8. Haloalkanes (Alkyl halides)
Learning Outcomes
By the end of the chapter, students should be able to:
- Describe the nomenclature, isomerism, and classification of monohaloalkanes.
- Explain the preparation of monohaloalkanes from alkanes, alkenes, and alcohols.
- State physical properties of haloalkanes.
- Describe chemical properties of haloalkanes: substitution reactions, SN1 and SN2 reactions (basic concept only).
- Explain the formation of alcohol, nitrile, amine, ether, thioether, carbylamines, nitrite, and nitroalkane using haloalkanes.
- Describe elimination reactions (dehydrohalogenation — Saytzeff’s rule), reduction reactions, and the Wurtz reaction.
- Show the preparation of trichloromethane from ethanol and propanone.
- Explain the chemical properties of trichloromethane: oxidation, reduction, action on silver powder, conc. nitric acid, propanone, and aq. alkali.
Introduction
Did you know that some of the most useful compounds in medicine, agriculture, and industry come from a simple idea: replacing hydrogen atoms in an alkane with halogen atoms? These compounds are called haloalkanes (also known as alkyl halides). They form the backbone of many drugs, refrigerants, solvents, and pesticides.
A haloalkane is an organic compound in which one or more hydrogen atoms of an alkane are replaced by halogen atoms (F, Cl, Br, or I).
General formula: R–XWhere R = alkyl group (e.g. –CH₃, –C₂H₅ …) and X = halogen atom (–F, –Cl, –Br, –I)
Nature of the C–X Bond
It is a polar covalent bond: Carbon is δ⁺, Halogen is δ⁻.
Since halogens are more electronegative than carbon, the shared electron pair is pulled towards the halogen atom. As a result, carbon becomes slightly positive (δ⁺) and the halogen becomes slightly negative (δ⁻).
Classification
A. Based on the Number of Halogen Atoms
- Monohaloalkanes → CH₃Cl
- Dihaloalkanes → CH₂Cl₂
- Trihaloalkanes → CHCl₃
- Polyhaloalkanes → CCl₄
B. Based on the Nature of the Carbon Atom
| Type | Description | Example |
|---|---|---|
| Primary (1°) | Carbon attached to 1 carbon | CH₃CH₂Cl |
| Secondary (2°) | Attached to 2 carbons | CH₃CHClCH₃ |
| Tertiary (3°) | Attached to 3 carbons | (CH₃)₃CCl |
A carbon bonded with only one or no other carbon is called a primary carbon. The carbon bonded with two next carbon atoms is called secondary, and carbons bonded with 3 and 4 next carbon atoms are respectively called tertiary and quaternary carbon atoms.
IUPAC and Common Nomenclature
IUPAC Rules
- Select the longest carbon chain.
- Number the chain to give the halogen the lowest position.
- Use prefixes: chloro-, bromo-, iodo-.
CH₃CH₂CHCH₃ (Br on C-2)
- Longest chain = 4 carbons, word root = but
- C–C single bonds; primary suffix = ane
- Numbering the prefix bromo = 2-bromo
- Name = 2-bromobutane
Common Names
Named as alkyl + halide. Example: CH₃Br → methyl bromide (IUPAC: bromomethane)
| Bromoalkane | IUPAC name | Common name | Degree |
|---|---|---|---|
| CH₃Br | Bromomethane | Methyl bromide | 1° |
| CH₃CH₂Br | Bromoethane | Ethyl bromide | 1° |
| CH₃CH₂CH₂Br | 1-bromopropane | n-propyl bromide | 1° |
| CH₃CHBrCH₃ | 2-bromopropane | iso-propyl bromide | 2° |
| CH₃CH₂CH₂CH₂Br | 1-bromobutane | n-butyl bromide | 1° |
| CH₃CH₂CHBrCH₃ | 2-bromobutane | sec-butyl bromide | 2° |
| (CH₃)₂CHCH₂Br | 1-bromo-2-methylpropane | iso-butyl bromide | 1° |
| (CH₃)₃CBr | 2-bromo-2-methylpropane | tert-butyl bromide | 3° |
🧠 Observe the molecular formula (C₄H₉Br) of the last four compounds in the table above — same formula, different structures.
