Hybridization	Bond	Bonding Energy
C(sp³)-C(sp³)	Sigma bond	85 kcal/mol
C(sp²)-C(sp²)	Sigma bond	90 kcal/mol
C(sp³)-H(s)	Sigma bond	100 kcal/mol
C(sp²)-H(s)	Sigma bond	105 kcal/mol
C(p)-C(p)	Pi bond	60 kcal/mol
C(sp³)-C(sp³)	Semi-pi bond	45 kcal/mol

Table 1. Summary of Bond Energies

④ Bond Lengths: Single bond > Double bond > Triple bond

○ Single bond: sp³ hybridized, Double bond: sp² hybridized, Triple bond: sp hybridized

○ As one moves from a single bond to a triple bond, the s-character increases, causing the electrons to stay closer to the nucleus, resulting in a shorter bond length.

⑶ Properties

① Boiling Point

○ Principle 1. Generally, longer chain length and larger surface area lead to higher boiling points, similar to alkanes

○ Principle 2. Higher dipole moment leads to higher boiling point

○ cis alkenes have higher dipole moments compared to trans alkenes, resulting in higher boiling points

○ Example: Boiling point of cis-2-butene is 4°C, while trans-2-butene is 1°C

○ Exception: trans-2-hexene has a higher boiling point than cis-2-hexene

○ Reason: Van der Waals forces. Evaluation needs to be done on a case-by-case basis

○ Principle 3. Alkynes have higher boiling points than structurally similar alkenes

○ Reason 1. Alkenes have a trigonal planar geometry, while alkynes are linear, favoring better intermolecular interactions in alkynes

○ Reason 2. The sp carbon in alkynes is more electronegative than the sp2 carbon in alkenes, leading to greater polarity

○ Reason 3. The strength of pi bonding in triple bonds is weaker than in alkenes, promoting greater polarization and increasing van der Waals forces.

○ Principle 4. Internal alkenes and alkynes have higher boiling points than terminal ones

○ Reason: Because terminal alkenes and alkynes are highly hydrophobic.

② Melting Point

○ Principle 1. Longer chain length and larger surface area lead to higher melting points, similar to alkanes

○ Principle 2. Higher lattice (symmetry) leads to higher melting points: note the difference in trend compared to boiling points

○ trans alkenes can form crystal lattices compared to cis alkenes, resulting in higher melting points

○ Example: Melting point of cis-2-butene is -139°C, while trans-2-butene is -106°C

○ Example: Melting point of n-hexane is higher than (E)-hex-2-ene

③ Unlike single bonds, rotation is restricted in double and triple bonds

○ Cycloalkanes have reduced degrees of freedom, leading to easy intermolecular interactions, resulting in higher boiling and melting points

⑷ Stability

① Method 1. Hydrogenation Reaction Enthalpy

○ Heat released by adding hydrogen maximally, higher heat release indicates a less stable form

○ Used frequently to assess stability of compounds with multiple bonds due to its precision

② Method 2. Heat of Combustion per Carbon

○ Stability is inversely proportional to the heat of combustion per carbon

③ Substituent Count: More substituents lead to greater stability in alkenes

○ Complex Explanation: Interaction between anti-π (π*) bonding orbitals and adjacent σ orbitals due to hyperconjugation

○ Simplified Explanation: Double bond carbons are sp2 hybridized, with 33% s-character, while substituent carbons are sp3 hybridized with 25% s-character, leading to substituent carbons having an affinity to donate electrons, and central carbons having an affinity to accept electrons

○ Even Simpler Explanation: Alkenes are more electronegative → they become stable by receiving electrons from substituents

④ cis-trans Stability Difference

○ Linear Alkenes: trans is more stable than cis due to steric hindrance.

○ Cyclic Alkenes

○ Cyclooctene [8] ~ Cycloundecene [11]: cis is more stable than trans

○ Reason 1: trans alkenes adopt twisted conformations, leading to increased ring strain

○ Reason 2: trans alkenes have hydrogen atoms within the ring, causing significant steric hindrance

○ Cyclododecene [12]: Similar stability for cis and trans structures

○ Cyclotridecene [13]: trans structure is more stable than cis

2. Nomenclature of Linear Alkenes and Alkynes

⑴ Selection of Main Chain

① Assuming functional groups are alkyl, halogen, alkene, and alkyne

② Alkenes and alkynes take priority over alkanes, so the longest carbon chain containing alkene or alkyne determines the main chain, and the rest are considered substituents

⑵ Naming Substituents

① Alkyl and halogen group nomenclature is the same as for alkanes

② For chain and cyclic alkenes and alkynes functioning as substituents, change -ene and -yne suffixes to -enyl and -ynyl

○ In this case, the nearest carbon to the substituent on the main chain is assigned as carbon 1

⑶ Numbering

① Sequentially assign numbers from one end of the main chain to the other

② Alkenes and alkynes take precedence over substituents, so assign lower numbers to them

○ The position of a multiple bond is indicated by the first carbon where the multiple bond begins.

