Alkene Reactions – Study Notes (Comprehensive)

Alkene and Reactions – Comprehensive Study Notes

Alkene fundamentals

• Alkenes (olefins) are hydrocarbons containing carbon–carbon double bonds (C=C).
• They are unsaturated hydrocarbons and do not have the maximum number of hydrogens due to the presence of the double bond.
• General formula: $\mathrm{C<em>nH</em>{2n}}$
• The C=C double bond is more reactive than a single C–C bond due to pi-bond electrons and the availability of a vacant p-orbital on each carbon when reacting.
• Carbon atoms in the double bond are sp2 hybridized.

Nomenclature of alkenes

• Determine the longest chain that contains the double bond; this becomes the parent chain.
• Number the chain so that the double bond receives the lowest possible locant (lowest number at the double bond).
• Use the root name of the parent chain and add the -ene ending.
• Substituents are named and numbered in the same way as for alkanes.
• Note: Alkenes can be cyclic or acyclic (e.g., cycloalkenes). For cyclic alkenes, the double bond is included in the ring naming (e.g., cycloalkene, cycloalkenyl substituents).

Examples and naming practice

• Linear example: 1-butene (double bond at C-1 in a four-carbon chain).
• More complex example: 3-propyl-1-heptene (double bond at C-1, with a propyl substituent at C-3 on a seven-carbon chain).
• For cyclic alkenes with substitutions, naming can yield forms like 1-methylcyclobutene or 1-methylcyclobut-1-ene depending on the preferred parent ring and unsaturation locant.
• For cycloalkenes with vinyl substituents: 1-ethenylcyclohexene or 1-ethenylcyclohex-1-ene are acceptable IUPAC forms.

Visual examples (from transcript)

• CH3–CH=CH2 naming example: 1-methylcyclobutene or 1-methylcyclobut-1-ene; 1-ethenylcyclohexene or 1-ethenylcyclohex-1-ene.

Stereoisomerism in alkenes

• Stereoisomers: same connectivity of atoms (same order of bonding) but different spatial arrangement.
• Two main types:
  • Geometric isomers (around double bonds): cis/trans nomenclature.
  • Optical isomers (enantiomers) due to chirality.
• Optical isomerism (Chirality): occurs when a carbon atom is stereogenic (attached to four different groups).
• Geometric isomers arise from restricted rotation around the C=C bond, giving cis (Z) and trans (E) forms, with distinct physical and chemical properties.

Cis–trans nomenclature (pre-CIP description)

• Step 1: Draw a line down the center of the double bond.
• Step 2: Circle the hydrogens and evaluate whether substituents are on the same side (cis) or opposite sides (trans).
• Example: If chlorines are on the same side with hydrogens on the same side, it is Cis; if on opposite sides, it is Trans.

Cahn–Ingold–Prelog (CIP) priority and E/Z nomenclature

• For more complex alkenes with four different substituents around the double bond, use the CIP system.
• Assign priority to the groups attached to each carbon of the double bond based on atomic number (Z): higher Z gets higher priority (rough order: S > P > O > N > C > H).
• (E) configuration: the higher-priority groups are on opposite sides of the double bond.
• (Z) configuration: the higher-priority groups are on the same side of the double bond.
• Nomenclature: E or Z precedes the alkene name (e.g., (E)-2-butene, (Z)-2-butene).

Mechanism: electrophilic addition to alkenes (general framework)

• Electrophile (Lewis acid): electron-poor species that accepts an electron pair.
• Nucleophile (Lewis base): electron-rich species that donates an electron pair.
• Typical addition of HX to alkenes proceeds in two steps:
  1) Formation of a carbocation intermediate after the electrophile adds to one carbon of the double bond (rate-determining slow step).
  2) Nucleophilic attack by the counterion (X−) or solvent to form the final product.
• An example mechanism is bromination of an alkene like ethene: Br2 is polarized by the alkene; formation of a carbocation-like intermediate and Br− capture yields 1,2-dibromoethane.
• Energy profile concept: slow formation of carbocation (rate-determining) followed by fast nucleophilic attack to form product.

Experimental evidence for the mechanism

• Reaction of ethene with bromine water yields a mixture of 1,2-dibromoethane and 2-bromoethanol, indicating formation of carbocation intermediates that can be trapped by bromide or hydroxide.
• This supports the two-step electrophilic-addition mechanism with carbocation intermediates.

