9.3.1 Nucleophilic substitution (SN) reactions of Alkyl halides

In the previous section, we learned that halogenoalkanes (alkyl halides) undergo two main types of reactions: nucleophilic substitution and elimination. This section focuses on the nucleophilic substitution (S_N) reactions, which are among the most important reactions in organic chemistry. These reactions are the key to synthesizing a wide variety of compounds from alkyl halides.

Definition

A nucleophilic substitution reaction (S_N) is a reaction in which a nucleophile replaces the halogen atom (leaving group) in an alkyl halide. The general representation of such reactions is:

Nu^- + R-X \rightarrow R-Nu + X^-

• Nucleophile (Nu⁻): An electron-rich species that donates a pair of electrons to form a new bond. It attacks the electron-deficient carbon atom.
• Substrate (R-X): The alkyl halide molecule on which the nucleophile attacks.
• Leaving Group (X⁻): The halogen atom that departs with a pair of electrons.

Core Concept

The core concept is that the polar carbon-halogen bond makes the carbon atom electrophilic (electron-deficient). A nucleophile, which is electron-rich, is attracted to this carbon and attacks it. The carbon-halogen bond breaks, and the halogen leaves as a halide ion. The nucleophile forms a new bond with the carbon, resulting in a substituted product. The reaction is driven by the formation of a more stable species and the good leaving ability of the halide ion.

Detailed Explanation

The textbook introduces the nucleophilic substitution reactions of alkyl halides with a clear explanation.

The General Reaction

The reaction in which a halogen atom of an alkyl halide is substituted or replaced by a strong nucleophile is known as the nucleophilic substitution reaction. The general representation is given as:

Nu^- + R-X \rightarrow R-Nu + X^-

• Attacking Nucleophile: The species that attacks the substrate.
• Substrate: The alkyl halide.
• Leaving Nucleophile (Leaving Group): The halogen that departs with an unshared pair of electrons.
• Product: The substituted product.

Substrate Molecule

The alkyl halide molecule on which a nucleophile attacks is called a substrate molecule.

Nucleophile

It is a species having a lone pair of electrons and donates a pair of electrons to the electrophilic carbon of the alkyl halide. (It may be neutral or negatively charged.)

• HO^- (hydroxide ion)
• NH_2^- (amino group)
• C_2H_5O^- (ethoxide ion)
• Cl^- (chloride ion)
• HS^- (hydrogen sulphide ion)
• Br^- (bromide ion)
• SCN^- (thiocyanate ion)
• NH_3 (ammonia)
• H_2O (water)

Leaving Group (LG)

The halogen of the alkyl halide is called the leaving group. The leaving group is also a nucleophile. It departs with an unshared pair of electrons. The incoming nucleophile must be stronger than the departing one.

• Good leaving groups: Cl^-, Br^-, I^-, HSO_4^-.
• Poor leaving groups: OH^-, OR^-, NH_2^-.
• Special case: Iodide ion (I^-) is a good nucleophile as well as a good leaving group.

Two Fundamental Events in S_N Reactions

There are two fundamental events in a nucleophilic substitution reaction:

• Breaking of the σ bond between carbon and the halogen.
• Formation of the new σ bond between carbon and the attacking nucleophile.

The substitution reaction is greatly influenced by the nature of the attacking nucleophile, the substrate, and the leaving group.

Two Mechanisms: S_N1 and S_N2

The textbook notes that in primary or secondary alkyl halides, attack by the incoming nucleophile and displacement of the leaving group occurs in one step, but in bulky tertiary alkyl halides, attack by the incoming nucleophile and displacement of the leaving group occurs in two steps. Thus, these substitution reactions follow two different mechanisms known as S_N1 and S_N2 reactions.

• S_N1 (Substitution Nucleophilic Unimolecular): A two-step mechanism involving a carbocation intermediate. Favored by tertiary alkyl halides.
• S_N2 (Substitution Nucleophilic Bimolecular): A one-step mechanism where the nucleophile attacks simultaneously as the leaving group departs. Favored by primary alkyl halides.

