Organic Chemistry: Alkenes and Reactions
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Questions and Answers

What type of strain is caused by repulsive interactions between electron clouds in a conformation?

  • Torsional strain (correct)
  • Steric strain
  • Bond strain
  • Van der Waals strain

    • The eclipsed form of a molecule has less torsional strain than the staggered form.

    False (B)

    What is the general formula for alkenes?

    CnH2n

    Isomers in which two identical atoms or groups lie on the same side of the double bond are called ______ isomers.

    <p>cis</p> Signup and view all the answers

    Match the terms with their descriptions:

    <p>Alkene = Unsaturated hydrocarbon with at least one double bond Cis isomer = Identical groups on the same side of the double bond Trans isomer = Identical groups on opposite Sides of the double bond Dehydrohalogenation = Removal of halogen acid from alkyl halides</p> Signup and view all the answers

    Which catalyst is used for the partial reduction of alkynes to alkenes?

    <p>Lindlar’s catalyst (C)</p> Signup and view all the answers

    Dehalogenation involves the removal of water from a molecule.

    <p>False (B)</p> Signup and view all the answers

    What type of reaction is dehydrohalogenation?

    <p>β-elimination reaction</p> Signup and view all the answers

    Alkanes are unsaturated hydrocarbons containing carbon-carbon double bonds.

    <p>False (B)</p> Signup and view all the answers

    What products are formed when sodium salts of carboxylic acids are heated with soda lime?

    <p>Alkanes and sodium carbonate</p> Signup and view all the answers

    A carbon atom attached to three other carbon atoms is known as a ______ carbon.

    <p>tertiary</p> Signup and view all the answers

    Match the following reaction types with their descriptions:

    <p>Hydrogenation = Addition of dihydrogen to unsaturated hydrocarbons Wurtz Reaction = Treatment of alkyl halides with sodium metal in dry ether to form higher Alkanes Decarboxylation = Elimination of carbon dioxide from a carboxylic acid Kolbe's Electrolytic Method = Electrolysis of an aqueous solution of sodium or potassium salt of a carboxylic acid to produce alkanes</p> Signup and view all the answers

    Which of the following metals is NOT used as a catalyst in the hydrogenation of alkenes and alkynes?

    <p>Magnesium (A)</p> Signup and view all the answers

    The H-C-H bond angle in alkanes is 120 degrees.

    <p>False (B)</p> Signup and view all the answers

    In Wurtz reaction, what type of solvent is used?

    <p>Dry ether (A)</p> Signup and view all the answers

    What type of mechanism does the peroxide effect proceed through?

    <p>Free radical chain (B)</p> Signup and view all the answers

    According to Markovnikov's rule, cold concentrated sulfuric acid adds to alkenes to form alkyl hydrogen sulfate.

    <p>True (A)</p> Signup and view all the answers

    Which of the following reagents can be used to dehydrate alcohols into alkenes?

    <p>Concentrated H2SO4 (D)</p> Signup and view all the answers

    What type of product is formed when alkenes react with acidified water, in accordance with Markovnikov's rule?

    <p>Alcohol</p> Signup and view all the answers

    Alkenes undergo substitution reactions with electrophiles.

    <p>False (B)</p> Signup and view all the answers

    What type of product is formed when alkenes react with halogens?

    <p>vicinal dihalides</p> Signup and view all the answers

    Alkenes react with cold, dilute, aqueous solution of potassium permanganate to produce ______ glycols.

    <p>vicinal</p> Signup and view all the answers

    According to Markovnikov's rule, during the addition of HBr to an unsymmetrical alkene, the negative part of the addendum gets attached to the carbon with the ______ number of hydrogen atoms.

    <p>lesser</p> Signup and view all the answers

    Match the following reagents with the products formed when they react with alkenes:

    <p>Cold, dilute, aqueous KMnO4 = Vicinal glycols Acidic KMnO4 or K2Cr2O7 = Ketones and/or acids O3 followed by Zn-H2O = Smaller molecules (aldehydes or ketones)</p> Signup and view all the answers

    Match the following reactants with the type of products formed during their addition to alkenes:

    <p>Dihydrogen (H2) = Alkanes Halogens (Br2, Cl2) = Vicinal dihalides Hydrogen halides (HCl, HBr, HI) = Alkyl halides</p> Signup and view all the answers

    Which of the following reagents is used in ozonolysis to cleave the ozonide?

    <p>Zn-H2O (B)</p> Signup and view all the answers

    Which carbocation is more stable?

