Unit 8

Organic Chemistry

Class 12 Chemistry

Haloalkanes

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NEB/CDC syllabus scope: This 8-teaching-hour chapter covers introduction, nomenclature, classification and isomerism of monohaloalkanes; preparation from alkanes, alkenes and alcohols; physical properties; nucleophilic substitution with basic SN1/SN2 concepts; preparation of alcohols, nitriles, amines, ethers, thioethers, isocyanides, nitrites and nitroalkanes; dehydrohalogenation with Saytzeff’s rule; reduction; Wurtz reaction; and preparation plus specified chemical reactions of trichloromethane (chloroform).

1. Introduction to Haloalkanes

Definition Haloalkanes or alkyl halides are organic compounds obtained when one or more hydrogen atoms of an alkane are replaced by halogen atoms such as fluorine, chlorine, bromine or iodine.

General representation R–X

R = alkyl group; X = F, Cl, Br or I.

For an open-chain saturated monohaloalkane, a common molecular formula is:

C_nH_2n+1X

The carbon bonded to halogen is sp³-hybridized. Because halogens are more electronegative than carbon, the C–X bond is polar:

Cδ⁺ — Xδ⁻

This polarization makes the carbon atom susceptible to attack by electron-rich species called nucleophiles.

Diagram 1: Polarity of the carbon–halogen bond

2. Classification of Haloalkanes

2.1 Based on Number of Halogen Atoms

Class	Meaning	Example
Monohaloalkane	One halogen atom	CH₃Cl, chloromethane
Dihaloalkane	Two halogen atoms	CH₂Cl₂, dichloromethane
Trihaloalkane	Three halogen atoms	CHCl₃, trichloromethane
Polyhaloalkane	Several halogen atoms	CCl₄

2.2 Primary, Secondary and Tertiary Haloalkanes

For monohaloalkanes, classification depends on the number of carbon atoms directly attached to the carbon bearing the halogen.

Type	General form	Example
Primary (1°)	R–CH₂–X	CH₃CH₂Cl
Secondary (2°)	R₂CH–X	CH₃CHClCH₃
Tertiary (3°)	R₃C–X	(CH₃)₃CCl

Diagram 2: Main classification scheme for haloalkanes

3. Nomenclature and Isomerism of Monohaloalkanes

3.1 IUPAC Nomenclature

Choose the longest carbon chain containing the carbon bonded to halogen.
Number the chain to give substituents the lowest possible set of locants.
Name halogens as prefixes: fluoro-, chloro-, bromo-, iodo-.
Arrange substituent prefixes alphabetically when needed.

Formula	IUPAC name	Common name
CH₃Cl	Chloromethane	Methyl chloride
CH₃CH₂Br	Bromoethane	Ethyl bromide
CH₃CHClCH₃	2-Chloropropane	Isopropyl chloride
(CH₃)₃CCl	2-Chloro-2-methylpropane	tert-Butyl chloride

3.2 Isomerism

Monohaloalkanes may exhibit:

chain isomerism — different carbon skeleton;
position isomerism — halogen at different positions on the same skeleton;
optical isomerism in suitable molecules containing a chiral carbon.

Example: C₃H₇Cl CH₃CH₂CH₂Cl = 1-chloropropane CH₃CHClCH₃ = 2-chloropropane

These are position isomers.

Common Mistake The terms primary, secondary and tertiary refer to the carbon bonded to X, not simply to where the halogen appears when the formula is written on paper.

4. Preparation of Monohaloalkanes

4.1 From Alkanes: Free-Radical Halogenation

Alkanes react with chlorine or bromine in the presence of light or heat by substitution.

CH₄ + Cl₂ → CH₃Cl + HCl (hν)

Further substitution is possible, so mixtures may form unless conditions are carefully controlled.

4.2 From Alkenes: Addition of Hydrogen Halide

CH₂=CH₂ + HBr → CH₃CH₂Br

For unsymmetrical alkenes and ordinary ionic addition of HX, orientation usually follows Markovnikov’s rule.

CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (major)

4.3 From Alcohols

Using Hydrogen Halides

ROH + HX → RX + H₂O C₂H₅OH + HBr → C₂H₅Br + H₂O

Using Thionyl Chloride

ROH + SOCl₂ → RCl + SO₂ + HCl

Using Phosphorus Halides

3ROH + PCl₃ → 3RCl + H₃PO₃ ROH + PCl₅ → RCl + POCl₃ + HCl

Diagram 3: Preparation of monohaloalkanes from alkanes, alkenes and alcohols

5. Physical Properties of Monohaloalkanes

Property	General trend / explanation
Physical state	Lower members may be gases or volatile liquids; higher members become less volatile.
Polarity	C–X bonds are polar because X is more electronegative than carbon.
Boiling point	Generally increases with molecular mass and polarizability; branching usually lowers boiling point among isomers.
Water solubility	Generally low because haloalkanes cannot form sufficiently strong hydrogen bonds with water to compensate for disrupting water–water interactions.
Organic-solvent solubility	Usually soluble in many organic solvents.
Density	Depends on halogen and structure; many bromo- and iodoalkanes are denser than corresponding chloroalkanes.

Bond strength and leaving group trend For comparable alkyl halides, the C–I bond is weaker than C–Br, which is weaker than C–Cl. Therefore iodoalkanes often undergo substitution more readily than bromoalkanes and chloroalkanes.

6. Chemical Properties: Nucleophilic Substitution

Nucleophilic substitution A reaction in which a nucleophile attacks the electron-deficient carbon of a haloalkane and replaces the halide leaving group.

General equation R–X + Nu⁻ → R–Nu + X⁻

Haloalkanes may react mainly through SN1 or SN2 pathways. At this level, the essential differences are reaction order, number of mechanistic steps, substrate preference and intermediate formation.

Feature	SN1	SN2
Meaning	Substitution nucleophilic unimolecular	Substitution nucleophilic bimolecular
Rate dependence	Rate mainly depends on haloalkane concentration	Rate depends on both haloalkane and nucleophile
Mechanism	Stepwise	One concerted step
Intermediate	Carbocation	No carbocation intermediate
Favoured substrate trend	3° > 2° > 1° for ordinary alkyl systems	Methyl > 1° > 2° ≫ 3°
Steric effect	Less important in nucleophile attack after ionization	Strong: crowded carbon slows backside attack

6.1 Basic SN1 Mechanism

SN1 usually proceeds in two major stages.

Step 1: Slow ionization

R₃C–X → R₃C⁺ + X⁻

Step 2: Fast nucleophilic attack

R₃C⁺ + Nu⁻ → R₃C–Nu

Rate concept The slow ionization step controls the rate, so a simple SN1 rate law is written qualitatively as: Rate ∝ [R–X]

Diagram 4: Basic SN1 mechanism

6.2 Basic SN2 Mechanism

In SN2, nucleophile attack and departure of the leaving group occur together in one concerted step. The nucleophile attacks from the side opposite the leaving group.

Nu⁻ + R–X → [Nu···R···X]‡ → R–Nu + X⁻

Rate concept A simple SN2 rate law is: Rate ∝ [R–X][Nu⁻]

Diagram 5: Basic SN2 backside-attack concept

Remember SN1 and SN2 are mechanisms, not two different overall products by definition. The mechanism depends on substrate structure, nucleophile, solvent and leaving-group ability.

7. Important Substitution Reactions and Conversions

7.1 Haloalkane to Alcohol

R–X + KOH(aq) → R–OH + KX

7.2 Haloalkane to Nitrile

R–X + KCN → R–C≡N + KX

The nitrile product contains one more carbon atom than the original alkyl group.

7.3 Haloalkane to Isocyanide / Carbylamine

R–X + AgCN → R–N≡C + AgX

7.4 Haloalkane to Primary Amine

R–X + 2NH₃ → R–NH₂ + NH₄X

Excess ammonia favours formation of the primary amine over further alkylation.

7.5 Haloalkane to Ether: Williamson-Type Substitution

R–X + R′ONa → R–O–R′ + NaX

7.6 Haloalkane to Thioether

R–X + R′SNa → R–S–R′ + NaX

7.7 Alkyl Nitrite vs Nitroalkane

R–X + KNO₂ → R–O–N=O + KX R–X + AgNO₂ → R–NO₂ + AgX

Exam Important Pair KCN gives nitrile (R–CN), whereas AgCN gives isocyanide (R–NC).
KNO₂ mainly gives alkyl nitrite (R–ONO), whereas AgNO₂ gives nitroalkane (R–NO₂).

