 12 Chemistry)
💯 Elimination vs Substitution Revision Q&A (Class 12 Chemistry)
Q: What is a substitution reaction? A: A reaction where an atom/group in a molecule is replaced by another atom/group.
Q: What is an elimination reaction? A: A reaction where atoms/groups are removed from a molecule to form a multiple bond.
Q: Which reagents favor substitution over elimination? A: Weak bases and strong nucleophiles favor substitution.
Q: Which reagents favor elimination over substitution? A: Strong bulky bases favor elimination.
Q: What is the leaving group in substitution/elimination reactions? A: Usually a halide ion (-X).
Q: What is the difference between E1 and E2 mechanisms? A: E1 has carbocation intermediate; E2 is concerted with base abstracting proton and leaving group leaving simultaneously.
Q: Which mechanism shows first order kinetics? A: SN1 and E1.
Q: Which mechanism shows second order kinetics? A: SN2 and E2.
Q: What does Zaitsev's rule state? A: The more substituted (stable) alkene forms preferentially in elimination.
Q: Why do tertiary alkyl halides undergo SN1 and E1? A: Due to stable tertiary carbocation formation.
Q: What is the kinetic difference between SN1/E1 and SN2/E2? A: SN1/E1 are unimolecular; SN2/E2 are bimolecular.
Q: What is the main product of SN2 reaction of a chiral substrate? A: Product with inverted configuration at chiral center.
Q: What influences whether a reaction undergoes substitution or elimination? A: Strength and bulkiness of base/nucleophile, substrate structure and reaction conditions.
Q: Give an example of SN2 reaction. A: Methyl bromide with OH– to methanol.
Q: Give an example of SN1 reaction. A: Tert-butyl chloride with water to tert-butyl alcohol.
Q: Give an example of E2 reaction. A: 2-bromo-butane with strong base (e.g. OH–) to butene.
Q: Give an example of E1 reaction. A: Tert-butyl bromide with weak base to isobutylene.
Q: Which halide is the poorest leaving group? A: Fluoride ion (F–).
Q: Which will overpower in competition: substitution or elimination? A: Depends on base strength, temperature, and substrate.
Q: What is “concerted” mechanism? A: Bond breaking and making happen simultaneously (SN2, E2).
Q: What is the carbocation intermediate? A: A positively charged carbon atom formed transiently in SN1 and E1.
Q: What is the effect of a strong base on substitution and elimination? A: Strong base favors elimination (especially E2).
Q: What is the effect of weak nucleophile? A: Favors SN1, E1 in polar protic solvents.
Q: What kind of substrates favor SN2 reactions? A: Methyl and primary alkyl halides.
Q: Which pathway is hindered by steric effects? A: SN2.
Q: Which type of alkyl halide usually undergoes E2? A: Primary or secondary with strong bulky base.
Q: Why does increasing temperature increase elimination products? A: Elimination reactions increase entropy more than substitution.
Q: How does leaving group ability affect reaction rate? A: Better leaving group increases rate of substitution and elimination.
Q: What is "anti-periplanar" geometry? A: Geometry where beta hydrogen and leaving group are opposite in the same plane, essential for E2.
Q: What is "Zaitsev product"? A: The more substituted, stable alkene formed preferentially in elimination.
Q: What is "Hofmann product"? A: The less substituted alkene formed due to sterics or bulky base effects.
Q: Can elimination and substitution occur simultaneously? A: Yes, in many reactions both compete.
Q: Name one factor favoring elimination over substitution. A: Use of strong, bulky base.
Q: Name one factor favoring substitution over elimination. A: Use of strong nucleophile but weak base.
Q: What is solvolysis? A: Substitution or elimination where solvent acts as nucleophile or base.
Q: What factor slows SN2 reaction? A: Steric hindrance to nucleophile approach.
Q: Which reaction is favored under high temperature? A: Elimination.
Q: Which reaction generally yields racemization? A: SN1.
Q: Is inversion observed in SN1? A: Partial inversion mixed with retention causing racemization.
Q: What is required for E2 mechanism besides base and substrate? A: Anti-periplanar beta hydrogen and leaving group.
Q: What is a concerted reaction? A: Bonds break and form in single step (SN2, E2).
Q: What is the major difference between SN1 and E1? A: SN1 forms substitution product; E1 forms elimination product from same carbocation intermediate.
Q: Which is a faster process between SN1 and SN2? A: SN2 is generally faster for suitable substrates.
Q: What does "base strength" influence? A: Mainly elimination reactions (especially E2).
Q: What does "nucleophile strength" influence? A: Mainly substitution (SN2).