Isomerism
- Different compounds having the same molecular formula are called isomers.
- Structural isomers are compounds that differ in structure and have the same molecular formula.
- Two types of structural isomerism are observed in monohaloalkanes:
Chain Isomerism (skeletal isomerism)
Chain isomers are compounds with the same molecular formula differing only in the main carbon chain or skeleton. Exhibited by haloalkanes having 4 or more carbon atoms.
e.g., 2-iodobutane and 2-iodo-2-methylpropane
Positional Isomerism
Position isomers are compounds with the same molecular formula differing only in the position of the halo group. Exhibited by haloalkanes having 3 or more carbon atoms.
e.g., 1-chloropropane and 2-chloropropane
General Methods of Preparation of Monohaloalkanes
Haloalkanes can be prepared from several organic compounds. The three methods included in the NEB syllabus are shown below.
1. From Alkanes (by direct halogenation)
Alkanes can be halogenated easily in the presence of sunlight (UV light). These reactions are free radical substitution reactions. Sunlight (UV light) provides the energy needed to break the X–X bond, producing highly reactive free radicals that initiate the reaction.
If chlorine is supplied in excess and the reaction is allowed to continue, all the hydrogen atoms may gradually be replaced by chlorine atoms:
2. From Alkene (by hydrohalogenation)
Alkenes undergo addition with hydrohalic acids (HCl, HBr, and HI), resulting in haloalkanes.
When an unsymmetrical reagent adds to an unsymmetrical alkene, the positive part of the reagent (e.g. H⁺) is attached to the carbon atom with more hydrogen atoms, and the negative part gets bonded to the carbon atom with a smaller number of hydrogen atoms.
Why? It operates because the reaction follows the pathway that forms the more stable carbocation intermediate.
When HBr is added to unsymmetrical alkenes in the presence of organic peroxide, the positive part (H) gets bonded with the carbon having a smaller number of hydrogens, and the negative part (Br) gets bonded with the carbon having a higher number of hydrogens.
3. From Alcohols
a. Reaction with halogen acids
- Alcohols, when treated with halogen acids (HCl, HBr, HI), have the –OH group substituted with the –X group (halide).
- The reaction of alcohols with hydrochloric acid is slow; it therefore requires the presence of ZnCl₂ as a catalyst.
b. Reaction with PCl₅ / PX₃ / SOCl₂
Alcohols also react with PCl₅, PCl₃, and SOCl₂, resulting in chloroalkanes.
Preparations Summary
| Starting compound | Reagent | Reaction type |
|---|---|---|
| Alkane | Cl₂ / Br₂ + UV light | Substitution |
| Alkene | HCl / HBr / HI | Addition |
| Alcohol | HX, PCl₅, SOCl₂ | Substitution |
Physical Properties
- Lower members are gases; higher members are liquids with a pleasant smell; even higher members are waxy solids.
- Boiling points increase with molecular mass in the homologous series and among different halogen atoms: RI > RBr > RCl
- Insoluble in water; soluble in organic solvents like benzene, ether, and acetone.
Chemical Properties
[A] Nucleophilic Substitution Reactions (SN1 / SN2)
The most common type of reaction that haloalkanes undergo — the halogen is replaced by a nucleophile.
where Y⁻ is a nucleophile like OH⁻, NH₂⁻, CN⁻, etc. Nucleophilic substitution reactions are of two types:
- SN1 — substitution nucleophilic unimolecular
- SN2 — substitution nucleophilic bimolecular
I) SN1 Reaction
The rate of reaction depends upon the concentration of only one of the reactants — the haloalkane substrate. Hence called unimolecular.
Rate ∝ [Haloalkane] → Rate law: Rate = k[R–X]
It is a two-step process:
Step I — Haloalkane undergoes heterolytic fission to form a carbocation and a halide ion. It’s slow, hence the rate-determining step (rds).