Types of addition reactions to alkenes

• There are four main types:
  • Hydrogenation: addition of H2 across the double bond to form alkanes.
  • Halogenation: addition of X2 (e.g., Cl2, Br2) to form dihaloalkanes.
  • Hydrohalogenation: addition of HX (X = Cl, Br, I) to form haloalkanes.
  • Hydration: addition of water (H2O) across the double bond to form alcohols.

Hydrohalogenation (HX) details

• Mechanism: the H end (partially positive) adds to the carbon that bears more hydrogens (more substituted carbon usually bears more hydrogens, forming a more stable carbocation on the other carbon).
• The halogen X− then adds to the carbocation, yielding haloalkanes.
• Regiochemistry: Markovnikov rule often governs HX additions; the hydrogen attaches to the carbon with more hydrogens, and the halogen attaches to the carbon with fewer hydrogens.
• Reactivity order of HX: HI > HBr > HCl > HF (lower bond strength in HI leads to easier formation of I− and faster reaction).

Hydration (water addition)

• Water is polar; the mechanism is similar to hydrohalogenation but with water as the nucleophile.
• The hydrogen adds first to the carbon with more hydrogens, forming a carbocation, which is then attacked by OH− from water to yield an alcohol after workup.
• General form for an unsubstituted alkene: $\mathrm{R-CH=CH<em>2 + H</em>2O \rightarrow R-CH(OH)-CH_3}$ (followed by possible proton transfers depending on conditions).

Other electrophilic additions to alkenes

• Halogenation: addition of X2 in a suitable solvent (often CCl4) to form dihaloalkanes; the halogen’s color typically disappears as the reaction proceeds.
• Hydration: as above, to form alcohols; can be acid-catalyzed or otherwise depending on conditions.
• Hydrogenation: H2 with a metal catalyst (e.g., Ni, Pd, Pt) at high temperature (about 180–200 °C) to produce alkanes.

Five common addition reagents and outcomes

• a) HX (X = Cl, Br, I): haloalkanes via Markovnikov-type addition; order HI > HBr > HCl > HF in reactivity.
• b) X2 in nonpolar solvent (e.g., CCl4): dihaloalkanes; color of X2 disappears as reaction proceeds.
• c) Aqueous halogen (HOX/HX + H2O): haloalcohol in addition to dihaloalkanes.
• d) H2 with a catalyst (Ni, Pd, Pt): hydrogenation to form alkanes.
• e) Acidic water (H2SO4 or hydration): alcohol formation via hydration.

Regioselectivity and the major/minor products

• In many additions, the more substituted carbon becomes more stabilized in the carbocation intermediate (if formed), leading to Markovnikov regioselectivity.
• Major product is typically the one from the more stable carbocation intermediate; minor product arises from alternative routes (e.g., anti-Markovnikov under specific conditions).

Markovnikov’s rule and carbocation stability

• Markovnikov’s Rule: In addition to HX (where X is a halogen or the OH group from water), the H adds to the carbon with the greater number of hydrogens, and the X (or OH) adds to the carbon with fewer hydrogens.
• Rationale: The carbocation intermediate is more stable when adjacent to more carbon substituents (i.e., tertiary > secondary > primary).
• Example (typical): Addition of HBr to an alkene forms the more substituted bromide as major product due to the more stable carbocation produced after H adds to the less substituted carbon.

Exceptions: Anti-Markovnikov rule (peroxide effect) for HBr

• In the presence of peroxides or oxygen, HBr addition to alkenes can proceed anti-Markovnikov (H adds to the carbon with fewer hydrogens; Br adds to the carbon with more hydrogens).
• This radical pathway is initiated by peroxides and involves bromine radicals rather than carbocation intermediates.
• Example (anti-Markovnikov): 2-methylpropene + HBr (with peroxide) yields 1-bromo-2-methylpropane instead of 2-bromo-2-methylpropane.

