Comprehensive Study Guide on Organohalides: Structure, Properties, and Reactions

Overview and Definition of Organohalides

Organohalides, also referred to as halogenoalkanes or haloalkanes, are organic compounds derived from alkanes in which one or more hydrogen atoms have been replaced by a halogen atom. These compounds can contain a single type of halogen or multiple different halogens. A prominent example of the latter is chlorofluorocarbons (CFCs), which contain both chlorine and fluorine atoms. This material is presented by Nurul Izza Husin from the Faculty of Applied Sciences at UiTM.

The scope of study for organohalides includes their definition and classification, structural formulas, IUPAC nomenclature, and specific chemical reactions. These reactions encompass the formation of alcohols, amines, alkenes, and alkanes from alkyl halides. Furthermore, the curriculum addresses specific organochlorine compounds, such as chlorinated hydrocarbon pesticides, 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), dibenzodioxins, and dibenzofurans.

Classification and Nomenclature of Halogenoalkanes

Halogenoalkanes are classified into four primary types based on the nature of the carbon atom to which the halogen is attached. A methyl halogenoalkane occurs when the halogen is bonded to a carbon that is only attached to hydrogen atoms (e.g., $CH_3Br$). A primary (1°) halogenoalkane exists when the carbon attached to the halogen is bonded to exactly one methyl or alkyl group. A secondary (2°) halogenoalkane occurs when that carbon is bonded to two methyl or alkyl groups. Finally, a tertiary (3°) halogenoalkane is characterized by the carbon attached to the halogen being bonded to three methyl or alkyl groups.

The IUPAC nomenclature system for haloalkanes treats halogens as substituents. These are named using the prefixes fluoro- for $-F$, chloro- for $-Cl$, bromo- for $-Br$, and iodo- for $-I$. These prefixes are placed at the beginning of the compound's name and are listed in alphabetical order, similar to alkyl substituents. Examples of nomenclature include:

• $CH_3Cl$: chloromethane
• $CH_3CH_2Br$: bromoethane
• $Cl-CH_2-CH_2-CH_2-Br$: 1-bromo-3-chloropropane
• $CH_3-CHI-CH_2I$: 1,2-diiodopropane
• 2-Chloro-2-methylpropane
• 2-Chloro-2,3-dimethylpentane
• 1-Chloro-2,3-dimethylpropane

When multiple functional groups are present, the IUPAC priority of substituents must be followed: $COOH > SO_3H > CHO > CN > OH > NH_2 > OR > C=C / C\equiv C$.

Physical Properties and Industrial Uses

At $20^\circ\text{C}$, haloalkanes exhibit specific densities. Chloroalkanes typically range from $0.9\,g\,cm^{-3}$ to $1.1\,g\,cm^{-3}$, while bromoalkanes and iodoalkanes have densities greater than $1.0\,g\,cm^{-3}$. Most haloalkanes are liquids at room temperature, with the exception of halomethanes. Their melting and boiling points are generally higher than those of alkanes with similar relative molecular masses because haloalkane molecules are polar. The boiling points increase according to the halogen's atomic mass in the following order: $RCH_2F < RCH_2Cl < RCH_2Br < RCH_2I$. In terms of solubility, haloalkanes are insoluble in water or highly polar solvents but are soluble in non-polar organic solvents.

Organohalides serve diverse industrial and medical roles. They are used as fire-resistant solvents, organic solvents, coolants, aerosols, and anaesthetics. Specific commercial compounds include:

• Trichloroethylene ($CCl_2=CHCl$): A solvent known as "Trichlor."
• Halothane ($CF_3CHClBr$): An inhaled anaesthetic.
• Dichlorodifluoromethane ($CCl_2F_2$): Used as a refrigerant.
• Bromomethane ($CH_3Br$): Used as a fumigant.
• Dichloromethane ($CH_2Cl_2$): Also known as methylene chloride.
• Trichloromethane ($CHCl_3$): Also known as chloroform.
• 1,1,1-Trichloroethane ($CH_3CCl_3$): Also known as methyl chloroform.