    <p>Tertiary carbocation (C)</p> Signup and view all the answers

    Alkynes contain at least one double bond between two carbon atoms.

    <p>False (B)</p> Signup and view all the answers

    The addition of HBr to unsymmetrical alkenes always follows Markovnikov's rule, regardless of the presence of peroxides.

    <p>False (B)</p> Signup and view all the answers

    What is the name given to the addition of HBr to unsymmetrical alkenes in the presence of peroxide?

    <p>Anti Markovnikov addition or peroxide effect or Kharash effect</p> Signup and view all the answers

    What type of catalyst is used in the hydrogenation of benzene to cyclohexane under vigorous conditions?

    <p>Nickel (D)</p> Signup and view all the answers

    The arenium ion is stabilized by resonance.

    <p>True (A)</p> Signup and view all the answers

    What product is formed when three chlorine molecules add to benzene under ultraviolet light?

    <p>benzene hexachloride</p> Signup and view all the answers

    To restore the aromatic character, σ -complex releases ______ from sp3 hybridised carbon.

    <p>proton</p> Signup and view all the answers

    Match the functional group with its directing influence in electrophilic aromatic substitution:

    <p>-OH = Ortho, para-directing -NO2 = Meta-directing -CH3 = Ortho, para-directing -CN = Meta-directing</p> Signup and view all the answers

    What are the products of benzene combustion in air?

    <p>Carbon dioxide and water (B)</p> Signup and view all the answers

    Meta-directing groups increase the overall electron density on the benzene ring.

    <p>False (B)</p> Signup and view all the answers

    What is the directive influence of substituents when monosubstituted benzene is subjected to further substitution?

    <p>either ortho and para products or meta product is predominantly formed</p> Signup and view all the answers

    What type of bonds are formed when two $sp^2$ hybrid orbitals of each carbon atom overlap with $sp^2$ hybrid orbitals of adjacent carbon atoms in benzene?

    <p>σ bonds (D)</p> Signup and view all the answers

    According to X-ray diffraction data, benzene exhibits two distinct C-C bond lengths, corresponding to single and double bonds.

    <p>False (B)</p> Signup and view all the answers

    What three conditions must a compound satisfy to be considered aromatic, according to Hückel's Rule?

    <p>Planarity, complete delocalization of π electrons in the ring, and presence of (4n + 2) π electrons</p> Signup and view all the answers

    Heating the sodium salt of benzoic acid with _______ gives benzene.

    <p>sodalime</p> Signup and view all the answers

    Match the preparation method with its corresponding reactant:

    <p>Cyclic polymerization = Ethyne Decarboxylation = Sodium salt of benzoic acid Reduction = Phenol with zinc dust</p> Signup and view all the answers

    Aromatic hydrocarbons are generally miscible with water.

    <p>False (B)</p> Signup and view all the answers

    The reluctance of benzene to undergo addition reactions under normal conditions is attributed to:

    <p>The absence of pure double bonds (B)</p> Signup and view all the answers

    What is released when phenol is heated with zinc dust to produce benzene?

    <p>Zinc oxide or ZnO</p> Signup and view all the answers

    Flashcards

    Staggered vs Eclipsed

    Staggered form has less torsional strain; eclipsed has maximum strain.

    Torsional strain

    Repulsive interaction between electron clouds affecting conformation stability.

    Alkenes

    Unsaturated hydrocarbons with at least one double bond, formula CnH2n.

    Isomerism in Alkenes

    Alkenes exhibit structural and geometrical isomerism.

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    Cis Isomer

    Identical atoms/groups on the same side of the double bond.

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    Trans Isomer

    Identical atoms/groups on opposite sides of the double bond.

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    Dehydrohalogenation

    Elimination of halogen acid from alkyl halides to form alkenes.

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    Vicinal Dihalides

    Dihalides with halogens on adjacent carbon atoms, forming alkenes upon dehalogenation.

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    Hydrocarbons

    Organic compounds made of hydrogen and carbon only.

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    Dehydration of Alcohols

    Alcohols lose water to form alkenes when treated with protic acids like H2SO4.

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    Saturated hydrocarbons

    Hydrocarbons with only single carbon-carbon bonds.

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    Physical Properties of Alkenes

    The first three alkenes are gases, the next fourteen are liquids, and higher members are solids.

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    Addition Reactions

    Alkenes react with electrophiles adding to the double bond to form new products.

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    Hydrogenation

    Alkenes add dihydrogen to form alkanes using catalysts like nickel or palladium.

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    Primary carbon atom

    A carbon bonded to one other carbon atom.

