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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?
What distinguishes cis isomers from trans isomers in alkenes?
What distinguishes cis isomers from trans isomers in alkenes?
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?
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?
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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?
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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?
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Which of the following statements correctly describes Saytzeff’s rule?
Which of the following statements correctly describes Saytzeff’s rule?
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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?
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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?
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What is one use of alkenes in technology?
What is one use of alkenes in technology?
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Which of the following correctly distinguishes Zaitsev and Hoffman products?
Which of the following correctly distinguishes Zaitsev and Hoffman products?
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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?
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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?
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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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Description
This quiz covers essential topics in Organic Chemistry Lecture 5, focusing on bonding in alkenes, geometric isomerism, and nomenclature. Dive into the intricacies of sp² hybridization and explore cis-trans isomerism, discovering how spatial arrangements affect chemical properties. Prepare to test your understanding of elimination mechanisms and preparation methods.