Elimination Reactions and Saytzeff's Rule

Haloalkanes undergo elimination reactions to form alkenes. This process typically involves reacting the haloalkane with a base such as potassium hydroxide ($KOH$) or potassium ethoxide ($C_2H_5OK$) dissolved in ethanol, accompanied by heating ($\Delta$). During elimination, a hydrogen halide ($HX$) is removed from the molecule. For example, the reaction of $CH_3CH_2CHClCH_3$ with $KOH$ in ethanol and heat yields $H_3C-CH=CH-CH_3 + HCl$.

The regioselectivity of the elimination reaction is governed by Saytzeff's rule. According to this rule, the major product is the alkene that has the lesser number of hydrogen atoms on the double-bonded carbon atoms (the more substituted alkene). This principle determines the distribution of products when multiple isomeric alkenes can be formed from a single haloalkane.

Nucleophilic Substitution Reactions

Haloalkanes participate in substitution reactions where the halogen atom is replaced by another functional group. One primary type is the hydrolysis of haloalkanes to form alcohols. This is achieved by reacting the haloalkane ($R-Cl$) with aqueous sodium hydroxide ($NaOH$) under reflux conditions. The general equation is $R-Cl + NaOH \rightarrow R-OH + NaCl$. An example is the conversion of bromoethane to ethanol: $CH_3CH_2Br + OH^-(aq) \rightarrow CH_3CH_2OH(aq) + Br^-(aq)$

Another significant substitution reaction is the formation of amines through reaction with ammonia. When a haloalkane reacts with ammonia ($NH_3$) in ethanol and is heated, a primary amine is formed. For instance, the reaction between bromoethane and ammonia produces ethylamine and hydrogen bromide: $CH_3CH_2Br + NH_3 \xrightarrow{\text{ethanol, heat}} CH_3CH_2NH_2 + HBr$

Organometallic Compounds and Grignard Reagents

Grignard reagents are organometallic compounds where a metal atom is bonded directly to an organic group. The general formula for a Grignard reagent is $RMgX$, where $R$ represents an alkyl group and $X$ represents a halogen. A common example is ethylmagnesium bromide ($CH_3CH_2MgBr$). To prepare a Grignard reagent, a haloalkane is reacted with magnesium metal in the presence of dry ether.

Grignard reagents are highly reactive and can be used to prepare alkanes. In the presence of water at room temperature and with an acid catalyst ($H^+_{(aq)}$), the reagent reacts to form an alkane and a basic magnesium halide. The general equation is: $RMgX + H_2O \xrightarrow{H^+} R-H + Mg(OH)X$ For example, methylmagnesium bromide reacts with water to produce methane and $Mg(OH)Br$: $CH_3MgBr + H_2O \rightarrow CH_4 + Mg(OH)Br$

Questions & Discussion

Try This Exercise 1: Identify the IUPAC names for the structures provided on Page 9. The molecules include substituted cyclohexanes and branched chain haloalkanes involving chlorine and methyl groups.

Try This Exercise 2: Determine the priority of substituents for a molecule containing both a bromine and a chlorine atom based on the priority chain ($COOH > SO_3H > CHO > CN > OH > NH_2 > OR > C=C / C\equiv C$).

Learning Check - Alcohol Formation: When 3-bromo-2,3-dimethylpentane reacts with aqueous sodium hydroxide, an alcohol is formed. The task is to outline the reaction mechanism and show the structure of the resulting alcohol.

Learning Check - Reaction Roadmap: Identify products A, B, C, D, and E in the following sequence:

1. $CHCl_3$ reacts with $HBr$ to form A.
2. Compound B reacts with $NH_3$ in ethanol under reflux to form C.
3. Reaction with $NaOH$ under reflux produces D.
4. Reaction with $KOH$ in ethanol under reflux produces E.

Major Product Prediction: Determine the major product when 2-bromo-2-methylbutane (represented by the structure on Page 14) reacts with alcoholic $KOH$. Applying Saytzeff’s rule, the major product will be the alkene with fewer hydrogens on the double-bonded carbons.