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    Halogen Addition

    Alkenes react with halogens to produce vicinal dihalides, like 1,2-dibromoethane.

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    Wurtz reaction

    Reaction using sodium to form higher alkanes from alkyl halides.

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    Markovnikov Rule

    In unsymmetrical alkenes, the hydrogen adds to the carbon with more hydrogens.

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    Hydrogenation

    Adding hydrogen to alkenes or alkynes to create alkanes.

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    Decarboxylation

    Elimination of CO2 from carboxylic acids to form alkanes.

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    Electrophilic Attack

    H+ from hydrogen bromide attacks the double bond forming a carbocation.

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    Anti Markovnikov Addition

    In the presence of peroxide, HBr adds to unsymmetrical alkenes contrary to Markovnikov's rule.

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    Kolbe’s electrolytic method

    Electrolysis of carboxylic acid salts giving alkanes.

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    sp2 hybrid orbitals

    Orbitals formed by mixing one s and two p orbitals, used in bonding.

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    C-C sigma bonds

    Single covalent bonds formed between carbon atoms in benzene.

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    π bond in benzene

    A bond formed by the lateral overlap of unhybridized p orbitals.

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    Hückel Rule

    Criteria that determine if a compound is aromatic, involving planarity and electron count.

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    Aromaticity

    Property of molecules that adhere to Hückel's rule and possess stability due to electrons.

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    X-ray diffraction of benzene

    Technique that shows benzene has equal C-C bond lengths, indicating resonance.

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    Decarboxylation of aromatic acids

    Method to prepare benzene by removing a carboxyl group from benzoic acid.

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    Physical properties of aromatic hydrocarbons

    Non-polar molecules that are colorless and have distinctive aromas, immiscible with water.

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    Peroxide effect

    A free radical chain mechanism leading to bromination.

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    1-bromopropane

    A major product formed from the addition of HBr to propene.

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    Oxidation with KMnO4

    Alkenes oxidized to vicinal glycols using cold, dilute potassium permanganate.

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    Ozonolysis

    The reaction of alkenes with ozone to form ozonides and smaller molecules.

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    Polymerisation

    The process of combining many ethene molecules to form polythene.

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    Calcium carbide method

    Industrial method for producing ethyne by reacting calcium carbide with water.

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    Arenium ion stabilization

    Arenium ion is stabilized by resonance during electrophilic aromatic substitution.

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    Removal of proton

    σ-complex releases a proton from sp3 carbon to restore aromaticity after electrophile attack.

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    Benzene hydrogenation

    Benzene converts to cyclohexane under high temperature/pressure with nickel catalyst.

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    Chlorination of benzene

    Benzene reacts with Cl2 under UV light to produce benzene hexachloride (gammaxane).

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    Directive influence of substituents

    The behavior of substituents that affect the position of further substitutions in benzene rings.

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    Ortho and para directing groups

    Groups that favor substitution at ortho and para positions due to higher electron density.

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    Meta directing groups

    Groups that direct substitution at meta position, often reducing electron density at o- and p-.

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    Deactivating groups

    Groups that reduce electron density on benzene, making substitutions harder.

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    Study Notes

    Hydrocarbons

    • Hydrocarbons are compounds composed of carbon and hydrogen.

    Classification of Hydrocarbons

    • Hydrocarbons are classified into three main categories based on the types of carbon-carbon bonds:
      • Saturated hydrocarbons
      • Unsaturated hydrocarbons
      • Aromatic hydrocarbons

    Alkanes

    • Alkanes are saturated open-chain hydrocarbons containing only carbon-carbon single bonds.
    • General formula: CnH2n+2
      • n stands for the number of carbon atoms
      • 2n+2 stands for the number of hydrogen atoms
    • All H-C-H bond angles are 109.5°.
    • Types of carbon atoms:
      • Primary (1°): Attached to one other carbon atom
      • Secondary (2°): Attached to two other carbon atoms
      • Tertiary (3°): Attached to three other carbon atoms
      • Quaternary (4°): Attached to four other carbon atoms

    Preparation of Alkanes

    • From unsaturated hydrocarbons: Dihydrogen gas adds to alkenes and alkynes in the presence of platinum, palladium or nickel catalysts which creates alkanes. This process is called hydrogenation.
    • From alkyl halides: Alkyl halides (except fluorides) with zinc and dilute hydrochloric acid yields alkanes.
    • Wurtz reaction: Alkyl halides with sodium metal in dry ether produce higher alkanes.
    • From carboxylic acids: Sodium salts of carboxylic acids undergo decarboxylation (eliminating carbon dioxide) by heating with soda lime (mixture of sodium hydroxide and calcium oxide) leading to a reduction of the alkane chain by one carbon.
    • Kolbe's electrolytic method: An aqueous solution of the sodium or potassium salts of a carboxylic acid produces an alkane via electrolysis.