○ Example: 4-methyl-2-pentene

③ When two numbering combinations are possible, assign numbers so that the alkene or alkyne has the lower number.

④ If numbers are the same, select the combination where the alkene (alk e ne) comes before the alkyne (alk y ne) based on alphabetical order

○ Example: 1-penten-4-yne

⑤ When both numbering combinations are still possible after ④ and there’s a single substituent, assign the lowest number to the carbon bearing the substituent

⑥ When both numbering combinations are still possible after ④ and there are multiple substituents, assign the lowest number to the carbon(s) bearing the substituents

⑦ If numbers are still the same after ⑥, assign the lower number to the substituent in alphabetical order

⑷ Suffix Naming

① When there’s only one alkene: Name as 《··· + alkene position-ene》

② When there’s only one alkyne: Name as 《··· + alkyne position-yne》

③ When there’s one alkene and one alkyne: Name as 《··· + alkene position-en-alkyne position-yne》

④ When there are multiple alkenes or alkynes

○ Alkenes: Change -ene to -diene(2), -triene(3), -tetraene(4), etc. based on the number of alkenes

○ Example: 4-methyl-1,3-pentadiene

○ Alkynes: Change -yne to -diyne(2), -triyne(3), -tetrayne(4), etc. based on the number of alkynes

⑸ Full Name Nomenclature

① Name as 《(substituent position-substituent name)_n + alk-alkene-alkyne position-suffix》

② For main chain ( alk ), usually end with a consonant; however, if consonant conflict occurs, leave -a

○ Example: hept-1-ene doesn’t end with -a, but hexa-1,3-diene ends with -a.

3. Nomenclature of Cyclic Alkenes and Alkynes

⑴ Selection of Main Chain

① If the cyclic alkene or alkyne has an alkane substituent: Priority given to alkene/alkyne, making the cyclic compound the main chain

② If a chain alkene or alkyne is substituent on a cyclic alkene or alkyne (to be updated later)

⑵ Naming Substituents

① Alkyl and halogen groups are named the same as for alkanes

② For chain and cyclic alkenes and alkynes functioning as substituents, change -ene and -yne suffixes to -enyl and -ynyl

○ In this case, the nearest carbon to the substituent on the main chain is assigned as carbon 1

⑶ Numbering

① Assign numbers so that alkene/alkyne is positioned between carbon 1 and carbon 2 of the main chain.

② If multiple alkenes/alkynes are present, assign numbers to minimize the numbers while not violating ①

③ If numbers are the same, select the combination with the lower-numbered alkene before alkyne based on alphabetical order

④ If two numbering combinations are still possible (clockwise, counterclockwise) and substituents are attached to only one alkene/alkyne carbon, designate that carbon as carbon 1

○ Example: 3-ethyl-2-methyl-cyclopent-1-ene (×), 5-ethyl-1-methylcyclopent-1-ene (○)

⑤ If two numbering combinations are still possible and not covered by ④, assign numbers to minimize the carbon bearing the substituent

○ If not covered by ④: When substituents are present or absent on all alkene/alkyne carbons

⑷ Suffix Naming: Same as 2-⑷

① Example: 5-methyl-1,3-cyclohexadiene

⑸ Full Name Nomenclature

① Name as 《(substituent position-substituent name)_n + cycloalk-alkene-alkyne position-suffix》

4. Electrophilic Reactions of Alkenes

⑴ Overview

① The nature of the reagent is electrophilic, and the reactant causing addition reactions is nucleophilic.

② Generally, addition reactions are exothermic and ΔG ≒ ΔH, so they are spontaneous due to thermodynamics.

③ Type 1: Halogen compounds: Halogenation of hydrogen, halogen addition reaction, formation of halohydrins.

④ Type 2: Hydration reactions: Acid-catalyzed hydration, oxymercuration-demercuration, hydroboration-induced hydration.

⑤ Type 3: Carbonyl compounds: Carbonyl addition reaction.

⑥ Type 4: Hydrogenation: Pd/C hydrogenation reaction, Pt/O2 hydrogenation reaction.

⑦ Type 5: Oxidation reactions: Epoxidation, 1,2-dihydroxylation, oxidative cleavage by KMnO4, ozonolysis.

⑧ Type 6: Other reactions.

⑨ Key Points

○ Position selectivity (Markovnikov’s rule): Selectivity of structural isomers.

○ Stereoselectivity (syn, anti addition): Selectivity of stereoisomers.

○ Racemization.

○ Ring formation reactions.

⑵ 1-1. Halogenation of Hydrogen Addition Reaction

① (Formula) Attach H+ to a carbon with more hydrogens, rearrange positions, and then add X- or proceed with intramolecular S_N2 reaction.

