Chapter 8: Haloalkanes
Unit 8: Haloalkanes (Class 12) covers its nomenclature, classification, isomerism, preparation from alkanes, alkenes, and alcohols, physical properties, chemical properties including SN1 and SN2 mechanisms, formation of alcohols, nitriles, amines, ethers, thioethers, carbylamines, nitrites, and nitroalkanes, elimination reactions following Saytzeff’s rule, reaction with sodium metal via Wurtz’s Reaction, and reduction reactions of haloalkanes.
| Unit 8 | Haloalkanes class 12 |
| 8.1 | Nomenclature |
| 8.2 | Classification |
| 8.3 | Isomerism |
| 8.4 | Preparation (alkanes, alkenes, and alcohols) |
| 8.5 | Physical properties |
| 8.6 | Chemical properties (SN1 and SN2) |
| 8.7 | Formation (alcohol, nitrile, amine, ether, thioether, carbylamine, nitrite, and nitroalkane) |
| 8.8 | Elimination Reaction ( Saytzeff ’s rule) |
| 8.9 | Reaction with Sodium metal (Wurtz’s Reaction) |
| 8.10 | Reduction Reaction of Haloalkane |
Introduction to Haloalkanes
Introduction
Haloalkanes are the derivatives of hydrocarbon because they are derived by replacing hydrogen with halogen atoms, therefore the halogen derivatives of alkane are called haloalkanes. Or
The organic compound containing halogen atom (X=-F,-Cl, -Br, -I) as a functional group is called Haloalkanes.
R-H + X⟶R-X+ HX
alkane haloalkane
CH₃-H + Cl₂⟶CH₃-Cl+ HCl
methane chloromethane
They are also called Alkyl halides.
Uses: Solvent, Medicine, Insecticide, etc.
Haloalkanes are formed by the replacement of one or more hydrogen atoms of alkane by the same number of halogen atoms and are bonded with the carbon atom of alkane through a strong covalent bond. They are presented by the general molecular formula
CnH2n+1X.
Nomenclature of haloalkanes
Prefix + Word root + Primary suffix
Prefix: side chain branches substituents
Word root: number of C-atoms
Primary suffix: nature of C-atoms (-ane or -one or -one)
(-X=halo,-F=fluoro, -Cl=chloro, -Br=bromo, -I=iodo)
| Formula | Common name (Alkyl+halide) | IUPAC name (Halo + word) |
|---|---|---|
| R-X | alkyl halide | halo alkane |
| CH₃F | methyl fluoride | fluoro methane |
| CH₃CH₂-Cl | ethyl chloride | chloro ethane |
| CH₃CH₂CH₂Br | propyl bromide | 1-Bromo propane |
| CH₃CH₂-I | ethyl iodide | iodo ethane |
| CHCl₃ | chloroform | trichloro methane |
| CHI₃ | iodoform | triiodo methane |
| CCl₄ | carbon tetrachloride | tetrachloro methane |
| CH₃-CH-Cl₂ | ethylidene chloride (geminal dichloride) | 1,1-dichloro ethane |
| Cl-CH₂-CH₂-Cl | ethylene dichloride (Vicinal dichloride) | 1,2-dichloro ethane |
| CH₃CH₂CH₂Cl | n-propyl chloride | 1-Chloro propane |
 | tertiary butyl bromide (Neo-butyl bromide) | 2-Bromo, 2-methyl propane |
| CH₃CH₂CH₂CH₂Br | n-butyl bromide | 1-Bromo butane |
 | iso-butyl bromide | 1-Bromo-2-methyl propane |
 | isopropyl chloride | 2-chloro propane |
 | secondary butyl bromide | 2-Bromo butane |
Classification of haloalkanes
[A] Based on the nature of the carbon atom
1. Primary haloalkane (1 ̊)
The haloalkane in which the halogen-containing carbon is further bonded to one carbon atom (one alkyl group) or primary carbon is called primary haloalkane.
2. Secondary haloalkane (2 ̊)
The haloalkane in which the halogen-containing carbon is further bonded to two carbon atoms (two alkyl groups) or secondary carbon is called secondary haloalkane.
