Alkene Structure and Reactions

Alkene Structure and Nomenclature

• Naming alkenes, including mono-, di-, tri-, tetra-substituted alkenes, and E/Z isomers.
• Determine the names of alkenes.

Alkenes from Petroleum

• Alkenes like ethylene and propylene are derived from petroleum and serve as building blocks for various industrial chemicals.
• Ethylene is used to produce polyethylene, ethanol, ethylene dichloride, acetaldehyde, acetic acid, ethylene oxide, vinyl chloride, ethylene glycol, and PVC.
• Propylene is used to produce polypropylene, cumene, isopropyl alcohol, propylene oxide, acetone, and propylene glycol.

Alkene Nomenclature (Linear)

1. Select the Longest Chain: Identify the longest continuous carbon chain containing the C=C double bond.
   • Use this chain as the parent name, changing the alkane suffix from '-ane' to '-ene'.
2. Number the Chain: Number the carbon chain to give the C=C double bond the lowest possible locants.
   • The location is determined by the first atom of C=C.
   • The locant for the alkene suffix can precede the parent name or be placed immediately before the suffix.
3. Indicate Substituent Locations: Identify and indicate the locations of substituent groups by the numbers of the carbon atoms to which they are attached.
4. Label (E) or (Z): If necessary, label the alkene as (E) or (Z) before the full name to indicate the stereochemistry around the double bond.
5. Multiple Double Bonds: If more than one double bond is present, name the compound as a diene, triene, etc., indicating the number of double bonds
   • Assign each double bond a locator number.
   • Examples: (R,E)-3,5-dimethylhept-3-ene, (E)-3-ethyl-2-methylhepta-1,3-diene

Alkene Nomenclature (Cyclic)

1. Number Substituted Cycloalkenes: Number the cycloalkene ring such that:
   • The carbon atoms of the C=C double bond are given the 1 and 2 positions.
   • The substituent groups receive the lowest possible numbers at the first point of difference.
   • Example: (S)-3-bromo-1-methylcyclohepta-1,4-diene

• Cycloalkanes with fewer than 7 carbons will always be cis (trans is too much strain).

Reactions

• Hydrohalogenation (Markovnikov): Addition of HX to an alkene following Markovnikov's rule (H adds to the carbon with more hydrogens).
• Hydrohalogenation (anti-Markovnikov): Addition of HBr using ROOR, leading to anti-Markovnikov addition.
• Acid-Catalyzed Hydration and Oxymercuration-Demercuration: Addition of water across the double bond, resulting in alcohol formation.
• Hydroboration-Oxidation: Anti-Markovnikov addition of water across the double bond.
• Hydrogenation: Addition of H2 across the double bond (reduction).
• Bromination: Addition of Br2 across the double bond.
• Halohydrin Formation: Addition of HO and X across the double bond.
• Anti-Dihydroxylation: Addition of two hydroxyl groups on opposite faces of the double bond.
• Syn Dihydroxylation: Addition of two hydroxyl groups on the same face of the double bond.
• Ozonolysis: Cleavage of the double bond using ozone, followed by a reducing agent.

Multistep Transformation Practice

• Examples provided for multi-step synthesis problems involving various reagents and conditions.

Synthetic Organic Chemists

• Utilize available tools to synthesize novel or target compounds for drug discovery, petrochemicals, food, materials, etc.
• Reactions include SN2, SN1, E1, E2, catalytic hydrogenation, hydro-halogenation, radical hydro-halogenation, halogenation, and halohydrin formation.

Key Reaction Properties

• Reaction: Catalytic Hydrogenation
  *Product and reagents: H2, Pt
  *Stereospecificity: Syn
  *Mechanism: No rearrangement.
  *Regioselectivity: Neither Markovnikov or Anti-Markovnikov
• Reaction: Hydro-halogenation
  *Product and reagents: HX
  *Stereospecificity: Neither.
  *Mechanism: Yes, potential rearrangement.
  *Regioselectivity: Markovnikov
• Reaction: Radical Hydro-halogenation
  *Product and reagents: HBr, ROOR
  *Stereospecificity: Neither
  *Mechanism: Chapter 10
  *Regioselectivity: Anti-Markovnikov
• Reaction: Halogenation
  *Product and reagents: Br_2
  *Stereospecificity: Anti
  *Mechanism: Yes
  *Regioselectivity: Neither
• Reaction: Halohydrin Formation
  *Product and reagents: Br2, H2O
  *Stereospecificity: Anti
  *Mechanism: Yes
  *Regioselectivity: Yes
• Reaction: Acid-catalyzed Hydration
  *Product and reagents: H_3O^+
  *Stereospecificity: Neither
  *Mechanism: Yes potential rearrangement.
  *Regioselectivity: Markovnikov
• Reaction: Hydroboration Oxidation
  *Product and reagents: 1) BH3 . THF, 2) H2O_2, NaOH
  *Stereospecificity: Syn.
  *Mechanism: Partial.
  *Regioselectivity: Anti-Markovnikov
• Reaction: Oxymercuration- Demercuration
  *Product and reagents: 1) Hg(OAc)2, H2O, 2) NaBH_4
  *Stereospecificity: Anti.
  *Mechanism: Partial.
  *Regioselectivity: Markovnikov
• Reaction: Syn dihydroxylation
  *Product and reagents: OsO_4
  *Stereospecificity: Syn.
  *Mechanism: No
  *Regioselectivity: Neither
• Reaction: Epoxidation
  *Product and reagents: RCO_2H
  *Stereospecificity: Syn.
  *Mechanism: Yes.
  *Regioselectivity: Neither
• Reaction: Anti-dihydroxylation
  *Product and reagents: KMnO_4, NaOH, cold
  *Stereospecificity: Anti.
  *Mechanism: Yes.
  *Regioselectivity: Neither
• Reaction: Ozonolysis
  *Product and reagents: 1) O_3, 2) DMS
  *Stereospecificity: Neither.
  *Mechanism: No.
  *Regioselectivity: Neither

Reaction Mechanisms

• Mechanism: Step-by-step process showing how reactants convert to products.
• Mechanisms are experimentally determined and based on accumulated evidence.