    Physical Properties of Alkanes

    • Alkanes are almost nonpolar molecules due to covalent C-C and C-H bonds and the small difference in electronegativity between carbon and hydrogen atoms.
    • First four members (C1 to C4) are gases.
    • C5 to C17 are liquids.
    • Those with 18 or more carbons are solids at room temperature (298 K).
    • Alkanes are colorless and odorless.

    Chemical Properties of Alkanes

    • Substitution reactions: One or more hydrogen atoms can be replaced by halogens, nitro group, or sulfonic acid group.
    • Halogenation: Halogenation occurs at high temperatures (573-773 K) or in the presence of diffused sunlight or ultraviolet light.
    • Reactions:
      • CH4 + Cl2 → CH3Cl + HCl (UV light)
      • CH3Cl + Cl2 → CH2Cl2 + HCl (UV light)
      • CH2Cl2 + Cl2 → CHCl3 + HCl (UV light)
      • CHCl3 + Cl2 → CCl4 + HCl (UV light)
    • Mechanism
      • Initiation: The reaction starts with the homolysis of chlorine molecules in presence of heat or light.
      • Propagation: Chlorine free radical attacks methane generating a methyl free radical with the formation of HCl
      • Termination: This reaction stops due reactant consumption and/or other side reactions.
    • Combustion: Alkanes, when heated in the presence of air or oxygen, are completely oxidized into carbon dioxide and water with the release of heat (used as fuel). CH4 + 2O2 → CO2 + 2H2O; ΔH° = -840 kJ/mol
    • Controlled oxidation: Alkanes react with oxygen/air at a controlled/regulated rate, and high temperature/pressure, and catalysts to yield a variety of oxidation products.
    • Isomerization: n-alkanes transformed to branched-chain alkanes upon heating in the presence of anhydrous aluminum chloride and hydrogen chloride.
    • Aromatization: n-alkanes with six or more carbon atoms on heating to 773K at 10-20 atmospheric pressure with vanadium, molybdenum or chromium oxides on alumina undergo dehydrogenation and cyclization to produce benzene and its homologues.
    • Reaction with steam: Methane reacts with steam at 1273 K in the presence of nickel catalysts to form carbon monoxide and dihydrogen. (Used for industrial preparation of dihydrogen).
    • Pyrolysis (Cracking): Higher alkanes decompose into lower alkanes/alkenes, etc. when heated to high temperatures (decomposition into smaller molecules using heat).

    Conformations

    • The rotation of C-C bonds results in different spatial arrangements called conformations, conformers, or rotamers.
      • Eclipsed: Hydrogen atoms are closest to each other.
      • Staggered: Hydrogen atoms are farthest apart for minimal repulsion.
      • Skew: Intermediate between eclipsed and staggered.

    Representations of Conformations

    • Line-wedge, Sawhorse, and Newman projections show different ways to depict the same conformation.

    Alkenes

    • Alkenes are unsaturated hydrocarbons with at least one carbon-carbon double bond.
    • General formula: CnH2n
    • Nomenclature: The suffix "ene" is used in alkenes

    Isomerism in Alkenes

    • Structural isomerism (position isomerism and chain isomerism)
    • Geometrical isomerism:
      • Cis: Identical groups on the same side of the double bond.
      • Trans: Identical groups on opposite sides of the double bond.

    Preparation of Alkenes

    • From alkynes: Partial reduction of alkynes (with a calculated amount of dihydrogen) in the presence of Lindlar's catalyst (partially deactivated palladium-on-charcoal catalyst) yields alkenes.
    • From alkyl halides (dehydrohalogenation): Alkyl halides with alcoholic potassium hydroxide, eliminate hydrogen halides, to produce alkenes. This is a β-elimination reaction.
    • From vicinal dihalides (dehalogenation): Vicinal dihalides react with zinc metal to form an alkene.

    Physical Properties of Alkenes

    • The first three members are gases.
    • The next fourteen are liquids.
    • Higher alkenes are solids.

