Derivatives of Hydrocarbons – Organohalides: Structure, Nomenclature, Properties, Preparation, and Reactions

Derivatives of Hydrocarbons – Organohalides

Introduction

Organohalides are organic compounds that contain one or more halogen atoms (F, Cl, Br, I) bonded to a carbon atom. These compounds are important in both industrial and laboratory chemistry due to their diverse reactivity and applications.

• Definition: Organohalides are compounds where halogen atoms replace hydrogen atoms in hydrocarbons.

• General Formula: R–X, where R is an alkyl or aryl group and X is a halogen.

• Example: Chloromethane (CH3Cl), Bromobenzene (C6H5Br)

Structure

The structure of organohalides depends on the type of carbon to which the halogen is attached. Alkyl halides are classified as primary (1°), secondary (2°), or tertiary (3°) based on the number of alkyl groups attached to the carbon bearing the halogen.

• Primary Alkyl Halide: Halogen attached to a carbon bonded to one other carbon.

• Secondary Alkyl Halide: Halogen attached to a carbon bonded to two other carbons.

• Tertiary Alkyl Halide: Halogen attached to a carbon bonded to three other carbons.

Nomenclature

Naming organohalides follows IUPAC rules, with halogen substituents named as prefixes to the parent hydrocarbon.

• Step 1: Identify the longest carbon chain containing the halogen.

• Step 2: Number the chain from the end nearest the halogen.

• Step 3: Name and number substituents, listing halogens alphabetically.

• Example: 2-Bromopropane, 1,2-Dichloroethane

Common Name	IUPAC Name
Methyl chloride	Chloromethane
Ethyl bromide	Bromoethane
Isopropyl iodide	2-Iodopropane

Properties

Physical and chemical properties of organohalides are influenced by the type of halogen and the structure of the hydrocarbon.

• Boiling Point: Increases with molecular weight and decreases with branching.

• Polarity: C–X bond is polar, with the carbon atom bearing a slight positive charge and the halogen a slight negative charge.

• Solubility: Generally insoluble in water but soluble in organic solvents.

Compound	Boiling Point (°C)	Density (g/mL)
Chloromethane	-24	0.91
Bromomethane	4	1.42
Iodomethane	42	2.28

Preparation of Alkyl Halides

Alkyl halides can be synthesized by several methods, including radical halogenation of alkanes, addition of halogens to alkenes, and substitution reactions of alcohols.

• Radical Halogenation: Reaction of alkanes with Cl2 or Br2 in the presence of light.

• Addition to Alkenes: Alkenes react with halogens (X2) or hydrogen halides (HX) to form alkyl halides.

• Substitution of Alcohols: Alcohols react with HX, PCl3, SOCl2, or other reagents to yield alkyl halides.

Example Equations:

• $CH_4 + Cl_2 \xrightarrow{hv} CH_3Cl + HCl$

• $C_2H_4 + Br_2 \rightarrow C_2H_4Br_2$

• $R-OH + HCl \rightarrow R-Cl + H_2O$

Reactions of Alkyl Halides

Alkyl halides undergo a variety of reactions, including nucleophilic substitution, elimination, and organometallic reactions.

• Nucleophilic Substitution: Replacement of the halogen by a nucleophile (e.g., OH-, CN-).

• Elimination: Formation of alkenes by removal of HX.

• Organometallic Reactions: Formation of Grignard reagents ($R-X + Mg \rightarrow R-MgX$) and organolithium compounds.

Example: Synthesis of alcohols using Grignard reagents:

• $R-MgX + R'CO \rightarrow R'COR$

Oxidation and Reduction in Organic Chemistry

Oxidation and reduction reactions are fundamental in organic chemistry, often involving changes in the oxidation state of carbon.

• Oxidation: Increase in the number of C–O bonds or decrease in C–H bonds.

• Reduction: Increase in the number of C–H bonds or decrease in C–O bonds.

• Example: Oxidation of alcohols to aldehydes/ketones; reduction of alkyl halides to alkanes.

Compound	Oxidation Level
CH4	Lowest
CH3Cl	Higher
CH2Cl2	Even Higher
CHCl3	Highest

Summary Table: Physical Properties of Common Organohalides

Compound	Boiling Point (°C)	Density (g/mL)	Solubility in Water
Chloromethane	-24	0.91	Low
Bromomethane	4	1.42	Low
Iodomethane	42	2.28	Low

Additional info:

• Organohalides are foundational in organic synthesis and are precursors to many pharmaceuticals, agrochemicals, and polymers.

• Understanding their nomenclature, properties, and reactivity is essential for further study in organic and biological chemistry.