Chapter: Electrophilic Addition to Alkenes

1. Introduction to Electrophilic Addition

• Alkenes as Nucleophiles: The carbon-carbon double bond (C=C) in alkenes consists of a strong σ bond and a weaker π bond. The π electrons are relatively loosely held and exposed, making the double bond a source of electron density. Therefore, alkenes act as nucleophiles (electron-rich species) and react with electrophiles (electron-deficient species).
• Definition: Electrophilic addition reactions are characteristic reactions of alkenes (and alkynes) where an electrophile adds across the π bond, breaking the π bond and forming two new σ bonds.

2. General Mechanism of Electrophilic Addition

Most electrophilic addition reactions proceed via a two-step mechanism (though some are concerted, like hydroboration):

1. Electrophilic Attack: The π electrons of the alkene attack the electrophile (E+). This breaks the π bond and forms a new σ bond between one of the alkene carbons and the electrophile. This step generates a carbocation intermediate. This is typically the rate-determining step.
2. Nucleophilic Attack: The carbocation intermediate (an electrophile itself) is then rapidly attacked by a nucleophile (Nu−), forming a second new σ bond and yielding the final product.

3. Carbocation Stability and Rearrangements

• Carbocation Stability: The stability of the carbocation intermediate is crucial as its formation is the rate-determining step. Carbocation stability is enhanced by:
  • Alkyl groups: Electron-donating inductive effect (pushing electron density towards the positive charge).
  • Hyperconjugation: Overlap of adjacent C-H σ bonds with the empty p-orbital of the carbocation, delocalizing the positive charge.
  • Resonance: Delocalization of the positive charge through a conjugated π system (e.g., allylic or benzylic carbocations are highly stable).
  • Order of Stability: Methyl (⊕CH3​) < Primary (⊕RCH2​) < Secondary (⊕R2​CH) < Tertiary (⊕R3​C) ≪ Allylic/Benzylic.
• Carbocation Structure: Carbocations are sp2 hybridized and planar around the positively charged carbon.
• Rearrangements: Carbocations can rearrange to form a more stable carbocation. This occurs through a 1,2-hydride shift (migration of a hydrogen with its bonding electrons) or a 1,2-alkyl shift (migration of an alkyl group with its bonding electrons). Rearrangements often lead to unexpected products.

4. Regioselectivity: Markovnikov’s Rule

• Markovnikov’s Rule: In the electrophilic addition of HX (or H2​O where H is the initial electrophile) to an unsymmetrical alkene, the hydrogen atom (or the electrophile’s most electropositive part) adds to the carbon atom of the double bond that already has the greater number of hydrogen atoms, and the X (or nucleophile) adds to the carbon atom with the fewer hydrogen atoms.
• Mechanistic Basis: This regioselectivity arises because the initial electrophilic attack forms the more stable carbocation intermediate, which is then attacked by the nucleophile.

5. Stereochemistry of Electrophilic Addition

• Syn Addition: Both parts of the adding molecule add to the same face of the double bond.
• Anti Addition: Both parts of the adding molecule add to opposite faces of the double bond.
• Carbocation Intermediate: Since carbocations are planar, the nucleophile can attack from either face, leading to a mixture of products (racemic if a new chiral center is formed).
• Cyclic Halonium Ion: In some reactions (e.g., halogenation, halohydrin formation), a bridged intermediate (like a halonium ion) is formed, which forces the subsequent nucleophilic attack to occur from the opposite face, leading to anti addition.

6. Specific Electrophilic Addition Reactions

6.1. Hydrohalogenation (Addition of HX, where X=Cl,Br,I)

• Reagents: HCl, HBr, HI (order of reactivity: HI>HBr>HCl).
• Mechanism: Two-step (carbocation intermediate).
  1. Protonation of the alkene to form the more stable carbocation.
  2. Nucleophilic attack by the halide ion (X−).
• Regioselectivity: Follows Markovnikov’s Rule.
• Stereochemistry: Not stereospecific (can be syn or anti), often leads to a mixture if chiral centers are formed due to planar carbocation.
• Rearrangements: Possible if a more stable carbocation can be formed.
• Peroxide Effect (for HBr only): In the presence of peroxides, HBr adds in an anti-Markovnikov fashion via a radical mechanism.

