Alkenes & Alkynes Nomenclature
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Questions and Answers

Which type of diene has the highest stability?

  • Conjugated dienes (correct)
  • Cumulated dienes
  • Isolated dienes
  • All dienes have equal stability

What factor contributes to the stability of an alkene?

  • Number of substituents on the double bond (correct)
  • Number of beta hydrogens
  • Number of alpha carbons
  • Presence of a hydroxyl group

Which of the following correctly describes Saytzeff's Rule?

  • Proton is removed from both beta and alpha carbons
  • Proton is removed from the most substituted carbon
  • Proton is removed from the least substituted carbon (correct)
  • Proton is removed from hydroxyl groups

What is the first step in the electrophilic addition reaction of alkenes?

<p>Pi electrons attack the electrophile (C)</p> Signup and view all the answers

Alkenes can be synthesized from which of the following?

<p>Alkyl halides and alcohols (D)</p> Signup and view all the answers

What are the products of dehydrohalogenation of an alkyl halide?

<p>Alkene and HX (C)</p> Signup and view all the answers

The alpha hydrogen is defined as which of the following?

<p>A hydrogen on the first carbon attached to a functional group (B)</p> Signup and view all the answers

Which of the following statements about carbocations is correct?

<p>Carbocations readily react with nucleophiles (A)</p> Signup and view all the answers

What suffix is used in the IUPAC naming system to indicate the presence of a triple bond in hydrocarbons?

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

How are compounds with two triple bonds named in the IUPAC system?

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

What is the common name for the simplest alkyne, H-C≡C-H?

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

What type of reaction is primarily used to synthesize alkynes from dihalides?

<p>E2 elimination (A)</p> Signup and view all the answers

During hydrohalogenation of alkynes, what product is initially formed after the first equivalent of HX is added?

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

What is the fate of the enol produced during hydration of an alkyne in the presence of a strong acid and Hg2+ catalyst?

<p>It converts to a carbonyl compound (B)</p> Signup and view all the answers

When numbering the carbon chain of a compound containing both double and triple bonds, what is emphasized in IUPAC nomenclature?

<p>The first site of unsaturation (C)</p> Signup and view all the answers

What is the general trend in the addition reactions of alkynes due to the presence of weak π bonds?

<p>They typically undergo multiple sequential additions (B)</p> Signup and view all the answers

What is the correct order of stability for carbocations from most stable to least stable?

<p>3o &gt; 2o &gt; 1o &gt; methyl (C)</p> Signup and view all the answers

What is the result of hydrating an alkene in the presence of concentrated sulfuric acid?

<p>An alcohol (C)</p> Signup and view all the answers

In the addition of HX to an alkene, which carbon atom does the proton attach to according to Markovnikov's Rule?

<p>The carbon with fewer substituents (A)</p> Signup and view all the answers

What is the main outcome of a halogenation reaction with alkenes?

<p>Formation of a vicinal dihalide (B)</p> Signup and view all the answers

What type of product is formed when HBr is added in the absence of peroxide?

<p>Markovnikov's product (C)</p> Signup and view all the answers

Which statement is true about alkynes?

<p>Terminal alkynes have the triple bond at the end of the carbon chain. (C)</p> Signup and view all the answers

Which step correctly identifies the electrophile in the addition of HBr in absence of peroxide?

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

What type of catalyst is commonly used in the hydrogenation of alkenes?

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

What product is formed when 1-ethylcyclopentene reacts with HCl?

<p>2-chloro-1-ethylcyclopentane (A)</p> Signup and view all the answers

In the presence of peroxide, what type of product is formed during the addition of HBr?

<p>Anti-Markovnikov's product (B)</p> Signup and view all the answers

What difference in hydrogen atoms can you expect when comparing alkenes and alkynes?

<p>Alkynes have four fewer hydrogens than alkenes. (A)</p> Signup and view all the answers

Which process describes the addition of elements X and OH to an alkene?