Diagram 6: High-yield haloalkane conversion map

8. Elimination Reaction: Dehydrohalogenation

Dehydrohalogenation Removal of H and X from adjacent carbon atoms of a haloalkane to form an alkene. It is commonly carried out with alcoholic KOH on heating.

R–CH₂–CHX–R′ + KOH(alc.) → alkene + KX + H₂O

Saytzeff’s Rule

Rule When more than one alkene can form by β-elimination, the more substituted alkene is generally the major product under ordinary Saytzeff conditions.

Example: 2-Bromobutane

CH₃CHBrCH₂CH₃ + KOH(alc.) → CH₃CH=CHCH₃ + KBr + H₂O

But-2-ene is the more substituted major alkene; but-1-ene is a minor product.

Diagram 7: Saytzeff elimination of 2-bromobutane

Aqueous vs Alcoholic KOH Aqueous KOH commonly favours substitution to an alcohol, while alcoholic KOH + heat commonly favours elimination to an alkene.

9. Reduction of Haloalkanes

Haloalkanes can be reduced to alkanes by replacing X with hydrogen.

R–X + 2[H] → R–H + HX

Reducing systems such as nascent hydrogen or suitable hydride reagents may be used depending on the laboratory context.

CH₃CH₂Cl + 2[H] → CH₃CH₃ + HCl

10. Wurtz Reaction

Wurtz reaction Two molecules of an alkyl halide react with sodium metal in dry ether to form a higher alkane.

2R–X + 2Na → R–R + 2NaX (dry ether)

Example

2CH₃Br + 2Na → C₂H₆ + 2NaBr

Limitation Wurtz reaction is most useful for preparing symmetrical alkanes using one type of haloalkane. Using two different haloalkanes can produce a mixture of coupling products.

Diagram 8: Wurtz reaction

11. Trichloromethane (Chloroform), CHCl₃

Trichloromethane Trichloromethane, commonly called chloroform, is a trihaloalkane with formula CHCl₃.

11.1 Preparation of Chloroform

A. From Ethanol

The classical laboratory route proceeds through oxidation of ethanol to ethanal, chlorination to chloral (trichloroethanal), followed by alkaline hydrolysis.

CH₃CH₂OH + [O] → CH₃CHO + H₂O CH₃CHO + 3Cl₂ → CCl₃CHO + 3HCl 2CCl₃CHO + Ca(OH)₂ → 2CHCl₃ + Ca(HCOO)₂

B. From Propanone (Acetone)

Propanone undergoes extensive α-chlorination followed by alkaline cleavage.

CH₃COCH₃ + 3Cl₂ → CCl₃COCH₃ + 3HCl 2CCl₃COCH₃ + Ca(OH)₂ → 2CHCl₃ + Ca(CH₃COO)₂

Diagram 9: Preparation of chloroform from ethanol and propanone

11.2 Physical Properties of Chloroform

Colourless, volatile liquid with a characteristic sweet odour.
Sparingly soluble in water but miscible with many organic solvents.
Denser than water.
Its vapour and decomposition products require careful handling.

Safety Chloroform is harmful and should not be inhaled or handled casually. It was historically used as an anaesthetic but is not used for routine self-administered anaesthesia because of serious toxicity risks.

11.3 Chemical Properties of Chloroform

A. Oxidation in Air and Light

Chloroform can be oxidized to highly toxic phosgene (carbonyl chloride), COCl₂.

2CHCl₃ + O₂ → 2COCl₂ + 2HCl

Important storage note Classical chloroform is protected from light and air to reduce phosgene formation. This is a chemical-safety fact, not an instruction for untrained handling.

B. Reduction

CHCl₃ + 6[H] → CH₄ + 3HCl

C. Action with Silver Powder

2CHCl₃ + 6Ag → C₂H₂ + 6AgCl

Acetylene (ethyne) is formed.

D. Action with Concentrated Nitric Acid

CHCl₃ + HNO₃ → CCl₃NO₂ + H₂O

The product chloropicrin (trichloronitromethane) is a strongly irritating toxic compound.