Intermediate Level

Nucleophilic substitution is a fundamental reaction of alkyl halides. The polar C-X bond makes the carbon electrophilic, attracting nucleophiles. The reaction involves two key events: breaking the C-X bond and forming the C-Nu bond. The mechanism can be S_N2, a concerted one-step process where the nucleophile attacks as the leaving group departs, or S_N1, a two-step process involving a carbocation intermediate. The preferred mechanism depends on the structure of the alkyl halide (primary favors S_N2, tertiary favors S_N1), the strength of the nucleophile, and the solvent. The leaving group ability is also important; good leaving groups (like Cl^-, Br^-, I^-) facilitate the reaction. The textbook lists several common nucleophiles, including OH^-, NH₃, and H₂O.

Important Points

• - Nucleophilic Substitution (S_N): A nucleophile replaces a leaving group (halogen).
• - General Equation: Nu^- + R-X \rightarrow R-Nu + X^-.
• - Nucleophile: Electron-pair donor (e.g., OH^-, NH₃, H₂O).
• - Leaving Group: Halogen atom (e.g., Cl^-, Br^-, I^-).
• - Good Leaving Groups: Cl^-, Br^-, I^-, HSO₄^-.
• - Poor Leaving Groups: OH^-, OR^-, NH₂^-.
• - Two Mechanisms: S_N1 (two-step, carbocation) and S_N2 (one-step, concerted).
• - Primary Alkyl Halides: Favor S_N2.
• - Tertiary Alkyl Halides: Favor S_N1.

Remember

• - S_N reaction: Nucleophile replaces halogen.
• - Nucleophile: Electron-rich, attacks electron-deficient carbon.
• - Leaving group: Halogen, departs with electrons.
• - Good leaving groups: Cl^-, Br^-, I^-.
• - Poor leaving groups: OH^-, NH₂^-.
• - S_N1: Two steps, carbocation intermediate, tertiary.
• - S_N2: One step, concerted, primary.
• - Iodide (I^-): Good nucleophile and good leaving group.

Common Mistakes

• - Forgetting that the nucleophile attacks the carbon atom, not the halogen. The nucleophile is attracted to the electrophilic carbon.
• - Assuming that all leaving groups are equally good. The leaving group ability depends on the halogen: I^- > Br^- > Cl^- > F^-.
• - Confusing S_N1 and S_N2 mechanisms. S_N1 is two-step (carbocation); S_N2 is one-step (concerted).
• - Not recognizing the role of the substrate structure. Primary alkyl halides favor S_N2, tertiary favor S_N1.
• - Thinking that a stronger nucleophile is always a better leaving group. The incoming nucleophile must be stronger than the leaving group, but the leaving group ability is a separate property.
• - Ignoring the competitive elimination reaction. S_N reactions often compete with elimination, especially with strong bases and tertiary substrates.

Quick Revision

Nucleophilic substitution (S_N) reactions are a fundamental class of reactions for alkyl halides. The polar carbon-halogen bond makes the carbon atom electrophilic, allowing a nucleophile to attack and replace the halogen. The reaction involves breaking the C-X bond and forming the C-Nu bond. The halogen acts as a leaving group, and its ability to leave depends on its nature (I⁻ > Br⁻ > Cl⁻ > F⁻). The reaction can proceed via two mechanisms: S_N1 (a two-step process involving a carbocation intermediate, favored by tertiary alkyl halides) and S_N2 (a one-step concerted process, favored by primary alkyl halides). Common nucleophiles include OH^-, NH₃, and H₂O. The reaction is influenced by the substrate structure, the nucleophile, and the leaving group.

60-Second Revision

Nucleophilic substitution (S_N) is a reaction where a nucleophile replaces a halogen on an alkyl halide. The polar C-X bond makes the carbon electrophilic. The reaction can be S_N1 (two steps, carbocation intermediate, tertiary substrates) or S_N2 (one step, concerted, primary substrates). Good leaving groups are Cl^-, Br^-, I^-; poor leaving groups are OH^-, NH₂^-. Common nucleophiles include OH^-, NH₃, and H₂O.