Q: Why does SN1 favor tertiary substrate? A: Stability of tertiary carbocation intermediate.
Q: What happens to elimination product selectivity with bulky bases? A: More Hofmann (less substituted) product.
Q: What type of solvent favors elimination reactions? A: Polar protic solvents favor E1; polar aprotic solvents facilitate E2.
Q: How does leaving group affect the mechanism? A: Better leaving groups favor both substitution and elimination by speeding up bond breaking.
Q: What substrate class simultaneously favors substitution and elimination? A: Secondary alkyl halides.
Q: What mechanistic feature is common to SN1 and E1? A: Carbocation intermediate species.
Q: What mechanistic feature is common to SN2 and E2? A: Concerted single step mechanism.
Q: Which of SN1 or SN2 produces optically inactive product when starting from chiral center? A: SN1 produces optically inactive or racemized product.
Q: Give an example of elimination reaction in industry. A: Preparation of alkenes from alcohols via dehydration.
Q: What is the role of temperature in elimination reaction? A: Higher temperature favors elimination over substitution.
Q: Name a bulky base used in elimination reactions. A: Potassium tert-butoxide (t-BuOK).
Q: Can elimination occur via carbocation rearrangement? A: Yes, in E1 mechanism.
Q: Which is favored by strong nucleophile and weak base? A: SN2.
Q: Which pathway is common for tertiary alkyl halides? A: SN1 and E1.
Q: What happens when primary alkyl halide reacts with strong bulky base? A: E2 elimination.
Q: What effect does the structure of alkyl halide have on elimination? A: More substituted carbons favor E1/E2 due to stability.
Q: What kind of nucleophile favors substitution? A: Strong, non-bulky nucleophile.
100 Exam-Style MCQs on Elimination vs Substitution Reactions
Which mechanism proceeds via a carbocation intermediate?
(A) SN1 & E1 (B) SN2 & E2 (C) SN2 only (D) E2 only
Which mechanism involves a single concerted step?
(A) SN1 & E1 (B) SN2 & E2 (C) SN1 only (D) E1 only
Which alkyl halide is most likely to undergo SN2?
(A) Methyl halide (B) Tertiary halide (C) Secondary halide (D) None of these
Which base favors elimination over substitution?
(A) Weak nucleophile, weak base (B) Strong nucleophile, weak base
(C) Strong bulky base (D) Weak nucleophile, strong base
What is the stereochemical outcome of an SN2 reaction?
(A) Racemization (B) Inversion of configuration (C) Retention of configuration (D) No stereochemical change
What stereochemistry results from SN1?
(A) Racemization (B) Complete inversion (C) Complete retention (D) None
The rate law for E2 reactions depends on:
(A) Concentration of base only (B) Concentration of substrate only
(C) Concentrations of both base and substrate (D) None of these
Tertiary alkyl halides predominantly undergo:
(A) SN1 and E1 (B) SN2 and E2 (C) SN2 only (D) E2 only
Which reaction is favored at higher temperatures?
(A) Substitution (B) Elimination (C) Both equally (D) Neither
Which of these solvents favors SN1?
(A) Polar protic (B) Polar aprotic (C) Nonpolar (D) None of these
Which is the better leaving group?
(A) F− (B) Cl− (C) Br− (D) I−
Which factor is more important in determining SN2 reaction rate?
(A) Strength of nucleophile
(B) Stability of carbocation intermediate
(C) Temperature
(D) Solvent polarity
What is the preferred base in E2 reactions?
(A) Small, strong base
(B) Bulky, strong base
(C) Weak base
(D) Neutral molecule
In an SN2 reaction, the nucleophile attacks:
(A) The same side as the leaving group
(B) The opposite side to the leaving group (backside attack)
(C) Either side
(D) None of the above
Which alkene is the major product in an elimination reaction according to Zaitsev's rule?
(A) Less substituted alkene
(B) More substituted alkene
(C) Cis alkene
(D) Trans alkene
Which reaction is unimolecular?
(A) SN1 and E1 (B) SN2 and E2 (C) SN2 only (D) E2 only
In an E2 reaction, elimination occurs only if:
(A) The beta hydrogen is syn-periplanar to the leaving group
(B) The beta hydrogen is anti-periplanar to the leaving group
(C) Beta hydrogen and leaving group are adjacent in any orientation
(D) None of these
What happens to optical purity in an SN2 reaction?
(A) It is retained
(B) It is inverted
(C) It is racemized
(D) It stays the same
Which mechanism is favored by a highly reactive nucleophile and primary substrate?