E. Action with Propanone

In the presence of alkali, chloroform adds to propanone to form chlorobutanol (chloretone).

CHCl₃ + (CH₃)₂CO → (CH₃)₂C(OH)CCl₃

F. Action with Aqueous Alkali

CHCl₃ + 4KOH → HCOOK + 3KCl + 2H₂O

Potassium formate is produced.

Diagram 10: Syllabus reactions of trichloromethane

12. Reaction Conditions: High-Yield Summary

Reagent / condition	Main product from R–X	Reaction type
Aqueous KOH	R–OH	Nucleophilic substitution
Alcoholic KOH, heat	Alkene	β-Elimination
KCN	R–CN	Substitution; nitrile
AgCN	R–NC	Substitution; isocyanide
Excess NH₃	R–NH₂	Substitution
R′ONa	R–OR′	Ether formation
R′SNa	R–SR′	Thioether formation
KNO₂	R–ONO	Alkyl nitrite formation
AgNO₂	R–NO₂	Nitroalkane formation
Reducing agent	R–H	Reduction
Na / dry ether	R–R	Wurtz coupling

13. Common Exam Mistakes

Confusing haloalkanes with haloarenes. In haloalkanes, X is bonded to an sp³ carbon.
Classifying 1°, 2° and 3° by the wrong carbon. Inspect the carbon directly bonded to X.
Using aqueous KOH and alcoholic KOH interchangeably.
Writing KCN and AgCN as giving the same product. KCN → nitrile; AgCN → isocyanide.
Writing KNO₂ and AgNO₂ as giving the same connectivity. KNO₂ → R–ONO; AgNO₂ → R–NO₂.
Calling SN1 a one-step mechanism. It proceeds through a carbocation intermediate.
Drawing a carbocation intermediate for ordinary SN2. SN2 is concerted.
Forgetting “dry ether” in the Wurtz reaction.
Applying Saytzeff’s rule without first identifying all β-carbons that contain removable hydrogen.
Writing the chloroform oxidation product as CO₂. The important syllabus product is phosgene, COCl₂.
Confusing trichloromethane (CHCl₃) with tetrachloromethane (CCl₄).
Forgetting that the classical ethanol route to chloroform proceeds through ethanal/chloral chemistry.

14. Worked Examples

Worked Example 1: Classify CH₃CHClCH₃

The carbon bonded to Cl is attached to two other carbon atoms.

Answer: it is a secondary (2°) haloalkane, 2-chloropropane.

Worked Example 2: Predict product with aqueous and alcoholic KOH

Starting compound: CH₃CH₂Br.

CH₃CH₂Br + KOH(aq) → CH₃CH₂OH + KBr CH₃CH₂Br + KOH(alc.) → CH₂=CH₂ + KBr + H₂O

Key distinction: aqueous medium favours substitution; alcoholic medium with heat favours elimination.

Worked Example 3: Nitrile or isocyanide?

For bromoethane:

C₂H₅Br + KCN → C₂H₅CN + KBr

Product: propanenitrile.

C₂H₅Br + AgCN → C₂H₅NC + AgBr

Product: ethyl isocyanide.

Worked Example 4: Wurtz reaction

What alkane forms from chloroethane?

2C₂H₅Cl + 2Na → C₄H₁₀ + 2NaCl

Answer: butane.

Worked Example 5: Major elimination product

2-bromobutane with alcoholic KOH can give but-1-ene and but-2-ene. Saytzeff’s rule predicts the more substituted but-2-ene as the major product.

15. Important Exam Questions

Short-Answer Questions

Define haloalkane and write its general representation.
Classify monohaloalkanes as primary, secondary and tertiary with examples.
Give IUPAC names of CH₃CH₂Br and CH₃CHClCH₃.
What types of isomerism are possible in monohaloalkanes?
How is a haloalkane prepared from an alkane?
How is bromoethane prepared from ethene?
Write two methods for converting an alcohol into a chloroalkane.
Why are haloalkanes only slightly soluble in water?
Define nucleophile and nucleophilic substitution.
State two differences between SN1 and SN2 reactions.
Why does a tertiary haloalkane more readily form a carbocation than a primary haloalkane?
Write the reaction of an alkyl halide with KCN and AgCN.
Write the reaction of a haloalkane with ammonia.
How is ether prepared from a haloalkane?
What is dehydrohalogenation?
State Saytzeff’s rule.
What is Wurtz reaction?
Write the formula and IUPAC name of chloroform.
What toxic compound forms when chloroform is oxidized in air/light?
What happens when chloroform is heated with aqueous KOH?