6.2. Hydration (Addition of Water, Acid-Catalyzed)

• Reagents: H2​O and a strong acid catalyst (e.g., H2​SO4​, H3​PO4​).
• Mechanism: Two-step (carbocation intermediate). Reverse of E1 dehydration of alcohols.
  1. Protonation of the alkene to form the more stable carbocation.
  2. Nucleophilic attack by water.
  3. Deprotonation of the resulting protonated alcohol to yield the neutral alcohol product.
• Regioselectivity: Follows Markovnikov’s Rule.
• Stereochemistry: Not stereospecific.
• Rearrangements: Possible.

6.3. Halogenation (Addition of Halogens, X2​=Cl2​,Br2​)

• Reagents: Cl2​, Br2​ (often in a non-nucleophilic solvent like CCl4​ or CH2​Cl2​). F2​ is too reactive, I2​ is too unreactive.
• Mechanism: Proceeds via a cyclic halonium ion intermediate (bromonium ion or chloronium ion).
  1. Electrophilic attack by X2​ to form a bridged halonium ion.
  2. Nucleophilic attack by X− (from the opposite face) to open the ring.
• Regioselectivity: Not an issue unless substituents are present.
• Stereochemistry: Strictly anti addition due to backside attack on the bridged halonium ion.

6.4. Halohydrin Formation (Addition of X2​ and H2​O)

• Reagents: Cl2​ or Br2​ in water (water acts as the nucleophile).
• Mechanism: Similar to halogenation, but water (a more abundant nucleophile) attacks the halonium ion.
  1. Electrophilic attack by X2​ to form a bridged halonium ion.
  2. Nucleophilic attack by water on the more substituted carbon of the halonium ion (due to greater partial positive charge on the more substituted carbon) from the opposite face.
  3. Deprotonation of the resulting protonated halohydrin.
• Regioselectivity: The halogen adds to the less substituted carbon, and the hydroxyl group adds to the more substituted carbon (appears anti-Markovnikov for the halogen, but is mechanistically driven by carbocation stability in the halonium ion).
• Stereochemistry: Strictly anti addition.

6.5. Oxymercuration-Demercuration

• Reagents:
  1. Mercuric acetate (Hg(OAc)2​) in water (or alcohol).
  2. Sodium borohydride (NaBH4​) (for demercuration).
• Overall Reaction: Converts an alkene to an alcohol (or ether if alcohol is used instead of water).
• Mechanism (Oxymercuration): Forms a bridged mercurinium ion intermediate (analogous to halonium ion), followed by nucleophilic attack by water on the more substituted carbon.
• Regioselectivity: Strictly Markovnikov addition (hydroxyl group on the more substituted carbon).
• Stereochemistry: Anti addition of OH and HgOAc.
• Key Advantage: No carbocation rearrangements, making it a very reliable method for Markovnikov hydration.

6.6. Hydroboration-Oxidation

• Reagents:
  1. Borane (BH3​, often as BH3​⋅THF or BH3​⋅S(CH3​)2​).
  2. Hydrogen peroxide (H2​O2​) and aqueous base (NaOH).
• Overall Reaction: Converts an alkene to an alcohol.
• Mechanism (Hydroboration): Concerted syn addition of B-H across the double bond, with boron adding to the less substituted carbon and hydrogen to the more substituted carbon. This is followed by two more additions to form a trialkylborane.
• Mechanism (Oxidation): The trialkylborane is then oxidized by H2​O2​/NaOH, replacing the C-B bond with a C-O bond, with retention of configuration.
• Regioselectivity: Anti-Markovnikov addition (hydroxyl group on the less substituted carbon).
• Stereochemistry: Overall syn addition of H and OH.
• Key Advantage: Complementary to acid-catalyzed hydration and oxymercuration; produces anti-Markovnikov product and no rearrangements.