Step II — The carbocation, being highly reactive, reacts with nucleophiles such as OH⁻ to form the product in a fast step.
Order of reactivity of 1°, 2°, 3°, and methyl haloalkanes: R₃C–X > R₂CHX > RCH₂X > CH₃–X i.e. 3° > 2° > 1° > methyl
Stability of carbocations: 3° > 2° > 1° > CH₃⁺
II) SN2 Reaction
This reaction occurs in a single step. The rate depends on the concentration of both the substrate (haloalkane) and the reagent (nucleophile) — a bimolecular reaction.
Rate law: Rate = k[R–X][Nu⁻]
In the transition state, the nucleophile and the leaving group (halogen) have partial bonds:
Primary halogenoalkanes tend to react via the SN2 mechanism; tertiary halogenoalkanes via the SN1 mechanism; and secondary halogenoalkanes by a mixture of the two, depending on structure.
Quick Comparison
| Feature | SN1 | SN2 |
|---|---|---|
| Steps | Two (stepwise) | One (concerted) |
| Intermediate | Carbocation | None (transition state only) |
| Rate law | k[R–X] | k[R–X][Nu⁻] |
| Favoured by | Tertiary (3°) substrates | Primary (1°) substrates |
| Stereochemistry | Racemisation (mixture of products) | Inversion of configuration (Walden inversion) |
1. Formation of Alcohols
Reaction with aqueous caustic alkali (substitution by –OH group). When treated with aq. NaOH or KOH, haloalkanes give alcohols.
2. Formation of Ether
Reaction with sodium alkoxide — substitution by –OR (Williamson’s etherification). The alkoxide ion (OR⁻) acts as a nucleophile and substitutes the halogen atom from the alkyl halide.
- It is an SN2 reaction of an alkoxide ion with a primary alkyl halide.
- Helps to prove the structure of ethers.
- Suitable for preparing a wide variety of symmetrical and unsymmetrical ethers.
3. Formation of Nitrile
Action of alcoholic potassium cyanide (substitution by –CN).
4. Formation of Carbylamine (Isocyanide)
Reaction with alcoholic AgCN.
Occurs due to the ambident nature of CN⁻.
5. Formation of Nitrites
Action of alcoholic potassium nitrite.
Reaction occurs via the oxygen of NO₂⁻ (ambident nucleophile).
6. Formation of Nitro Compound
Action of alcoholic silver nitrite.
7. Formation of Amines
Reaction with ammonia (Hoffmann’s ammonolysis) — forms different degrees of amines. When an alkyl halide is heated with an aqueous or alcoholic solution of ammonia, a 1° amine is formed. Excess of alkyl halide gives a mixture of primary, secondary, and tertiary amines and quaternary ammonium salts.
8. Formation of Thioether
Action of thioalcohol.
[B] Elimination Reaction
Dehydrohalogenation (reaction with alc. KOH). Haloalkanes undergo elimination with alcoholic KOH or NaOH, giving alkenes.
When two different alkenes are possible from an elimination reaction, the alkene with the highest number of side chains (alkyl groups) is formed as the major product — the more substituted alkene is the major product.
CH₃–CH₂–CHBr–CH₃ → CH₃–CH=CH–CH₃ (but-2-ene, major)
→ CH₃–CH₂–CH=CH₂ (but-1-ene, minor)
But-2-ene is more substituted → major product (Saytzeff’s rule).
Competition: Substitution vs Elimination
- Aq. KOH / NaOH → substitution
- Alc. KOH / NaOH → elimination
[C] Reduction
Haloalkanes are reduced to alkanes with lithium aluminium hydride.
or,
[D] Reaction with Metals — Wurtz Reaction
Haloalkanes react with sodium metal in the presence of dry ether, forming an alkane with a double number of carbon atoms.
Preparation of Chloroform
Chloroform is prepared in the laboratory by distilling ethanol (ethyl alcohol) or propanone (acetone) with an aqueous paste of bleaching powder. In the reaction, aqueous bleaching powder acts as an oxidising, chlorinating, and hydrolysing agent.
