Alkenes and Their Properties

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Questions and Answers

What is the smallest isolable trans-cycloalkene?

  • Cyclodecene
  • Cyclohexene
  • Cyclopentene
  • Cyclooctene (correct)

Which of the following statements is TRUE about trans-cycloalkenes?

  • They are more stable than their _cis_-isomers only when containing more than 8 carbons.
  • They are less stable than their _cis_-isomers only when containing less than 8 carbons. (correct)
  • They are always more stable than their _cis_-isomers.
  • They are always less stable than their _cis_-isomers.

How does the presence of a heteroatom, like oxygen, affect the degree of unsaturation calculation?

  • It increases the degree of unsaturation by one.
  • It depends on the type of heteroatom present.
  • It has no effect on the degree of unsaturation. (correct)
  • It decreases the degree of unsaturation by one.

What is the equivalent formula for C6H10OCl3N, considering the impact of heteroatoms on degree of unsaturation?

<p>C6H12 (D)</p> Signup and view all the answers

Which of the following conditions contributes to the stability of a trans-cycloalkene?

<p>Having more than 8 carbon atoms in the ring. (A)</p> Signup and view all the answers

What does the term 'degrees of unsaturation' refer to in the context of organic molecules?

<p>The number of hydrogen atoms that are missing compared to a saturated alkane with the same number of carbons. (B)</p> Signup and view all the answers

How many degrees of unsaturation are present in a molecule with the molecular formula C5H8?

<p>2 (D)</p> Signup and view all the answers

Which of the following statements about alkenols is INCORRECT?

<p>Alkenols are always cyclic compounds. (D)</p> Signup and view all the answers

A molecule with the molecular formula C6H10 has how many degrees of unsaturation?

<p>2 (B)</p> Signup and view all the answers

What is the main difference between cis and trans alkene isomers in terms of physical properties?

<p>Cis isomers typically have a higher boiling point than trans isomers. (A)</p> Signup and view all the answers

Which of the following compounds would be classified as an alkenol?

<p>CH3CH=CHCH2OH (B)</p> Signup and view all the answers

What causes cis isomers to have a higher boiling point than trans isomers?

<p>The presence of a net molecular dipole in cis isomers. (B)</p> Signup and view all the answers

Why does the number of double bonds in a fatty acid affect its melting point?

<p>An increase in double bonds reduces the rigidity of the molecule, resulting in a lower melting point. (A)</p> Signup and view all the answers

What is the general structure of a fatty acid?

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

Which type of fatty acids have no double bonds in their hydrocarbon chains?

<p>Saturated fatty acids (A)</p> Signup and view all the answers

What are triacylglycerols hydrolyzed into?

<p>Glycerol and three fatty acids (B)</p> Signup and view all the answers

Which of the following is NOT a consequence of the alkene dipole in cis and trans isomers?

<p>Different reactivity with oxidizing agents (D)</p> Signup and view all the answers

What is the primary factor responsible for the differences in physical properties between cis and trans isomers?

<p>The orientation of the substituents around the double bond (B)</p> Signup and view all the answers

Why do simple alkenes not react with nucleophiles or bases?

<p>The double bond in alkenes is electron-poor and difficult to attack by electron-rich nucleophiles/bases. (D)</p> Signup and view all the answers

Which of the following can be used to prepare alkenes?

<p>All of the above (D)</p> Signup and view all the answers

What does the term 'syn addition' refer to in alkene reactions?

<p>The addition of two groups to the same side of the alkene. (D)</p> Signup and view all the answers

What is the key feature of a double bond that allows for syn and anti addition?

<p>The pi bond. (C)</p> Signup and view all the answers

What happens to the pi bond during an addition reaction?

<p>It gets converted to a sigma bond. (A)</p> Signup and view all the answers

When drawing the products of an addition reaction, which bond is broken in the reagent?

<p>The sigma bond. (B)</p> Signup and view all the answers

What is the geometry of the carbon atoms in a double bond before addition?

<p>Trigonal planar. (A)</p> Signup and view all the answers

How can you differentiate between syn and anti addition products?

<p>Syn addition products have two identical groups on the same side of the molecule. (C)</p> Signup and view all the answers

What is the enthalpy change, in kilojoules per mole, for the breaking of the C-H bond in the reactant?

<p>368 (D)</p> Signup and view all the answers

Which structural feature is present in both 1-vinylcyclopentene and 2-allylcycloheptanol?

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

What is the significance of the wavy line in the structures of the methylene, vinyl, and allyl groups?

<p>It symbolizes a point of attachment to another molecule or fragment. (B)</p> Signup and view all the answers

Based on the enthalpy calculations, what can you conclude about the overall reaction?