Radical mechanism for anti-Markovnikov HBr addition (overview)

• Initiation: Decomposition of benzoyl peroxide to generate radicals (e.g., PhCOO• or related radicals).
• Propagation: The radical abstracts a hydrogen from HBr to generate Br• radicals.
• The Br• radical adds to the alkene to form a more stable carbon-centered radical (often a secondary radical).
• The carbon-centered radical then abstracts a hydrogen atom from another molecule of HBr to yield the anti-Markovnikov haloalkane and regenerate Br•.
• This chain propagation continues until termination.

Carbocation rearrangements in electrophilic additions

• Carbocations formed during electrophilic additions can rearrange to more stable carbocations.
• Whitmore proposed that rearrangements occur to give more stable intermediates.
• Common rearrangements:
  • Hydride shift: a hydride (H−) moves from a neighboring carbon to form a more stable carbocation.
  • Methyl shift: a methyl group migrates with its electron pair to form a more stable carbocation.
• These rearrangements favor the formation of the most stable carbocation, which then undergoes further reaction.

Oxidation of alkenes (KMnO4) and related processes

• Oxidation with potassium permanganate (KMnO4) can proceed under different conditions to yield different products:
  a) Cold, dilute KMnO4 (alkaline or acidic, room temperature): syn-diol formation (vicinal diols) from syn addition across the double bond.
  Example: $\mathrm{CH<em>2=CH</em>2 + KMnO<em>4 + H</em>2O \rightarrow HO-CH<em>2-CH</em>2OH}$ (ethylene diol).
  b) Hot, concentrated KMnO4 (often acidic): oxidative cleavage of the C=C bond to give aldehydes, ketones, carboxylic acids, and possibly CO2, depending on substitutions.
  Mechanistic outline: alkene oxidized to a diol, diol cleaved into carbonyl compounds; further oxidation of aldehydes/primary products yields carboxylic acids and CO2.
• Overall, complete oxidation of alkenes by hot KMnO4 in acidic solution can be summarized as:
  $\mathrm{R-CH=CH-R' + [O] \rightarrow R-COOH + R'-COOH}$
  with possible CO2 formation when oxidation proceeds to completion for certain substituents.
• Note on diol formation: during KMnO4 oxidation, the purple color of KMnO4 is deactivated as it becomes MnO2 or other oxidation states, indicating reaction progress.

Oxygen-induced epoxidation (epoxides) with silver catalyst

• In the presence of a catalyst such as silver (Ag) at elevated temperature with oxygen, alkenes can form epoxides (oxiranes). For example:
  $\mathrm{C<em>2H</em>4 + O_2 \rightarrow \text{epoxyethane (ethylene oxide)}}$
• Epoxides can further react with water to form vicinal diols:
  $\mathrm{C<em>2H</em>4O + H<em>2O \rightarrow HO-CH</em>2-CH_2OH}$
• Industrially, epoxides like ethylene oxide are important as intermediates for further chemical synthesis (e.g., ethylene oxide → ethylene glycol).

Industrial importance of alkenes and polymerization

• Ethene (ethylene) is a key industrial feedstock.
• Uses of ethene include:
  • Manufacture of poly(ethylene) plastics (polyethylene).
  • Production of ethylene oxide (epoxide) and subsequently ethylene glycol for antifreeze and antifungal materials.
  • Production of chloroethane used to prepare tetraethyl lead (anti-knock agent for car engines).
  • Formation of 1,2-dichloroethane used to make vinyl chloride (chloroethene) for PVC plastic via thermal cracking at high temperature (e.g., 500 °C).
• Plastic manufacturing and polymerization:
  • Polymers are long chain molecules formed by addition polymerization of monomers (common mechanism is free-radical or ionic polymerization).
  • General polymerization equation (radical or ionic):
    $\mathrm{n\, CH<em>2=CH</em>2 \rightarrow [-CH<em>2-CH</em>2-]_n}$
  • Common polymers and monomers:
  • Polyethylene (PE): Monomer $\mathrm{CH<em>2=CH</em>2}$
    • Variants: Low-density (LDPE) and High-density (HDPE) with differing properties (soft/flexible vs robust).
  • Polyvinyl chloride (PVC): Monomer $\mathrm{CH_2=CHCl}$
  • Polystyrene (PS): Monomer $\mathrm{CH_2=CHPh}$ (phenyl-ethylene unit)
  • Poly(methyl methacrylate) (PMMA): Monomer $\mathrm{CH<em>2=CH-COOCH</em>3}$ (perspective polymer name in notes)
  • Uses of polymers: bags, bottles, buckets, chairs, toys, insulation, pipes, tiles, optical equipment, etc.

