Unit 10 – Haloalkanes and Haloarenes

10.1 Classification
10.2 Nomenclature
10.3 Nature of C–X Bond
10.4 Methods of Preparation of Haloalkanes
10.5 Preparation of Haloarenes
10.6 Physical Properties
10.7 Chemical Reactions
10.8 Polyhalogen Compounds

Introduction to Haloalkanes and Haloarenes

Haloalkanes (alkyl halides) and haloarenes (aryl halides) are organic compounds formed when one or more hydrogen atoms in an aliphatic or aromatic hydrocarbon are replaced by halogen atoms.

Haloalkanes contain halogen atom(s) attached to an sp3 hybridised carbon atom of an alkyl group.
Haloarenes contain halogen atom(s) attached to sp2 hybridised carbon atom(s) of an aryl group.

These compounds are widely used in industry and daily life as solvents for non-polar compounds and as starting materials for synthesizing various organic compounds. Many naturally occurring halogen compounds are clinically useful, such as chloramphenicol (an antibiotic for typhoid fever), thyroxine (an iodine-containing hormone), chloroquine (for malaria treatment), and halothane (an anesthetic). Fully fluorinated compounds are even being considered as potential blood substitutes in surgery.

Classification

Haloalkanes and haloarenes can be classified based on the number of halogen atoms and the hybridization of the carbon atom to which the halogen is bonded.

1. On the Basis of Number of Halogen Atoms

Monohalogen compounds: Contain one halogen atom.
Dihalogen compounds: Contain two halogen atoms.
Polyhalogen compounds: Contain three or more halogen atoms (e.g., tri-, tetra-).

2. Compounds Containing sp3 C—X Bond (X = F, Cl, Br, I)

This class includes:

Alkyl halides or haloalkanes (R—X):
- The halogen atom is bonded to an alkyl group (R).
- They form a homologous series represented by CnH2n+1X.
- Further classified as:
  - Primary (1°): Halogen attached to a primary carbon atom.
  - Secondary (2°): Halogen attached to a secondary carbon atom.
  - Tertiary (3°): Halogen attached to a tertiary carbon atom.
Allylic halides:
- The halogen atom is bonded to an sp3-hybridised carbon atom adjacent to a carbon-carbon double bond (C=C). This sp3 carbon is known as an allylic carbon.
Benzylic halides:
- The halogen atom is bonded to an sp3-hybridised carbon atom attached to an aromatic ring.

3. Compounds Containing sp2 C—X Bond

This class includes:

Vinylic halides:
- The halogen atom is bonded directly to an sp2-hybridised carbon atom of a carbon-carbon double bond (C=C).
Aryl halides:
- The halogen atom is directly bonded to the sp2-hybridised carbon atom of an aromatic ring.

Nomenclature

Haloalkanes and haloarenes can be named using common names or the IUPAC system.

Common names of alkyl halides are derived by naming the alkyl group followed by the name of the halide (e.g., sec-Butyl chloride).
In the IUPAC system, alkyl halides are named as halosubstituted hydrocarbons (e.g., 2-Chlorobutane).
For monohalogen substituted derivatives of benzene, common and IUPAC names are often the same (e.g., o-Chlorotoluene or 1-Chloro-2-methylbenzene).
For dihalogen derivatives, prefixes o-, m-, p- are used in the common system, while numerals (1,2; 1,3; 1,4) are used in the IUPAC system.

Nature of C-X Bond

The carbon-halogen (C—X) bond in alkyl halides is polarised because halogen atoms are more electronegative than carbon. This results in the carbon atom bearing a partial positive charge and the halogen atom bearing a partial negative charge.

Bond length: Increases as you go down the group from C—F to C—I, as the size of the halogen atom increases (Fluorine is the smallest, Iodine is the largest). For example, CH3—F has a bond length of 139 pm, while CH3—I has 214 pm.
Bond enthalpies: Decrease from C—F to C—I, meaning the C—X bond gets weaker down the group. CH3—F has a bond enthalpy of 452 kJ mol-1, while CH3—I has 234 kJ mol-1.
Dipole moment: Values vary, but generally, the polarity allows for certain chemical reactions.

Methods of Preparation of Haloalkanes

Alkyl halides are typically prepared from alcohols, which are readily available.