<p>The reaction is exothermic and releases energy. (D)</p> Signup and view all the answers

Which of the following statements correctly describes the relationship between the enthalpy change and the energy released in the formation of bonds?

<p>The enthalpy change is always less than the energy released in bond formation. (A)</p> Signup and view all the answers

What is the name of the product formed in the reaction?

<p>2-bromobutane (C)</p> Signup and view all the answers

Which functional group is present in methylenecyclohexane?

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

What type of reaction is described in the text?

<p>Addition reaction (C)</p> Signup and view all the answers

Which of these statements accurately describes the stereochemistry of trans-1,2-dichlorocyclopentane formation through chlorination of cyclopentene?

<p>The reaction proceeds through a chloronium ion intermediate, leading to the formation of only the <em>trans</em> isomer. (C)</p> Signup and view all the answers

During halohydrin formation, what is the primary reason for the formation of a halohydrin product? What is the main reason for the formation of the halohydrin product over a dihaloalkane?

<p>The high concentration of water (H₂O) compared to the halide ion (X⁻). (B)</p> Signup and view all the answers

In the mechanism of halohydrin formation, what is the role of the halide ion (X⁻) formed in the first step?

<p>It is a spectator ion and does not participate in the reaction. (A)</p> Signup and view all the answers

Why does the electrophile (X⁺) in halohydrin formation preferentially attach to the less substituted carbon of an unsymmetrical alkene?

<p>The more substituted carbon is sterically hindered. (B)</p> Signup and view all the answers

What is the main driving force for the formation of the halonium ion intermediate in the chlorination of cyclopentene?

<p>The formation of a three-membered ring with a partial positive charge on the chlorine atom. (A)</p> Signup and view all the answers

In the formation of a halohydrin, which of the following is the correct order of events?

<p>Addition of electrophile (X⁺), formation of halonium ion, attack by nucleophile (H₂O), deprotonation (B)</p> Signup and view all the answers

What is the stereochemical outcome of the addition of Br₂ and H₂O to a cis-alkene?

<p>The product will be a racemic mixture of <em>trans</em>-halohydrin. (D)</p> Signup and view all the answers

Why is the product of halohydrin formation preferred to have the electrophile (X⁺) bonded to the less substituted carbon and the nucleophile (H₂O) bonded to the more substituted carbon in unsymmetrical alkenes?

<p>It is a result of the steric effect, with the nucleophile favoring the less hindered position. (D)</p> Signup and view all the answers

Flashcards

Cyclic Alkenes

Alkenes in a ring structure that can exhibit cis or trans geometry.

Cis Geometry

Configuration where substituents are on the same side of the double bond in cyclic alkenes.

Trans Geometry

Configuration where substituents are on opposite sides of the double bond in cyclic alkenes.

Degree of Unsaturation

Measure of the number of rings and double bonds in a molecule.

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Adjustment for Heteroatoms

When calculating unsaturation, adjust H count based on halogens and nitrogens present.

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Acyclic Alkene Formula

The general formula for acyclic alkenes is CnH2n, indicating unsaturation.

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Cycloalkane Formula

Cycloalkanes also follow the formula CnH2n, similar to acyclic alkenes.

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Calculating Degrees of Unsaturation

Comparison of actual H atoms to maximum H atoms in a hypothetical alkane.

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Alkenols

Compounds with both a double bond and a hydroxy group (–OH).

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Cis/Trans Isomers

Cis and trans isomers differ in the arrangement of atoms around a double bond, impacting their physical properties.

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Boiling Points of Isomers

Cis-2-butene has a higher boiling point than trans-2-butene due to molecular dipole interactions.

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Alkene Dipole

The presence of a double bond in alkenes leads to dipole moments influencing physical properties.

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Saturated vs. Unsaturated Fatty Acids

Saturated fatty acids have no double bonds; unsaturated fatty acids contain one or more double bonds.

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Effect of Double Bonds on Melting Point

Increasing double bonds in fatty acids leads to a decrease in melting point.

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Triacylglycerols Hydrolysis

Triacylglycerols are broken down into glycerol and three fatty acids when hydrolyzed.

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Cis Isomer Dipole Interaction

In the cis isomer, Csp3-Csp2 bond dipoles enhance each other, leading to a net molecular dipole.

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Trans Isomer Characteristics

In trans isomers, bond dipoles cancel out, resulting in no net dipole moment.

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Alkenes preparation methods

Alkenes can be made from alkyl halides, tosylates, and alcohols through elimination reactions.

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Electrostatic Potential Plot

A graphical representation showing regions of electron density in a molecule, illustrating nucleophilicity.

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Syn Addition

A reaction where two substituents add to the same side of a double bond in alkenes.