② Mechanism

Figure 1. Electrophilic Substitution Reaction

○ Carbocation: Rate-determining step is the first step, racemization, rearrangement, electron-donating resonance effect, hyperconjugation.

○ More alkyl groups on carbocation result in more stability due to electron-donating resonance effect, hyperconjugation.

○ X^- can attack from either the front or back side, allowing racemization.

○ Significance: Reaction of achiral molecules leading to achiral intermediates and chiral products.

○ In the transition state of the first step, hydrogen forms two bonds (C-H, H-X).

③ Markovnikov’s rule

Figure 2. Markovnikov’s Rule

○ Description: The hydrogen of H-X adds to the carbon with more hydrogens in the alkene’s double bond, forming the major product.

○ The 3° carbocation on the left is more stable than the 2° carbocation on the right (∵ more substituents).

○ The pathway through the 3° carbocation has a lower activation energy, favoring the major product formation.

④ Markovnikov’s Rule Exception: Rearrangement Reaction

Figure 3. Markovnikov Exception

○ Types: 1,2-hydride shift, 1,2-methyl shift.

○ Cause: Due to hyperconjugation.

○ Rearrangement occurs due to hyperconjugation, and the path through the 3° carbocation generates the major product.

○ While Markovnikov’s rule doesn’t always apply, the formation of a more stable intermediate leading to the major product is invariant.

⑤ 1,2-Addition, 1,4-Addition: Illustrated with 1,3-butadiene

○ Mechanism of 1,2-addition and 1,4-addition

Figure 4. Halogenation of Hydrogen Addition Reaction in 1,3-butadiene

○ At 0°C, 1,2-addition: 1,4-addition = 71:29.

○ At 40°C, 1,2-addition: 1,4-addition = 15:85.

○ 1,2-addition: Kinetic product, favored at lower temperatures due to stable 1,2-addition via carbocation.

○ 1,4-addition: Thermodynamic product, favored at higher temperatures; reverse reaction occurs, so thermodynamically stable 1,4-addition prevails.

⑶ 1-2. Halogenation Addition Reaction: anti Addition

① (Formula) Add X⁺ considering steric hindrance, then anti addition of X^-.

○ In the halonium ring intermediate of an asymmetrical alkene, the carbon atom with more substituents, as well as the halogen atom, carries partial positive charge.

○ Carbon with more substituents is partially positively charged due to S_N1-like reaction.

○ X^- attacks on carbons with multiple substituents.

② Mechanism

○ X-X is polarized, with one side having δ+ charge and the other side having δ^- charge.

○ X^- can exist independently in a stable manner.

○ Mechanism of halogenation in linear alkenes

Figure 5. Halogenation Addition Reaction Mechanism in Linear Alkenes

○ Mechanism of halogenation in cyclic alkenes

Figure 6. Halogenation Addition Reaction Mechanism in Cyclic Alkenes

③ Halonium ring intermediate

○ Due to instability of the halonium ring intermediate, other paths are chosen if available.

○ In the case of unsymmetrical reagents, the reaction occurs to stabilize the halonium ring intermediate.

○ In the case of halogenation of hydrogen addition reaction, the limited number of hydrogen orbitals (2) prevents the formation of a ring intermediate.

○ The reason halogen addition reactions occur with alkenes before alkynes is that the halonium ring intermediate of an alkyne is too unstable.

④ Stereochemistry

○ Backside attack: X^- proceeds with a backside attack due to steric hindrance.

○ However, anti-addition and syn-addition are possible.

○ Mesocompounds or racemic mixtures can form, but one side can be favored due to steric hindrance.

○ Halogenation of cyclohexene

○ Due to the ring intermediate, the stable conformation of cyclohexene is boat-form, leading to unexpected outcomes in steric hindrance and major product.

○ Incorrect intuition

Figure 7. Incorrect Intuition

○ Correct approach

Figure 8. Correct Approach

⑤ Application 1. cis halogenation of alkene: Developed by GIST

Figure 9. cis halogenation of alkene

⑷ 1-3. Formation of Halohydrins

① (Formula) Similar to halogen addition reaction: Attach X⁺ first, then anti addition of OH.

② Mechanism

Figure 10. Formation of Halohydrins Reaction

○ Since HX is a stronger acid than H₂O, X^- is less basic than OH^-, so water adds if present.

○ In the halonium ring intermediate of an asymmetrical alkene, the carbon atom with more substituents, as well as the halogen atom, carries partial positive charge.

○ Carbon atoms with more substituents are partially positively charged due to S_N1-like reaction.

○ OH adds to partially positively charged carbon atoms.

○ NBS can be used as the source of Br₂ similarly to NIS for I₂.

⑸ 2-1. Acid-Catalyzed Hydration: Markovnikov Hydration

① Characteristics

○ Since OH^- is not a good leaving group, the following reaction may be thermodynamically possible but cannot occur kinetically.

