Alkenes

Alkenes Overview

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  • Definition of Alkenes

    • Unsaturated hydrocarbons with at least one carbon-to-carbon double bond.

    • More reactive than alkanes due to the double bond and exposed pi electrons.

    • General formula: CnH2n.

  • Common Characteristics

    • Known as 'Olefines' due to oily products formed with chlorine or bromine.

    • Ethylene in fruits and vegetables speeds up ripening.

    • Lycopene in tomatoes gives them a bright red color.

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  • Classification of Alkenes

    • Monosubstituted, disubstituted, tri/tetrasubstituted based on alkyl groups attached to sp2 hybridized carbon atoms.

    • Alkyl groups affect stability and reactivity of alkenes.

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  • Nomenclature of Alkenes

    • Common and IUPAC names derived from corresponding alkanes by changing -ane to -ylene or -ene.

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  • Nomenclature of Alkenes Continued

    • Naming conventions for alkenes with varying carbon chains.

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  • IUPAC Naming for Higher Alkenes

    • Steps for assigning systematic names for higher alkenes.

    • Replacement of -ane with -adiene or -atriene for multiple double bonds.

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  • Nomenclature of Alkenes Continued

    • Determining the IUPAC name based on the longest carbon chain containing the C=C bond.

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  • IUPAC Nomenclature of Alkenes Continued

    • Examples of naming alkenes using IUPAC conventions.

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  • Structure of Alkenes

    • Orbital makeup of alkenes illustrated using ethylene.

    • Formation of sigma and pi bonds in ethylene.

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  • Isomerism in Alkenes

    • Chain isomers, position isomers, and geometrical isomers explained.

    • Cis and trans isomers as stereoisomers with different orientations in three-dimensional space.

E-Z Configuration in Alkenes

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  • E-Z Configuration Overview

    • Describes absolute stereochemistry of double bonds in organic chemistry.

    • Extension of cis-trans isomer notation.

    • Can describe double bonds with two, three, or four substituents.

  • Cahn–Ingold–Prelog Priority Rules

    • Assigns priority to substituents on a double bond.

    • Compares positions of higher priority substituents on each carbon.

  • E and Z Configuration

    • E: Higher priority groups are on opposite sides of the double bond.

    • Z: Higher priority groups are on the same side of the double bond.

  • Example: 1-Bromo-1,2-dichloroethane application of CIP rules.

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  • Assigning Group Priorities

    • Follow Cahn, Ingold, Prelog rules.

    • Rank atoms attached to each carbon of the double bond by atomic number.

  • Examples: (Z)-3-methyl-2-pentene, (E)-2-chloro-2-butene.

  • Designation: Z for same side, E for opposite sides of the double bond.

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  • Alitretinoin

    • Form of vitamin A with E-Z configuration in alkenes.

    • Chemical name indicates E and Z configurations at specific positions.

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  • Ranking Atoms for E-Z Configuration

    • Rank atoms by atomic number for priority.

  • Examples: (E)-1,2-dichloroethene, (Z)-but-2-ene.

  • Considerations: Triple bonds count three times in priority.

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  • Priority Determination

    • Compare atoms directly attached to the pi bond by atomic number.

  • Example: (Z)-1-chloro-2-ethyl-1,3-butadiene.

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  • Priority Determination Continued

    • Identify high priorities on opposite sides for E configuration.

  • Examples: (Z)-form, atoms attached directly to the carbon.

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  • Priority Determination and Tie-Breaking

    • List atoms directly bonded to each carbon for priority.

  • Example: (1E,4Z)-1,5-dichloro-1,4-hexadiene.

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  • Priority Group Comparison

    • Highlight higher priority groups for E-Z determination.

  • Examples: E or Z configurations based on priority groups.

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  • Naming alkenes

    • (Z)-4-ethyl-5-methyloct-3-ene or (Z)-4-ethyl-5-methyl-3-octene

    • 3-ethyl-6-methyl-4-propylhept-3-ene or 3-ethyl-6-methyl-4-propyl-3-heptene

    • (E)-2-chloro-4-bromo-5-ethyl-7-methyldec-4-ene or (E)-2-chloro-4-bromo-5-ethyl-7-methyl-4-decene

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  • Dehydration of alcohols

    • Dehydration can be done with dehydrating agents like P2O5, H3PO4, Al2O3

    • Unsymmetrical alcohols form a mixture of alkenes, following Saytzeff rule

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  • Dehydrohalogenation of alkyl halides

    • 3° alkyl halide > 2° alkyl halide > 1° alkyl halide in ease of dehydrohalogenation

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  • Dehalogenation of vicinal dihalides

    • Zn/CH3OH can dehalogenate vicinal dihalides to form alkenes

    • Example: 1,2-dibromoethane to ethene

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  • Controlled hydrogenation of alkynes

