Notes on Chapter 5 Organohalides

Terminology: Organohalides are alternatively referred to as halogenoalkanes or haloalkanes.
Structural Definition: Halogenoalkanes are structurally similar to alkanes, but they have one or more of their hydrogen atoms replaced by a halogen substituent ( $-F$ , $-Cl$ , $-Br$ , or $-I$ ).
Multiple Substituents: Halogenoalkanes can contain more than one type of halogen atom within the same molecule. A notable example is CFCs (chlorofluorocarbons), which contain both chlorine and fluorine atoms.

The classification of a haloalkane is determined by the specific carbon atom to which the halogen is attached. There are four primary types:

Methyl: The halogen is attached to a carbon atom that is bonded only to hydrogen atoms. * Example: $H-C(H)(H)-Br$ (bromomethane).
Primary (1°): The carbon atom attached to the halogen is bonded to exactly one methyl or alkyl group. * Example: $CH_3-C(H)(H)-Br$ .
Secondary (2°): The carbon atom attached to the halogen is bonded to two methyl or alkyl groups. * Example: $CH_3-C(H)(CH_3)-Br$ .
Tertiary (3°): The carbon atom attached to the halogen is bonded to three methyl or alkyl groups. * Example: $(CH_3)_3C-Br$ .

Halogen Groups: Halogens are named as prefixes in the IUPAC system: * $-F$ : fluoro * $-Cl$ : chloro * $-Br$ : bromo * $-I$ : iodo
Naming Rules: * Halogens are treated as substituents similarly to alkyl groups. * Substituents are placed at the beginning of the name and must be ordered alphabetically. * The carbon chain is numbered to give the substituents the lowest possible locants.
Specific Examples: * Chloromethane: $H-C(H)(Cl)-H$ * Bromoethane: $CH_3-CH_2-Br$ * 1-bromo-3-chloropropane: $Br-CH_2-CH_2-CH_2-Cl$ * 1,2-diiodopropane: $I-CH_2-CH(I)-CH_3$ * 2-Chloro-2-methylpropane: A tertiary haloalkane where a chlorine and a methyl group are both on the second carbon of a propane chain. * 2-Chloro-2,3-dimethylpentane: A five-carbon chain with chlorine and methyl at position 2, and another methyl at position 3. * 1-Chloro-2,3-dimethylpropane: A propane chain with chlorine at position 1 and methyls at positions 2 and 3 (Note: The longest chain would actually make this 1-chloro-2-methylbutane, but it is listed in the transcript as stated).
Priority of Substituents: When multiple functional groups are present, priority follows this order: * $COOH > SO_3H > CHO > CN > OH > NH_2 > OR > C=C / C\equiv C$ (Halogens generally rank below these groups).

State of Matter: All haloalkanes are liquids at room temperature except for halomethanes.
Density at $20\,^{\circ}\text{C}$ : * Haloalkanes: $0.9\text{ to }1.1\,g\,cm^{-3}$ . * Chloroalkanes: Approximately $> 1.0\,g\,cm^{-3}$ . * Bromoalkanes and Iodoalkanes: Significantly $> 1.0\,g\,cm^{-3}$ .
Melting and Boiling Points: * Boiling points (b.p.) and melting points (m.p.) are higher than those of alkanes with similar relative molecular masses because haloalkane molecules are polar. * The points increase following the order of the halogen's atomic mass: $RCH_2F < RCH_2Cl < RCH_2Br < RCH_2I$ .
Solubility: * In Water/Polar Solvents: Insoluble. * In Non-polar Organic Solvents: Soluble.

