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Questions and Answers
What are alkenes also called?
What are alkenes also called?
Olefins
What does the suffix '-ene' indicate in an alkene name?
What does the suffix '-ene' indicate in an alkene name?
What does 'cyclo.....ene' indicate in an alkene name?
What does 'cyclo.....ene' indicate in an alkene name?
The rotation around the C=C double bond in alkenes is free and unrestricted.
The rotation around the C=C double bond in alkenes is free and unrestricted.
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In cis isomers of alkenes, where are the two alkyl groups located?
In cis isomers of alkenes, where are the two alkyl groups located?
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Cycloalkenes with fewer than eight carbon atoms typically have a trans geometry.
Cycloalkenes with fewer than eight carbon atoms typically have a trans geometry.
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Which of the following statements is true regarding the stability of alkenes?
Which of the following statements is true regarding the stability of alkenes?
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What are the priority rules used for E/Z nomenclature?
What are the priority rules used for E/Z nomenclature?
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The E isomer of an alkene has the two higher-priority groups on the same side of the double bond.
The E isomer of an alkene has the two higher-priority groups on the same side of the double bond.
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What is the common name for the simplest alkene, CH2=CH2?
What is the common name for the simplest alkene, CH2=CH2?
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Which of the following is NOT a common name for an alkene or alkenyl substituent?
Which of the following is NOT a common name for an alkene or alkenyl substituent?
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What is the name of the polymer formed from the addition reaction of ethylene molecules?
What is the name of the polymer formed from the addition reaction of ethylene molecules?
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Addition reactions are generally endothermic.
Addition reactions are generally endothermic.
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When an alkene reacts with an electrophile, what type of compound typically results?
When an alkene reacts with an electrophile, what type of compound typically results?
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Alkenes readily react with nucleophiles.
Alkenes readily react with nucleophiles.
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What is the difference between syn addition and anti addition in alkene reactions?
What is the difference between syn addition and anti addition in alkene reactions?
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Which of the following reactions is NOT an addition reaction of cyclohexene?
Which of the following reactions is NOT an addition reaction of cyclohexene?
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What is the general pattern of addition for hydrohalogenation?
What is the general pattern of addition for hydrohalogenation?
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What is the major product of the addition reaction of cyclohexene with HCl?
What is the major product of the addition reaction of cyclohexene with HCl?
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The addition of HX to an alkene is always an exothermic reaction.
The addition of HX to an alkene is always an exothermic reaction.
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What is the name of the intermediate that forms during the electrophilic addition of HX to an alkene?
What is the name of the intermediate that forms during the electrophilic addition of HX to an alkene?
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The rate-determining step in the mechanism of electrophilic addition of HX to an alkene is the formation of the carbocation.
The rate-determining step in the mechanism of electrophilic addition of HX to an alkene is the formation of the carbocation.
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Which step in the electrophilic addition mechanism is typically endothermic?
Which step in the electrophilic addition mechanism is typically endothermic?
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What is the term used to describe the shift of a hydrogen or an alkyl group in a carbocation rearrangement?
What is the term used to describe the shift of a hydrogen or an alkyl group in a carbocation rearrangement?
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Carbocation rearrangements can only occur with primary carbocations.
Carbocation rearrangements can only occur with primary carbocations.
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What is the name of the rule that predicts the regiochemistry of the addition of HX to an unsymmetrical alkene?
What is the name of the rule that predicts the regiochemistry of the addition of HX to an unsymmetrical alkene?
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Which of the following is the reason for the regioselectivity observed in Markovnikov's rule?
Which of the following is the reason for the regioselectivity observed in Markovnikov's rule?
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What is the name of the reaction that involves the addition of water to an alkene?
What is the name of the reaction that involves the addition of water to an alkene?
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The mechanism of hydration is similar to the mechanism of hydrohalogenation with HX.
The mechanism of hydration is similar to the mechanism of hydrohalogenation with HX.
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Which of the following is a key difference between halogenation and hydrohalogenation?
Which of the following is a key difference between halogenation and hydrohalogenation?
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What is the name of the intermediate formed in the halogenation reaction?
What is the name of the intermediate formed in the halogenation reaction?
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Carbocation rearrangements are common in halogenation reactions.