6.7. Catalytic Hydrogenation

• Reagents: H2​ gas with a metal catalyst (e.g., Pd, Pt, Ni, Rh).
• Overall Reaction: Reduces an alkene to an alkane.
• Mechanism: Involves adsorption of both alkene and H2​ onto the metal surface, followed by sequential transfer of hydrogen atoms to the same face of the alkene.
• Stereochemistry: Strictly syn addition.

6.8. Epoxidation

• Reagents: Peroxyacids (peroxycarboxylic acids, e.g., mCPBA – meta-chloroperoxybenzoic acid).
• Overall Reaction: Forms an epoxide (oxirane), a three-membered cyclic ether.
• Mechanism: Concerted syn addition of oxygen to the double bond.
• Stereochemistry: Retention of alkene stereochemistry (cis-alkene gives cis-epoxide, trans-alkene gives trans-epoxide).

6.9. Dihydroxylation (Formation of Diols)

• Syn Dihydroxylation: Adds two hydroxyl groups to the same face of the double bond.
  • Reagents:
    • Cold, dilute, neutral KMnO4​ (potassium permanganate)
    • OsO4​ (osmium tetroxide) followed by NaHSO3​/H2​O or H2​S.
• Anti Dihydroxylation: Adds two hydroxyl groups to opposite faces of the double bond.
  • Reagents:
    1. Epoxidation (using peroxyacid)
    2. Acid-catalyzed ring opening of the epoxide with water (H3​O+).

6.10. Oxidative Cleavage (Ozonolysis)

• Reagents:
  1. Ozone (O3​).
  2. Reductive workup (e.g., Zn/H+ or DMS – dimethyl sulfide) or Oxidative workup (H2​O2​).
• Overall Reaction: Cleaves the carbon-carbon double bond, forming carbonyl compounds.
  • Reductive workup: Aldehydes and/or ketones are formed.
  • Oxidative workup: Carboxylic acids (from aldehydes) and/or ketones are formed.
• Application: Useful for determining the position of a double bond in an unknown alkene.

Multiple Choice Questions (MCQ) on Electrophilic Addition to Alkenes

Instructions: Choose the best answer for each question.

1. In electrophilic addition reactions, alkenes act as: a) Electrophiles b) Nucleophiles c) Leaving groups d) Catalysts

2. What is the key intermediate formed in the first step of most electrophilic addition reactions to alkenes? a) A carbanion b) A radical c) A carbocation d) An enolate

3. Which of the following is the most stable carbocation? a) Methyl carbocation b) Primary carbocation c) Secondary carbocation d) Tertiary carbocation

4. According to Markovnikov’s Rule, in the addition of HBr to an unsymmetrical alkene, the hydrogen adds to the carbon of the double bond that: a) Has fewer hydrogen atoms. b) Has more hydrogen atoms. c) Is more substituted. d) Is less substituted.

5. What is the stereochemical outcome of the addition of Br2​ to an alkene? a) Syn addition b) Anti addition c) Racemization d) Random addition

6. Which of the following reagents is used for Hydroboration-Oxidation? a) H2​SO4​, H2​O b) Hg(OAc)2​, H2​O then NaBH4​ c) BH3​⋅THF then H2​O2​, NaOH d) OsO4​ then NaHSO3​

7. The hydration of an alkene using acid and water yields an alcohol. This reaction generally follows: a) Anti-Markovnikov’s Rule. b) Markovnikov’s Rule. c) Hofmann’s Rule. d) Zaitsev’s Rule.