<p>Formation of Halohydrin (B)</p> Signup and view all the answers

Which statement correctly describes the fission of the HBr bond in the presence of peroxide?

<p>Homolytic fission occurs (D)</p> Signup and view all the answers

What is the IUPAC name of the product formed from HBr and 2-methylbutene in presence of peroxide?

<p>2-Bromo-3-methylbutane (A)</p> Signup and view all the answers

In the context of alkenes, what does Markovnikov's rule state?

<p>The hydrogen will attach to the more substituted carbon. (D)</p> Signup and view all the answers

What type of bond hybridization is present in alkynes?

<p>sp hybridization (C)</p> Signup and view all the answers

What is the general molecular formula for alkenes?

<p>C_nH_{2n} (B)</p> Signup and view all the answers

How are substituents and functional groups prioritized when numbering the carbon chain in an alkene?

<p>Hydroxyl groups take precedence over both double bonds and alkyl groups. (B), Double bonds take precedence over alkyl groups and halogens. (D)</p> Signup and view all the answers

When naming a cyclic alkene, what is specified about the double bond?

<p>The double bond is numbered as 1 but the '1-' is omitted from the name. (A)</p> Signup and view all the answers

In the nomenclature of alkenes, what suffix is used to replace the '-ane' ending of alkanes?

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

When numbering carbon atoms in a chain with a double bond, what is the primary goal?

<p>To assign the double bond the lowest possible numbers. (C)</p> Signup and view all the answers

Which term describes hydrocarbons that contain at least one carbon-carbon double bond?

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

What must be indicated when naming alkenes with more than one double bond?

<p>Use the suffix '-diene' for two double bonds and '-triene' for three. (C)</p> Signup and view all the answers

Which of the following steps is NOT part of naming alkenes?

<p>Changing the suffix '-ene' to '-ane' for naming. (B)</p> Signup and view all the answers

What type of bond is formed by the side-by-side overlap of two 2p orbitals in alkenes?

<p>Pi (π) bond (A)</p> Signup and view all the answers

Which of the following correctly describes the geometry of a carbon-carbon double bond in alkenes?

<p>Trigonal planar with a bond angle of 120° (C)</p> Signup and view all the answers

In which scenario would a cis isomer of an alkene be formed?

<p>Both substituents on the double bond are on the same side (D)</p> Signup and view all the answers

Why is the cis isomer of an alkene generally less stable than the trans isomer?

<p>It experiences steric interactions between substituents (B)</p> Signup and view all the answers

What does the E,Z designation system in alkene stereoisomerism rely on?

<p>The ranking of substituent groups by atomic number (B)</p> Signup and view all the answers

What is the consequence of the lack of rotation around a carbon-carbon double bond in alkenes?

<p>It locks the molecule in a specific conformation (C)</p> Signup and view all the answers

How would you correctly name the compound with the structure 2-Chloro-4-Methyl-3-hexene?

<p>2-Chloro-4-Methyl-3-hexene (A)</p> Signup and view all the answers

Which of the following statements is true regarding alkenes?

<p>They contain at least one carbon-carbon double bond (B)</p> Signup and view all the answers

Flashcards

Alkene

Hydrocarbons containing a carbon-carbon double bond.

CnH2n

General molecular formula for alkenes, representing the number of carbon and hydrogen atoms.

Unsaturated hydrocarbons

Alkenes are unsaturated because they have double or triple bonds, indicating potential for more hydrogen atoms.

Alkene class suffix

The ending '-ene' designates an alkene when naming organic compounds.

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Step 1: Parent hydrocarbon naming

Find the longest chain containing the double bond and replace the '-ane' with '-ene'.

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Step 2: Numbering the carbon chain

Number the chain so that the double bond carbons have the lowest possible numbers.

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Step 3: Specifying double bond position

Add a number before '-ene' indicating the position of the double bond.

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Step 4: Double bond precedence

The double bond gets the lowest number when both double bonds and substituents are present.