Frequently Asked Questions (FAQs)

Q1. What is a nucleophilic substitution reaction? Answer: A nucleophilic substitution reaction is a reaction in which a nucleophile replaces a leaving group (usually a halogen) on an alkyl halide. The nucleophile attacks the electron-deficient carbon atom, and the leaving group departs with a pair of electrons. The general equation is: Nu^- + R-X → R-Nu + X^-

Q2. What is a nucleophile? Give examples. Answer: A nucleophile is an electron-rich species that donates a pair of electrons to an electron-deficient carbon atom to form a new bond. Examples include OH^- (hydroxide ion), NH₃ (ammonia), H₂O (water), CN^- (cyanide ion), and NH₂^- (amino group).

Q3. What is a leaving group? Give examples of good and poor leaving groups. Answer: A leaving group is an atom or group of atoms that departs with a pair of electrons during a substitution or elimination reaction. In alkyl halides, the halogen is the leaving group. Good leaving groups include Cl^-, Br^-, I^-, and HSO₄^-. Poor leaving groups include OH^-, OR^-, and NH₂^-.

• Breaking of the σ bond between carbon and the halogen (the leaving group).
• Formation of the new σ bond between carbon and the attacking nucleophile.

• S_N1 is a two-step mechanism that involves the formation of a carbocation intermediate. The rate depends only on the concentration of the alkyl halide. It is favored by tertiary alkyl halides and polar protic solvents.
• S_N2 is a one-step mechanism where the nucleophile attacks as the leaving group departs. The rate depends on both the alkyl halide and the nucleophile concentrations. It is favored by primary alkyl halides and polar aprotic solvents.

Q6. Which alkyl halides favor S_N1 and which favor S_N2? Answer: Tertiary alkyl halides favor S_N1 reactions because they form stable carbocation intermediates. Primary alkyl halides favor S_N2 reactions because they are less sterically hindered and the nucleophile can attack more easily. Secondary alkyl halides can undergo both mechanisms, depending on the conditions.

Q7. Why is iodide a good leaving group and also a good nucleophile? Answer: Iodide (I^-) is a good leaving group because the C-I bond is relatively weak and the iodide ion is stable (large, polarizable, and its negative charge is dispersed). It is also a good nucleophile because it is highly polarizable, which allows it to donate its electrons effectively to an electrophilic carbon.

• Q8. Write the equation for the reaction of chloroethane with hydroxide ion.
• Answer: The reaction is:
• CH₃CH₂Cl + OH^- → CH₃CH₂OH + Cl^-
• This is a nucleophilic substitution reaction where the hydroxide ion replaces the chloride ion to form ethanol.

Q9. What is the role of the substrate in a nucleophilic substitution reaction? Answer: The substrate is the alkyl halide molecule on which the nucleophile attacks. Its structure (primary, secondary, or tertiary) determines the preferred mechanism (S_N1 or S_N2). The nature of the C-X bond (polarity and strength) also affects the reactivity of the substrate.

Q10. Why is the C-X bond polar? Answer: The C-X bond is polar because the halogen atom is more electronegative than the carbon atom. This causes the halogen to pull electron density towards itself, creating a partial negative charge on the halogen (δ-) and a partial positive charge on the carbon (δ+). The electrophilic carbon is then susceptible to attack by nucleophiles.

Multiple Choice Questions (MCQs)

• Q1. What is the general representation of a nucleophilic substitution reaction?
• A. R-X + Nu^- → R-Nu + X^-
• B. R-X + H₂ → R-H + HX
• C. R-X + H₂O → R-OH + HX
• D. R-CH₂-CH₂-X → R-CH=CH₂ + HX
• Correct Answer: A
• Explanation: The general equation is R-X + Nu^- → R-Nu + X^-.

• Q2. Which of the following is a good leaving group?
• A. OH^-
• B. NH₂^-
• C. Cl^-
• D. CH₃^-
• Correct Answer: C
• Explanation: Cl^- is a good leaving group.

• Q3. Which of the following is a poor leaving group?
• A. I^-
• B. Br^-
• C. Cl^-
• D. OH^-
• Correct Answer: D
• Explanation: OH^- is a poor leaving group.

