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 SN1 and SN2 mechanisms?
A: SN1 is unimolecular with carbocation intermediate; SN2 is bimolecular and concerted. - 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: Which haloalkanes usually undergo SN2?
A: Primary and methyl haloalkanes. - Q: Which haloalkanes usually undergo SN1 and E1?
A: Tertiary haloalkanes. - Q: What stereochemical change occurs in SN2?
A: Inversion of configuration (Walden inversion). - Q: What stereochemical result occurs in SN1?
A: Racemization. - Q: What stereochemical result is seen in elimination?
A: Formation of alkene with possible E/Z isomers. - Q: Which factors affect the mechanism of substitution or elimination?
A: Substrate structure, nucleophile/base strength, solvent, and temperature. - Q: How does temperature influence E vs S?
A: Higher temperature favors elimination. - Q: What type of base favors E2?
A: Strong, bulky bases. - Q: What type of nucleophile favors SN2?
A: Strong nucleophile, small size, low steric hindrance. - Q: How does steric hindrance influence SN2?
A: More steric hindrance lowers SN2 rate. - Q: What intermediate is common in SN1 and E1?
A: Carbocation. - Q: Which solvent type favors SN1 and E1?
A: Polar protic solvents e.g., water, alcohol. - Q: Which solvent type favors SN2?
A: Polar aprotic solvents e.g., acetone, DMSO. - Q: Is E2 stereospecific?
A: Yes, E2 requires antiperiplanar geometry. - Q: What products form from elimination?
A: Alkenes or alkynes. - 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 solvent polarity favors carbocation formation?
A: Highly polar protic solvents. - Q: What is the role of nucleophile strength in SN2?
A: Strong nucleophile increases SN2 rate. - Q: What is beta-elimination?
A: Removal of hydrogen from beta carbon and leaving group from alpha carbon forming alkene. - Q: How does substrate structure affect elimination?
A: Tertiary substrates favor elimination due to carbocation stability and steric hindrance. - Q: Name a bulky base used in elimination reactions.
A: Potassium tert-butoxide (t-BuOK). - Q: What happens to optical activity in an SN2 reaction?
A: It undergoes inversion. - Q: What is the significance of carbocation rearrangement?
A: In SN1 or E1 carbocation intermediates can rearrange to more stable carbocations. - Q: What is the difference between E1 and SN1 intermediates?
A: Both share same carbocation intermediate. - Q: Can a weak base cause elimination?
A: Yes, via E1 mechanism. - Q: Describe the rate equation for E2.
A: Rate = k[substrate][base]. - Q: What is typical outcome of elimination reactions?
A: Formation of alkenes. - Q: Name typical strong bases facilitating elimination.
A: OH–, OR–, NH2–. - 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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