3. Tertiary haloalkane (3 ̊)
The haloalkane in which the halogen-containing carbon is further bonded to three carbon atoms (three alkyl groups) or tertiary carbon is called tertiary haloalkane.
[B] based on the number of halogen atoms
1. Mono-halo alkane
Haloalkane contains only one halogen atom.
2. Di-haloalkane
Haloalkane contains two halogen atoms.
3. Poly-haloalkane
Haloalkane contains three or more halogen atoms.
Isomerism in haloalakanes
1. Chain isomerism
Haloalkanes have the same number of carbon atoms but the different number in carbon chain length is called chain isomers.
2. Position isomerism
Haloalkanes have the same molecular formula but the different positions of halogen atoms on the carbon chain are called position isomers.
Self-test:
Q.Write all possible isomers with the molecular formula C4H9I and give their IUPAC name.
General methods of preparation of haloalkane
1. From alkanes (Halogenation of alkanes)
The haloalkanes are prepared by treating alkane with a limited amount of halogen in presence of halogen carriers and sunlight or heat.
On excess supply of chlorine poly-substituted product is formed.
In the case of higher alkanes two or more possible products are formed.
The bromination is carried out in presence of FeBr₃ under sunlight or heat.
CH₃CH₃+Br₂ —- Δ, FeBr3⟶ CH₃CH₂Br + HBr
ethane ⟶ Bromo ethane
The iodination of an alkane is a reversible reaction. So to obtain iodoalkane strong oxidizing agent like conc.HNO₃ or HIO₃ is used to increase the rate of the forward reaction.
Thus, formed iodine increases the rate of forwarding reaction. Hence the iodination of alkane must be carried out in presence of a strong oxidizing agent.
Reactivity of alkane=3 ̊ > 2 ̊ > 1 ̊
Iswori Education
Reactivity of halogen= F > Cl > Br > I
2. From alkenes(Hydrohalogenation of alkenes)
The haloalkanes are prepared by the reaction of an alkene with halogen acid (HF, HCl, HBr, HI). The reaction is called the Hydrohalogenation reaction.
CH₂=CH₂+ HCl ⟶ CH₃CH₂Cl
ethene chloroethane
If an unsymmetrical alkene is taken then two possible products are formed.
The Formation and stability of these two products can be explained by following two rules:
[A] Markovnikov’s rule:
According to this rule “when an unsymmetrical alkene reacts with an unsymmetrical reagent than the positive part of reagent goes to that double bonded carbon containing greater number of the hydrogen atoms”. For example
[B] Peroxide effect (Anti- Markovnikov’s rule)
According to this rule “when an unsymmetrical alkene reacts with the unsymmetrical reagent in presence of organic peroxide (R-O-O-R)then the positive part of the reagent goes to that double bonded carbon containing less number of the hydrogen atom”. For example,
It is also called the Kharasch effect.
HCl and HI do not give Markovnikov’s addition, why?
Ans: H-Cl is highly polar and hence does not undergo hemolysis easily. HI undergoes homolysis to give iodine free radicals which instantly combine to give I2.
3. From Alcohol
Generally, haloalkanes are prepared by the reaction of alcohols with haloacids or phosphorous halide, or thionyl chloride.
(a) Reaction with halogen acid(HX)
The chloroalkane is prepared by the reaction of alcohol with HCl in presence of anhydrous zinc chloride (ZnCl₂).
The mixture of conc. HCl and anhydrous ZnCl₂ is called Lucas reagent.
Reactivity of Alcohol: 3° > 2° > 1°
Reactivity of Halo acid: HI> HBr > HCl
(b) Reaction with a phosphorous halide (PX₅ or PX₃)
The haloalkanes are prepared by the action of alcohol with PX₅ or PX₃.
R-OH+ PX₅ –Δ⟶ R-X+POX₃+HX
3R-OH+ PX₃ —Δ⟶ 3R-X +H₃PO₃
alcohol haloalkane
CH₃CH₂OH+PCl₅ –Δ⟶ CH₃CH₂Cl+ POCl₃+HCl
ethanol Chloroethne+phosphoryl chloride
3CH₃OH +PCl₃ –Δ⟶ 3CH₃Cl +H₃PO₃
methanol chloromethane + phosphorous acid
Since PBr₃ and PI₃ are unstable compounds. So they are prepared in the reaction mixture (In-situ form) by the action of red phosphorous with Br₂ or I₂.