(A) SN1
(B) SN2
(C) E1
(D) E2
How does increased branching near the reaction center affect an SN2 reaction?
(A) Rate increases
(B) Rate decreases
(C) No effect
(D) Reaction mechanism changes
Which mechanism is common when stable carbocations can form?
(A) SN1 and E1
(B) SN2 and E2
(C) SN2 only
(D) E2 only
Which of the following bases is bulky and favors elimination?
(A) OH−
(B) MeO−
(C) t-BuO−
(D) NH2−
In which reaction do the bonds break and form simultaneously?
(A) SN1
(B) SN2
(C) E1
(D) Both SN1 and E1
Which mechanism generally gives a mixture of stereoisomers?
(A) SN1
(B) SN2
(C) E2
(D) None
Which kind of solvent stabilizes carbocations and favors SN1?
(A) Polar aprotic
(B) Polar protic
(C) Nonpolar
(D) None of the above
Which halide is expected to be least reactive in nucleophilic substitution?
(A) Alkyl iodide
(B) Alkyl bromide
(C) Alkyl chloride
(D) Alkyl fluoride
Which elimination mechanism requires the formation of an intermediate?
(A) E1
(B) E2
(C) Both E1 and E2
(D) SN1
Which factor does NOT affect SN1 reaction rate?
(A) Strength of nucleophile
(B) Concentration of substrate
(C) Solvent polarity
(D) Temperature
What major product is expected when 2-bromopropane reacts with OH− at room temperature?
(A) 2-propanol (SN2)
(B) Propene (E2)
(C) 2-propanol (SN1)
(D) Propene (E1)
Which of the following halides reacts exclusively by SN2?
(A) 1-bromopropane
(B) 2-bromopropane
(C) 2-bromo-2-methylpropane
(D) All
In an E1 reaction, the rate depends on:
(A) Substrate only
(B) Base only
(C) Base and substrate
(D) Neither
How does a polar aprotic solvent affect SN2 reactions?
(A) Increases rate
(B) Decreases rate
(C) No effect
(D) Favors elimination
Which of these favors an E2 but not SN2?
(A) Strong nucleophile, weak base
(B) Weak nucleophile, strong base (bulky)
(C) Weak nucleophile, weak base
(D) Strong nucleophile, strong base
What is the stereochemical requirement for E2 elimination?
(A) Syn-periplanar
(B) Anti-periplanar
(C) No stereochemistry requirement
(D) Planar substrate
During SN2 reaction, what is the hybridization of the reactive carbon at the transition state?
(A) sp3
(B) sp2
(C) sp
(D) None
What is the main difference between substitution and elimination reactions?
(A) Substitution replaces atom/group; elimination forms double bond
(B) Both replace atom/group
(C) Both form double bonds
(D) None
Which product is formed predominantly with a bulky base and a primary alkyl halide?
(A) Substitution
(B) Elimination
(C) Both equally
(D) None
Which reaction pathway involves carbocation rearrangement?
(A) SN1 and E1
(B) SN2 and E2
(C) SN2 only
(D) E2 only
Which base/nucleophile set will favor elimination over substitution?
(A) OH−, primary substrate
(B) t-BuOK, primary substrate
(C) CN−, methyl substrate
(D) NH3, tertiary substrate
Which halide would be most reactive in an SN1 reaction?
(A) Methyl chloride
(B) Ethyl chloride
(C) Tert-butyl chloride
(D) Vinyl chloride
Which of the following is a poor nucleophile usually involved in SN1?
(A) Cl−
(B) CN−
(C) H2O
(D) OH−
What is the major product when 2-bromo-2-methylpropane reacts with water?
(A) 2-methyl-2-propanol (SN1)
(B) 2-methylpropene (E1)
(C) Both (A) and (B)
(D) Neither
In E2 mechanism, a strong base will:
(A) Attack the carbocation
(B) Abstract beta hydrogen
(C) Leave
(D) Replace halide
Which is the most reactive substrate for E2 mechanism?
(A) Methyl halide
(B) Primary halide
(C) Secondary halide
(D) Tertiary halide
What is the major difference between SN2 and E2?
(A) SN2 substitution; E2 elimination
(B) Both substitution
(C) Both elimination
(D) None
Which substrate is most hindered in SN2 reactions?
(A) Methyl
(B) Primary
(C) Secondary
(D) Tertiary
Which is a better nucleophile in aprotic solvent?
(A) I–
(B) Cl–
(C) F–
(D) Br–
Which is favored by polar protic solvents?
(A) SN1 and E1
(B) SN2 and E2
(C) Both
(D) Neither
In elimination reactions, what is the typical role of the base?