1. From Alcohols

The hydroxyl (-OH) group of an alcohol can be replaced by a halogen using:

Concentrated halogen acids (HCl, HBr, HI):
- For primary (1°) and secondary (2°) alcohols, a catalyst like ZnCl2 is required with HCl.
- For tertiary (3°) alcohols, reaction occurs simply by shaking with concentrated HCl at room temperature, indicating a higher reactivity for 3° alcohols (3° > 2° > 1°).
- Alkyl bromides are prepared using constant boiling HBr (48%).
- Alkyl iodides are obtained by heating alcohols with NaI or KI in 95% orthophosphoric acid. Sulphuric acid should not be used with KI as it can oxidize HI to I2.
Phosphorus halides (PX3, PX5): Phosphorus tribromide (PBr3) and triiodide (PI3) are usually generated in situ (in the reaction mixture) by reacting red phosphorus with bromine and iodine, respectively.
Thionyl chloride (SOCl2): This is a preferred method as it produces alkyl halide along with gaseous SO2 and HCl, which escape, leading to pure alkyl halides.

2. From Hydrocarbons

From alkanes by free radical halogenation:
- Chlorination or bromination of alkanes via free radical mechanism yields a complex mixture of isomeric mono- and polyhaloalkanes, which are difficult to separate, resulting in a low yield of any single compound.
From alkenes:
- Addition of hydrogen halides (HX): Alkenes react with HCl, HBr, or HI to form alkyl halides. For unsymmetrical alkenes, the addition follows Markovnikov’s rule, where the hydrogen atom adds to the carbon atom of the double bond that already has more hydrogen atoms.
- Addition of halogens (X2): Addition of bromine in CCl4 to an alkene leads to vic-dibromides (halogens on adjacent carbons), which are colorless. This reaction is used to detect double bonds as the reddish-brown color of bromine disappears.

3. Halogen Exchange

Finkelstein reaction: Alkyl iodides are prepared by reacting alkyl chlorides or bromides with NaI in dry acetone. The precipitation of NaCl or NaBr drives the reaction forward according to Le Chatelier’s Principle.
Swarts reaction: Alkyl fluorides are synthesized by heating an alkyl chloride or bromide with a metallic fluoride like AgF, Hg2F2, CoF2, or SbF3.

Preparation of Haloarenes

1. From Amines by Sandmeyer’s Reaction

Primary aromatic amines are first converted to a diazonium salt by treatment with sodium nitrite in cold aqueous mineral acid.
Mixing this freshly prepared diazonium salt solution with cuprous chloride (CuCl) or cuprous bromide (CuBr) results in the replacement of the diazonium group by a chlorine or bromine atom, respectively.
For iodine substitution, potassium iodide (KI) is used, and it does not require cuprous halide.

Physical Properties

1. Appearance and Smell

Alkyl halides are colorless when pure.
Bromides and iodides develop color when exposed to light.
Many volatile halogen compounds have a sweet smell.
Methyl chloride, methyl bromide, ethyl chloride, and some chlorofluoromethanes are gases at room temperature, while higher members are liquids or solids.

2. Melting and Boiling Points

Haloalkanes are polar molecules and have stronger intermolecular forces of attraction (dipole-dipole and van der Waals forces) compared to their parent hydrocarbons. Therefore, their boiling points are considerably higher than those of hydrocarbons of comparable molecular mass.
For the same alkyl group, boiling points of alkyl halides decrease in the order: RI > RBr > RCl > RF. This is because the magnitude of van der Waals forces increases with the increasing size and mass of the halogen atom.
The boiling points of isomeric haloalkanes decrease with increasing branching. For example, 2-bromo-2-methylpropane has the lowest boiling point among its isomers.

3. Density

Bromo, iodo, and polychloro derivatives of hydrocarbons are heavier than water.
Density increases with an increase in the number of carbon atoms, halogen atoms, and the atomic mass of the halogen atoms.

Chemical Reactions of Haloalkanes

The reactions of haloalkanes can be categorized into nucleophilic substitution, elimination reactions, and reactions with metals.

1. Nucleophilic Substitution Reactions

Definition: A reaction where a nucleophile (electron-rich species) replaces an already existing nucleophile (the halogen atom, which acts as a leaving group) in a molecule. The nucleophile attacks the electron-deficient carbon atom bonded to the halogen.
Haloalkanes as substrates: Halogen is bonded to an sp3 hybridised carbon.