○ Acid catalyst (e.g., H₂SO₄): H₃O⁺ acts both as a reactant (electrophile) and as a product, which makes it appear as though the acid is a catalyst.

Figure 11. Incorrect Mechanism for Acid-Catalyzed Hydration

Figure 12. Correct Mechanism for Acid-Catalyzed Hydration

② (Formula) Attach H⁺ to carbon with more hydrogens, consider carbocation rearrangement and ring closure.

○ Carbocation: Rate-determining step is the first step, racemization, rearrangement, electron-donating resonance effect, hyperconjugation.

③ If an alkene and -OH group coexist within one molecule, the alkene reacts first.

○ The E1 reaction of alcohols proceeds through an oxonium ion intermediate.

⑹ 2-2. Oxymercuration-Demercuration

① (Formula) Attach H⁺ to carbon with more hydrogens, add -OH: -OH comes from H₂O, and H comes from NaBH₄.

Figure 13. Overall Reaction for Oxymercuration-Demercuration

② 1^st Step: Oxymercuration (Oxymercuration)

○ Reaction progresses under THF solvent: Hg(OAc)₂ → Hg(OAc)⁺ + OAc^-

○ Oxymercuration Reaction Mechanism

Figure 14. Oxymercuration Reaction Mechanism

○ 1-1. HgOAc⁺ acts as a Lewis acid.

○ 1-2. Pi electrons of alkene donate to mercury cation, forming mercury-bridged carbocation.

○ 1-3. The mercury-bridged carbocation possesses a positive charge that doesn’t cause rearrangement.

○ When in the carbocation state, having more alkyl groups stabilizes the carbon. Therefore, the carbon with more alkyl groups carries a partial positive charge in this reaction.

○ Major product is formed when nucleophile attacks carbon with more substituents.

○ Reaction resembles S_N2 but also exhibits characteristics of S_N1 reaction.

③ 2nd Step: Demercuration (Demercuration)

Figure 15. Demercuration Reaction Mechanism

④ Features

○ anti addition

○ Markovnikov Hydration

○ No carbocation rearrangement

○ Reaction occurs at room temperature with good yield.

⑤ Stereoselectivity and Stereospecificity

Figure 16. Stereoselectivity and Stereospecificity in Oxymercuration-Demercuration

○ Stereoselectivity: -OH adds to the side with more substituents because the more substituted carbon bears a stronger δ⁺ charge.

○ Stereospecificity: -OH is axial due to proximity to 1,3-diaxial interactions and the t-Bu group, while the opposite side only experiences the influence of distant 1,3-diaxial interactions.

Figure 17. Reason for the Stereospecificity of 1-methyl-4-tert-butylcyclohexene

⑥ Other Reaction Variations

○ If ROH is provided instead of H₂O in the 1^st step, -OR substitutes for -OH.

○ If NaBD₄ is used instead of NaBH₄ in the 2^nd step, -H substitutes for -D.

○ When NaBH₄ and NaBD₄ compete, NaBH4 reacts first: (Mnemonic tip) Deuterium D is heavier, so its reaction occurs more slowly.

○ Hg(O₂CCF₃)₂ can be used instead of Hg(OAc)₂: The latter is more electrophilic.

⑺ 2-3. Hydroboration-Oxidation Reaction

① Role of THF: Stabilizes BH₃ in solution.

○ Reaction equation: B₂H₆ (diborane) + 2 THF → 2 BH₃:THF

② (Formula) The conditions 1. BH₃, THF; 2. H₂O₂, OH^- involve syn addition of -OH and -H, considering H₂B-H analogous to HO-H.

Figure 18. Example of Hydroboration-Oxidation Reaction

③ 1^st Step: Hydroboration

Figure 19. Hydroboration Mechanism

○ syn addition of BH₃ to alkene.

○ -BH₂ group is located opposite to the CH₃ group: Indicates anti -Markovnikov’s rule.

○ Reason 1. Steric hindrance.

○ Reason 2. The substituent stabilization effect for the quasi-carbocation.

④ 2^nd Step: Oxidation by H₂O₂

Figure 20. Oxidation Mechanism by H₂O₂

○ The retention of configuration is important.

⑤ Step 3: OH^- hydrolysis mechanism

Figure 21. OH^- hydrolysis mechanism

○ BH₂OH acts as an acid, releasing H+ and becoming ROH when attached to RO^-

⑥ Characteristics

○ syn-addition

○ anti-Markovnikov hydration: Hydrogen adds to the side with fewer hydrogens to reduce steric hindrance

○ Role of THF: Stabilizes BH₃ in solution

○ Reaction equation: B₂H₆ (diborane) + 2 THF → 2 BH₃:THF

○ Does not involve rearrangement: No carbocation intermediates

○ Racemic mixture is produced

○ Majority of reactions proceed from the exo direction in bicyclic compounds

○ Exhibits reversible nature at 150-160 ℃: Imparts the effect of shifting the position of the double bond

⑦ Other Reaction Types

○ The less substituted alkenes with smaller steric hindrance react faster (∵ steric hindrance is important).