8. What is a common side reaction observed in electrophilic addition reactions involving carbocation intermediates? a) Elimination b) Substitution c) Rearrangements (e.g., hydride or alkyl shifts) d) Radical formation

9. The mechanism of halogenation (X2​) of an alkene proceeds through which intermediate? a) Carbocation b) Carbanion c) Cyclic halonium ion d) Alkyl radical

10. What is the product of the reaction of propene with HBr (no peroxides)? a) 1-bromopropane b) 2-bromopropane c) 1,2-dibromopropane d) Propane

11. Which reaction forms a halohydrin from an alkene? a) HBr addition b) Br2​ addition in CCl4​ c) Br2​ in H2​O d) Hydroboration-oxidation

12. Hydroboration-oxidation results in the overall addition of H and OH in an: a) Anti-Markovnikov fashion with syn stereochemistry. b) Markovnikov fashion with syn stereochemistry. c) Anti-Markovnikov fashion with anti stereochemistry. d) Markovnikov fashion with anti stereochemistry.

13. Which reagent is used for catalytic hydrogenation of alkenes? a) H2​O and acid b) LiAlH4​ c) H2​ with Pd (or Pt, Ni) catalyst d) NaBH4​

14. What is the stereochemical outcome of catalytic hydrogenation of alkenes? a) Anti addition b) Syn addition c) Racemization d) Random addition

15. What is the product when an alkene is treated with a peroxyacid (e.g., mCPBA)? a) Alcohol b) Ketone c) Epoxide d) Diol

16. Which reagents are used for syn dihydroxylation of an alkene? a) KMnO4​, cold, dilute, neutral b) OsO4​ followed by NaHSO3​/H2​O c) Both a and b d) mCPBA then H3​O+

17. Ozonolysis followed by reductive workup (e.g., Zn/H+) of a disubstituted alkene produces: a) An alkane. b) An alcohol. c) Aldehydes and/or ketones. d) Carboxylic acids.

18. What is the common regioselectivity of Oxymercuration-Demercuration? a) Anti-Markovnikov b) Markovnikov c) Hofmann d) No specific regioselectivity

19. A key advantage of Oxymercuration-Demercuration over acid-catalyzed hydration is that it: a) Produces the anti-Markovnikov product. b) Is stereospecific. c) Avoids carbocation rearrangements. d) Uses less toxic reagents.

20. Which of the following halogens is typically too unreactive for standard electrophilic addition to alkenes? a) Cl2​ b) Br2​ c) I2​ d) F2​

21. In the halohydrin formation, where does the hydroxyl group preferentially add? a) To the less substituted carbon of the original alkene. b) To the more substituted carbon of the original alkene. c) To the carbon that already has the most hydrogens. d) Randomly to either carbon of the double bond.

22. Which reaction results in the overall anti-Markovnikov addition of HBr to an alkene? a) HBr addition (no peroxides) b) HBr addition with peroxides c) Br2​ addition d) H2​O / acid hydration

23. Which of the following statements about carbocations is FALSE? a) They are planar. b) They are sp2 hybridized. c) They are electron-rich. d) They are electrophilic.

24. The rate-determining step in the acid-catalyzed hydration of an alkene is: a) The protonation of the alkene to form a carbocation. b) The nucleophilic attack of water on the carbocation. c) The deprotonation of the protonated alcohol. d) The formation of water.

25. If 1-methylcyclohexene undergoes reaction with Br2​ in CCl4​, what type of stereochemistry would be observed in the product? a) Only syn-addition b) Only anti-addition c) Racemic mixture of syn and anti products d) Retention of configuration

26. Which reaction is complementary to acid-catalyzed hydration in terms of regioselectivity? a) Halogenation b) Oxymercuration-Demercuration c) Hydroboration-Oxidation d) Catalytic hydrogenation

27. What functional group is formed on each carbon of the original double bond after dihydroxylation? a) Ketone b) Aldehyde c) Ether d) Hydroxyl (OH)

28. What is the maximum number of new σ bonds formed in a typical electrophilic addition reaction to an alkene? a) 1 b) 2 c) 3 d) 4

29. The term “π bond” in alkenes refers to: a) A bond formed by direct overlap of orbitals along the internuclear axis. b) A weaker bond formed by sidewise overlap of p-orbitals above and below the internuclear axis. c) A highly localized bond within a ring. d) A bond that is highly resistant to reaction.