(a) SN2 Mechanism (Substitution Nucleophilic Bimolecular)

Kinetics: Follows second-order kinetics, meaning the rate depends on the concentration of both the alkyl halide and the nucleophile.
Mechanism: A single-step process where the incoming nucleophile simultaneously attacks the carbon atom from the side opposite to the leaving halogen group, while the carbon-halogen bond breaks. No intermediate is formed.
Transition State: The carbon atom is simultaneously bonded to five atoms (incoming nucleophile, three original bonds, and outgoing leaving group). This structure is unstable and cannot be isolated.
Stereochemistry: Leads to inversion of configuration, much like an umbrella turning inside out in strong wind. If the reactant is optically active, the product will have an inverted configuration.
Reactivity Order: Methyl halides > Primary (1°) > Secondary (2°) > Tertiary (3°). This order is due to steric hindrance; bulky substituents on or near the carbon atom inhibit the approach of the nucleophile, making tertiary halides the least reactive.
Ambident Nucleophiles: Groups like cyanides (KCN vs AgCN) and nitrites (KNO2 vs AgNO2) possess two nucleophilic centers and can attack through either atom.
- KCN is mostly ionic, providing free cyanide ions. Attack occurs mainly through the carbon atom to form alkyl cyanides because the C—C bond is more stable than the C—N bond.
- AgCN is mainly covalent. Attack occurs through the nitrogen atom to form isocyanides because nitrogen's electron pair is freer to donate.

(b) SN1 Mechanism (Substitution Nucleophilic Unimolecular)

Kinetics: Follows first-order kinetics, meaning the rate depends only on the concentration of the alkyl halide.
Mechanism: Occurs in two steps.
- Step I (slow): The polarised C—X bond undergoes cleavage to produce a carbocation and a halide ion.
- Step II (fast): The carbocation is rapidly attacked by the nucleophile to form the product.
Solvent: Generally carried out in polar protic solvents (like water, alcohol, acetic acid).
Stereochemistry: Characterized by racemisation if the carbon center is chiral. This occurs because the carbocation intermediate is planar, allowing the nucleophile to attack from either face, leading to a mixture of enantiomers [No explicit source provided in the text for the racemization mechanism in SN1, but it is stated as a characteristic in summary].
Reactivity Order: Tertiary (3°) > Secondary (2°) > Primary (1°) > Methyl. This order is due to the stability of the carbocation intermediate; more substituted carbocations are more stable.

2. Competition between Substitution and Elimination

Alkyl halides with β-hydrogen atoms can undergo both substitution (SN1 and SN2) and elimination reactions when reacted with a base or nucleophile.
The preferred reaction pathway depends on:
- Nature of alkyl halide: Primary alkyl halides prefer SN2. Tertiary halides prefer SN1 or elimination. Secondary halides can undergo SN2 or elimination depending on other factors.
- Strength and size of base/nucleophile: A bulkier nucleophile tends to act as a base, abstracting a proton (leading to elimination) rather than approaching the carbon atom for substitution (due to steric reasons).
- Reaction conditions.
Zaitsev's Rule: In dehydrohalogenation (an elimination reaction), if more than one alkene can form, the preferred product is the alkene with the greater number of alkyl groups attached to the doubly bonded carbon atoms.

3. Reaction with Metals

Organic halides react with certain metals to form organo-metallic compounds, which contain carbon-metal bonds.
Grignard Reagents (RMgX): An important class of organo-metallic compounds, alkyl magnesium halides, discovered by Victor Grignard. They are formed by reacting haloalkanes with magnesium metal in dry ether. Grignard reagents must be prepared under anhydrous conditions.

Stereochemistry (as a tool for understanding reaction mechanisms)

1. Molecular Asymmetry, Chirality, and Enantiomers

Chirality: The property of objects that are non-superimposable on their mirror images, like left and right hands.
Chiral molecules: Molecules that are non-superimposable on their mirror images. They are optically active, meaning they rotate plane-polarized light.
Achiral molecules: Objects or molecules that are superimposable on their mirror images (e.g., a sphere, a cube, propan-2-ol). These molecules are optically inactive.
Asymmetric carbon atom: A common aid in identifying chiral molecules is the presence of a single carbon atom bonded to four different groups.
Enantiomers: Stereoisomers that are related to each other as non-superimposable mirror images.
- They possess identical physical properties (melting point, boiling point, refractive index).
- They only differ in their rotation of plane-polarized light. One enantiomer is dextrorotatory (rotates light clockwise), and the other is levorotatory (rotates light counter-clockwise). The sign of optical rotation is not necessarily related to the absolute configuration.