○ Greater selectivity with CyBH₂ (cyclohexyl BH₂), 9-BBN (9-borabicyclo(3.3.1)nonane), Sia₂BH (disiamylborane)

○ 1 equivalent of borane can react with 3 equivalents of alkene

○ To react 1 equivalent of alkene, 9-BBN or Sia2BH can be used

⑧ 2-3-1. brown hydroboration

○ syn addition

○ Hydration reaction

⑻ 3. Carbene addition reaction

① Carbene

○ Characteristics

○ R₂C:

○ 6 outer electrons

○ sp² hybridization

○ High reactivity

○ Possesses an empty p orbital, acts as a nucleophile

Figure 22. Carbene structure

○ Carbene-forming reagents

○ diazomethane: H₂C: is generated

○ chloroform (CHCl₃): Cl₂C: is generated

○ Simmons-Smith reagent (carbenoid): H₂C: is generated

○ diazocyclopentadiene: Orange liquid

○ diazofluorene: Orange solid

○ 4,4-dimethyldiazocyclohexa-2,5-diene: Purple liquid

○ methyl diazoacetate: Yellow liquid

② Reaction 1. Simmons-Smith reaction

○ Step 1: Carbenoid formation reaction: CH₂I₂ (catalyst: Zn(Cu) + ether) → ICH₂ZnI (carbenoid) → H₂C:

○ Step 2: Carbon-carbon coupling reaction

Figure 23. Carbene reaction type 1

③ Reaction 2.

○ Step 1: CH₂Cl₂ + strong base (e.g., LDA) → CHCl + Cl^-

Figure 24. Carbene reaction type 2

○ Step 2: Carbon-carbon coupling reaction

④ Reaction 3.

○ Step 1: Carbenoid formation reaction: CHBr₃ + strong base (e.g., LDA) → Br₂C: + Br^-

Figure 25. Carbene reaction 3

○ Step 2: Carbon-carbon coupling reaction

⑤ Reaction 4.

○ For diazomethane (CH₂N₂), under hν or Δ conditions, N₂ is expelled, forming a carbene

⑥ Characteristics

○ Carbene favors electron-rich compounds due to not satisfying the octet rule

○ Reacts with electron-rich alkenes, forms rings to prevent carbocation formation from donating alkene carbons

○ Carbenes avoid reacting with alkynes if a ring forms due to double bond strain

○ Carbene and peroxy acids prefer electron-rich multisubstituted alkenes.

○ Carbene singlet undergoes concerted reactions, thus showing syn addition

○ Carbene triplet doesn’t undergo concerted reactions, yielding two different products

⑼ 4. Pd/C hydrogenation, Pt/O₂ hydrogenation

① (General) H₂ in H-H state attaches to Pd/C, undergoes syn addition to alkene

② Stereospecific reaction: syn addition

Figure 26. Hydrogenation reaction example

③ Reactivity

○ Alkene selectivity in hydrogenation reactions: The fewer the substituents, the faster the reaction rate (due to substituents being proportional to steric hindrance).

○ The higher the electron density on the alkene, the faster the rate of reduction reactions: Electron-donating substituents increase the reaction rate when substituted.

○ Generally, hydrogenation reactions catalyzed by palladium do not reduce aromatic rings or ester functional groups.

○ When both an alkene and a ketone are present, the alkene is hydrogenated first (because the reactivity of the alkene is greater).

○ Under conditions of excess hydrogen and high temperature and pressure, carbonyl groups can also be reduced.

Figure 27. Pd/C hydrogenation with H₂ (1 eq.)

Figure 28. Pd/C hydrogenation with H₂ (excess)

⑽ 5. Oxidation reactions

① Epoxidation: Alkene oxidation by RCO₃H

○ Examples of RCO₃H: HOOH, _m_CPBA, HCO₃H, CH₃CO₃H, CF₃CO₃H, MMPP, DMDO

○ m-_CPBA: _meta -chloroperoxybenzoic acid

○ CH₃CO₃H: peroxyacetic acid

○ MMPP: magnesium monoperoxyphtalate

○ DMDO: dimethyldioxirane

○ Epoxidation reaction mechanism

Figure 29. Epoxidation reaction mechanism

○ Carbenes and _m_CPBA react rapidly in multisubstituted alkenes as substituents enhance alkene electron density

○ Henbest epoxidation: Epoxide formation is directed by hydrogen bonding, such as OH and amino groups

② Payne Epoxidation: Formation of epoxide from alkene while maintaining stereochemistry