    Chemical Properties of Alkenes

    • Addition reactions: Electrophiles add to the carbon-carbon double bond to form addition products.
      • Addition of dihydrogen (hydrogenation): Alkenes react with dihydrogen gas in the presence of nickel, palladium, or platinum to form alkanes.
      • Addition of halogens: Halogens like bromine or chlorine react to form vicinal dihalides.
      • Addition of hydrogen halides (Markovnikov and Anti-Markovnikov addition): Hydrogen halides add to unsymmetrical alkenes either by Markovnikov (the negative part of the addendum attacks carbon with fewest hydrogens) or Anti-Markovnikov (peroxide effect) mechanisms.
        • Addition of water (acidic conditions): Alkenes react with acidified water to form alcohols, in accordance with Markovnikov's rule.
      • Oxidation: Alkenes react with cold, dilute, aqueous potassium permanganate to produce vicinal glycols. Acidic potassium permanganate or potassium dichromate oxidize alkenes to ketones or acids, depending on the alkene structure.
      • Ozonolysis: Ozone adds to alkenes to form ozonides, which are then cleaved by Zn-H2O to yield smaller molecules.
    • Polymerization: Large numbers of ethene molecules combine at high temperatures, pressure, and in the presence of a catalyst (forming polythene).

    Alkynes

    • Alkynes contain at least one carbon-carbon triple bond.
    • General formula: CnH2n-2
    • Nomenclature: The suffix "yne" is used.
    • Position isomerism exists.
    • Preparation:
    • From calcium carbide by reaction with water.
    • From vicinal dihalides by reaction with alcoholic potassium hydroxide undergoing dehydrohalogenation.

    Physical Properties of Alkynes

    • First three are gases.
    • Next 8 are liquids. Higher are solids.
    • All are colourless.

    Chemical Properties of Alkynes

    • Acidic character: Sodium and sodamide react with alkynes to liberate dihydrogen gas, forming sodium acetylides.
    • Addition reactions:
      • Addition of dihydrogen (hydrogenation): React with hydrogen gas with a catalyst (Pt or Pd/C) to form alkenes.
      • Addition of halogens: React with halogens—forming di-halides.
      • Addition of hydrogen halides: React with hydrogen halides, producing alkyl halides. The addition follows Markovnikov's rule in normal conditions.
      • Addition of water: React with water under certain conditions to produce a carbonyl compound.
    • Polymerization: Can polymerize.

    Aromatic Hydrocarbons

    • Commonly known as arenes.
    • Structure of benzene: Kekule suggested an oscillating nature of double bonds. Experimentally, the bond lengths are equal, indicating delocalised electrons.
    • Benzene is a planar molecule.
    • Aromatic compounds have sp2 hybridized carbon atoms.
    • Aromaticity: A compound is aromatic if it meets Huckel's rule: Planar, complete delocalization of π electrons, has (4n+2) π electrons in the ring where n is an integer (0,1,2...).
    • Preparation of benzene: From cyclic polymerisation of ethyne, decarboxylation of aromatic acids (such as reacting sodium salt of benzoic acid with soda lime).
    • Reactions of benzene:
      • Reaction with zinc dust: Convert phenol to benzene.
      • Electrophilic substitution reactions (nitration, halogenation, sulphonation, Friedel-Crafts alkylation, acylation).

    Physical Properties of Aromatic Hydrocarbons

    • Usually colorless liquids or solids.
    • Characteristic aroma.
    • Immiscible with water, but miscible with organic solvents.

    Chemical Properties of Aromatic Hydrocarbons

    • Electrophilic substitution reactions.
      • Nitration: Introduction of a nitro group (using a mixture of concentrated nitric acid and concentrated sulfuric acid).
      • Sulphonation: Replacement of a hydrogen atom with a sulfonic acid group (using fuming sulfuric acid).
      • Halogenation: Reaction with halogens (e.g., chlorine) use a Lewis acid catalyst.
      • Friedel-Crafts alkylation: Reaction of benzene with alkyl halides using aluminum chloride as a catalyst.
      • Friedel-Crafts acylation: Reaction of benzene with acyl halides or acid anhydrides using aluminum chloride as a catalyst.
    • Addition reactions: Rare in normal conditions but can happen under very vigorous conditions (e.g. hydrogenation under Pt catalyst at high temperature and pressure to produce cyclohexane) or reactions with chlorine in the presence of UV light.

    Other relevant information

    • Various reactions of these compounds are possible including combustion (burning in air to give carbon dioxide and water) and numerous other specific reactions that are not explicitly detailed in the provided summaries.

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    Test your knowledge on alkenes and their reactions in organic chemistry. This quiz covers concepts such as strain types, isomers, catalysts, and reaction mechanisms. Perfect for students looking to reinforce their understanding of this key topic.

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