30. Which of these reactions involves a concerted syn addition? a) Acid-catalyzed hydration b) Halogenation c) Hydroboration d) Oxymercuration

31. What is the major product of the reaction of 2-methylpropene with HBr (no peroxides)? a) 1-bromo-2-methylpropane b) 2-bromo-2-methylpropane c) 2-methylpropane d) 2-methylpropene (unreacted)

32. What type of workup is needed after ozonolysis to produce carboxylic acids from aldehydes? a) Reductive workup (e.g., Zn/H+) b) Oxidative workup (e.g., H2​O2​) c) Acidic workup only d) Basic workup only

33. If an alkene is treated with H2​ over a Pd catalyst, which carbons are directly involved in the addition? a) Only one carbon of the double bond. b) Only carbons adjacent to the double bond. c) Both carbons of the double bond. d) All carbons in the molecule.

34. In the mechanism of Br2​ addition to an alkene, why does the bromide ion attack from the opposite face of the bromonium ion? a) To avoid steric hindrance. b) Because of the electrophilic nature of the bromide ion. c) To form a more stable carbocation. d) To maximize the anti-addition stereochemistry.

35. How many electrons are in the π bond of an alkene? a) 1 b) 2 c) 3 d) 4

36. Which reaction of alkenes is useful for determining the exact position of the original double bond in an unknown compound? a) Catalytic hydrogenation b) Hydration c) Ozonolysis d) Halogenation

37. If 3,3-dimethyl-1-butene reacts with HBr (no peroxides), what would be the major product, considering possible rearrangements? a) 2-bromo-3,3-dimethylbutane b) 2-bromo-2,3-dimethylbutane c) 1-bromo-3,3-dimethylbutane d) 3,3-dimethyl-1-butene (unreacted)

38. The addition of BH3​ to an alkene proceeds with which stereochemistry? a) Anti addition b) Syn addition c) Random addition d) Only when a chiral catalyst is present

39. Which of the following reagents is used to convert an alkene directly to an alcohol with Markovnikov regioselectivity and without rearrangements? a) H2​O / H2​SO4​ b) BH3​⋅THF then H2​O2​, NaOH c) Hg(OAc)2​, H2​O then NaBH4​ d) HBr

40. Why are F2​ and I2​ generally not used for halogenation of alkenes? a) F2​ is too unreactive, I2​ is too reactive. b) F2​ is too reactive, I2​ is too unreactive. c) Both are too reactive. d) Both are too unreactive.

