Alkenes and Hydrocarbon Nomenclature
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Questions and Answers

What characteristic differentiates alkenes from other hydrocarbons?

  • Presence of a carbon-carbon double bond (correct)
  • Absence of carbon-carbon bonds
  • Presence of a carbon-carbon single bond
  • Presence of a carbon-carbon triple bond

When naming alkenes, how should the chain of carbon atoms be numbered?

  • From either end of the chain without considering the double bond
  • Randomly to avoid confusion with substituents
  • So that double bond carbons have the lowest possible numbers (correct)
  • So that the carbon with the highest atomic number is the first

Which statement about the p bond in alkenes is true?

  • It has a lower energy barrier than a single bond
  • It can be easily broken by low amounts of energy
  • It limits rotation around the double bond (correct)
  • It allows for free rotation between bonded carbons

What prefix is used when naming cyclic alkenes?

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

What is the orientation of sp² orbitals in a double bond of an alkene?

<p>120º relative to each other (A)</p> Signup and view all the answers

What is the general naming suffix for hydrocarbons that contain carbon-carbon triple bonds?

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

In naming a compound with both double and triple bonds, which bond receives priority for lower numbering?

<p>Double bonds always receive lower numbers than triple bonds. (C)</p> Signup and view all the answers

What occurs when a 1,2-dihaloalkane is treated with KOH or NaOH?

<p>A two-fold elimination of HX occurs. (C)</p> Signup and view all the answers

How many types of compounds are named with the suffix '-yne'?

<p>Any compound with at least one triple bond. (D)</p> Signup and view all the answers

What is the electronic structure of a carbon-carbon triple bond?

<p>Formed by sp hybrid orbitals and unhybridized px and py orbitals. (B)</p> Signup and view all the answers

What determines whether an alkene is classified as cis or trans?

<p>The arrangement of similar groups around the double bond (D)</p> Signup and view all the answers

In the context of alkene stability, which of the following statements is accurate?

<p>Cis alkenes give off more heat during hydrogenation than trans alkenes (D)</p> Signup and view all the answers

Which designation indicates that the higher priority groups are on opposite sides of the double bond?

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

According to the Cahn-Ingold-Prelog priority rules, how should atoms be ranked at a comparison point?

<p>By the highest atomic number (A)</p> Signup and view all the answers

When considering stability, which order of alkene substitution is correct from most to least stable?

<p>tetrasubstituted &gt; trisubstituted &gt; disubstituted &gt; monosubstituted (A)</p> Signup and view all the answers

What does the term 'hyperconjugation' refer to in the context of alkene stability?

<p>The stabilization due to the overlap of sigma bonds with empty p-orbitals (A)</p> Signup and view all the answers

Which of the following conditions is necessary for cis-trans isomerism to occur in alkenes?

<p>Each carbon atom in the double bond must have two different substituents (B)</p> Signup and view all the answers

What does the 'Z' designation indicate in the context of alkene stereochemistry?

<p>The two higher priority groups are on the same side (C)</p> Signup and view all the answers

What type of bond is present in acetylene (HCCH) which makes it shorter and stronger than single or double bonds?

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

Which addition reaction of alkynes results in a tetrahalide product?

<p>Initial addition with excess reagent (A)</p> Signup and view all the answers

What is the outcome of hydroboration-oxidation of terminal alkynes?

<p>Formation of aldehydes (D)</p> Signup and view all the answers

How are alkynes reduced to form alkanes?

<p>By adding two equivalents of H2 (D)</p> Signup and view all the answers

Which catalyst is used to obtain cis alkenes from alkynes?

<p>Hydrogenated palladium on calcium carbonate (C)</p> Signup and view all the answers

What happens during the oxidative cleavage of internal alkynes?

<p>Formation of two carboxylic acids (D)</p> Signup and view all the answers

Which reaction produces trans alkenes from alkynes?

<p>Reduction with sodium or lithium in liquid ammonia (C)</p> Signup and view all the answers

What distinguishes a terminal alkyne from an internal alkyne?

<p>Location of the triple bond at the end of a carbon chain (D)</p> Signup and view all the answers

Why is the keto form usually the only form observed in keto-enol tautomerism?

<p>Due to lower energy stability compared to enol (C)</p> Signup and view all the answers

What is the pKa of terminal alkynes, indicating their acidity level?

<p>Approximately 25 (B)</p> Signup and view all the answers

What is the primary requirement for the thermal cracking of alkenes?

<p>Very high temperatures without a catalyst (B)</p> Signup and view all the answers

Which process is used for the dehydrohalogenation of alkyl halides?

<p>Application of heat and KOH (D)</p> Signup and view all the answers

What occurs during the electrophilic addition of alkenes with HBr?

<p>A carbocation and bromide ion are produced (A)</p> Signup and view all the answers

What does Markovnikov's rule state regarding the addition of HX to unsymmetrical alkenes?

<p>H attaches to the more substituted carbon (B)</p> Signup and view all the answers

What is the major outcome when 2-methylpropene reacts with HCl?

