Nomenclature and Reactivity of Haloalkanes

Study Notes

Nomenclature of Haloalkanes

Haloalkanes: Organic compounds containing carbon, hydrogen, and halogen (F, Cl, Br, I).
Common names: Based on alkane parent structure, naming halogen as a prefix (e.g., methyl chloride for CH3Cl).
IUPAC naming:
- Identify longest carbon chain.
- Number carbon chain to give the halogen the lowest number.
- Name halogen as a substituent (bromo-, chloro-, iodo-, or fluoro-).
- Example: 2-bromopentane (Br on the second carbon of a five-carbon chain).

Reactivity of Haloarenes

Haloarenes: Aromatic compounds where one or more hydrogen atoms are replaced by halogens.
Key factors influencing reactivity:
- Resonance: Halogen atoms stabilize the aromatic system, making haloarenes less reactive than haloalkanes.
- Electrophilicity: Halogen atoms are weakly deactivating; they withdraw electrons but can still participate in electrophilic aromatic substitution.
Substitution reactions: Commonly proceed via electrophilic aromatic substitution rather than nucleophilic substitution.

Preparation Methods

From alkanes:
- Halogenation (free radical substitution) using Cl2 or Br2, typically in ultraviolet (UV) light.
From alkenes:
- Electrophilic addition of HX (HX = HCl, HBr, HI) to alkenes.
From alcohols:
- Converting alcohols to haloalkanes using PCl3, SOCl2, or reagents like PBr3.
From arenes:
- Nucleophilic aromatic substitution with strong nucleophiles (e.g., NaI) or via direct halogenation with halogens in the presence of a catalyst (e.g., FeBr3).

Mechanisms of Substitution

Haloalkanes:
- SN1 Mechanism:
  - Two-step process: formation of carbocation followed by nucleophile attack.
  - Favored in tertiary haloalkanes.
- SN2 Mechanism:
  - One-step process: nucleophile attacks the electrophilic carbon while the leaving group departs.
  - Favored in primary haloalkanes; results in inverted stereochemistry.
Haloarenes:
- Electrophilic Aromatic Substitution:
  - The aromatic ring acts as a nucleophile, attacking an electrophile.
  - Halogen acts as a deactivating group, influencing the rate and position of substitution.
- Nucleophilic Aromatic Substitution:
  - Occurs under specific conditions, often involving an electron-withdrawing group (e.g., nitro group) ortho or para to the halogen.
  - Mechanism includes a Meisenheimer complex intermediate.

Nomenclature of Haloalkanes

Haloalkanes are organic compounds containing carbon, hydrogen, and a halogen (F, Cl, Br, I).
Common names are based on the parent alkane structure with the halogen named as a prefix (e.g., methyl chloride for CH3Cl).
IUPAC naming involves:
- Identifying the longest carbon chain.
- Numbering the chain to assign the halogen the lowest possible number.
- Naming the halogen as a substituent using prefixes like "bromo-", "chloro-", "iodo-", or "fluoro-".
- Example: 2-bromopentane (Br on the second carbon of a five-carbon chain).

Reactivity of Haloarenes

Haloarenes are aromatic compounds where one or more hydrogen atoms are replaced by halogens.
Factors influencing reactivity:
- Resonance: Halogen atoms stabilize the aromatic system, making haloarenes less reactive than haloalkanes.
- Electrophilicity: Halogen atoms are weakly deactivating; they withdraw electrons but can still participate in electrophilic aromatic substitution.
Substitution reactions: Haloarenes typically undergo electrophilic aromatic substitution rather than nucleophilic substitution.

Preparation Methods

From alkanes: Halogenation (free radical substitution) using Cl2 or Br2, typically in ultraviolet (UV) light.
From alkenes: Electrophilic addition of HX (HX = HCl, HBr, HI) to alkenes.
From alcohols: Converting alcohols to haloalkanes using PCl3, SOCl2, or reagents like PBr3.
From arenes: Nucleophilic aromatic substitution with strong nucleophiles (e.g., NaI) or via direct halogenation with halogens in the presence of a catalyst (e.g., FeBr3).

Mechanisms of Substitution

Haloalkanes:
- SN1 Mechanism: Two-step process; formation of carbocation followed by nucleophile attack. Favored in tertiary haloalkanes.
- SN2 Mechanism: One-step process; nucleophile attacks the electrophilic carbon while the leaving group departs. Favored in primary haloalkanes, results in inverted stereochemistry.
Haloarenes:
- Electrophilic Aromatic Substitution: Aromatic ring acts as a nucleophile, attacking an electrophile. Halogen acts as a deactivating group, influencing the rate and position of substitution.
- Nucleophilic Aromatic Substitution: Occurs under specific conditions, often involving an electron-withdrawing group (e.g., nitro group) ortho or para to the halogen. Involves a Meisenheimer complex intermediate.

Description

This quiz covers the nomenclature rules for haloalkanes and haloarenes, including IUPAC naming conventions. Understand the reactivity of these organic compounds, focusing on key factors such as resonance and electrophilicity. Test your knowledge on how to identify and name various haloalkanes and haloarenes.