2. Racemic Mixture and Racemisation

Racemic mixture (or racemic modification): A mixture containing two enantiomers in equal proportions.
Optical Rotation of Racemic Mixture: It has zero optical rotation because the rotation caused by one enantiomer is canceled by the opposite rotation of the other.
Racemisation: The process of converting an enantiomer into a racemic mixture. SN1 reactions of chiral alkyl halides are characterized by racemisation.

3. Retention and Inversion of Configuration

Configuration: The spatial arrangement of functional groups around a carbon atom.
Retention of Configuration: The preservation of the spatial arrangement of bonds to an asymmetric center during a chemical reaction. If no bond to the stereocenter is broken, the product maintains the same general configuration as the reactant.
Inversion of Configuration: When the configuration of the carbon atom under attack is inverted during a reaction, similar to an umbrella turning inside out. SN2 reactions of chiral alkyl halides are characterized by the inversion of configuration.

Chemical Reactions of Haloarenes

Haloarenes are generally less reactive than haloalkanes towards nucleophilic substitution reactions due to several factors.

1. Reasons for Lower Reactivity in Nucleophilic Substitution

Resonance Effect: The halogen atom's lone pairs of electrons are in conjugation with the benzene ring, leading to partial double bond character for the C—X bond. This makes the C—X bond shorter (e.g., C—Cl bond in haloalkane is 177 pm, in haloarene is 169 pm) and harder to break than a single bond.
Difference in Hybridization of Carbon Atom: In haloalkanes, the carbon attached to halogen is sp3-hybridized, while in haloarenes, it is sp2-hybridized. The sp2-hybridized carbon, having greater s-character, is more electronegative and holds the electron pair of the C—X bond more tightly, making it stronger.
Instability of Phenyl Cation: If an SN1 mechanism were to occur, the resulting phenyl cation would not be stabilized by resonance, thus ruling out the SN1 mechanism for haloarenes.
Possible Repulsion: The electron-rich nucleophile is less likely to approach the electron-rich arenes due to repulsion.

Despite their lower reactivity, nucleophilic substitution can occur under harsh conditions, such as converting chlorobenzene to phenol by heating with aqueous NaOH at high temperature (623K) and pressure (300 atm). The presence of electron-withdrawing groups, especially nitro groups at ortho- and para-positions, significantly increases the reactivity towards nucleophilic substitution by stabilizing the intermediate carbanion through resonance. A nitro group at the meta-position has no such effect.

2. Electrophilic Substitution Reactions

Haloarenes undergo typical electrophilic reactions of the benzene ring, including halogenation, nitration, sulphonation, and Friedel-Crafts reactions.
The halogen atom, while slightly deactivating the ring, is ortho- (o-) and para- (p-) directing. This directing influence is explained by the resonating structures of halobenzene, which show higher electron density at the ortho and para positions.

Important Polyhalogen Compounds and Their Environmental Effects

Several polyhalogen compounds have significant industrial and agricultural applications, but some also pose environmental hazards due to their persistence.

Dichloromethane (CH2Cl2):
- Uses: Solvent (paint remover), propellant in aerosols, process solvent in drug manufacturing, metal cleaning and finishing.
- Health Effects: Harms the central nervous system, impairs hearing and vision at low levels, causes dizziness, nausea, tingling, and numbness at higher levels. Direct skin contact causes burning and redness; eye contact can burn the cornea.
Chloroform (CHCl3):
- Uses: Solvent for fats, alkaloids, iodine. Major use is in the production of Freon refrigerant R-22. Formerly used as a general anesthetic.
- Health Effects: Inhaling vapors depresses the central nervous system, causing dizziness, fatigue, and headache. Chronic exposure can damage the liver (metabolized to phosgene) and kidneys, and skin immersion can cause sores.
- Storage: Stored in closed dark-colored bottles completely filled to prevent oxidation by air in the presence of light, which forms highly poisonous carbonyl chloride (phosgene).
Iodoform (CHI3):
- Uses: Formerly used as an antiseptic, but its antiseptic properties are due to the liberation of free iodine. Due to its objectionable smell, it has been replaced.
Carbon Tetrachloride (CCl4):
- Uses: Manufacture of refrigerants and propellants, feedstock in chlorofluorocarbon synthesis, pharmaceutical manufacturing, general solvent. Formerly used as cleaning fluid and fire extinguisher.
- Health Effects: Evidence suggests it causes liver cancer in humans. Common effects include dizziness, light-headedness, nausea, and vomiting, potentially leading to permanent nerve cell damage, stupor, coma, unconsciousness, or death in severe cases. Can irritate eyes and cause irregular heartbeat.
- Environmental Impact: When released into the air, it rises to the atmosphere and depletes the ozone layer.
p,p'-Dichlorodiphenyltrichloroethane (DDT):
- Discovery: First prepared in 1873; its effectiveness as an insecticide was discovered by Paul Muller in 1939, earning him a Nobel Prize in 1948.
- Initial Uses: Widely used after World War II for its effectiveness against mosquitoes (malaria) and lice (typhus).
- Problems and Environmental Impact: Problems emerged in the late 1940s due to:
  - Insect resistance.
  - High toxicity towards fish.
  - Chemical stability and fat solubility: It is not rapidly metabolized by animals and accumulates in fatty tissues, leading to bioaccumulation if ingestion continues.
- Regulation: Banned in the United States in 1973, though still used in some other parts of the world.