Carbocation rearrangements are common in halogenation reactions.
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What is the name of the reaction that involves the addition of a halogen and water to an alkene?
What is the name of the reaction that involves the addition of a halogen and water to an alkene?
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Halohydrin formation is analogous to halogenation, with water acting as the nucleophile.
Halohydrin formation is analogous to halogenation, with water acting as the nucleophile.
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What is the stereochemistry of the addition of X and OH in halohydrin formation?
What is the stereochemistry of the addition of X and OH in halohydrin formation?
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What is the name of the reaction that involves the addition of H and BH2 to an alkene?
What is the name of the reaction that involves the addition of H and BH2 to an alkene?
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Hydroboration is an anti-Markovnikov addition reaction.
Hydroboration is an anti-Markovnikov addition reaction.
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Which factor determines the regiochemistry of hydroboration?
Which factor determines the regiochemistry of hydroboration?
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What is the name of the reaction that follows hydroboration to convert the alkylborane to an alcohol?
What is the name of the reaction that follows hydroboration to convert the alkylborane to an alcohol?
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Study Notes
Chapter 10: Alkenes - Addition Reactions
- Alkenes are also called olefins.
- Addition reactions are methods to add substituents to multiple bonds, primarily double bonds (alkenes). Other types of double bonds have similar basic principles.
- Slides up to 16 cover previously discussed concepts.
- Naming: ...ene denotes a chain with a double bond. Cyclo... has a double bond in a ring.
- Cis-Trans Isomers: Alkenes having one alkyl group bonded to each carbon atom can be differentiated using cis/trans prefixes.
- Cis: alkyl groups on the same side of the double bond.
- Trans: alkyl groups on opposite sides of the double bond.
- Cyclic Alkenes: Cycloalkenes with fewer than eight carbon atoms have a cis geometry.
- Alkenes Summary:
- Restricted Rotation: Rotation around the C=C double bond is restricted. Unfavorable bond breaking and reforming is required for rotation.
- Stereoisomerism: Different groups on a C=C double bond result in cis/trans isomers.
- Stability: Trans alkenes are generally more stable than cis alkenes, and stability increases with the number of R groups on the C=C.
- Prefixes E and Z: Step 1: Assign priorities to substituents on each end using Cahn-Ingold-Prelog rules. Divide the double bond into two parts, and assign 1 and 2 to indicate relative priority. Also assign priority separately to each side.
- E-isomer: Higher priority groups are on opposite sides.
- Z-isomer: Higher priority groups are on the same side.
- Common Names: Some alkenes have common names (e.g., ethylene for ethene). Also alkenyl substituents have common names: methylene, vinyl, allyl.
- Useful Products from Ethylene: Ethylene is an important industrial starting material for various products like polyethylene, alcohols (ethanol, ethylene glycol), PVC, styrene, and poly(vinyl acetate).
- Naturally Occurring Alkenes: Some alkenes are naturally produced, like β-carotene (pigment in carrots) and zingiberene (oil of ginger).
- Fatty Acids and Alkenes: Increasing double bonds in fatty acids decreases melting points.
- Preparation of Alkenes: Alkyl halides, tosylates, and alcohols are used for preparation via elimination reactions. These processes create a suitable leaving group.
- Regioselectivity and Stereoselectivity of Alkene Formation: The most stable alkene predominates (Zaitsev product), and this usually includes a higher number of alkyl group substitutions.
- Addition Reactions: Addition reactions of alkenes generally involve a π bond breaking and two new σ bonds forming. Alkenes act mainly as nucleophiles and react with electrophiles (but not nucleophiles themselves).
- Syn Addition: Both groups add to the same side of the double bond.
- Anti Addition: Groups add to opposite sides of the double bond.
- Addition Reactions of Cyclohexene: Addition to cyclohexene demonstrates similar principles of π-bond breaking and new σ bond formation. This shows general principles for any alkene.
- Hydrohalogenation: Electrophilic addition of HX (HCl, HBr, HI) to the double bond, with the H bonding to the less substituted carbon. This depends on the preference for forming carbocations.
- Examples of Hydrohalogenation: Reaction examples are shown with different starting materials, illustrating the concept.
