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Questions and Answers
What is a requirement for an alkene to exhibit geometric isomerism?
What is a requirement for an alkene to exhibit geometric isomerism?
- There must be only one double bond present in the alkene.
- Neither of the double bond carbons can be bonded to identical groups. (correct)
- The alkene must have at least three double bonds.
- Both double bond carbons must be bonded to identical groups.
What distinguishes cis isomers from trans isomers in alkenes?
What distinguishes cis isomers from trans isomers in alkenes?
- Cis isomers have the same groups on opposite sides, while trans isomers have them on the same side.
- Cis isomers have the same groups on the same side, while trans isomers have them on opposite sides. (correct)
- Cis and trans isomers have identical physical and chemical properties.
- Cis isomers can easily change into trans isomers by rotation around the double bond.
In the E/Z system for nomenclature, how is priority assigned to atoms or groups attached to the double bond?
In the E/Z system for nomenclature, how is priority assigned to atoms or groups attached to the double bond?
- Based solely on the alphabetical order of the groups.
- By comparing the weight of the molecules involved.
- Based on the higher atomic number of the attached atoms. (correct)
- According to the structural formula of the molecules.
What is a characteristic feature of both cis-trans isomers and E/Z isomers?
What is a characteristic feature of both cis-trans isomers and E/Z isomers?
How are atoms attached by double or triple bonds treated when determining priority in the E/Z system?
How are atoms attached by double or triple bonds treated when determining priority in the E/Z system?
What is the IUPAC name for the compound with a methyl substituent of higher priority than H on carbon a and a bromine substituent of higher priority than H on carbon b?
What is the IUPAC name for the compound with a methyl substituent of higher priority than H on carbon a and a bromine substituent of higher priority than H on carbon b?
Which of the following statements correctly describes Saytzeff’s rule?
Which of the following statements correctly describes Saytzeff’s rule?
Which reagents can be used for the dehydrohalogenation synthesis of alkenes from primary alkyl halides?
Which reagents can be used for the dehydrohalogenation synthesis of alkenes from primary alkyl halides?
What occurs during dehydrohalogenation with a bulky base according to Hoffman’s method?
What occurs during dehydrohalogenation with a bulky base according to Hoffman’s method?
What is one use of alkenes in technology?
What is one use of alkenes in technology?
Which of the following correctly distinguishes Zaitsev and Hoffman products?
Which of the following correctly distinguishes Zaitsev and Hoffman products?
What is the priority sequence of substituents on carbon c in the given structural information?
What is the priority sequence of substituents on carbon c in the given structural information?
What is the significance of the electron donating properties of alkyl groups in alkene stability?
What is the significance of the electron donating properties of alkyl groups in alkene stability?
Flashcards
What is an alkene?
What is an alkene?
Alkenes are hydrocarbons containing a carbon-carbon double bond. The general formula for an alkene is CnH2n, where "n" represents the number of carbon atoms. They are unsaturated hydrocarbons, meaning they have fewer hydrogen atoms than a corresponding alkane. Examples include ethene (C2H4), propene (C3H6), and butene (C4H8).
What is the hybridization of carbons in an alkene?
What is the hybridization of carbons in an alkene?
The carbons involved in the double bond in an alkene are sp2 hybridized. This means that each carbon atom forms three sigma bonds (one with the carbon-carbon double bond and two with hydrogen or other atoms) and one pi bond from the overlap of p orbitals.
What are geometric isomers?
What are geometric isomers?
Geometric isomers are stereoisomers that have the same molecular formula and the same connectivity of atoms, but differ in the spatial arrangement of atoms due to restricted rotation around a double bond or a cyclic structure. They are also known as cis-trans isomers.
What are cis isomers?
What are cis isomers?
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What are trans isomers?
What are trans isomers?
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E/Z Nomenclature
E/Z Nomenclature
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Saytzeff's Rule
Saytzeff's Rule
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Hoffman Product
Hoffman Product
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Zaitsev Product
Zaitsev Product
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Dehydrohalogenation
Dehydrohalogenation
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Alkene Stability
Alkene Stability
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Elimination Reaction
Elimination Reaction
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Primary Alkyl Halide
Primary Alkyl Halide
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Study Notes
Organic Chemistry Lecture 5
- Lecture outline covers bonding in alkenes, geometric isomerism (alkenes/cycloalkanes), nomenclature, and preparation methods (from 1º and 2°/3° alkyl halides). Elimination mechanisms are also discussed.
Bonding in Alkenes (Ethene)
- Ethene (H₂C=CH₂) exhibits sp² hybridization.
- Three equivalent sp² hybrid orbitals are involved in bonding.
Geometric Isomerism
- Geometric isomers arise when neither carbon of the double bond is bonded to two identical groups.
- If both carbons have identical groups, geometric isomers are not possible.
- Designated as cis or trans isomers.
Cis-Trans Isomerism
- Cis-trans isomers differ in the spatial arrangement of atoms/groups.
- Cis isomers have identical groups on the same side.
- Trans isomers have identical groups on opposite sides.
- Different physical and chemical properties due to varied interactions with enzymes and receptors.
- Rotation around single bonds can change cis-trans configurations, but rotation around double bonds is restricted.
The E/Z System
- Based on assigning priorities to atoms/groups on each carbon of the double bond.
- Higher priority groups on opposite sides of the pi bond result in (E) configuration.
- Higher priority groups on the same side of the pi bond result in (Z) configuration.
E/Z Sequence Rules
- Higher atomic number atoms have higher priority.
- If atoms are identical, move to the next atom to determine priority.
- Priority is determined at the first differing point along the chain if atoms have directly attached identical groups.
- Double and triple bonds are treated as single-bonded equivalents for assigning priorities.
Examples of Priority Assignments
- Methyl > Hydrogen; Bromine > Hydrogen
- Chloroethyl > Methyl; Bromine > Chlorine
E/Z Nomenclature - Other Priority Assignments
- Various groups can have higher priorities depending on atomic number, structural complexity, etc.
Alkenes in Technology
- Alkenes have applications in fruit ripening, anesthesia (e.g., Methoxyflurane), manufacture of poisonous mustard gas, and ethylene-oxygen flames.
Preparation of Alkenes from Alkyl Halides
- Elimination reaction is the predominant method
- Strong base is used to remove Hydrogen halide (e.g., HBr)
- Process involves dehydrohalogenation to eliminate HX from the Alkyl halide.
Saytzeff's Rule
- In elimination reactions, the most substituted (more alkyl groups) alkene is the major product.
- This is due to increased stability from electron donation of adjacent alkyl groups.
Dehydrohalogenation from Secondary/Tertiary Alkyl Halides
- Saytzeff`s rule applies, creating more stable alkenes.
- Formation of less substituted alkenes (Hoffman products) can occur with bulky bases.
Preparation of Alkenes from Alcohols (Dehydration)
- Elimination reaction producing alkene upon heating with acidic conditions.
- Results in water loss
- Some primary and secondary alcohols undergo rearrangements.
Mechanism (E1) and (E2)
- E1 requires protonation using acid (e.g., H₂SO₄ or H₃PO₄), loss of water to form a carbocation, followed by deprotonation to create the alkene.
- E2 mechanism involves simultaneous protonation and loss of HX using a strong base, leading directly to alkene formation.
Carbocation Stability
- Carbocation stability increases with delocalization of charge across multiple atoms. – More highly substituted carbocations are generally more stable.
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