Alkyl Halides: Nomenclature, Structure, and Preparation

Classification and Examples

Alkyl halides are organic compounds in which a halogen atom (X) is bonded to an alkyl group. They are classified based on the carbon to which the halogen is attached:

• Primary (1°) halide: Halogen attached to a carbon bonded to one other carbon.

• Secondary (2°) halide: Halogen attached to a carbon bonded to two other carbons.

• Tertiary (3°) halide: Halogen attached to a carbon bonded to three other carbons.

Other types include:

• Geminal dihalide: Two halogens on the same carbon.

• Vicinal dihalide: Two halogens on adjacent carbons.

Examples:

• 2-bromobutane (secondary halide)

• 2,2-dichlorobutane (geminal dihalide)

• 2,3-dibromobutane (vicinal dihalide)

Alkyl halides are important reagents in organic synthesis and can also be used as solvents (e.g., CH2Cl2).

Because halogens are more electronegative than carbon, the carbon-halogen bond is polar:

• Carbon: partial positive ()

• Halogen: partial negative ()

Preparation of Alkyl Halides

Alkyl halides can be synthesized from alkanes by free-radical halogenation:

• Example:

The SN2 Reaction (Substitution, Nucleophilic, Bimolecular)

Mechanism

The SN2 reaction involves a one-step, concerted mechanism where the nucleophile attacks the electrophilic carbon as the leaving group departs:

• Nucleophile: Electron-rich species that donates a pair of electrons (Lewis base).

• Electrophile: Electron-deficient species that accepts a pair of electrons (Lewis acid).

General reaction:

Rate law:

Reaction-Energy Diagram

The SN2 reaction has a single transition state and no intermediates. The energy diagram shows a direct path from reactants to products.

Stereochemistry of the SN2 Reaction

SN2 reactions proceed with inversion of configuration at the carbon center ("backside attack"). If the carbon is chiral, the product will have the opposite configuration:

• Pure stereoisomer (R) → transition state → pure stereoisomer (S)

Example: (R) + → (S)

Factors Affecting the SN2 Reaction

The rate and outcome of SN2 reactions depend on several factors:

The Nucleophile

• Strong nucleophiles are more effective in attacking the electrophilic carbon.

• Nucleophilicity is influenced by charge, size, and polarizability.

• Trend (from strong to weak): (RO) > > > > > (ROH)

The Solvent

• Protic solvents (with OH or NH groups) solvate anions strongly, decreasing nucleophilicity.

• Aprotic solvents (without OH or NH groups) do not solvate anions as strongly, increasing nucleophilicity.

Common protic solvents: CH3OH (methanol), CH3CH2OH (ethanol)

Common polar aprotic solvents: acetone, acetonitrile, DMF, DMSO

The Substrate

• Steric hindrance slows down the SN2 reaction.

• Rate order: methyl > primary > secondary > tertiary (no reaction)

The Leaving Group

• Good leaving groups are weak bases (conjugate bases of strong acids).

• Trend:

• Poor leaving groups: , , ,

How to Turn OH into a Better Leaving Group

• Protonation: Mixing an alcohol with a strong acid protonates the OH group, allowing it to leave as water.

• Example:

The SN1 Reaction (Substitution, Nucleophilic, Unimolecular)

Mechanism

The SN1 reaction involves a two-step mechanism:

1. Loss of the leaving group to form a carbocation intermediate.

2. Nucleophilic attack on the carbocation.

General reaction:

Rate law:

This is a first-order reaction (unimolecular).

Stereochemistry of the SN1 Reaction

• SN1 reactions produce a racemic mixture if the carbon is chiral, due to attack from either side of the planar carbocation.

• Both inversion and retention of configuration occur.

Factors Affecting the SN1 Reaction

The Nucleophile

• The nucleophile does not affect the rate-determining step (formation of the carbocation).

The Substrate

• Carbocation stability is crucial. More substituted carbocations are more stable.

• Stability order: tertiary > secondary > primary > methyl

• Rate order: tertiary > secondary > primary > methyl (no reaction)

The Solvent

• Polar solvents stabilize the carbocation intermediate, increasing the reaction rate.

The Leaving Group

• Better leaving groups increase the reaction rate.

Carbocation Rearrangements

• Carbocations may rearrange via hydride or alkyl shifts to form more stable intermediates.

Summary Table: SN2 vs SN1 Reactions

Key Terms and Concepts

• Nucleophile: Species that donates an electron pair.

• Electrophile: Species that accepts an electron pair.

• Leaving group: Atom or group that departs with an electron pair.

• Steric hindrance: Physical crowding that slows reactions.

• Carbocation: Positively charged carbon intermediate.

• Protic solvent: Solvent with OH or NH groups.

• Aprotic solvent: Solvent without OH or NH groups.

Practice Problems

• Predict the product and mechanism for the following reactions:

• (SN2)

• (SN2)

• (SN2)

• (SN1)

Additional info: These notes cover the essential features of alkyl halides and nucleophilic substitution reactions, including mechanisms, stereochemistry, and factors affecting SN2 and SN1 reactions, as well as practical examples and summary tables for comparison.