- Carbocation rearrangements: Less stable carbocations transform into more stable ones through hydrogen and alkyl shifts.
- Markovnikov's Rule: In unsymmetrical alkenes, the H atom in HX adds to the less substituted carbon, leading to a more stable carbocation in the rate-determining step of the mechanism.
- Carbocation Stability and Markovnikov's Rule: The stability of carbocations (primary, secondary, tertiary) reflects why Markovnikov addition occurs. The mechanism of the reaction favors the more stable carbocation as a product,
- Stereochemistry of Electrophilic Addition: Trigonal planar atoms react equally in two directions. Hydrohalogenation sometimes creates new stereogenic centers.
- Stereochemistry of Carbocation Formation: Initial H⁺ addition to a double bond may occur from either side. Both modes yield the same achiral carbocation.
- Hydration: Adding water to alkenes forms alcohols. Addition of water to an alkene creates an alcohol. The mechanism involves a carbocation intermediate with the acid catalysing the reaction.
- Hydration Example & Mechanism: Examples of hydration are shown, and the detailed mechanism is explained.
- Mechanism of Hydration: Step-by-step mechanism of the hydration reaction, with an emphasis on the rate-determining step and the formation of the carbocation intermediate.
- Use of Alkenes in Synthesis: Retrosynthetic analysis is used to determine the starting material needed.
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Halogenation: Addition of X₂ to an alkene forms a vicinal dihalide. This reaction follows a different mechanism than hydrohalogenation; the double bond creates a dipole in the halogen and addition occurs.
- Halogenation Details: Halogens are polarizable, so X₂ forms a dipole facilitating the reaction. This also shows how addition is different than the Markovnikov rule, due to the mechanism.
- Halogenation Mechanism: This is a two-step process involving a bridged halonium ion, followed by a nucleophilic attack of halide.
- Stability of Cation Intermediates: Carbocations are unstable due to lack of a full octet on the carbon. Bridged halonium ions are unstable due to ring strain.
- Stereochemistry of Halonium Formation: Chlorination of cyclic alkenes produces only trans-dihalides because the intermediate chloronium ion is achiral,
- Stereochemistry of Halonium Ring Opening: Substitution occurs via a backside attack resulting in a trans product.
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Halohydrin Formation: Addition of X₂ and H₂O to an alkene yields a halohydrin (an alcohol with a halogen). This is an electrophilic addition of the electrophilic molecule halogen followed by a nucleophile (OH).
- Mechanism of Halohydrin Formation: Mechanism of halohydrin formation shows three steps, with the rate-determining step generating a bridged halonium ion.
- Anti Stereochemistry in Halohydrin Formation: Bridged halonium opening is a backside attack and favors a trans product.
- Regiochemistry of Halohydrin Formation: Mechanism preferences the more stable carbocation intermediate. The nucleophile (water) will preferentially attack the more substituted carbocation.
- Summary of Halohydrin Formation: Halohydrin mechanism involves three steps: forming a bridge, adding the nucleophile, and forming the product. The electrophile attacks the less substituted carbon. Addition is anti.
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Borane and Hydroboration: Hydroboration-oxidation adds BH₃ to the alkene, creating an alkylborane.
- Hydroboration Mechanism: The reaction proceeds in one step without any carbocation formation. Borane is added to the less substituted carbon, and the product follows syn addition.
- Regiochemistry of Hydroboration: Borane attacks the less substituted carbon. This is called "anti-Markovnikov addition."
- Electronic Factors of Hydroboration: More stable transition state occurs when partially positive charge is on the less substituted carbon, which is favored due to alkyl groups stabilizing the positive charge.
- Oxidation Following Hydroboration: Alkylboranes are oxidized to the alcohol without isolating the intermediate. The reaction follows syn addition, retaining the original stereochemical properties.
- Summary of Hydroboration-Oxidation: Alkylboranes are oxidized. OH group joins the less substituted carbon in the syn addition.
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Description
Explore the fascinating world of alkenes and their addition reactions in this quiz based on Chapter 10. Learn about the naming conventions, cis-trans isomerism, and the intricacies of cyclic alkenes. Test your understanding of stereoisomerism and restricted rotation around double bonds.