First, bleaching powder reacts with water:
Preparation from Ethyl Alcohol
Step 1 — Oxidation of ethyl alcohol into acetaldehyde:
Step 2 — Chlorination of acetaldehyde into trichloroacetaldehyde (chloral):
Step 3 — Hydrolysis of chloral into chloroform:
Preparation from Propanone
Involves only 2 steps.
Step 1 — Chlorination of propanone into 1,1,1-trichloropropanone:
Step 2 — Hydrolysis of 1,1,1-trichloropropanone into chloroform:
Properties of Trichloromethane
- It is a sweet-smelling colourless liquid.
- It is heavier than water with sp. gr. 1.485.
- It boils at 61 °C and freezes at −63 °C.
- It dissolves nonpolar compounds like fat, oil, and wax.
- If inhaled in a small amount, it causes temporary unconsciousness.
1) Oxidation — Reaction with Air
When exposed to light and air, chloroform undergoes slow oxidation to give highly poisonous phosgene gas, which can cause death in higher amounts. This is one of the reasons for discarding chloroform as an anaesthetic.
To prevent the formation of carbonyl chloride, the following precautions should be taken:
- Store in a dark bottle to cut off the light.
- Fill the chloroform up to the stopper to exclude air.
- 1% ethanol is also added, which reacts with any phosgene gas formed and changes it into a nontoxic compound — diethyl carbonate.
Q1. Why does chloroform not give a white precipitate with aqueous silver nitrate? (1 mark)
Solution: Chloroform and other chloroalkanes contain covalently bonded chlorine, which does not ionise easily in an aqueous solution. Therefore, they do not give a white precipitate with aqueous silver nitrate.
Unlike inorganic chlorides like NaCl, HCl, etc., which ionise into Cl⁻ (chloride ion), which reacts with AgNO₃, giving a white precipitate of AgCl:
(But actually, chloroalkanes do give a white precipitate very slowly — over about an hour — due to hydrolysis.)
2) Reduction
Chloroform can be reduced to dichloromethane and methane, respectively, by Zn/HCl in the presence of ethanol and Zn dust in water.
3) Reaction with Silver Powder
Chloroform and iodoform both, when heated with silver powder, give ethyne (acetylene).
Q2. How would you obtain ethylene from trichloromethane?
4) Reaction with Aqueous Caustic Alkali (Hydrolysis)
Chloroform reacts with aqueous NaOH or KOH, forming a tri-alcohol — a typical substitution reaction like that of monohaloalkanes. But the triol, being very unstable, undergoes decomposition, losing a molecule of water and resulting in methanoic acid.
5) Reaction with Concentrated HNO₃
Chloroform reacts with conc. nitric acid, resulting in chloropicrin, which is used as a broad-spectrum antimicrobial, fungicide, herbicide, insecticide, and a component of tear gas.
6) Reaction with Propanone (Acetone)
Chloroform condenses with propanone (acetone) in the presence of KOH to give chloretone, which is used as a hypnotic drug (sleep-inducing).
Chapter Summary
| Topic | Key points |
|---|---|
| Preparation | (1) Alkane + X₂/hν (2) Alkene + HX (Markovnikov) (3) Alcohol + HX / PX₃ / SOCl₂ |
| SN1 | Two steps; carbocation intermediate; rate = k[R–X]; favoured by 3° R–X |
| SN2 | One step; no intermediate; rate = k[R–X][Nu⁻]; favoured by 1° R–X |
| Substitution products | Alcohols, ethers, nitriles, isocyanides, nitrites, nitroalkanes, amines, thioethers |
| Elimination | Alc. KOH → alkene; major product = more substituted alkene (Saytzeff) |
| Reduction | LiAlH₄ / dry ether → alkane |
| Wurtz reaction | 2R–X + 2Na → R–R; doubles the carbon chain |
| Chloroform prep | Ethanol or propanone + bleaching powder + water (distillation) |
| Chloroform oxidation | Air + light → phosgene (toxic); prevent by dark bottle + 1% ethanol |