• Q4. Which of the following is a nucleophile?
• A. H⁺
• B. NO₂⁺
• C. OH^-
• D. AlCl₃
• Correct Answer: C
• Explanation: OH^- is a nucleophile.

• Q5. What is the mechanism called when the nucleophile attacks simultaneously as the leaving group departs?
• A. S_N1
• B. S_N2
• C. E1
• D. E2
• Correct Answer: B
• Explanation: S_N2 is the one-step concerted mechanism.

• Q6. Which type of alkyl halide favors the S_N1 mechanism?
• A. Primary
• B. Secondary
• C. Tertiary
• D. Methyl
• Correct Answer: C
• Explanation: Tertiary alkyl halides favor S_N1 because they form stable carbocations.

• Q7. Which type of alkyl halide favors the S_N2 mechanism?
• A. Primary
• B. Secondary
• C. Tertiary
• D. Quaternary
• Correct Answer: A
• Explanation: Primary alkyl halides favor S_N2 due to less steric hindrance.

• Q8. What is the first fundamental event in a nucleophilic substitution reaction?
• A. Formation of the new bond.
• B. Breaking of the C-X bond.
• C. Formation of a carbocation.
• D. Removal of a proton.
• Correct Answer: B
• Explanation: The first event is the breaking of the C-X bond.

• Q9. Which of the following is a good nucleophile?
• A. H₂O
• B. NH₃
• C. Cl^-
• D. OH^-
• Correct Answer: D
• Explanation: OH^- is a strong nucleophile.

• Q10. What is the product of the reaction between chloroethane and hydroxide ion?
• A. Ethene
• B. Ethanol
• C. Ethanoic acid
• D. Ethane
• Correct Answer: B
• Explanation: The product is ethanol.

Multiple Choice Questions (MCQs)

• Q1. What is the general representation of a nucleophilic substitution reaction?
• A. R-X + Nu^- → R-Nu + X^-
• B. R-X + H₂ → R-H + HX
• C. R-X + H₂O → R-OH + HX
• D. R-CH₂-CH₂-X → R-CH=CH₂ + HX
• Correct Answer: A
• Explanation: The general equation is R-X + Nu^- → R-Nu + X^-.

• Q2. Which of the following is a good leaving group?
• A. OH^-
• B. NH₂^-
• C. Cl^-
• D. CH₃^-
• Correct Answer: C
• Explanation: Cl^- is a good leaving group.

• Q3. Which of the following is a poor leaving group?
• A. I^-
• B. Br^-
• C. Cl^-
• D. OH^-
• Correct Answer: D
• Explanation: OH^- is a poor leaving group.

• Q4. Which of the following is a nucleophile?
• A. H⁺
• B. NO₂⁺
• C. OH^-
• D. AlCl₃
• Correct Answer: C
• Explanation: OH^- is a nucleophile.

• Q5. What is the mechanism called when the nucleophile attacks simultaneously as the leaving group departs?
• A. S_N1
• B. S_N2
• C. E1
• D. E2
• Correct Answer: B
• Explanation: S_N2 is the one-step concerted mechanism.

• Q6. Which type of alkyl halide favors the S_N1 mechanism?
• A. Primary
• B. Secondary
• C. Tertiary
• D. Methyl
• Correct Answer: C
• Explanation: Tertiary alkyl halides favor S_N1.

• Q7. Which type of alkyl halide favors the S_N2 mechanism?
• A. Primary
• B. Secondary
• C. Tertiary
• D. Quaternary
• Correct Answer: A
• Explanation: Primary alkyl halides favor S_N2.

• Q8. What is the first fundamental event in a nucleophilic substitution reaction?
• A. Formation of the new bond.
• B. Breaking of the C-X bond.
• C. Formation of a carbocation.
• D. Removal of a proton.
• Correct Answer: B
• Explanation: The first event is the breaking of the C-X bond.

• Q9. Which of the following is a good nucleophile?
• A. H₂O
• B. NH₃
• C. Cl^-
• D. OH^-
• Correct Answer: D
• Explanation: OH^- is a strong nucleophile.

• Q10. What is the product of the reaction between chloroethane and hydroxide ion?
• A. Ethene
• B. Ethanol
• C. Ethanoic acid
• D. Ethane
• Correct Answer: B
• Explanation: The product is ethanol.

