Chapter 7: Alkyl Halides & Nucleophilic Substitution

Types of Organic Halides

Organic halides are compounds in which a halogen atom (X) is bonded to an sp3 hybridized carbon. The nature of the carbon-halogen bond and its environment determines the properties and reactivity of the halide.

• Alkyl halides: Halogen attached to an sp3 carbon (simple alkyl chain).

• Aryl halides: Halogen attached directly to an aromatic ring (sp2 carbon).

• Allylic halides: Halogen attached to a carbon adjacent to a double bond (sp3 carbon next to a π-system).

• Benzylic halides: Halogen attached to a carbon adjacent to a benzene ring (sp3 carbon next to aromatic system).

Key Point: Allylic and benzylic halides are generally more reactive than simple alkyl halides due to the proximity of the C–X bond to a π-system.

1°, 2°, 3° Alkyl Halides & Noteworthy Examples

The classification of alkyl halides depends on the degree of substitution of the carbon bonded to the halogen:

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

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

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

Applications: Alkyl halides are used as alkylating agents (e.g., methyl iodide), solvents (e.g., chloroform), and as materials (e.g., Teflon, PVC) or bio-active compounds (e.g., DDT, CFCs).

IUPAC Naming of Alkyl Halides

Alkyl halides are named as haloalkanes, following these steps:

1. Find the longest carbon chain (parent chain).

2. Number the chain to give the halogen the lowest possible number.

3. Name and number substituents, listing halogens as prefixes (e.g., 2-chloro-5-methylheptane).

Common names are often used for simple alkyl halides (e.g., tert-butyl iodide, chloroform, carbon tetrachloride, halothane).

Additional info: IUPAC naming is not always required for exams, but understanding the system aids in communication and identification of compounds.

Defining Characteristic of X-atom: Leaving Group Ability

The halogen atom in alkyl halides acts as a leaving group in substitution reactions. The polar C–X bond makes the carbon atom electron-deficient (electrophilic), allowing nucleophilic attack.

• Electrophile: The carbon bonded to the halogen.

• Nucleophile: Species that donates an electron pair to form a new bond.

• Leaving group (LG): The halide ion (X−) that departs during substitution.

Competing elimination reactions can occur, producing alkenes via base-induced removal of HX.

Mechanisms of Nucleophilic Substitution: SN1 versus SN2

Nucleophilic substitution can proceed via two main mechanisms:

• SN2 (Substitution, Nucleophilic, Bimolecular): Concerted mechanism; bond breaking and bond making occur simultaneously. Rate depends on both substrate and nucleophile.

• SN1 (Substitution, Nucleophilic, Unimolecular): Stepwise mechanism; bond breaking occurs before bond making, forming a carbocation intermediate. Rate depends only on substrate.

Rate equations:

• SN2:

• SN1:

Additional info: SN2 reactions are stereospecific (inversion of configuration), while SN1 reactions can lead to racemization.

General Features of Nucleophilic Substitution: Overview

Three components are required for nucleophilic substitution:

1. An alkyl electrophile containing an sp3 hybridized carbon.

2. A leaving group (LG), typically a halide.

3. A nucleophile, which can be negatively charged or neutral.

Substitution occurs readily when both the leaving group and nucleophile are 'good'. Poor leaving groups or nucleophiles result in no reaction.

General Features of Nucleophilic Substitution: Leaving Group

Leaving group ability depends on:

• Strength (Bond Dissociation Energy, BDE) of the C–X bond

• Stability of the X− anion (measured by the pKa of HX)

Good leaving groups are weak bases; the lower the pKa of HX, the better the leaving group.

Order of leaving group ability: I− > Br− > Cl− >> F− (worst)

Additional info: Alcohols (OH−) and amines (NR2−) are poor leaving groups due to their high basicity and strong C–O/N bonds.