<p>One product predominates due to regioselectivity (B)</p> Signup and view all the answers

How do carbocations generally react in terms of stability?

<p>Greater substitution increases carbocation stability (A)</p> Signup and view all the answers

What is the process of hydration of alkenes typically used for?

<p>Addition of H2O to form an alcohol in acidic conditions (A)</p> Signup and view all the answers

What describes the structure of carbocations?

<p>They are planar with a vacant p-orbital (B)</p> Signup and view all the answers

Which reagent is used for oxymercuration-demercuration in hydration of alkenes?

<p>Mercuric acetate in THF (B)</p> Signup and view all the answers

What is the result of adding ozone to an alkene in ozonolysis?

<p>Generation of ketones and/or aldehydes (B)</p> Signup and view all the answers

What is the main function of hydrogenation in the context of alkenes?

<p>Addition of H2 across the double bond (B)</p> Signup and view all the answers

In oxidation reactions, how do alkenes react with potassium permanganate?

<p>Cleavage to form carboxylic acids and carbon dioxide (C)</p> Signup and view all the answers

Which of the following describes the method of alkene oxidation through epoxidation?

<p>Formation of a cyclic ether via addition of an oxygen atom (A)</p> Signup and view all the answers

Flashcards

Alkene

A hydrocarbon containing a carbon-carbon double bond. They are often found in natural materials like flavors, fragrances, and vitamins.

Alkene Naming

A naming system for alkenes that ensures the double bond carbons have the lowest possible numbers in the chain.

Cis-Trans Isomerism

The type of isomerism found in alkenes where substituents on the double bond can be on the same side (cis) or opposite sides (trans) of the double bond.

Pi Bond

A chemical bond formed by the overlap of atomic orbitals that prevents rotation around the carbon-carbon double bond.

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Rotation of Pi Bond

The process of breaking the pi bond in an alkene, allowing rotation around the C-C bond.

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Cis Isomer

Two similar groups are on the same side of the double bond. Think of it like two passengers sitting next to each other on a double-decker bus.

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Trans Isomer

Two similar groups are on opposite sides of the double bond. Imagine two passengers sitting on opposite sides of a double-decker bus.

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E,Z System

A system for naming alkene isomers based on the priority of groups linked to each carbon of the double bond. It's like a road map for organizing alkenes.

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Cahn-Ingold-Prelog Rule 1

A set of rules to determine the priority of substituents on double bond carbons; higher atomic number gets higher priority. This allows us to compare the substituents like in a competition - who's the strongest?

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Cahn-Ingold-Prelog Rule 2

If the atomic numbers are the same, continue comparing the next connection point at the same distance. It's like looking at a family lineage to determine who's the oldest.

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Cahn-Ingold-Prelog Rule 3

Multiple bonds are treated as the same number of single bonds. Imagine a double bond as two single bonds holding hands.

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Stability of Alkenes

Trans alkenes are more stable than cis alkenes. It's like having a more comfortable arrangement of objects - less energy, more stability.

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Hydrogenation Heat (DHo)

The heat given off when an alkene is converted to an alkane. It's a measure of the alkene's stability - the less heat released, the more stable.

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Acetylene

The simplest alkyne, produced industrially from methane and steam at high temperatures.

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Organic Synthesis

The process of creating more complex organic molecules from simpler ones, often starting with alkynes as building blocks.

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Preparation of Alkynes through Elimination

The elimination of HX from a 1,2-dihaloalkane using KOH or NaOH, resulting in the formation of a carbon-carbon triple bond.

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Numbering Alkynes

The process of numbering a carbon chain containing a triple bond, starting from the end closest to the triple bond to ensure the smallest possible numbers for the carbons in the triple bond.

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Elimination Reaction

A chemical reaction where a molecule loses a small molecule, like water or hydrogen halide, forming a double bond.

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Dehydrohalogenation

A type of elimination reaction where a hydrogen halide (HX) is removed from an alkyl halide, resulting in the formation of an alkene.

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Dehydration

A type of elimination reaction where water (H2O) is removed from an alcohol, leading to the formation of an alkene.

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Regioselectivity

The regioselectivity of an addition reaction describes which product is formed when multiple products are possible. It refers to the preference for one product over another.

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Addition Reaction

A type of reaction where a molecule adds to a double or triple bond, breaking the pi bond and forming a new single bond.

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Halogenation of Alkenes

The addition of a halogen (like bromine or chlorine) to an alkene, resulting in a saturated molecule with two halogen atoms attached to adjacent carbons.

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Electrophilic Addition

A special type of addition reaction involving an electrophile and a nucleophile, where the electrophile attacks the alkene's double bond and forms a carbocation intermediate.

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Markovnikov's Rule

The tendency for a reagent to add to the more substituted carbon of a double bond in an unsymmetrical alkene.

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Carbocation

An intermediate formed in many reactions involving alkenes, it is a positively charged carbon, and the stability of the carbocation depends on the number of alkyl substituents.

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Hydration of Alkenes

The process of adding water to an alkene, typically in the presence of an acid catalyst, to form an alcohol.