Multiple Choice Questions (MCQs)

• Q1. What is the role of the nucleophile in an S_N reaction?
• A. It donates an electron pair to the substrate.
• B. It accepts an electron pair from the substrate.
• C. It acts as a leaving group.
• D. It acts as a catalyst.
• Correct Answer: A
• Explanation: The nucleophile donates an electron pair to the electrophilic carbon.

• Q2. What is the role of the leaving group in an S_N reaction?
• A. It attacks the substrate.
• B. It departs with an electron pair.
• C. It donates an electron pair.
• D. It acts as a catalyst.
• Correct Answer: B
• Explanation: The leaving group departs with an electron pair.

• Q3. Which of the following is an example of a nucleophile?
• A. Br⁺
• B. NO₂⁺
• C. NH₃
• D. H⁺
• Correct Answer: C
• Explanation: NH₃ is a nucleophile.

• Q4. Which of the following is an example of a leaving group?
• A. OH^-
• B. NH₂^-
• C. Cl^-
• D. CH₃^-
• Correct Answer: C
• Explanation: Cl^- is a good leaving group.

• Q5. What is the hybridization of the carbon atom in the substrate of an S_N2 reaction?
• A. sp
• B. sp²
• C. sp³
• D. sp³d
• Correct Answer: C
• Explanation: The carbon atom is sp³ hybridized.

• Q6. What is the stereochemical outcome of an S_N2 reaction?
• A. Retention of configuration.
• B. Inversion of configuration.
• C. Racemization.
• D. No change.
• Correct Answer: B
• Explanation: S_N2 proceeds with inversion of configuration.

• Q7. What is the stereochemical outcome of an S_N1 reaction?
• A. Retention of configuration.
• B. Inversion of configuration.
• C. Racemization.
• D. No change.
• Correct Answer: C
• Explanation: S_N1 proceeds with racemization due to the planar carbocation intermediate.

• Q8. Which of the following factors favors S_N1 over S_N2?
• A. Primary alkyl halide.
• B. Tertiary alkyl halide.
• C. Strong nucleophile.
• D. Polar aprotic solvent.
• Correct Answer: B
• Explanation: Tertiary alkyl halides favor S_N1 due to the stability of the carbocation.

• Q9. Which of the following factors favors S_N2 over S_N1?
• A. Tertiary alkyl halide.
• B. Primary alkyl halide.
• C. Polar protic solvent.
• D. Weak nucleophile.
• Correct Answer: B
• Explanation: Primary alkyl halides favor S_N2 due to less steric hindrance.

• Q10. What is the rate law for an S_N2 reaction?
• A. Rate = k[substrate]
• B. Rate = k[nucleophile]
• C. Rate = k[substrate][nucleophile]
• D. Rate = k[substrate]²
• Correct Answer: C
• Explanation: S_N2 is a bimolecular reaction, so the rate depends on both the substrate and nucleophile concentrations.

Multiple Choice Questions (MCQs)

• Q1. What is the role of the nucleophile in an S_N reaction?
• A. It donates an electron pair to the substrate.
• B. It accepts an electron pair from the substrate.
• C. It acts as a leaving group.
• D. It acts as a catalyst.
• Correct Answer: A
• Explanation: The nucleophile donates an electron pair.

• Q2. What is the role of the leaving group in an S_N reaction?
• A. It attacks the substrate.
• B. It departs with an electron pair.
• C. It donates an electron pair.
• D. It acts as a catalyst.
• Correct Answer: B
• Explanation: The leaving group departs with an electron pair.

• Q3. Which of the following is an example of a nucleophile?
• A. Br⁺
• B. NO₂⁺
• C. NH₃
• D. H⁺
• Correct Answer: C
• Explanation: NH₃ is a nucleophile.

• Q4. Which of the following is an example of a leaving group?
• A. OH^-
• B. NH₂^-
• C. Cl^-
• D. CH₃^-
• Correct Answer: C
• Explanation: Cl^- is a good leaving group.