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Conjugated Dienes

Dienes with alternating double and single bonds. The double bonds are adjacent to each other.

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Isolated Dienes

Dienes where the double bonds are separated by at least one single carbon atom.

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Cumulated Dienes

Dienes with two double bonds connected to the same carbon atom.

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Alkene Stability: Substitution

The more substituted an alkene is, the more stable it is.

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Alkene Stability: Alpha Hydrogens

The more alpha hydrogens an alkene has, the more stable it is.

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Dehydrohalogenation

A reaction that removes a hydrogen halide (HX) from an alkyl halide to form an alkene.

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Dehydration

A reaction that removes water (H2O) from an alcohol to form an alkene.

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Saytzeff Rule

A rule that states that in an elimination reaction, the proton is removed from the carbon atom that has fewer substituents. This leads to the more stable (Saytzeff) product.

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Double bond structure

The two carbon atoms of a double bond are joined by one sigma bond and one pi bond, resulting in a planar geometry with 120-degree bond angles.

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Restricted Rotation in Alkenes

Alkenes have restricted rotation around the C=C double bond due to the pi bond. This prevents free rotation and leads to the existence of cis and trans isomers.

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Cis-Trans Isomers (Geometric Isomers)

Isomers that differ in the spatial arrangement of substituents around a double bond.

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Cis Isomer

The configuration of a double bond where the two highest-priority substituents on each carbon are on the same side of the double bond.

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Trans Isomer

The configuration of a double bond where the two highest-priority substituents on each carbon are on opposite sides of the double bond.

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E,Z Designation

A system used to describe the configuration of double bonds based on the priority of substituents. The higher priority substituents are assigned E ( entgegen, opposite) if they are on opposite sides and Z ( zusammen, together) if they are on the same side.

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Priority Rules for E,Z Designation

Atoms are ranked based on their atomic number. Higher atomic number = higher priority. For example, Br > Cl > C > H.

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Stability of Cis/Trans Isomers

The cis isomer is often less stable due to steric hindrance (repulsion between electron clouds of substituents), leading to more crowded and strained geometry.

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Acid-Catalyzed Hydration

Addition reaction where water is added to an alkene in the presence of concentrated sulfuric acid, yielding an alcohol.

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Markovnikov's Rule

A rule that states that in the addition of a protic acid (HX) to an alkene, the hydrogen atom of the acid attaches to the carbon atom in the double bond that already has more hydrogen atoms.

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Halogenation

Addition of a halogen (e.g., bromine or chlorine) to an alkene, resulting in a vicinal dihalide.

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Halohydrin Formation

An alkene reacts with a halogen (X2) and water to form a halohydrin, where X and OH are added across the double bond.

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Hydrogenation

Reaction of an alkene with hydrogen gas (H2) in the presence of a catalyst (e.g., Pt, Pd, Ni), forming an alkane.

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Terminal Alkynes

Alkynes where the triple bond is at the end of the carbon chain.

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Internal Alkynes

Alkynes where the triple bond is not at the end of the carbon chain.

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Carbocation Stability

The stability of carbocations increases with the number of alkyl groups attached to the positively charged carbon. Tertiary carbocations are the most stable, followed by secondary, primary, and methyl carbocations.

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Electrophilic Addition

Addition reactions involve the breaking of a double or triple bond and the formation of two new single bonds.

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Step 1: Protonation

The first step in the addition of HX to an alkene involves the protonation of the double bond, forming a carbocation.

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Step 2: Nucleophilic Attack

The second step in the addition of HX to an alkene involves the attack of the halide ion (X-) on the carbocation, forming the final product.

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HBr Addition: Peroxide Effect

In the absence of peroxides, the addition of HBr to an alkene follows Markovnikov's rule. However, in the presence of peroxides, the addition of HBr proceeds in an anti-Markovnikov fashion.

