Haloalkanes and Haloarenes Practice Flashcards

Introduction to Haloalkanes and Haloarenes

• Environmental Persistence: Halogenated compounds persist in the environment because they are resistant to breakdown by soil bacteria.
• Definition: The formation of alkyl halides (haloalkanes) and aryl halides (haloarenes) occurs when hydrogen atom(s) in an aliphatic or aromatic hydrocarbon, respectively, are replaced by halogen atom(s).
• Chemical Bonding and Hybridization:
  • Haloalkanes: These contain halogen atom(s) attached to the $sp^3$ hybridised carbon atom of an alkyl group.
  • Haloarenes: These contain halogen atom(s) attached to the $sp^2$ hybridised carbon atom(s) of an aryl group.
• General Applications:
  • Naturally occurring halogen compounds are often clinically useful.
  • They are used widely in industry and daily life as solvents for relatively non-polar compounds.
  • They serve as starting materials for the synthesis of a broad range of organic compounds.

Clinical and Biological Significance

• Chloramphenicol: A chlorine-containing antibiotic produced by microorganisms. It is highly effective for treating typhoid fever.
• Thyroxine: An iodine-containing hormone produced by the human body. A deficiency of thyroxine causes goiter.
• Chloroquine: A synthetic halogen compound used in the treatment of malaria.
• Halothane: Used as an anaesthetic during surgical procedures.
• Fluorinated Compounds: Certain fully fluorinated compounds are currently being considered as potential blood substitutes in surgery.

Classification of Halogenated Compounds

On the Basis of Number of Halogen Atoms

Compounds are classified according to whether they contain one, two, or more halogen atoms in their structure:

• Monohaloalkane: Contains one halogen atom (e.g., $CH_3X$).
• Dihaloalkane: Contains two halogen atoms (e.g., $CH_2X-CH_2X$).
• Trihaloalkane: Contains three halogen atoms (e.g., $CH_2X-CHX-CH_2X$).
• Monohaloarene: One halogen attached to an aromatic ring.
• Dihaloarene: Two halogens attached to an aromatic ring.
• Trihaloarene: Three halogens attached to an aromatic ring.

Compounds Containing $sp^3$ C-X Bond (X = F, Cl, Br, I)

(a) Alkyl halides or haloalkanes (R-X)

• The halogen is bonded to an alkyl group (R).
• They form a homologous series with the general formula $C_nH_{2n+1}X$.
• Classification by Carbon Type:
  1. Primary (1°): Halogen is attached to a primary carbon atom (e.g., $R'-CH_2-X$).
  2. Secondary (2°): Halogen is attached to a secondary carbon atom (e.g., $R'R''-CH-X$).
  3. Tertiary (3°): Halogen is attached to a tertiary carbon atom (e.g., $R'R''R'''-C-X$).

(b) Allylic halides

• Compounds where the halogen atom is bonded to an $sp^3$-hybridised carbon atom that is adjacent to a carbon-carbon double bond ($C=C$).
• This adjacent carbon is known as the allylic carbon.

(c) Benzylic halides

• Compounds where the halogen atom is bonded to an $sp^3$-hybridised carbon atom that is itself attached to an aromatic ring.

Compounds Containing $sp^2$ C-X Bond

(a) Vinylic halides

• Compounds in which the halogen atom is bonded to an $sp^2$-hybridised carbon atom of a carbon-carbon double bond ($C=C$).

(b) Aryl halides

• Compounds in which the halogen atom is directly bonded to the $sp^2$-hybridised carbon atom of an aromatic ring.

Nomenclature

Naming Conventions

• Common Names: Derived by naming the alkyl group followed by the name of the halide (e.g., n-Propyl bromide).
• IUPAC System: Alkyl halides are named as halo-substituted hydrocarbons (e.g., 1-Bromopropane).
• Benzene Derivatives:
  • For mono-halogen substituted benzenes, common and IUPAC names are identical (e.g., Bromobenze).
  • For dihalogen derivatives, the common system uses prefixes o- (ortho), m- (meta), and p- (para).
  • The IUPAC system uses numerals 1,2; 1,3; and 1,4.

Examples of Names and Structures

• $CH_3CH_2CH_2Br$: Common: n-Propyl bromide; IUPAC: 1-Bromopropane.
• $(CH_3)_2CHCl$: Common: Isopropyl chloride; IUPAC: 2-Chloropropane.
• $(CH_3)_2CHCH_2Cl$: Common: Isobutyl chloride; IUPAC: 1-Chloro-2-methylpropane.
• m-Dibromobenzene: IUPAC: 1,3-Dibromobenzene.
• sym-Tribromobenzene: IUPAC: 1,3,5-Tribromobenzene.
• $(CH_3)_3CCH_2Cl$: IUPAC: 1-Chloro-2,2-dimethylpropane.

Geminal and Vicinal Dihalides

Dihaloalkanes with the same type of halogen atoms are classified as:

1. Geminal halides (gem-dihalides): Both halogen atoms are present on the same carbon atom. Also called alkylidene halides in the common system.
   • Example: $CH_3CHCl_2$ (Common: Ethylidene chloride; IUPAC: 1,1-Dichloroethane).
2. Vicinal halides (vic-dihalides): Halogen atoms are present on adjacent carbon atoms. Also called alkylene dihalides in the common system.
   • Example: $CH_2Cl-CH_2Cl$ (Common: Ethylene dichloride; IUPAC: 1,2-Dichloroethane).

Table 6.1: Common and IUPAC Names of Selected Halides

Structural Isomerism (Example 6.1)

Molecular Formula: $C_5H_{11}Br$ There are eight structural isomers. Listed examples include:

1. $CH_3CH_2CH_2CH_2CH_2Br$ (1-Bromopentane; 1°)
2. $CH_3CH_2CH_2CH(Br)CH_3$ (2-Bromopentane; 2°)
3. $CH_3CH_2CH(Br)CH_2CH_3$ (3-Bromopentane; 2°)
4. $(CH_3)_2CHCH_2CH_2Br$ (1-Bromo-3-methylbutane; 1°)