• Q5. What is the hybridization of the carbon atom in the substrate of an S_N2 reaction?
• A. sp
• B. sp²
• C. sp³
• D. sp³d
• Correct Answer: C
• Explanation: The carbon atom is sp³ hybridized.

• Q6. What is the stereochemical outcome of an S_N2 reaction?
• A. Retention of configuration.
• B. Inversion of configuration.
• C. Racemization.
• D. No change.
• Correct Answer: B
• Explanation: S_N2 proceeds with inversion of configuration.

• Q7. What is the stereochemical outcome of an S_N1 reaction?
• A. Retention of configuration.
• B. Inversion of configuration.
• C. Racemization.
• D. No change.
• Correct Answer: C
• Explanation: S_N1 proceeds with racemization.

• Q8. Which of the following factors favors S_N1 over S_N2?
• A. Primary alkyl halide.
• B. Tertiary alkyl halide.
• C. Strong nucleophile.
• D. Polar aprotic solvent.
• Correct Answer: B
• Explanation: Tertiary alkyl halides favor S_N1.

• Q9. Which of the following factors favors S_N2 over S_N1?
• A. Tertiary alkyl halide.
• B. Primary alkyl halide.
• C. Polar protic solvent.
• D. Weak nucleophile.
• Correct Answer: B
• Explanation: Primary alkyl halides favor S_N2.

• Q10. What is the rate law for an S_N2 reaction?
• A. Rate = k[substrate]
• B. Rate = k[nucleophile]
• C. Rate = k[substrate][nucleophile]
• D. Rate = k[substrate]²
• Correct Answer: C
• Explanation: The rate law for S_N2 is Rate = k[substrate][nucleophile].

Multiple Choice Questions (MCQs)

• Q1. What is the main reason for the reactivity of alkyl halides in S_N reactions?
• A. The C-H bond is polar.
• B. The C-X bond is polar.
• C. The C-C bond is polar.
• D. The C-X bond is non-polar.
• Correct Answer: B
• Explanation: The C-X bond is polar.

• Q2. Which of the following is a good leaving group?
• A. F^-
• B. Cl^-
• C. OH^-
• D. NH₂^-
• Correct Answer: B
• Explanation: Cl^- is a good leaving group.

• Q3. Which of the following is a poor leaving group?
• A. I^-
• B. Br^-
• C. Cl^-
• D. OH^-
• Correct Answer: D
• Explanation: OH^- is a poor leaving group.

• Q4. What type of reaction occurs when a nucleophile replaces a halogen in an alkyl halide?
• A. Elimination
• B. Substitution
• C. Addition
• D. Oxidation
• Correct Answer: B
• Explanation: It is a substitution reaction.

• Q5. What is the mechanism called when the nucleophile attacks simultaneously as the leaving group departs?
• A. S_N1
• B. S_N2
• C. E1
• D. E2
• Correct Answer: B
• Explanation: S_N2 is the one-step concerted mechanism.

• Q6. Which type of alkyl halide favors S_N1?
• A. Primary
• B. Secondary
• C. Tertiary
• D. Methyl
• Correct Answer: C
• Explanation: Tertiary alkyl halides favor S_N1.

• Q7. Which type of alkyl halide favors S_N2?
• A. Primary
• B. Secondary
• C. Tertiary
• D. Quaternary
• Correct Answer: A
• Explanation: Primary alkyl halides favor S_N2.

• Q8. What is the product of the reaction between chloroethane and hydroxide ion?
• A. Ethene
• B. Ethanol
• C. Ethanoic acid
• D. Ethane
• Correct Answer: B
• Explanation: The product is ethanol.

• Q9. What is the role of the leaving group?
• A. It attacks the substrate.
• B. It departs with an electron pair.
• C. It donates an electron pair.
• D. It acts as a catalyst.
• Correct Answer: B
• Explanation: The leaving group departs with an electron pair.

• Q10. Which of the following is a nucleophile?
• A. H⁺
• B. NO₂⁺
• C. OH^-
• D. AlCl₃
• Correct Answer: C
• Explanation: OH^- is a nucleophile.