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Anti-Markovnikov Product

The product formed in the presence of peroxides is called the anti-Markovnikov product, where the bromine atom (Br) attaches to the carbon atom with fewer alkyl substituents.

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Anti-Markovnikov Mechanism

The free radical mechanism involving homolytic fission of the HBr bond and bromine free radical as an electrophile leads to the formation of the anti-Markovnikov product.

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Alkynes Nomenclature

Alkyne nomenclature follows a similar naming convention to alkenes, with the -ane ending of the parent alkane name being changed to -yne, and the triple bond being assigned the lowest possible number in the chain.

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Acetylene

The simplest alkyne, H-C≡C-H, is commonly referred to as acetylene, although IUPAC names it ethyne.

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Alkynes Synthesis

Alkynes can be synthesized through elimination reactions by removing two equivalents of HX from a vicinal or geminal dihalide using a strong base like KOH or NaNH2.

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Alkynes Reactions

Alkynes undergo addition reactions as they contain a weak π bond. Two sequential reactions can occur by adding one equivalent of reagent to form an alkene, which can then react with another equivalent of reagent to create a product with four new bonds.

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Halogenation of Alkynes

Halogenation of alkynes involves the addition of halogens like Cl2 or Br2 to the triple bond. The reaction proceeds in two steps and involves the intermediate formation of a tetrahaloalkane.

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Hydration of Alkynes

Alkynes undergo hydration in the presence of strong acids and Hg²⁺ catalysts, adding the elements of water to the triple bond. This forms an enol, which rearranges to a more stable ketone or aldehyde product with a carbonyl group.

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Ethynyl Group

The two-carbon alkyl group derived from acetylene, H-C≡C-, is known as an ethynyl group.

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Study Notes

Alkene & Alkyne

  • Alkenes are hydrocarbons containing a carbon-carbon double bond.
  • General molecular formula: CnH2n
  • Unsaturated hydrocarbons
  • Suffix for alkenes is "ene"

Alkene Nomenclature

  • Step 1: Identify the longest carbon chain containing the double bond and name the compound using the suffix "-ene" instead of "-ane".
  • Step 2: Number the carbon atoms in the chain to give the double bond the lowest possible numbers.
  • Step 3: Indicate the position of the double bond with a number.
  • Step 4: If there is more than one double bond, use "diene," "triene," etc. in the name. Add prefixes to name substituents and list alphabetically. Substituents with the lowest numbers on the chain take priority.
  • Step 5: Name any substituents and arrange them alphabetically.
  • Step 6: Write the full name of the alkene.

Special Nomenclature

  • Vinyl group: CH2=CH-
  • Allyl group: CH2=CHCH2-
  • Vinyl carbons: RCH=CH-CH2
  • Allyl carbons: RCH2CH=CH-CH2R

Electronic Structure of Alkenes

  • Sp2 hybridized carbon atoms.
  • One sigma (σ) bond formed by linear overlap.
  • One pi (π) bond formed by side-by-side overlap of p orbitals.
  • All atoms on the double bond are in one plane.
  • Bond angle between atoms is 120°.

Cis-Trans Isomers of Alkenes

  • Alkenes can exhibit cis-trans isomerism.
  • Cis isomers have identical groups on the same side of the double bond.
  • Trans isomers have identical groups on opposite sides of the double bond.
  • The cis isomer is generally less stable due to steric interactions.

Naming Stereoisomerism of Alkenes by E,Z Designation

  • E/Z system uses sequence rules to rank substituents on each double-bond carbon.
  • Rank substituents based on atomic number.
  • E: higher-ranked substituents are on opposite sides.
  • Z: higher-ranked substituents are on the same side.

Cahn-Ingold-Prelog Rules (CIP System)

  • Rule 1: Higher atomic number outranks lower atomic number.
  • Rule 2: If a decision cannot be reached by the first atoms, move further out until difference is found.
  • Rule 3: Multiple-bonded atoms are equivalent to the same number of single-bonded atoms.
  • Apply the rules to determine E or Z configuration.