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Anti Addition

A reaction where two substituents add to opposite sides of a double bond in alkenes.

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Trigonal Planar

The geometric arrangement of carbon atoms in a double bond, making them all in the same plane.

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Breaking σ bond

The step in addition reactions where the σ bond of the reagent is cleaved to form new bonds with alkenes.

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Adding Groups to Alkenes

During reactions, new substituents attach to the carbon atoms of a double bond via syn or anti addition.

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Elimination Reactions

Chemical reactions that remove elements from compounds, resulting in the formation of alkenes.

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Halonium Ion

A three-membered cyclic structure formed during halogen addition to alkenes, such as Cl⁺.

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Halohydrin Formation

The addition of halogen (X) and water (H2O) to an alkene forming a halohydrin.

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Methylene Group

A carbon atom double bonded to another carbon atom.

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Vinyl Group

A carbon atom double bonded to another carbon atom, part of a chain.

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Stereochemistry of Halohydrins

The spatial arrangement of groups in halohydrin products. Preferred formation is anti when involving unsymmetrical alkenes.

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Electrophilic Addition

A reaction where an electrophile forms a bond with an alkene or alkyne, initially forming a carbocation.

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Allyl Group

A three-carbon chain with a double bond between the first and second carbon.

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Exothermic Reaction

A reaction where energy is released, resulting in a negative delta H.

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Nucleophile in Halohydrin Formation

In halohydrin formation, H2O acts as the nucleophile attaching to the more substituted carbon.

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Bonds Broken

Energy required to break chemical bonds in a reaction.

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Unsymmetrical Alkenes

Alkenes with different groups attached to the carbons of the double bond, leading to varied product outcomes.

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Product Preference in Halohydrins

With unsymmetrical alkenes, products favor X⁺ on the less substituted carbon and H2O on the more substituted one.

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Bonds Formed

Energy released when new bonds are created during a reaction.

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Chlorination Mechanism

The reaction mechanism for chlorination involves initial addition to form a chloronium ion leading to dichloride products.

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Delta H Degrees

The change in energy during a reaction, measured in kilojoules per mole.

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Total Delta H Calculation

Sum of bonds broken and formed to determine overall energy change.

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Study Notes

  • Alkenes are also called olefins
  • Alkenes contain a carbon-carbon double bond
  • Terminal alkenes have the double bond at the end of the carbon chain
  • Internal alkenes have at least one carbon atom bonded to each end of the double bond
  • Cycloalkenes contain a double bond in a ring
  • The double bond consists of a sigma (σ) bond and a pi (π) bond
  • Each carbon is sp² hybridized and trigonal planar, with bond angles of approximately 120°
  • Bond dissociation energies of C-C bonds in ethane (σ bond only) and ethylene (σ + π bond) can estimate the strength of the π component of the double bond
  • The π bond is much weaker than the σ bond, making it more easily broken.
  • Alkenes undergo many more reactions than alkanes
  • Cis-2-Butene has a higher boiling point (4°C) than trans-2-butene (1°C)
  • Cis isomers have a net molecular dipole; trans isomers do not
  • Saturated fatty acids have no double bonds
  • Unsaturated fatty acids have one or more double bonds
  • As the number of double bonds in the fatty acid increases, the melting point decreases
  • The larger the number of Z double bonds, the more kinks in the hydrocarbon chain
  • This causes poorer stacking and less van der Waals interactions, leading to lower melting points
  • Alkenes can be prepared from alkyl halides, tosylates, and alcohols via elimination reactions
  • Addition reactions occur when the π bond is broken and two new σ bonds are formed
  • Regioselectivity and stereoselectivity are important concepts in alkene formation
  • Markovnikov's rule applies to addition reactions of unsymmetrical alkenes
  • Carbocation stability determines the regiochemistry of addition reactions
  • The more stable carbocation is formed preferentially
  • Halohydrin formation involves addition of X₂ and H₂O to the double bond
  • The reaction often produces two stereoisomers (e.g., cis-but-2-ene) or one (e.g., trans-but-2-ene)
  • Halohydrins have been used in the synthesis of many naturally occurring compounds
  • Hydroboration-oxidation converts an alkene into an alcohol in two steps
  • The intermediate alkylborane is oxidized to form the alcohol
  • Syn addition refers to the addition of two groups from the same side of the double bond
  • Anti addition refers to the addition of two groups from opposite sides of the double bond
  • Regioselectivity often favors the less substituted carbon atom in hydroboration
  • Steric factors play a crucial role in determining the regiochemistry of hydroboration reactions
  • Degree of unsaturation helps determine the number of pi bonds and rings in a molecule
  • For molecules containing heteroatoms, ignore O, add number of halogens to H, subtract 1 H for each N

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