Multiple Choice Questions (MCQs)

• Q1. What is the main reason for the reactivity of alkyl halides in S_N reactions?
• A. The C-H bond is polar.
• B. The C-X bond is polar.
• C. The C-C bond is polar.
• D. The C-X bond is non-polar.
• Correct Answer: B
• Explanation: The C-X bond is polar.

• Q2. Which of the following is a good leaving group?
• A. F^-
• B. Cl^-
• C. OH^-
• D. NH₂^-
• Correct Answer: B
• Explanation: Cl^- is a good leaving group.

• Q3. Which of the following is a poor leaving group?
• A. I^-
• B. Br^-
• C. Cl^-
• D. OH^-
• Correct Answer: D
• Explanation: OH^- is a poor leaving group.

• Q4. What type of reaction occurs when a nucleophile replaces a halogen in an alkyl halide?
• A. Elimination
• B. Substitution
• C. Addition
• D. Oxidation
• Correct Answer: B
• Explanation: It is a substitution reaction.

• Q5. What is the mechanism called when the nucleophile attacks simultaneously as the leaving group departs?
• A. S_N1
• B. S_N2
• C. E1
• D. E2
• Correct Answer: B
• Explanation: S_N2 is the one-step concerted mechanism.

• Q6. Which type of alkyl halide favors S_N1?
• A. Primary
• B. Secondary
• C. Tertiary
• D. Methyl
• Correct Answer: C
• Explanation: Tertiary alkyl halides favor S_N1.

• Q7. Which type of alkyl halide favors S_N2?
• A. Primary
• B. Secondary
• C. Tertiary
• D. Quaternary
• Correct Answer: A
• Explanation: Primary alkyl halides favor S_N2.

• Q8. What is the product of the reaction between chloroethane and hydroxide ion?
• A. Ethene
• B. Ethanol
• C. Ethanoic acid
• D. Ethane
• Correct Answer: B
• Explanation: The product is ethanol.

• Q9. What is the role of the leaving group?
• A. It attacks the substrate.
• B. It departs with an electron pair.
• C. It donates an electron pair.
• D. It acts as a catalyst.
• Correct Answer: B
• Explanation: The leaving group departs with an electron pair.

• Q10. Which of the following is a nucleophile?
• A. H⁺
• B. NO₂⁺
• C. OH^-
• D. AlCl₃
• Correct Answer: C
• Explanation: OH^- is a nucleophile.

Multiple Choice Questions (MCQs)

• Q1. What is the general formula for an S_N reaction?
• A. R-X + Nu^- → R-Nu + X^-
• B. R-X + H₂ → R-H + HX
• C. R-X + H₂O → R-OH + HX
• D. R-CH₂-CH₂-X → R-CH=CH₂ + HX
• Correct Answer: A
• Explanation: The general equation is R-X + Nu^- → R-Nu + X^-.

• Q2. Which of the following is a good leaving group?
• A. OH^-
• B. NH₂^-
• C. Cl^-
• D. CH₃^-
• Correct Answer: C
• Explanation: Cl^- is a good leaving group.

• Q3. Which of the following is a poor leaving group?
• A. I^-
• B. Br^-
• C. Cl^-
• D. OH^-
• Correct Answer: D
• Explanation: OH^- is a poor leaving group.

• Q4. Which of the following is a nucleophile?
• A. H⁺
• B. NO₂⁺
• C. OH^-
• D. AlCl₃
• Correct Answer: C
• Explanation: OH^- is a nucleophile.

• Q5. What is the mechanism called when the nucleophile attacks simultaneously as the leaving group departs?
• A. S_N1
• B. S_N2
• C. E1
• D. E2
• Correct Answer: B
• Explanation: S_N2 is the one-step concerted mechanism.

• Q6. Which type of alkyl halide favors the S_N1 mechanism?
• A. Primary
• B. Secondary
• C. Tertiary
• D. Methyl
• Correct Answer: C
• Explanation: Tertiary alkyl halides favor S_N1.

• Q7. Which type of alkyl halide favors the S_N2 mechanism?
• A. Primary
• B. Secondary
• C. Tertiary
• D. Quaternary
• Correct Answer: A
• Explanation: Primary alkyl halides favor S_N2.

Q8. What is the first fundamental event in a