Answer Key with Explanations

1. b) Nucleophiles.
   • Explanation: The π electrons of the double bond are electron-rich and readily donate to electron-deficient species (electrophiles), making alkenes nucleophiles.
2. c) A carbocation.
   • Explanation: In most electrophilic addition reactions, the initial attack of the alkene on the electrophile results in the formation of a carbocation intermediate.
3. d) Tertiary carbocation.
   • Explanation: Carbocation stability increases with increasing alkyl substitution (tertiary > secondary > primary > methyl) due to the electron-donating inductive effect and hyperconjugation from the alkyl groups.
4. b) Has more hydrogen atoms.
   • Explanation: Markovnikov’s Rule states that the hydrogen adds to the carbon of the double bond that already has the greater number of hydrogen atoms, leading to the formation of the more stable carbocation intermediate.
5. b) Anti addition.
   • Explanation: Halogenation of alkenes proceeds through a cyclic halonium ion intermediate, which forces the second halogen atom to attack from the opposite face, resulting in anti addition.
6. c) BH3​⋅THF then H2​O2​, NaOH.
   • Explanation: This two-step reagent sequence is characteristic of Hydroboration-Oxidation, which results in anti-Markovnikov hydration with syn stereochemistry.
7. b) Markovnikov’s Rule.
   • Explanation: Acid-catalyzed hydration involves a carbocation intermediate and therefore follows Markovnikov’s Rule, with the hydroxyl group adding to the more substituted carbon.
8. c) Rearrangements (e.g., hydride or alkyl shifts).
   • Explanation: Carbocation intermediates are prone to rearrangements (1,2-hydride or 1,2-alkyl shifts) if a more stable carbocation can be formed, leading to rearranged products.
9. c) Cyclic halonium ion.
   • Explanation: The mechanism of halogenation (Cl2​ or Br2​) involves the formation of a bridged, three-membered cyclic halonium ion (e.g., bromonium ion) intermediate.
10. b) 2-bromopropane.
    • Explanation: Propene is an unsymmetrical alkene. According to Markovnikov’s Rule, the hydrogen from HBr adds to the C1​ (less substituted) to form a more stable secondary carbocation at C2​, which then reacts with Br−.
11. c) Br2​ in H2​O.
    • Explanation: When Br2​ is added in the presence of water, water acts as the nucleophile to open the bromonium ion, leading to the formation of a halohydrin (contains a halogen and a hydroxyl group).
12. a) Anti-Markovnikov fashion with syn stereochemistry.
    • Explanation: Hydroboration-oxidation is unique in that it adds H and OH in an anti-Markovnikov fashion (OH on the less substituted carbon) and proceeds with syn stereochemistry (H and OH add to the same face).
13. c) H2​ with Pd (or Pt, Ni) catalyst.
    • Explanation: Catalytic hydrogenation involves the addition of H2​ gas to an alkene in the presence of a metal catalyst to reduce the double bond to a single bond.
14. b) Syn addition.
    • Explanation: In catalytic hydrogenation, both hydrogen atoms add to the same face of the double bond because the reaction occurs on the surface of the metal catalyst.
15. c) Epoxide.
    • Explanation: Peroxyacids are commonly used reagents for the epoxidation of alkenes, forming a three-membered cyclic ether called an epoxide (or oxirane).
16. c) Both a and b.
    • Explanation: Both cold, dilute, neutral KMnO4​ (Baeyer test) and OsO4​ followed by a reducing workup are standard reagents for achieving syn dihydroxylation (adding two hydroxyl groups to the same face).
17. c) Aldehydes and/or ketones.
    • Explanation: Ozonolysis followed by a reductive workup (e.g., with Zn/H+ or DMS) cleaves the double bond and forms aldehydes and/or ketones, depending on the substitution pattern of the original alkene.
18. b) Markovnikov.
    • Explanation: Oxymercuration-Demercuration is a Markovnikov addition of H and OH, with the hydroxyl group ending up on the more substituted carbon.
19. c) Avoids carbocation rearrangements.
    • Explanation: Oxymercuration-Demercuration proceeds through a bridged mercurinium ion intermediate rather than an open carbocation, thus preventing rearrangements.
20. c) I2​.
    • Explanation: F2​ is too reactive (and difficult to control), while I2​ is generally too unreactive for practical electrophilic addition reactions with alkenes.
21. b) To the more substituted carbon of the original alkene.