○ Step 1: H₂O₂ → H⁺ + OOH^-

○ Step 2: PhCN + OOH- → Ph(OOH)-C=NH

○ Step 3: Epoxidation

Figure 30. Payne Epoxidation

③ 1,2-Dihydroxylation cis-diol: syn addition

(Formula 1) 1. OsO₄, 2. NaHSO4, H2O conditions add syn-diol to both carbons forming the alkene

Figure 31. OsO₄ alkene oxidation

○ syn addition results from two OH groups originating from one molecule of OsO₄

○ syn -diol is added to alkenes with lower steric hindrance

○ (Formula 2) 1. OsO₄ (0.2%), NMO (N-methylmorpholine N-oxide), 25 ℃, 2. NaSO₃, H₂O

Figure 32. Reaction using NMO

○ OsO₄ (osmium tetroxide) is toxic, volatile, and expensive

○ NMO (N-methylmorpholine N-oxide) is added to regenerate OsO4 even when a catalytic amount of OsO4 is used

○ Catalytic amount is very small compared to the reactants

○ NMO: Functions to regenerate OsO4, slightly excess is necessary

○ (Formula 3) Oxidative decomposition by KMnO4

④ 1,2-Dihydroxylation trans-diol

○ (Formula 1) 1. RCO₃H, 2. H₂O (H⁺ or OH^-)

⑤ Oxidative decomposition by KMnO4

Figure 33. KMnO4 alkene oxidation

○ **syn-diol addition: 1. KMnO₄ (cold), 2. OH^-, H₂O: Bonds are cleaved at the double bond, and each carbon is connected to an -OH group. syn addition

○ KMnO₄ requires cold, dilute conditions (H₂O) due to its strong reactivity

○ syn addition is due to two OH groups originating from one molecule of KMnO₄

○ Note that chromium (Cr) oxidants do not oxidize alkenes or alkynes

○ Cheap

○ (Formula 1) If any of the conditions don’t match, each can be turned into a carbonyl group

⑥ Ozonolysis

○ (Formula 1) 1. O₃, 2. Me₂S (or Zn): Bonds are cleaved at the double bond, and each carbon is connected to a carbonyl group (=O)

○ Ozonolysis proceeds through a 1,3-dipolar cycloaddition reaction

○ Ozonolysis starts from electron-rich alkenes, i.e., multisubstituted alkenes

○ Condition 2 is reducing condition, so an aldehyde can be formed

Figure 34. Example of ozonolysis

○ (Formula 2) 1. O₃, 2. H₂O₂: Condition 2 is oxidative, an aldehyde can be formed and then replaced by a carboxylic group

○ Ozonolysis proceeds through a 1,3-dipolar cycloaddition reaction

○ Ozonolysis starts from electron-rich alkenes, i.e., multisubstituted alkenes

○ Aldehyde is not formed under this condition: Instead, a carboxylic group is formed

⑾ 6. Other reactions

① Radical addition reactions of alkenes

② Alkene metathesis (alkene cross-metathesis): Reaction where two alkenes exchange carbons with a double bond

③ Shapiro reaction: Forms alkenide anion (vinyl anion), which is R₂C=CR^-.

Figure 35. Shapiro reaction

④ Grubbs reaction (Olefin metathesis)

Figure 36. Grubbs reaction

Figure 37. Grubbs reaction

○ Mechanism: Specifically follows Chauvin mechanism. Wittig reaction has similar appearance

○ Significance: The double bond (C=C) can be broken and formed in one step under mild conditions.

5. Electrophilic Reactions of Alkynes

⑴ Overview

① Alkenes undergo reactions similar to those of alkenes, except for having one additional pi bond compared to alkenes.

② Type 1: Halogens: Hydrogen Halide Addition Reaction, Halogen Addition Reaction, Halo Hydrin Formation Reaction

③ Type 2: Hydration Reaction: Hydration Reaction: Acid-Catalyzed Hydration, Oxymercuration-Demercuration, Hydroboration-oxidation Reaction

④ Type 3: Acid·Base Reaction

⑤ Type 4: Hydrogenation: Lindlar Catalyst Hydrogenation Reaction

⑥ Type 5: Oxidation-Reduction Reaction: Ozonolysis, Hydroxylation

⑦ Type 6: Other Reactions

⑵ 1-1. Halogenation of Hydrogen Addition Reaction

① Mechanism

○ Hydrogen Halide Addition Reaction in Alkynes: The resulting alkene is reactive and undergoes halogenation of hydrogen addition reaction again.

Figure 38. Hydrogen Halide Addition Reaction Step 1

Figure 39. Hydrogen Halide Addition Reaction Step 2

○ In reality, it doesn’t go through a vinyl carbocation.

○ Resultantly equivalent, depicted as shown above.

○ In reality, hydrogen forms a weak bridge with the two alkene carbons.

○ In step 2, the carbocation forms on the carbon with X^- due to resonance stabilization.

○ Since it’s a carbocation, X^- can approach from any direction.

