Haloalkanes and Haloarenes — Study Notes

1. Introduction

Haloalkanes and haloarenes are organic compounds in which one or more halogen atoms are attached to an alkyl or aryl group. These compounds are of great industrial and synthetic importance, serving as solvents, refrigerants, anaesthetics, and pesticides. The chapter covers their nomenclature, classification, methods of preparation, physical properties, and the mechanisms of nucleophilic substitution and elimination reactions that are fundamental to organic chemistry.

The halogen atom is an electron-withdrawing group through its inductive effect, but in haloarenes it donates electron density to the ring through resonance. This dual character explains many of the differences between haloalkanes and haloarenes, particularly their reactivity. Haloalkanes are generally more reactive than haloarenes because the carbon-halogen bond in haloalkanes is more easily broken, whereas the aryl halides require more vigorous conditions for substitution.

The chapter deals systematically with the reactions of these compounds: nucleophilic substitution (SN1 and SN2 mechanisms), elimination reactions, and reactions with metals such as magnesium to form Grignard reagents. The important role of these compounds in the synthesis of other organic molecules makes this chapter a cornerstone of organic chemistry study.

2. Nomenclature and Classification

Haloalkanes are named by prefixing the halogen name to the parent alkane: for example, CH3Cl is chloromethane, CH3CH2Br is bromoethane, and CH3CH2CH2Cl is 1-chloropropane. Haloarenes are named as halogen derivatives of benzene: C6H5Cl is chlorobenzene and C6H5Br is bromobenzene.

Classification is based on the position of the halogen: - Primary (1°) haloalkane: Halogen attached to a carbon attached to one other carbon. - Secondary (2°) haloalkane: Halogen attached to a carbon attached to two other carbons. - Tertiary (3°) haloalkane: Halogen attached to a carbon attached to three other carbons.

Haloalkanes are also classified according to the number of halogen atoms as mono, di, tri, or tetra haloalkanes. Compounds such as methyl chloride (CH3Cl), ethyl bromide (C2H5Br), allylic, benzylic, vinylic, and aryl halides are distinguished by the environment of the halogen-bearing carbon.

3. Methods of Preparation

From Alcohols

Haloalkanes are prepared by treating alcohols with halogen acids, phosphorus halides, or thionyl chloride: $$ROH + HX \rightarrow RX + H_2O$$ $$3ROH + PX_3 \rightarrow 3RX + H_3PO_3$$ $$ROH + SOCl_2 \xrightarrow{\text{pyridine}} RCl + SO_2 + HCl$$

The reaction with thionyl chloride is preferred because the by-products SO2 and HCl are gases that escape, leaving a pure product.

From Hydrocarbons

• Free radical halogenation: Alkanes react with halogens in the presence of UV light. Chlorination of methane gives a mixture of products; bromination is more selective.
• Addition of HX to alkenes: Follows Markovnikov's rule; hydrogen adds to the carbon with more hydrogen atoms.
• Addition of HX to alkynes: Similarly, follows Markovnikov's rule.

From Haloarenes

Haloarenes are prepared by electrophilic substitution of benzene with halogens in the presence of a Lewis acid catalyst: $$C_6H_6 + Cl_2 \xrightarrow{FeCl_3} C_6H_5Cl + HCl$$

Sandmeyer Reaction

Aniline is converted to the diazonium salt, which reacts with cuprous halides to give haloarenes: $$C_6H_5N_2^+Cl^- + CuCl \rightarrow C_6H_5Cl + N_2$$

Finkelstein Reaction

Alkyl iodides are prepared from alkyl chlorides or bromides by treating with sodium iodide in dry acetone.

Swarts Reaction

Alkyl fluorides are prepared by treating alkyl chlorides or bromides with metallic fluorides such as AgF, Hg2F2, or CoF3.

4. Physical Properties

• Haloalkanes are generally colourless liquids at room temperature, with the exception of the lighter members such as methyl chloride which are gases.
• The boiling point of haloalkanes increases with the size of the halogen atom: RI > RBr > RCl > RF. It also increases with the length of the carbon chain and the degree of branching.
• The density of halocompounds is greater than water, and it increases with the atomic mass of the halogen. Dihalo and polyhalo compounds are denser than water.
• Haloalkanes are sparingly soluble in water but soluble in organic solvents; the C-X bond is covalent and cannot hydrogen bond with water.
• Haloarenes have higher boiling points than the corresponding hydrocarbons.

5. Chemical Reactions

Nucleophilic Substitution Reactions

The carbon-halogen bond in haloalkanes is polarised, making the carbon atom electrophilic. Nucleophiles attack this carbon and replace the halogen.

$$CH_3CH_2Br + OH^- \rightarrow CH_3CH_2OH + Br^-$$ $$CH_3CH_2Cl + CN^- \rightarrow CH_3CH_2CN + Cl^-$$ $$CH_3CH_2Br + NH_3 \rightarrow CH_3CH_2NH_2 + HBr$$

Mechanism of SN1 and SN2 Reactions

• SN2 mechanism: A bimolecular, one-step reaction where the nucleophile attacks from the back side while the leaving group departs. The rate depends on both the substrate and the nucleophile. It shows inversion of configuration. Reactivity order: CH3X > 1° > 2° > 3°.
• SN1 mechanism: A unimolecular, two-step reaction. In the first step, the C-X bond breaks to form a carbocation; in the second, the nucleophile attacks the carbocation. The rate depends only on the concentration of the substrate. It shows racemisation and is favoured by tertiary substrates, polar protic solvents, and weak nucleophiles. Reactivity order: 3° > 2° > 1° > CH3X.