    • Lindlar’s catalyst for controlled hydrogenation

    • Alkynes can be fully hydrogenated into alkanes with platinum or Palladium catalyst

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  • Cracking of alkanes

    • Decomposition of alkanes at high temperatures without air

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  • Physical properties of alkenes

    • Alkenes' solubility, boiling points, and reactivity compared to alkanes

    • Differences in properties between trans and cis isomers

    • IR spectrum absorption peaks for C-H and C=C bonds

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  • Electrophilic addition reactions of alkenes

    • Addition of hydrogen halides to alkenes

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  • Addition reactions of alkenes

    • Markovnikov’s rule in HX addition to unsymmetrical alkenes

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  • Mechanism of formation of Markovnikov's product

    • Addition of hydrogen halides to unsymmetrical alkenes

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  • Mechanism of formation of anti-Markovnikov’s product

    • Peroxide Effect causing anti-Markovnikov addition of HBr

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  • Addition of hypohalous acids

    • Formation of halohydrins with Br2 or Cl2 in the presence of water

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  • Addition of sulphuric acid

    • H2SO4 addition to alkenes to form alkyl hydrogen sulphate

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  • Addition of water (Hydration)

    • Hydration of alkenes in the presence of dilute Sulphuric acid

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  • Addition of halogens

    • Rapid reaction of alkenes with bromine and chlorine to form vicinal-dihalides

Chemical Properties of Alkenes

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  • Electrophilic Addition Reactions

    • Alkylation

      • Alkanes add to alkenes in the presence of H2SO4 or HF.

      • Used in the manufacture of isooctane.

    • Addition of Hydrogen

      • Alkenes add hydrogen under pressure with Ni, Pt, or Pd catalyst.

      • Catalytic Hydrogenation reaction.

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  • Electrophilic Addition Reactions

    • Oxymercuration-demercuration

      • Alkene treated with mercuric acetate, then reduced with sodium borohydride to form alcohol.

      • Markovnikov addition of H2O to a double bond.

    • Hydroboration

      • Diborane reacts with alkenes to form trialkylboranes.

      • Trialkylboranes used for synthesizing primary alcohols.

      • Anti-Markovnikov addition of H2O to a double bond.

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  • Electrophilic Addition Reactions

    • Oxidation with Cold KMnO4 Solution

      • Alkenes react with cold dilute potassium permanganate to form glycols.

      • Test for the Presence of a Double Bond (Baeyer's Test).

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  • Electrophilic Addition Reactions

    • Oxidation with Hot KMnO4 Solution

      • Alkenes split at the double bond to form ketones and/or acids.

    • Catalytic Oxidation

      • Alkenes react with oxygen in the presence of silver catalyst to form Epoxides.

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  • Electrophilic Addition Reactions

    • Oxidation with Ozone

      • Ozone adds across the double bond to form an Ozonide.

      • Ozonolysis process to get carbonyl compounds.

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  • Electrophilic Addition Reactions

    • Combustion

      • Alkenes oxidized to CO2 and water when burnt in air.

    • Polymerization

      • Simple alkenes polymerize to form long chain addition polymers.

      • Catalyzed by HF, H2SO4, or organic peroxides.

    • Substitution of Alkenes by Halogens

      • Allylic Substitution with Cl2 or Br2 at high temperatures.

Chemical Properties of Alkenes

  • Alkenes Reactions

    • Alkenes react with hydrogen in the presence of a nickel catalyst to form alkanes.

    • Reaction with bromine in the presence of light or heat leads to the addition of bromine across the double bond.

    • Alkenes can undergo halogenation with bromine to form dibromo compounds.

    • Alkenes react with hydrogen bromide to form alkyl bromides.

    • Alkenes can react with hypochlorous acid to form chlorohydrins.

Dienes

  • Dienes Definition

    • Dienes are alkenes with two carbon-carbon double bonds.

    • Different types of dienes include isolated delocalized, conjugated, and cumulated dienes.

  • 1,3-Butadiene

    • 1,3-Butadiene is an example of a conjugated diene.

    • The delocalization of pi electrons in 1,3-butadiene contributes to its stability.

Preparation of 1,3-Butadiene

  • Methods of Preparation

    • 1,3-Butadiene can be prepared from acetylene, 1-butene, 1,4-butanediol, and n-butane through specific reactions.

Chemical Properties of 1,3-Butadiene

  • Addition Reactions

    • Addition of halogen acids can lead to 1,2-addition or 1,4-addition based on temperature conditions.

    • Addition of halogens, water, and hydrogen to 1,3-butadiene can result in different products.

    • Polymerization of 1,3-butadiene can lead to the formation of polybutadiene, known as Buna Rubber