Organic Chem Chapter 8

Class Announcement

  • Today, continuation of Chapter 8.

  • Change in assessment style:

    • No quiz given today.

    • Assignment instead, to be done over the weekend online.

  • Note on assignment difficulty:

    • Some questions may be more challenging than typical quiz questions.

    • Multiple attempts allowed for the online assignment.

  • Weight of assignment:

    • 20 points assigned, counted towards quiz points.

Upcoming Test Information

  • Test approaching; scheduling considerations discussed:

    • Acknowledgment of students having another exam on Wednesday.

  • Possibility of delaying the test discussed:

    • May cut off the material covered in the test at Chapter 7 or include up to Chapter 8.

    • Details TBD.

    • Confirmed: No test scheduled for next Friday.

Topic Focus for Today

  • Continuing discussion on alkenes and alkynes.

  • Shift from elimination reactions to addition reactions:

    • Elimination: removal of two atoms/groups, creation of a double bond.

    • Addition:

    • Involves adding components across a double bond.

    • Electrophiles (Y) and nucleophiles (Z) present in reactions.

Mechanism of Addition Reactions

  • Addition reactions explained with examples:

    • Hydrohalogenation with HX: Addition of HX across the double bond increases alkyl halide formation.

    • Hydration:

    • Instead of dehydration, H and OH added to create hydration.

    • Halogenation:

    • Addition of two halogens (e.g., X_2), yielding dihaloalkanes.

Process Insight

  • Process of breaking pi bonds and forming sigma bonds:

    • Breaking a pi bond creates two sigma bonds, making the reaction energetically favorable.

    • Characteristics of pi bonds:

    • Positioned above and below the plane of carbon atoms (less stable compared to sigma bonds).

    • More reactive than sigma bonds; pi electrons are generally at higher energy states.

Hydrohalogenation Detailed Mechanism

  • Initial setup includes a generic alkene and a generic HX.

  • First step:

    • Electrons from p orbitals attack the electrophile (H), forming a carbocation:

    • Partial positive charge on H due to electronegativity of X, attracting pi electrons.

    • Formation of carbocation leads to potential charge deficiency.

    • Hybridization of carbocation: SP2 hybridization (not SP3).

  • Orientation of attack:

    • Nucleophile (X-) can attack either from above or below the plane of the carbocation, leading to different stereochemical outcomes.

    • If two identical groups present, no chiral center created, thus no stereochemistry implications arise.

Energetics of Reaction

  • Reaction coordinate illustrates energy requirements:

    • Step where carbocation forms (first step) requires more energy than nucleophile attack (second step).

    • Carbocation formation is rate-determining step (requires highest energy).

Reactivity Trends of HX

  • Reactivity of HX follows acidity trends:

    • More acidic HX means weaker H-X bond, allowing for easier H removal during addition reaction.

    • Example of acid strength: HI and HBr are more reactive than HCl; HF is the least reactive due to its stronger H-F bond.

Markovnikov's Rule and Product Formation

  • Definition of Markovnikov's Rule:

    • In hydrohalogenation reactions, the hydrogen adds to the carbon with more hydrogen atoms; halogen (X) adds to the more stable carbocation.

  • Preference in product formation:

    • Secondary carbocation is favored over primary due to stability.

  • Comparison of two different carbocations shows why one path may dominate depending on stability:

    • Example: H addition leading to secondary versus primary carbocation demonstrates preference for the more stable product.

Examples of Hydrohalogenation

  • Consider a typical hydrohalogenation reaction involving 2-methylpropane:

    • Two pathways exist:

    • Hydrogen on one carbon → secondary carbocation.

    • Hydrogen on the other carbon → primary carbocation.

    • Majority product favors the secondary carbocation due to greater stability.

  • Energy comparisons outline that pathways relying on secondary intermediates have lower energy barriers and are favored over less stable primary pathways, leading to a selective product outcome.

Regioselectivity and Stereochemistry

  • Distinction between regioselective addition (following Markovnikov's Rule) and stereoselective outcomes.

  • Overview of regioselectivity mechanisms:

    • Result shows potentially stable carbocations dictate product formation.

    • When substituents differ on the carbocation, it could lead to enantiomeric products, and stereogeneity should be considered.

  • Introduction of peroxides creates conditions for anti-Markovnikov addition, deviating from classical addition pathways:

    • Anti-Markovnikov addition routes do not follow the usual mechanism leading to different types of products.

Conclusion and Future Steps

  • Summary of session highlights:

    • Discussion of addition reactions emphasizing hydrohalogenation and mechanisms.

    • Emphasis on understanding regioselectivity and reactivity trends for exam preparation.

  • Reminder of assignment requirements and the importance of thorough comprehension for upcoming tests.

  • Follow-up scheduled for Monday. Enjoy the weekend!