Alkene Stability

  • More substituted alkenes are more stable.
  • The more alpha-hydrogens, the more stable.
  • The stability of alkenes depends on the number of alkyl groups attached to the carbons forming the double bond.

Synthesis of Alkenes

  • Alkenes can be prepared from alkyl halides by elimination reactions (dehydrohalogenation) using alcoholic potassium hydroxide (KOH).
  • Alkenes can be prepared from alcohols by dehydration.
  • Alcoholic KOH and concentrated sulfuric acid are used in the process.

Saytzeff Rule

  • The most stable alkene product is formed in elimination reactions.
  • Proton is removed from the carbon atom with fewest hydrogens.

Alkene Reaction-Electrophilic Addition

  • Pi electrons attack the electrophile.
  • Nucleophile attacks the carbocation.
  • Electrophiles are poor in electrons, while nucleophiles are rich in electrons.
  • Carbocation stability order: Tertiary > Secondary > Primary > Methyl.

Electrophilic Addition

  • Hydrogen atom from electrophile HBr attaches to one carbon in the double bond.
  • Bromide anion attaches to the other carbon.

Markovnikov's Rule

  • In addition reactions of HX to an alkene, the H atom adds to the carbon with fewer alkyl substituents while X adds to the carbon with more alkyl substituents.
  • If the double bond carbons have the same degree of substitution, a mixture of addition products is formed.

Hydration

  • Addition of water to alkenes using concentrated sulfuric acid according to Markovnikov’s rule.
  • The product is an alcohol.

Halogenation

  • Addition of X2 (Cl2 or Br2) to an alkene to form a vicinal dihalide.
  • The double bond breaks and X2 adds to the carbons.

Formation of Halohydrins

  • Addition of a halogen (Cl2 or Br2) and water (H2O) to an alkene, to form a halohydrin.
  • This reaction forms OH on one carbon and halogen on the other carbon of the double bond.

Addition of Hydrogen (Hydrogenation)

  • Transition metal catalysts (e.g., Pd/C, PtO2) are used to add hydrogen to an alkene, reducing it to an alkane in the presence of a catalyst (called catalytic reduction or catalytic hydrogenation).

Incomplete Reduction(Conversion of Alkynes to Cis Alkenes)

  • Addition of Hydrogen using Lindlar catalyst produces a cis alkene.
  • Lindlar catalyst is heterogeneous catalyst consisting of palladium on calcium carbonate poisoned with lead acetate and quinoline.

Incomplete Reduction(Conversion of Alkynes to Trans Alkenes)

  • Addition of H2using the sodium/lithium catalyst in presence of NH3 produces a Trans alkene.

Alkynes

  • Hydrocarbons containing a carbon-carbon triple bond.
  • General molecular formula: CnH2n-2, Four less hydrogens than maximum possible.
  • Sp hybridized, linear geometry, bond angles of 180°.

Alkynes nomenclature

  • Alkynes are named similarly to alkenes, changing the suffix to "-yne".
  • Numbering the carbon chain gives the triple bond the lowest number.
  • Diynes, triynes, enynes are named using prefixes.

Alkynes Reactions

  • Addition reactions are characteristic (two sequential steps).
  • Addition of one equivalent of reagent forms an alkene
  • Addition of a second equivalent of reagent yield product with four new bonds.
  • Hydrohalogenation: Addition of HX, two moles of HX are added.
  • Halogenation: Addition of X2, two halogen atoms add.
  • Hydration: Addition of H2O, yields ketone product when using HgSO4 catalyst.

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Lecture 3 - Alkene & Alkyne Lecture Notes PDF

Description

Test your knowledge on the nomenclature of alkenes and alkynes. This quiz will cover the rules for naming these unsaturated hydrocarbons, including identifying carbon chains and positioning double bonds. Perfect for chemistry students wanting to strengthen their understanding of organic compounds.

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