② Carbocation: First step is rate-determining, racemization, rearrangement, electron-donating effect, hyperconjugation

③ Resonance Stabilization: In rearrangement, the carbocation moves toward the carbon with X^- even if the carbon with X^- is less substituted.

④ Stereoechemistry

○ In the addition of 1 equivalent of HX, trans product is preferred.

○ When X^- ion is in excess with HX, the yield of trans-oriented product increases.

⑶ 1-2. Halogen Addition Reaction: anti Addition, The resulting alkene is reactive and undergoes halogen addition reaction again.

Figure 40. Halogen Addition Reaction Step 1

Figure 41. Halogen Addition Reaction Step 2

① In the halogen addition reaction in alkynes, the generated halonium ring intermediate has a double bond and is quite unstable.

② When both an alkene and an alkyne are present, halogen addition to the alkene is preferred due to the above reason.

⑷ 1-3. Halo Hydrin Formation Reaction

① Mechanism

Figure 42. Halo Hydrin Reaction

② Unsymmetrical alkynes: After the formation of the halonium ion, when the nucleophilic water attacks, it attacks the carbon that holds more carbocation character, determined by the direction of the stereogenic center.

⑸ 2. Hydration Reaction

① Enol and Keto Forms

○ Enol form: Structure with a double bond and an alcohol group, i.e., R2C=C(OH)R

○ Keto form: Structure with a carbonyl group (or aldehyde group), i.e., R2HC-CO-R

○ Tautomerization: Equilibrium reaction between enol and keto forms

○ π bonding: C=O bond is much stronger than C=C bond, favoring the keto form, making enol unstable

○ Exception 1. 4-aminopyridine undergoes tautomeric reactions; as the amine group is basic, enol-like form is favored.

○ Exception 2. When the enol form satisfies aromaticity and the keto form does not, the enol form predominates.

② Basic Mechanism: Follows Markovnikov’s rule, OH^- adds to the more substituted carbon.

○ Step 1. Alkyne Hydration Reaction

Figure 43. Alkyne Hydration Reaction

○ Step 2. Alkene Hydroboration Reaction

Figure 44. Alkene Hydration Reaction

○ Step 3. Tautomerization

○ Reverse reaction occurs readily, so the final products which proceed through tautomerization are racemic.

○ Since it’s not an oxidation-reduction reaction, the sum of oxidation states remains unchanged.

Figure 45. Tautomerization

③ Acid Catalyzed Hydration Reaction: Markovnikov Hydration, Carbocation

○ (Formula) For alkynes, the acid-catalyzed hydration requires HgSO₄ (catalyst) in addition to the conditions of the acid-catalyzed hydration of alkenes.

○ Especially for terminal alkynes, since their reactivity is lower, they rely considerably on HgSO₄ catalyst.

○ Carbocation should have a 120° bond angle due to sp² hybridization, so the reaction where alkene becomes a carbocation is highly unstable.

○ When both a reactive alkyne and alkene are present, the priority for acid-catalyzed hydration lies with the alkene.

Figure 46. Acid-Catalyzed Hydroboration Reaction

④ Oxymercuration-Demercuration: Markovnikov Hydration, Carbocation

Figure 47. Alkyne’s Oxymercuration-demercuration

⑤ Hydroboration Oxidation Reaction for Internal Alkynes: anti-Markovnikov Hydration, syn Addition

Figure 48. Hydroboration Oxidation Reaction for Internal Alkynes

⑥ Hydroboration Oxidation Reaction for Terminal Alkynes: anti-Markovnikov Hydroboration, syn Addition

Figure 49. Hydroboration Oxidation Reaction for Terminal Alkynes

⑹ 3. Acid · Base Reaction

① Unlike alkanes and alkenes, alkynes have sufficiently low pK_a values to undergo acid · base reactions.

○ Alkanes: pK_a = ~50

○ Alkenes: pK_a = 45

○ Alkynes: pK_a = 25

② Representative Type: S_N2 Reaction (Backside Attack, Inversion of Configuration)

Figure 50. Alkyne’s S_N2 Reaction

○ (Formula) Primary alkyne + strong base = H⁺ release.

○ Methyl halide alkyl or 1° halide alkyl undergo S_N2 reaction as described above.

○ 2° halide alkyl undergoes E2 elimination reaction.

⑺ 4. Hydrogenation Reaction

① H₂ / Pt, Pd, or Ni: Reduces alkynes to alkanes.

② syn Addition Reaction: Lindlar Catalyst (P-2 Catalyst) Hydrogenation Reaction

Figure 51. syn Hydrogenation Reaction

○ (Formula) H₂ is adsorbed on Lindlar catalyst as H-H, then undergoes syn addition to the alkyne.

○ Lindlar Catalyst: Catalyst composed of Pd, CaCO₃ (or BaSO₄), lead (Ⅱ) acetate ((CH₃COO^-)₂Pb²⁺), quinoline, etc.

