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.

This quiz covers the fundamentals of alkyl halides, including their classification, naming conventions, and common names. Additionally, it explores the significance of polar bonds and nucleophilic substitution reactions within alkyl halides. Perfect for students studying organic chemistry.