    • Explanation: In halohydrin formation, the nucleophilic water attacks the more substituted carbon of the cyclic halonium ion intermediate (due to greater partial positive charge there), leading to the hydroxyl group on the more substituted carbon.
22. b) HBr addition with peroxides.
    • Explanation: The presence of peroxides initiates a radical mechanism for HBr addition, which reverses the regioselectivity to anti-Markovnikov (bromine adds to the less substituted carbon). This is known as the peroxide effect.
23. c) They are electron-rich.
    • Explanation: Carbocations are electron-deficient species (having only six valence electrons around the carbon) and are therefore electrophilic.
24. a) The protonation of the alkene to form a carbocation.
    • Explanation: The initial attack of the alkene on the proton (from the acid catalyst) to form the carbocation intermediate is typically the slowest, rate-determining step.
25. b) Only anti-addition.
    • Explanation: Halogenation of cyclohexenes (and other alkenes) proceeds via a cyclic halonium ion, which dictates anti addition.
26. c) Hydroboration-Oxidation.
    • Explanation: Hydroboration-Oxidation produces the anti-Markovnikov product, making it complementary to acid-catalyzed hydration (Markovnikov product).
27. d) Hydroxyl (OH).
    • Explanation: Dihydroxylation reactions add two hydroxyl groups to the two carbons that were originally part of the double bond.
28. b) 2.
    • Explanation: In a typical electrophilic addition, the π bond is broken, and two new σ bonds are formed to the two carbons of the original double bond.
29. b) A weaker bond formed by sidewise overlap of p-orbitals above and below the internuclear axis.
    • Explanation: The π bond is formed by the overlap of unhybridized p-orbitals, leading to electron density above and below the plane of the σ bond. This makes it weaker and more accessible to electrophiles.
30. c) Hydroboration.
    • Explanation: The addition of BH3​ to an alkene in the first step of hydroboration-oxidation is a concerted process that occurs with syn stereochemistry.
31. b) 2-bromo-2-methylpropane.
    • Explanation: This is Markovnikov addition. The H from HBr adds to the C1​ (less substituted) to form a tertiary carbocation at C2​, which then reacts with Br−. No rearrangement is needed here as the tertiary carbocation is formed directly.
32. b) Oxidative workup (e.g., H2​O2​).
    • Explanation: To obtain carboxylic acids from aldehydes (or ketones from highly substituted carbons), an oxidative workup using H2​O2​ is required after ozonolysis. Reductive workup yields aldehydes.
33. c) Both carbons of the double bond.
    • Explanation: In catalytic hydrogenation, two hydrogen atoms are added across the double bond, one to each carbon of the original C=C bond.
34. d) To maximize the anti-addition stereochemistry.
    • Explanation: The bridged halonium ion intermediate is sterically protected from one face. The nucleophilic bromide ion must attack from the opposite face (backside attack), leading exclusively to anti addition.
35. b) 2.
    • Explanation: A π bond consists of two electrons occupying the π molecular orbital.
36. c) Ozonolysis.
    • Explanation: Ozonolysis cleaves the double bond, and by identifying the carbonyl products, one can deduce the original position of the double bond in the parent alkene.
37. b) 2-bromo-2,3-dimethylbutane.
    • Explanation:
      1. Protonation of 3,3-dimethyl-1-butene forms a secondary carbocation at C2​.
      2. This secondary carbocation can undergo a 1,2-methyl shift to form a more stable tertiary carbocation at C3​.
      3. Nucleophilic attack by Br− on the rearranged tertiary carbocation at C3​ yields 2-bromo-2,3-dimethylbutane as the major product.
38. b) Syn addition.
    • Explanation: The addition of BH3​ to an alkene (hydroboration) is a concerted process where both the hydrogen and boron atoms add to the same face of the double bond (syn addition).
39. c) Hg(OAc)2​, H2​O then NaBH4​.
    • Explanation: Oxymercuration-Demercuration provides Markovnikov hydration with no carbocation rearrangements, making it a reliable method for specific alcohol synthesis. Acid-catalyzed hydration allows rearrangements. Hydroboration-oxidation gives anti-Markovnikov. HBr gives alkyl halides.
40. b) F2​ is too reactive, I2​ is too unreactive.
    • Explanation: Fluorine is extremely reactive and difficult to control for selective addition. Iodine is generally too unreactive to add to alkenes under typical conditions.