Classes of Alkyl Halides

Introduction to Alkyl Halides

Alkyl halides are organic compounds in which a halogen atom is bonded to an sp3 hybridized carbon. They are important intermediates in organic synthesis and participate in various substitution and elimination reactions.

• Alkyl halide: Halogen attached to an sp3 carbon (e.g., ethyl chloride).

• Vinyl halide: Halogen attached to an sp2 carbon of an alkene.

• Aryl halide: Halogen attached to an sp2 carbon of a benzene ring.

Example: Alkyl halide: CH3CH2Cl Vinyl halide: CH2=CHCl Aryl halide: C6H5Cl

Nomenclature of Alkyl Halides

Systematic Naming and Classification

Alkyl halides are named by identifying the parent hydrocarbon and the halogen substituent. The position of the halogen is indicated by a number.

• Alpha carbon: The carbon directly bonded to the halogen.

• Geminal dihalide: Two halogen atoms bonded to the same carbon.

• Vicinal dihalide: Two halogen atoms bonded to adjacent carbons.

Common Names: Some alkyl halides are known by their common names, such as methyl chloride (CH3Cl) and chloroform (CHCl3).

Structure and Physical Properties of Alkyl Halides

Electronegativity and Bonding

Halogens are more electronegative than carbon, resulting in a polar C–X bond. The carbon atom bonded to the halogen has a partial positive charge, making it susceptible to nucleophilic attack.

• Electronegativity order: F > Cl > Br > I

• Bond length: Increases as the size of the halogen increases (C–F < C–Cl < C–Br < C–I).

• Bond dipole: Increases with electronegativity difference.

Boiling Points and Density

Boiling points of alkyl halides increase with stronger intermolecular forces and larger halogen atoms. Spherical shape leads to higher boiling points.

• Density: Alkyl fluorides and alkyl chlorides (with one chlorine atom) are less dense than water.

Preparation of Alkyl Halides

Free Radical Halogenation

Alkyl halides can be synthesized by halogenation of alkanes. Bromination is highly selective, especially at the allylic position (carbon adjacent to a double bond).

• Allylic radical stabilization: Resonance stabilizes the radical formed at the allylic position.

• Bromination: Occurs with good yield at the allylic position.

• N-Bromosuccinimide (NBS): Used as an allylic brominating agent to keep Br2 concentration low and reduce side reactions.

Example: Bromination of cyclohexene at the allylic position using NBS.

Nucleophilic Substitution and Elimination Reactions

Substitution vs. Elimination

Alkyl halides undergo two main types of reactions: nucleophilic substitution and elimination.

• Nucleophilic substitution: A nucleophile replaces the leaving group (halide).

• Elimination: Two or more groups are lost, forming a double bond.

Example: Sodium ethoxide reacts with ethyl bromide to form diethyl ether (substitution) or ethylene (elimination).

Nucleophiles and Bases

Definitions and Properties

Nucleophiles are Lewis bases that donate an electron pair. Bases refer to Brønsted bases which accept a proton. All bases are nucleophiles, but not all nucleophiles are basic.

• Strong nucleophile: High electron density, often an anion.

• Resonance: Nucleophiles without resonance are stronger.

• Polarizability: More polarizable atoms are stronger nucleophiles.

Example: OH- is a stronger nucleophile than H2O.

Classification of Alkyl Halides

Primary, Secondary, and Tertiary Alkyl Halides

Alkyl halides are classified based on the number of carbons attached to the carbon bearing the halogen.

• Primary: Halogen attached to a carbon bonded to one other carbon.

• Secondary: Halogen attached to a carbon bonded to two other carbons.

• Tertiary: Halogen attached to a carbon bonded to three other carbons.

Example: 2-bromopropane is a secondary alkyl halide.

Mechanistic Pathways: SN1 and SN2 Reactions

Substitution Mechanisms

There are two main mechanistic pathways for nucleophilic substitution: SN1 and SN2.

• SN1: Two-step process involving a carbocation intermediate. Favored by tertiary alkyl halides and weak nucleophiles.

• SN2: One-step process without an intermediate. Favored by primary alkyl halides and strong nucleophiles.

Example: SN2 reaction of methyl bromide with hydroxide ion.

Key Equations and Concepts

• General nucleophilic substitution:

• Rate law for SN2:

• Rate law for SN1:

Additional info: Some handwritten notes clarify that resonance stabilization and steric hindrance are important factors in determining reaction pathways. The notes also highlight the importance of leaving group ability and nucleophile strength in substitution reactions.