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Oxymercuration-Demercuration

A two-step process involving mercuric acetate and sodium borohydride, used to hydrate alkenes specifically in laboratory settings.

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Hydroboration

A reaction involving borane (BH3) adding to an alkene, forming an organoborane intermediate.

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Hydrogenation

A reaction involving the addition of hydrogen gas (H2) across a C=C double bond, catalyzed by metals like platinum or palladium, resulting in a saturated alkane.

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Epoxidation

A reaction where a cyclic ether (epoxide) is formed by adding an oxygen atom to the double bond of an alkene.

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Hydroxylation

A reaction involving osmium tetroxide, which adds two hydroxyl groups (OH) to the alkene, forming a syn-diol.

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Triple Bond

A triple bond between two carbon atoms, shorter and stronger than single or double bonds.

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Acetylene vs. Ethylene Bond Strength

The energy required to break a π bond in acetylene is greater than in ethylene, indicating that the triple bond is stronger.

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Addition Reactions of Alkynes

Addition reactions of alkynes follow the same pattern as those of alkenes, with the addition of a molecule across the triple bond.

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Markovnikov's Rule in Alkynes

The addition of HX or X2 to an alkyne occurs in a way that the hydrogen or halogen atom attaches to the carbon with the most hydrogens. For example, bromine or chlorine adds to the terminal carbon in a terminal alkyne.

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Bromination and Chlorination of Alkynes

The initial addition of bromine or chlorine to an alkyne results in a trans intermediate, where the halogens are on opposite sides of the newly formed double bond. With excess reagent, both double bonds are attacked, forming a tetrahalide.

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Keto-Enol Tautomerism

Isomers that rapidly interconvert by the movement of a proton. The keto form is usually more stable.

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Hydration of Unsymmetrical Alkynes

Hydration of alkynes with different alkyl groups on either end of the triple bond can result in a mixture of products. Hydration of terminal alkynes, with hydrogen attached to one carbon, always produces a methyl ketone.

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Comparison of Hydration of Terminal Alkynes

Hydroboration/oxidation converts terminal alkynes to aldehydes, while mercury(II) catalyzed hydration converts them to methyl ketones. Both reactions involve the addition of water, but the regiochemistry differs due to different mechanisms.

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Reduction of Alkynes

Addition of hydrogen gas (H2) over a metal catalyst, like palladium on carbon (Pd/C). This leads to the formation of alkanes due to complete reduction of the triple bond to single bonds.

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Study Notes

Organic Chemistry: Alkenes and Alkynes

  • Alkenes: Hydrocarbons containing a carbon-carbon double bond.
  • Alkynes: Hydrocarbons containing a carbon-carbon triple bond.
  • Nomenclature: Alkenes and alkynes are named using IUPAC nomenclature, which prioritizes the location of the double or triple bond. Lower numbers are assigned to the carbons of the multiple bond.
  • Cis-Trans Isomers: Alkenes with different substituents on each carbon of the double bond can exist as cis (same side) or trans (opposite sides) isomers. Rotation about a double bond is restricted, unlike rotation around a single bond. This results in different geometric isomers.
  • E,Z Designation: For alkenes with more than two substituents on the double bond, E/Z isomerism is used.
  • Stability: The stability of alkenes depends on the number of alkyl substituents on the double bond. More substituted alkenes are more stable.
  • Synthesis: Alkenes and alkynes can be synthesized through various reactions, including elimination reactions. Elimination reactions involve removal of atoms or groups from adjacent carbon atoms to form a double or triple bond.
  • Reactions: Alkenes and alkynes participate in electrophilic addition reactions. Addition reactions are specific reactions that add atoms or functional groups across a carbon-carbon multiple bond (double or triple bond).
  • Markovnikov's Rule: During addition reactions to unsymmetrical alkenes, the hydrogen preferentially bonds to the carbon with the greater number of hydrogen or alkyl substituents. A higher substituted carbocation is preferred.
  • Hydration: Alkenes can be hydrated under specific conditions to yield alcohols.
  • Hydroboration-Oxidation: This method gives the anti-Markovnikov product during hydration of alkenes.
  • Reduction: Addition of H2 to alkenes or alkynes using catalysts such as Pd/C, Lindlar's catalyst, or alkali metals in liquid ammonia can result in alkanes or specific cis or trans isomers.
  • Oxidation: Ozonolysis and permanganate can cleave the multiple bond of alkenes, yielding carbonyl compounds.

Alkynes

  • Nomenclature: Alkynes use the suffix -yne. Numerical identifiers position the triple bond.
  • Acidity: Terminal alkynes react with strong bases to form acetylide anions. This is due to the high electronegativity of the triple bond.
  • Alkylation: Acetylide anions are strong enough nucleophiles and bases to directly react with primary halides to create longer alkyne chains.
  • Reactions: Addition reactions for alkynes follow similar principles to alkenes, producing stereoisomers.

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This quiz explores the unique characteristics of alkenes compared to other hydrocarbons. Questions cover naming conventions, structural features, and the electronic properties of alkenes. Perfect for chemistry students looking to test their knowledge on this topic.

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