○ Reaction is overly strong under Pd catalyst conditions, leading to a single bond in the final alkane products

○ Reaction rate slows down with more substituted alkynes (∵ steric hindrance is significant).

③ syn Addition Reaction

○ Ni₂B (P-2) can perform alkyne hydrogenation reaction instead of Lindlar catalyst.

○ Reaction rate slows down with more substituted alkynes (∵ steric hindrance is significant).

④ anti Addition Reaction

Figure 52. anti Hydrogenation Reaction

○ (Formula) After proceeding with R-C⁺=C^--R + (·) → R-C(·)=C^--R and adding H⁺ to C^-, anti addition of H⁺ occurs to the remaining carbon (∵ steric hindrance).

○ When Na(Li) is melted or solvated, it releases radical electrons through a highly powerful ionization reaction.

○ Solvent: NH₃. Unusually NH₃ acts as an acid by donating a proton due to steric repulsion.

○ Amines like NH₃ have a pK_a of around 40, while alkenes have a pK_a of 45, so reaction can occur spontaneously.

○ Using alcohol as a solvent reacts with Na, so it’s inappropriate.

○ Reaction rate slows down with more substituted alkynes (∵ steric hindrance).

○ Birch reduction and reaction conditions are similar.

⑻ 5. Oxidative decomposition

① Ozonolysis reaction: (Formula) 1. O₃, 2. H₂O: Separation of triple bonds followed by connection to carboxyl groups (-COOH) each.

Figure 53. Ozonolysis reaction of alkynes

○ Memorize that CO₂ is emitted.

○ The mechanism of ozonolysis reaction of alkynes is not precisely elucidated.

○ Ozonolysis reaction of alkynes occurs starting from multi-substituted alkynes.

② When treated with KMnO₄, K₂CrO₇ and heated

○ Same products as the ozonolysis reaction are obtained.

③ Hydroxylation reaction of alkynes

○ Treating alkynes with dilute KMnO4 under cold conditions results in a dihydroxylation reaction occurring twice.

○ Subsequent dehydration leads to the formation of a 1,2-dicarbonyl structure.

⑼ 6. Other internal reactions

① Interconversion between alkynes and terminal alkynes

○ 2-butyne + NH₃ + NaNH₂ → 1-butyne

○ 1-butyne + KOH or NaOH → 2-butyne

② Radical addition reactions of alkynes

6. Synthesis of Alkenes

⑴ Elimination reactions of alkyl halides (E1, E2)

① Reaction with 1^st order RX by t-BuOK or LDA leads to E2 reaction.

② 2^nd and 3^rd order RX with strong base result in E2 reaction.

③ 2^nd and 3^rd order RX with weak base lead to E1 reaction: Competes with S_N1 reaction.

⑵ Elimination reactions of vicinal dibromo alkanes

⑶ Dehydration reactions of alcohols (E1, E2): Concentrated sulfuric acid, phosphoric acid, diluted sulfuric acid, alumina (Al₂O₃)

① 2^nd and 3^rd alcohols undergo elimination reaction via E1 mechanism.

② 1^st alcohols undergo elimination reaction via E2 mechanism.

③ In the case of E1 reactions, rearrangement reactions can occur to form a more stable carbocation.

⑷ Elimination reaction of alcohols + POCl₃: E2 reaction

⑸ Reduction reactions of alkynes

① Alkyne hydrogenation with metal catalysts reduces alkynes to alkanes.

② Reduction to cis-alkenes: Lindlar catalyst, P-2 catalyst, quinoline, or lead as poison.

③ Reduction to trans-alkenes with the reduction of dissolved metals.

⑹ Wittig reaction

① Alkene synthesis through the reaction of nucleophilic ylides and electrophilic carbonyl compounds.

⑺ Hofmann elimination reaction

① Reaction for generating alkenes from amines.

⑻ Cope elimination reaction: Alkene with fewer substituents is the major product.

7. Synthesis of Alkynes

⑴ Elimination reactions of vicinal dihalo alkanes or gem dihalo alkanes

① Internal alkynes require 2 equivalents of strong base per mole (e.g., NaNH₂, KO(CH₃)₃).

② Terminal alkynes require 3 equivalents of strong base per mole (e.g., NaNH₂, KO(CH₃)₃).

③ Reason for the need of relatively strong base: It’s necessary to go through a process of hydrogen removal from alkenes.

⑵ Produce gem dihalo alkanes through methyl ketone + PCl₅, then follow ⑴.

Input: 2019-01-11 14:07

Modified: 2022-02-01 17:36

1363

Chapter 8. Alkenes and Alkynes

1. Overview

2. Nomenclature of Linear Alkenes and Alkynes

3. Nomenclature of Cyclic Alkenes and Alkynes

4. Electrophilic Reactions of Alkenes

5. Electrophilic Reactions of Alkynes

6. Synthesis of Alkenes

7. Synthesis of Alkynes

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