Industrial Applications: Used as fire-resistant solvents, organic solvents, coolants, aerosols, anesthetics, and fumigants.
Named Examples: * Trichloroethylene (Trichlor) ( $CCl_2=CHCl$ ): Used as a fire-resistant and organic solvent. * Halothane ( $CF_3CH(Cl)Br$ ): An inhaled anesthetic. * Dichlorodifluoromethane/Freon-12 ( $CCl_2F_2$ ): Used as a refrigerant/coolant. * Bromomethane ( $CH_3Br$ ): Used as a fumigant. * Dichloromethane (Methylene chloride) ( $CH_2Cl_2$ ): Organic solvent. * Trichloromethane (Chloroform) ( $CHCl_3$ ): Organic solvent. * 1,1,1-Trichloroethane (Methyl chloroform) ( $CH_3CCl_3$ ): Organic solvent.

General Reaction: Elimination involves the removal of a hydrogen halide ( $HX$ ) to produce an alkene.
Conditions: Reaction with potassium hydroxide ( $KOH$ ) or sodium ethoxide ( $C_2H_5OK$ ) in ethanol under reflux (heat symbolized as $\Delta$ ).
Reaction Equation: $R-CH(X)-CH_3 \xrightarrow{\text{KOH, ethanol, } \Delta} R-CH=CH_2 + HX$
Saytzeff's Rule: In an elimination reaction, the major product is the alkene that has the lesser number of hydrogen atoms on the double-bonded carbon atoms (the more substituted alkene). * Example: Hydrochloric acid elimination from 2-chlorobutane ( $CH_3-CH_2-CHCl-CH_3$ ) primarily yields but-2-ene ( $CH_3-CH=CH-CH_3$ ).

Process: Hydrolysis of haloalkanes using sodium hydroxide ( $NaOH$ ).
Conditions: Aqueous $NaOH$ under reflux.
General Equation: $R-Cl + NaOH \rightarrow R-OH + NaCl$
Specific Example: $CH_3CH_2Br + OH^{-}(aq) \rightarrow CH_3CH_2OH(aq) + Br^{-}(aq)$

Process: Reaction with ammonia ( $NH_3$ ).
Conditions: $NH_3$ in ethanol under heat ( $\Delta$ ) or reflux.
General Equation: $R-Cl + NH_3 \xrightarrow{\text{ethanol, } \Delta} R-NH_2 + HCl$
Specific Example: $CH_3CH_2Br + NH_3 \xrightarrow{\text{heat}} CH_3CH_2NH_2 + HBr$ * The product is Ethylamine (a primary or 1° amine).

Halogenoalkanes react to form nitriles via substitution ( $R-X + CN^{-} \rightarrow R-CN$ ).

Definition: A Grignard reagent is an organometallic compound where a metal atom is bonded to an organic group.
General Formula: $RMgX$ , where $X$ is a halogen and $R$ is an alkyl group.
Formation: Haloalkanes react with magnesium metal in the presence of dry ether to form the reagent. * Example: $CH_3CH_2MgBr$ (ethylmagnesium bromide).
Preparation of Alkanes: Grignard reagents react with water ( $H_2O$ ) at room temperature, often in the presence of an acid catalyst ( $H^{+}(aq)$ ), to yield an alkane.
Reaction Equations: * $RMgX + H-OH \xrightarrow{H^{+}(aq)} R-H + Mg(OH)X$ * Specific Example: $CH_3MgBr + H_2O \rightarrow CH_4 + Mg(OH)Br$ (Conversion of methylmagnesium bromide to methane).

Specific organochlorine compounds discussed include:

Learning Check 1: Name the alcohol formed when 3-bromo-2,3-dimethylpentane, $(CH_3)_2CHCBr(CH_3)CH_2CH_3$ , reacts with aqueous sodium hydroxide. The mechanism should show the structure of the resulting alcohol.
Elimination Challenge: Identify the major product formed when $CH_3-C(Br)(CH_3)-CH_2-CH_3$ reacts with alcoholic $KOH$ . According to Saytzeff's rule, the more substituted alkene (2-methylbut-2-ene) is favored.
Reaction Map: Consider the starting material $CHCl_3$ . What are the products (A, B, C, D, E) when reacting with: * $HBr$ ? * $NH_3$ /ethanol/reflux? * $NaOH$ /reflux? * $KOH$ /ethanol/reflux?