General Overview of Reactions

1. Analyze Reagents: Determine which groups will be added across the C=C double bond based on the reagents used.
2. Determine Regioselectivity: Identify whether the reaction follows Markovnikov or anti-Markovnikov addition.
3. Determine Stereospecificity: Identify whether the reaction is syn- or anti-addition.

• Addition is the opposite of elimination.
• The C=C π bond is converted to two new sigma bonds.
• The π bond acts as a nucleophile (electron-pair donor).

Catalytic Hydrogenation

• Addition of H_2 across C=C pi bond.
• No carbocation generation (no rearrangement).
• Stereospecific (syn) addition.
• No Regioselectivity.
• Stereochemistry: Racemization.
• Heterogeneous catalysis: Catalyst is an insoluble component of the reaction (e.g., Pt, Pd, Ni metal).

Hydrogenation Energy of Alkenes

• The most stable alkene isomer releases the smallest amount of heat upon hydrogenation.
• Adding hydrogen decreases the units of unsaturation.
• Fully hydrogenated hydrocarbons (alkanes) are called “saturated hydrocarbons”.

Heats of Hydrogenation of Some Alkenes

Units: kJ/mol and (kcal/mol)
Ethylene: 136 (32.6)
Propene: 125 (29.9)
1-Butene: 126 (30.1)
1-Hexene: 126 (30.2)
cis-2-Butene: 119 (28.4)
cis-2-Pentene: 117 (28.1)
trans-2-Butene: 115 (27.4)
trans-2-Pentene: 114 (27.2)
2-Methyl-2-pentene: 112 (26.7)
2,3-Dimethyl-2-butene: 110 (26.4)

Hydrohalogenation & Radical Hydro-halogenation

1. Hydro-halogenation

• Addition of H-X across C=C pi bond (X = Br, Cl).
• Carbocation generation (possible rearrangement).
• Not stereospecific.
• Regioselective (Markovnikov addition).
• Stereochemistry: Racemization.
  *First step: H+ is the electrophile, pi bond is nucleophile
  *Possible carbocation rearrangement
  *Second step: X- is the nucleophile, carbocation is electrophile

2. Radical Hydro-bromination

*Mechanism: Chapter 10

*Carbocations are achiral and trigonal planar

Markovnikov’s Rule

• Hydrogen atom adds to the carbon atom of the double bond that has more hydrogen atoms.
• Heteroatom adds to the carbon atom of the double bond with the more carbon atoms.
• Carbocation resides on the carbon atom where it would be most stable.

Carbocation - Stereochemistry and Rearrangements

• Carbocations can undergo rearrangements via 1,2-hydride or 1,2-alkyl shifts to form more stable carbocations.

Regioselectivity and Cation Stability

• Regioselectivity is determined by the relative stability of the carbocation intermediate.
• Tertiary carbocations (3°) are more stable than secondary (2°) and primary (1°) carbocations.
• Methyl and primary carbocations are rarely intermediates in reactions due to their low stabilities.

Rearrangement of Carbocations

1,2-Hydride Shift:

*Example:
CH3CH-CH=CH2 + H-Br \rightarrow CH3C-CHCH3
$H

1,2-Alkyl Shift:

*Example:
CH3C-CH=CH2 + HCl \rightarrow CH3C-CH2CH3 CH3

Ring Expansion (1,2-Alkyl Shift):

CH=CH_2
+H^+ \rightarrow Expansion Or No Ring Expansion

Halogenation

• Addition of X_2 across a C=C bond (X = Br, Cl).
• Halonium ion (bromonium or chloronium intermediate).
• No Carbocation generation (No rearrangement).
• Stereospecific (anti-addition).
• Stereochemistry (racemization).
  --First step : Br_2 is the electrophile, pi bond is nucleophile
  --Br^- is lost as a LG, and a bridged intermediate Bromonium ion is formed
  --Second step (nucleophilic attack): Br^- (LG) is nucleophile, carbon with bromonium ion is electrophile

Halo-hydrin Formation

• Addition of –OH and -X across a C=C bond (X = Br, Cl).
• Halonium ion (bromonium or chloronium intermediate).
• No carbocation generation (No rearrangement).
• Regioselective (Markovnikov’s rule).
• Stereospecific (anti-addition).
• Stereochemistry (racemization).
  --First step : Br2 is the electrophile, pi bond is nucleophile --Br^- is lost as a LG, and a bridged intermediate Bromonium ion is formed --Second step (nucleophilic attack): H2O is nucleophile, carbon with bromonium ion is electrophile
  --NA at the most substituted carbon.
  --Third step (PT): solvent (H_2O) is the base, to deactivate the alcohol

Acid-Catalyzed Hydration

• Addition of H–OH across a C=C pi bond.
• Carbocation generation (possible rearrangement).
• Regioselective (Markovnikov’s rule).
• No stereospecificity.
• Stereochemistry (racemization).
  -First step (proton transfer): H^+ is the electrophile, pi bond is nucleophile
  -Carbocation generation (possible rearrangement)
  -Second step (nucleophilic attack): H_2O is the nucleophile, carbocation is electrophile
  -Third step (proton transfer): regenerate acid