Analogy for Chirality and Enantiomers: Think of your hands. They are mirror images of each other, but you cannot superimpose your left hand perfectly on your right hand without bending them out of shape. They are "chiral" objects. If you wear a left-handed glove, it will only fit your left hand, not your right. In chemistry, chiral molecules act similarly, interacting differently with other chiral molecules (like enzymes in your body), much like a left-handed glove only fitting a left hand.

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✅ NCERT Solutions Summary – Haloalkanes and Haloarenes

Important Concepts Covered in NCERT:

IUPAC Nomenclature
- Naming of haloalkanes and haloarenes
- Classification as primary, secondary, tertiary, allylic, benzylic, vinylic, and aryl halides
Nature of C–X Bond
- Polarity and bond strength
- Reactivity based on halogen size and electronegativity
Preparation Methods
- From alcohols (using HX, SOCl₂, PCl₅, etc.)
- From hydrocarbons (halogenation, electrophilic substitution, Sandmeyer reaction)
Physical and Chemical Properties
- Boiling/melting point trends
- Nucleophilic substitution reactions (SN1 and SN2 mechanisms)
- Elimination reactions (E1, E2)
Stereochemical Aspects
- Optical activity and chirality
- Walden inversion in SN2 reactions
Uses and Environmental Effects
- Freons and DDT: applications and environmental concerns

📝 Frequently Asked CBSE Exam Questions (PYQs)

🧠 1-Mark Questions

Give the IUPAC name of:
– CH₃CHClCH₂CH₃
– CH₃CH₂CHBrCH₃
Identify chiral compounds among given structures
Why is the C–Cl bond in chlorobenzene stronger than in CH₃Cl?

🧪 2-Mark Questions

Differentiate between SN1 and SN2 mechanisms with suitable examples
Explain why tertiary halides prefer SN1 while primary halides prefer SN2
Write chemical equations for the preparation of:
– 1-bromobutane from butanol
– Chlorobenzene from benzene

🔄 3-Mark Questions

Predict the product and mechanism for:
– Reaction of 2-bromo-2-methylpropane with aqueous KOH
– Elimination of 2-chlorobutane using alcoholic KOH
Compare boiling points of CH₃Cl, CH₃Br, CH₃I, and give reason

🧬 5-Mark Questions

Write short notes on:
– Ambident nucleophiles
– Optical isomerism in halogenated compounds
Give complete mechanism of SN1 reaction using 2-bromo-2-methylpropane
Explain chlorination of methane with free radical mechanism

📌 HOTs & Conceptual Questions

Why are aryl halides less reactive towards nucleophilic substitution?
Give reasons: Vinyl chloride does not undergo SN1/SN2 reactions
Explain the difference in reactivity of alkyl halides vs aryl halides

📚 Tips for Board Exam Preparation:

Focus on reaction mechanisms , particularly SN1/SN2 and elimination.
Practice conversions between alcohols ↔ haloalkanes ↔ alkenes.
Revise isomerism and stereochemistry – very likely in 1–3 mark questions.
Write complete reaction equations with conditions and reagents.

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Haloalkanes and Haloarenes Class 12 Notes – Reactions, Mechanisms & NCERT Solutions