Polymerization and properties recap

• Addition polymerization involves the opening of the double bond in monomers, linking unit after unit to form a polymer.
• Free-radical polymerization is common for many plastics; ionic polymerization is another route for certain polymers.
• Polymers have repetitive units that derive from the monomer; the overall properties of the polymer depend on monomer structure, tacticity, branching, and molecular weight.

Key concepts and connections

• Relationship between alkene structure and reactivity: electron-rich double bond interacts with electrophiles; the stability of intermediates (carbocations or radicals) governs regioselectivity and stereoselectivity.
• Markovnikov vs anti-Markovnikov rules: guided by carbocation stability or radical pathways; peroxide effect provides a radical mechanism for anti-Markovnikov HBr additions.
• Stereochemistry (cis/trans, E/Z) affects physical properties and reactivity; CIP rules allow disambiguation of complex cases.
• Oxidation chemistry shows how alkenes can be selectively transformed under different conditions (diol formation, cleavage to carbonyls/acid, or epoxidation).
• Industrial relevance: alkenes are not only central to basic organic transformations but also to polymer science and large-scale materials production.

Formulas and representative reactions (summary with LaTeX)

• General formula for alkenes: $ext{C}<em>n ext{H}</em>{2n}$
• Addition of HX (hydrohalogenation):
  $\mathrm{R-CH=CH<em>2 + HX \rightarrow R-CHX-CH</em>3}$
• Halogenation (X2):
  $\mathrm{R-CH=CH<em>2 + X</em>2 \rightarrow R-CHX-CH_2X}$
• Hydration (water addition):
  $\mathrm{R-CH=CH<em>2 + H</em>2O \rightarrow R-CH(OH)-CH_3}$
• Hydrogenation:
  $\mathrm{R-CH=CH<em>2 + H</em>2 \rightarrow R-CH<em>2-CH</em>3}$
• Epoxidation (ethene to ethylene oxide):
  $\mathrm{C<em>2H</em>4 + O<em>2 \rightarrow \text{epoxyethane (C}</em>2\text{H}_4\text{O)}}$
• KMnO4 oxidation (diol formation):
  $\mathrm{R-CH=CH-R' + KMnO<em>4 + H</em>2O \rightarrow \text{diol}}$
• KMnO4 oxidation (cleavage to carbonyls/CO2):
  $\mathrm{R-CH=CH-R' + [O] \rightarrow \text{R-COOH} + \text{R'-COOH}}$ (simplified)
• Polymerization (polymer from ethene):
  $\mathrm{n\ CH<em>2=CH</em>2 \rightarrow [-CH<em>2-CH</em>2-]_n}$

Key terms to remember

• Alkene, alkene naming, -ene suffix, parent chain, optional prefixes/substituents
• Stereoisomerism, cis/trans, E/Z nomenclature, CIP priority
• Electrophile, Nucleophile (Lewis acid/base)
• Carbocation, carbocation rearrangements (hydride/methyl shifts)
• Markovnikov’s rule and anti-Markovnikov exceptions (peroxide effect)
• Addition reactions: hydrogenation, halogenation, hydrohalogenation, hydration
• Oxidation of alkenes with KMnO4 (diols vs carbonyl/acid products)
• Epoxidation with oxygen/Ag catalysts
• Industrial relevance: plastics, epoxy intermediates, PVC, and polymerization mechanisms

Notes and study tips

• Practice naming: given a structural formula, determine the parent chain containing the double bond, then assign locants for double bond and substituents.
• Distinguish between cis/trans (older method) and E/Z (CIP-based) nomenclature; be able to apply both depending on substituents.
• For electrophilic additions, identify the electrophile and nucleophile, determine the major product via Markovnikov or anti-Markovnikov pathways, and consider possible rearrangements.
• For KMnO4 oxidation, remember the two regimes: cold/dilute gives diols; hot/concentrated gives cleavage to carbonyl-containing products and possibly CO2.
• For polymerization, remember that step-growth vs chain-growth mechanisms define polymer properties; polyethylene is the classic example of addition polymerization from ethene.