(A) Substitute halide
(B) Abstract proton to form double bond
(C) Leave molecule
(D) Attack carbocation
What does Hofmann’s rule predict?
(A) Formation of less substituted alkene as major product in elimination with bulky base
(B) Formation of more substituted alkene
(C) Formation of alcohol
(D) None
Which reaction would favor SN1?
(A) 3° haloalkane with weak nucleophile
(B) 1° haloalkane with strong nucleophile
(C) 2° haloalkane with strong bulky base
(D) Methyl halide with weak base
Which factor favors elimination over substitution?
(A) Strong nucleophile
(B) Strong bulky base
(C) Polar aprotic solvent
(D) Low temperature
Which mechanism is favored by low steric hindrance?
(A) SN2
(B) SN1
(C) E1
(D) E2
Which of the following undergoes elimination most readily?
(A) Tertiary haloalkane
(B) Primary haloalkane
(C) Methyl halide
(D) Vinyl halide
Which base is commonly used for E2 eliminations?
(A) NaOH
(B) KOH
(C) t-BuOK
(D) NH3
What is the effect of temperature on elimination reactions?
(A) Increase
(B) Decrease
(C) No effect
(D) Stops reaction
How does a poor leaving group affect elimination/substitution?
(A) Lowers rate
(B) Raises rate
(C) No effect
(D) Changes product
Backside attack is associated with which mechanism?
(A) SN1
(B) SN2
(C) E1
(D) E2
What is the stereochemical outcome of an E2 elimination?
(A) Both E and Z isomers possible
(B) Only E isomer
(C) Only Z isomer
(D) No stereochemistry
Which mechanism is preferred for secondary halides with strong base?
(A) E2
(B) SN1
(C) SN2
(D) E1
Possible to have both substitution and elimination in one reaction?
(A) Yes
(B) No
(C) Only substitution
(D) Only elimination
What does a strong base do in elimination?
(A) Donates electron
(B) Abstracts proton
(C) Leaves
(D) Forms carbocation
What is the rate-determining step in SN1?
(A) Formation of carbocation
(B) Attack by nucleophile
(C) Departure of leaving group
(D) None
What happens to reactivity of haloalkane in SN1 with increased carbocation stability?
(A) Rate increases
(B) Rate decreases
(C) No change
(D) Unpredictable
In SN1 mechanism, what kind of solvent stabilizes the intermediate?
(A) Polar aprotic
(B) Polar protic
(C) Nonpolar
(D) None
Which type of elimination is E1cb?
(A) Elimination via carbanion intermediate
(B) Elimination via carbocation intermediate
(C) Concerted elimination
(D) SN2 reaction
In haloalkanes, what increases substitution over elimination?
(A) Strong bulky base
(B) Lower temperature
(C) Strong base
(D) Higher temperature
Which process is faster in SN1?
(A) Rate-determining step is formation of carbocation
(B) Attack of nucleophile
(C) Leaving group departure
(D) Proton abstraction
How does concentration of nucleophile affect SN1?
(A) No effect
(B) Increases rate
(C) Decreases rate
(D) Stops reaction
What is the preferred product in elimination of 2-bromopropane by strong base?
(A) Propene
(B) 2-propanol
(C) 2-bromopropane
(D) None
Which reagent is commonly used to convert alkyl halide to alkene by elimination?
(A) NaOH
(B) KOH
(C) t-BuOK
(D) NH3
Which species commonly acts as base in elimination reactions?
(A) Nucleophile
(B) Leaving group
(C) Solvent
(D) Base
Which halide is expected to have the fastest SN2 rate?
(A) Alkyl iodide
(B) Alkyl bromide
(C) Alkyl chloride
(D) Alkyl fluoride
Which elimination process is bimolecular?
(A) E1
(B) E2
(C) Both
(D) Neither
Which elimination process involves carbocation formation?
(A) E1
(B) E2
(C) Both
(D) Neither
What is the rate law of SN1?
(A) Rate depends on substrate only
(B) Rate depends on nucleophile only
(C) Rate depends on both
(D) None
Which elimination often competes with SN2 in primary haloalkanes with strong bases?
(A) E1
(B) E2
(C) Both equally
(D) SN1
Which solvent is best for SN2?
(A) Polar protic
(B) Polar aprotic
(C) Nonpolar
(D) None
Why do tertiary haloalkanes give mainly elimination with bulky base?
(A) Steric hindrance prevents substitution
(B) Carbocation forms easily
(C) Strong nucleophile
(D) Prefer SN2
It's really helpful, thanks for the questions
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