Elimination Reactions

Haloalkanes undergo dehydrohalogenation, losing HX to form alkenes: $$CH_3CH_2Br + KOH \xrightarrow{\text{alc.}} CH_2=CH_2 + KBr + H_2O$$

The reaction follows Saytzeff's rule: the major product is the alkene with the more substituted double bond.

Reaction with Metals

• Wurtz reaction: Two alkyl halides react with sodium in dry ether to form a higher alkane. $$2R-X + 2Na \rightarrow R-R + 2NaX$$
• Grignard reagent: Alkyl halides react with magnesium in dry ether to form organomagnesium halides: $$R-X + Mg \xrightarrow{\text{dry ether}} RMgX$$ Grignard reagents are extremely versatile and react with water, carbon dioxide, aldehydes, and ketones.

6. Haloarenes: Properties and Reactions

The carbon-halogen bond in haloarenes is shorter and stronger than in haloalkanes because of resonance stabilisation, which gives the C-Cl bond partial double bond character. Consequently, haloarenes are much less reactive towards nucleophilic substitution than haloalkanes.

Reactions

• Haloarenes undergo nucleophilic substitution only under drastic conditions (high temperature and pressure), such as the conversion of chlorobenzene to phenol with aqueous NaOH at high temperature.
• In the presence of strong electron-withdrawing groups (nitro group) at ortho and para positions, the reactivity increases dramatically because the carbanion intermediate is stabilised.
• Electrophilic substitution reactions of chlorobenzene produce ortho and para products because the halogen directs the incoming electrophile to these positions through resonance.

7. Uses and Environmental Concerns

Haloalkanes and haloarenes are used as: 1. Solvents (carbon tetrachloride, chloroform). 2. Refrigerants (chlorofluorocarbons, CFCs). 3. Anaesthetics (chloroform, halothane). 4. Pesticides and insecticides (DDT, BHC). 5. Fire extinguishers (carbon tetrachloride).

CFCs are responsible for the depletion of the ozone layer, and DDT is a persistent environmental pollutant, which has led to restrictions on their use.

Quick Revision Tables

Table 1: Methods of Preparation

Table 2: SN1 versus SN2

Mind Map

Important Diagrams (SVG)

Diagram 1: SN2 Mechanism with Inversion of Configuration

Diagram 2: Resonance in Chlorobenzene

Common Mistakes

1. Believing haloarenes are as reactive as haloalkanes; resonance stabilisation of the C-Cl bond makes aryl halides far less reactive.
2. Applying the wrong mechanism; tertiary halides favour SN1, methyl halides favour SN2.
3. Confusing Finkelstein (iodide from chloride) with Swarts (fluoride) reactions.
4. Forgetting Saytzeff's rule in elimination; the major product is the more substituted alkene.
5. Stating SN2 gives racemisation; SN2 gives inversion, and racemisation is characteristic of SN1.
6. Naming haloalkanes with the locant before the halogen but misordering the substituents alphabetically.
7. Believing that Grignard reagents can be prepared in water; they require strictly dry ether and react violently with water.

Exam Tips

1. Memorise the reactivity order for SN2 (CH3X > 1° > 2° > 3°) and SN1 (3° > 2° > 1° > CH3X); these are directly asked.
2. For preparation of haloarenes, remember the Sandmeyer reaction with the diazonium salt and CuCl/CuBr.
3. Learn the stereochemical outcomes: SN2 inversion, SN1 racemisation.
4. Know the uses of common compounds: CHCl3 (anaesthetic and solvent), CCl4 (fire extinguisher), DDT and BHC (pesticides), CFCs (refrigerants causing ozone depletion).
5. For Grignard reactions, write the reaction with CO2 and with water as examples.
6. In dehydrohalogenation, always identify the more substituted alkene as the major product per Saytzeff's rule.

Conclusion

Haloalkanes and haloarenes are versatile compounds that serve as key intermediates in organic synthesis and as the basis of many industrially important products. The chapter clarifies the fundamental concepts of nucleophilic substitution, distinguishing the mechanistic features of SN1 and SN2 reactions, and introduces elimination as a competing pathway governed by Saytzeff's rule. The differences between haloalkanes and haloarenes, explained through resonance and inductive effects, highlight how electronic structure controls reactivity. Reactions such as the Wurtz reaction and the formation of Grignard reagents extend the synthetic utility of these compounds. Understanding the chemistry of halocarbons is essential not only for the board examinations but also for higher studies in organic chemistry, where these reactions form the toolkit for building complex molecules.

Test Your Understanding

1. Give the IUPAC names of CH3CH2Br and C6H5Cl.
2. Explain why SN2 reactions proceed with inversion of configuration.
3. What is the Sandmeyer reaction? Write its equation for the preparation of chlorobenzene.
4. Why are haloarenes less reactive than haloalkanes towards nucleophilic substitution?
5. State and explain Saytzeff's rule in elimination reactions.