Long-Answer Questions

Explain nomenclature, classification and isomerism of monohaloalkanes with suitable examples.
Describe preparation of monohaloalkanes from alkanes, alkenes and alcohols.
Explain the basic SN1 mechanism with a suitable example and rate concept.
Explain the basic SN2 mechanism and backside attack.
Compare SN1 and SN2 reactions in tabular form.
Show how haloalkanes are converted into alcohols, nitriles, amines, ethers, thioethers, isocyanides, nitrites and nitroalkanes.
Explain dehydrohalogenation and Saytzeff’s rule with 2-bromobutane.
Explain reduction and Wurtz reactions of haloalkanes.
Describe laboratory preparation of trichloromethane from ethanol.
Describe preparation of trichloromethane from propanone.
Explain the chemical properties of chloroform with oxygen, reducing agents, silver powder, concentrated nitric acid, propanone and aqueous alkali.

Conversion / Product Questions

CH₃CH₂Br → CH₃CH₂OH
CH₃CH₂Br → CH₃CH₂CN
CH₃CH₂Br → CH₃CH₂NH₂
CH₃CH₂Br → CH₂=CH₂
CH₃Br → C₂H₆
CH₃CH₂OH → CH₃CH₂Cl
CH₃CH₂OH → CHCl₃
CHCl₃ → COCl₂
CHCl₃ → HCOOK using aqueous KOH
2-bromobutane → major alkene using alcoholic KOH

Mechanism / Diagram Questions

Draw the polar C–X bond and show partial charges.
Draw a classification tree of haloalkanes.
Draw a preparation map from alkane, alkene and alcohol.
Draw the basic two-step SN1 mechanism.
Draw SN2 backside attack.
Draw the Saytzeff elimination products of 2-bromobutane.
Draw the reaction map of haloalkane conversions.
Draw a flow diagram for chloroform preparation from ethanol and propanone.

Exam Strategy For conversion questions, write the reagent and condition above the arrow, then give the correctly connected organic product. For mechanism questions, label the nucleophile, leaving group, carbocation or transition state as applicable.

16. One-Minute Revision

Haloalkanes have halogen bonded to an sp³ carbon and are represented as R–X.
The C–X bond is polar: carbon is δ⁺ and halogen is δ⁻.
Monohaloalkanes are classified as 1°, 2° or 3° by the carbon carrying X.
They can be prepared from alkanes, alkenes and alcohols.
Aqueous KOH converts R–X to R–OH.
Alcoholic KOH + heat favours β-elimination to an alkene.
SN1 is stepwise and forms a carbocation intermediate.
SN2 occurs in one concerted step by backside nucleophilic attack.
KCN gives R–CN; AgCN gives R–NC.
KNO₂ gives R–ONO; AgNO₂ gives R–NO₂.
Excess NH₃ converts haloalkanes to primary amines.
Sodium alkoxide gives ethers; sodium thiolate gives thioethers.
Saytzeff’s rule generally gives the more substituted alkene as the major elimination product.
Reduction replaces X by H.
Wurtz reaction: 2R–X + 2Na → R–R + 2NaX in dry ether.
Trichloromethane is CHCl₃.
Oxidation of CHCl₃ can produce toxic phosgene, COCl₂.
CHCl₃ with Ag powder forms ethyne; with conc. HNO₃ it forms chloropicrin.
CHCl₃ with aqueous KOH gives potassium formate.

17. Diagram Practice

Students should practice these diagrams for the NEB examination:

Polar C–X bond showing δ⁺ and δ⁻.
Classification of haloalkanes.
Preparation routes from alkanes, alkenes and alcohols.
SN1 mechanism with carbocation intermediate.
SN2 backside-attack mechanism.
Haloalkane conversion/reaction map.
Saytzeff elimination of 2-bromobutane.
Wurtz reaction scheme.
Preparation of chloroform from ethanol and propanone.
Chemical reaction map of chloroform.

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