Organic Chemistry (CHM 221) Chapter 7

Alkyl Halides

• Alkyl halides are halogen atoms bonded to an sp³ hybridized carbon atom.
• The halogen atom in halides is often denoted by the symbol "X".
• Alkyl halides are classified as primary (1°), secondary (2°), or tertiary (3°) depending on the number of carbons bonded to the carbon with the halogen atom.
• C is sp³ hybridized.

Naming Alkyl Halides

• Follow the same rules as naming alkanes.
• Find the parent chain.
• Name and number the substituents.
• Alphabetize the substituents.

Common Names of Alkyl Halides

• Common names are often used for simple alkyl halides.
• For example, "ine" becomes "ide" (e.g. iodine becomes iodide).
• tert-butyl iodide is an example

Polar Carbon-Halogen Bond

• Halogens create polar bonds in alkyl halides, making the carbon atom electron deficient.
• This slightly positive carbon becomes a reactive site for alkyl halides.
• Allows for substitution and elimination reactions.

Nucleophilic Substitution

• A nucleophile substitutes for the halide in a reaction.
• Nucleophiles can be negatively charged or neutral and donate an electron pair.
• The halide leaves, and the bond is heterolytically cleaved.
• The more stable the halide, the better it is at accepting the electron pair.
• The halide is known as a leaving group.

Nucleophiles and Bases

• Nucleophiles and bases are structurally similar, both having a lone pair or a bond.
• They differ in what they attack.
• Bases attack protons.
• Nucleophiles attack other electron-deficient atoms (usually carbons).

Nucleophilicity vs. Basicity

• Basicity measures how easily an atom donates its electron pair to a proton.
• Nucleophilicity measures how easily an atom donates its electron pair to other atoms.

Nucleophilicity Parallels Basicity

• For nucleophiles with the same atom, stronger bases are stronger nucleophiles.
• The relative nucleophilicity of different nucleophiles can be determined by comparing the pKa values of their conjugate acids.

Nucleophilicity in Polar Protic Solvents

• Smaller, more electronegative anions are solvated more strongly in polar protic solvents, effectively shielding them from reaction.
• Nucleophilicity increases down a column of the periodic table as the size of the anion increases (opposite to basicity).

Nucleophilicity in Polar Aprotic Solvents

• Polar aprotic solvents solvate cations by ion-dipole interactions.
• Anions are not well solvated in polar aprotic solvents, making them more reactive.
• Generally, nucleophilicity decreases as the structure becomes bulkier.

Common Nucleophiles

• A table listing common negatively charged and neutral nucleophiles.

Bond Breaking and Making in Nucleophilic Substitution Mechanisms

• Bond making and breaking can occur simultaneously.
• Bond breaking can occur first.
• Bond making can occur first.

SN2 Reactions

• Substitution Nucleophilic bimolecular reactions.
• Involves the inversion of configuration
• A second order reaction (rate depends on both reactants).
• A bimolecular reaction (one step mechanism).

SN1 Reactions

• A substitution nucleophilic unimolecular reaction.
• Has two steps (bond breaking occurs first, followed by bond forming), and a carbocation intermediate.
• A first order reaction (rate depends on the concentration of only the alkyl halide).

Carbocation Stability

• Carbocation stability increases with increasing alkyl substitution.
• Alkyl groups are electron-donating (better than hydrogen) and stabilize the positive charge.
• Stability is due to the inductive effect and hyperconjugation.

Hyperconjugation

• Hyperconjugation is the spreading out of charge by the overlap of an empty p orbital with an adjacent σ bond, stabilizing the carbocation intermediate.

The Hammond Postulate

• The transition state structure resembles the species (reactant or product) to which it is closer in energy.
• The closer the transition state is in energy to the product, the faster the reaction.

Transition State, Endothermic Reactions

• Transition states in endothermic reactions resemble the products.
• Lowering the product's energy helps to lower the transition state's energy which increases the reaction rate.

Transition State, Exothermic Reactions

• Transition states in exothermic reactions resemble the reactants.
• Lowering the product energy has little effect on transition state energy, so reaction rate is not affected.

Application of the Hammond postulate to SN1 reactions

• The rate-determining step in SN1 reactions is carbocation formation, an endothermic process.
• More stable carbocations (3° > 2° > 1°) form faster because their formation is less energetically demanding.

Predicting the Mechanism

• Factors to consider: Alkyl halide type, nucleophile strength, leaving group ability, and solvent type (protic vs. aprotic).

• The alkyl halide type, nucleophile, leaving group, and solvent influence the choice of the reaction mechanism.