Nomenclature and Reactions of Alkyl Halides & Alcohols

Chapter 5 & 6 Notes: Nomenclature and Reactions of Alkyl Halides & Alcohols

Part 1 - Nomenclature & Properties of Alkyl Halides & Alcohols (5.1 - 5.4)

  • Alkyl Halides: Compounds containing halogen atoms (F, Cl, Br, I) that replace hydrogen atoms in alkanes. They can be specified by the prefix "halo" when named using IUPAC nomenclature.

  • Alcohols: Organic compounds containing hydroxyl groups (-OH). Named as alkanols with a number denoting the position of the hydroxyl group.

Nomenclature Rules:

  1. IUPAC Nomenclature of Alkyl Halides (5.2)

    • The halogen is treated as a "halo" substituent on the alkyl chain.

    • When identifying the alkane chain, the substituent is numbered to give the lowest number to the substituent closest to the end of the chain.

    • Example: i-bromo-5-methyl-2-heptane.

  2. IUPAC Nomenclature of Alcohols (5.3)

    • Use substitutive nomenclature; the alcohol is named as an alkanol with a number before "-ol" indicating the position of the -OH group.

    • The suffix "-e" of the alkane name is replaced with "-ol" (e.g., hexane becomes hexanol).

    • Priority in numbering:

      • Alkyl substituents and halogens take precedence over the hydroxyl group.

      • Identify the longest carbon chain that includes the carbon atom connected to the -OH group.

Memorization:

  • Memorize Table 5.14 containing important nucleophilic reagents.

  • Key functional groups include:

    • Alkane: R-CH3

    • Alcohol: R-OH

    • Aldehyde: R-CHO

    • Carboxylic Acid: R-COOH

    • Ester: R-COOR'

    • Ketone: R-CCOR'

    • Alkoxide: R-O⁻

Part 2 - SN1 Reactions (5.7 - 5.12, 6.2, 6.6 - 6.9)

  • SN1 Reactions: Unimolecular nucleophilic substitution where the rate depends only on the concentration of the substrate.

    • The rate equation: extRate=k[extSubstrate]ext{Rate} = k[ ext{Substrate}]

    • Reactivity order: ext{3° > 2° > 1°} (due to stability of carbocations).

    • Mechanism Outline:

    1. Formation of carbocation (rate-determining step).

    2. Nucleophilic attack and rearrangement (if applicable).

Characteristics of SN1 Reactions:

  • Stability of Carbocations:

    • Tertiary carbocations are favored due to hyperconjugation and inductive effects.

    • Stability Trend: ext{3° > 2° > 1° > Methyl}

  • Solvent Effects: Polar protic solvents (e.g., H₂O, ROH) stabilize the ions formed in the process, influencing the reactivity and mechanism.

  • Stereochemistry: The reaction may lead to inversion or retention of configuration; often, the nucleophile attacks the opposite side of the leaving group.

Good vs Poor Leaving Groups:

  • Good Leaving Groups: Halides (X), H₂O.

  • Poor Leaving Groups: Hydroxides (OH⁻).

Part 3 - SN2 Reactions (5.13, 6.1, 6.3, 6.5, 6.9)

  • SN2 Reactions: Bimolecular nucleophilic substitution requiring both the substrate and nucleophile for the rate; characterized by a concerted mechanism.

    • The rate equation: extRate=k[extNucleophile][extSubstrate]ext{Rate} = k[ ext{Nucleophile}][ ext{Substrate}]

    • Reactivity order: ext{1° > 2° > 3°} due to steric hindrance in the transition state.

    • The reaction involves direct nucleophilic attack resulting in inversion of configuration (R to S or vice versa).

Reactivity Trends and Carbocation Rearrangement

  • Carbocation rearrangements can occur via:

    • Hydride Shifts: Involving the migration of a hydrogen atom from a neighboring carbon.

    • Methyl Shifts: Involving the migration of a methyl group to create a more stable carbocation.

  • Stability order confirmed through experimental observation of reaction rates and mechanisms involved.

Practical Applications in Organic Synthesis (Chapter 6.11)

  • Synthetic Transformations: Involves disconnecting bonds to functional groups and analyzing how those groups can be synthesized via nucleophilic substitution reactions.

  • Common reagents and their applications include:-

    • HCl (SN1/SN2)

    • NaOH (SN2)

    • PBr₃ for bromination.

  • Reactivity Comparisons: Examining which functional groups react fastest in SN1 or SN2 contexts based on the substrate structure, nucleophile strength, and solvent effects.

Conclusion

  • Comprehensive understanding of nomenclature, reaction mechanisms, and the chemical properties of alkyl halides and alcohols is crucial for advancing in organic chemistry. Constant practice and application in lab settings aid in solidifying these concepts.