P₄ + 6Br₂ –Δ⟶4PBr₃
CH₃CH₂OH+ PBr₃ –Δ⟶ CH₃CH₂-Br+ H₃PO₃
ethanol bromoethane
Similarly,
P₄ + 6I₂ –Δ⟶ 4PI₃
CH₃CH₂OH+PI₃ –Δ⟶ CH₃CH₂-I+ H₃PO₃
ethanol iodoethane
(c) Reaction with thionyl chloride (SOCl₂) (Darzen’s reaction)
The chloroalkanes are prepared by heating alcohol with SOCl₂ in presence of pyridine. Only chloroalkane is prepared by this method. From this method, pure chloroalkane can be prepared because SO₂ and HCl evolved as gases.
Physical properties of haloalkanes
- Lower members of haloalkane methyl chloride and methyl bromide are colorless gases, higher are colorless and sweet-smelling liquids and next higher are colorless solids.
- They are insoluble in water and soluble in almost all organic solvents like ether, alcohol, etc.
- They burn with green-edged flame in the air.
- The boiling point of haloalkanes is higher than corresponding parent alkanes.
- The boiling point of haloalkane having the same alkyl group is RI>RBr > RCl due to the large size of the halogen atom.
- Branched-chain haloalkane has a lower boiling point than straight-chain haloalkane due to its spherical nature.
- The B.P. increase as the increase in the alkyl group.
Chemical properties of haloalkanes
The haloalkanes are more reactive than alkanes due to the presence of polar C-X bonds. The polarity arises due to the difference in electronegativity value between carbon and a halogen atom.
[A] Nucleophilic substitution reaction
The nucleophile is electron-rich species having lone pairs of electrons or negative charges and can attack to electron-deficient center. When a nucleophile is substituted by another nucleophile then the reaction is called a nucleophilic substitution reaction.
Nuc: + R-LG → R-Nuc + LG:
Nuc- nucleophile LG- Leaving group
Example: R-Br + OH− → R-OH + Br−
Here, the existing nucleophile has been substituted by an incoming nucleophile.
The alkyl halides undergo nucleophilic substitution reaction (SN- reaction ) by following two mechanisms.
S stands for Substitution
N stand for Nucleophile
The number represents Kinetic order
SN1 reaction:
SN1 stands for nucleophilic substitution unimolecular. When the rate of SN depends upon the concentration of substrate only (alkyl halide), then the reaction follows first-order kinetics and is represented as SN1.
The alkaline hydrolysis of t-butyl bromide by aq. NaOH to give t-butyl alcohol is an example of SN1. The reaction completes in two steps.
Step 1)
Alkyl halide ionizes to give carbocation. Then step is slow and hence it is the rate-determining step.
Step 2)
In the second step, the nucleophile attacks the carbonium ion to give t-butyl alcohol. (ionic reaction, fast)
SN2 reaction:
SN2- stands for nucleophilic substitution bimolecular. When the rate of SN-reaction depends upon the concentration of substrate (alkyl halide) and nucleophile (Nu), then the reaction follows second-order kinetics and is represented as SN2.
Consider the alkaline hydrolysis of methyl bromide by aq. NaOH to give alcohol. It completes in one step through the formation of the intermediate.
Difference Between SN1 and SN2 Reaction
| SN1 | SN2 |
| This follows the Unimolecular Rate of Reaction mechanism | This follows the Bimolecular Rate of Reaction mechanism |
| Follows 1st order Kinetic Reaction | Follows 2nd order Kinetic Reaction |
| Two-Step Mechanism | One Step Mechanism |
| A carbocation is formed as an intermediate part | No Carbocation |
| Racemization occurs | Inversion occurs |
| Order: 3°>2°>1° | Order: 1°>2°>3° |
| RoR∝ [ substituent] | RoR∝ [ substituent].[ Nucleophile] |
1. Reaction with aqueous NaOH or KOH (Formation of alcohol)
When haloalkane reacts with an aqueous solution of NaOH or KOH then alcohol is formed.
R-X+aq.NaOH -Δ⟶ R-OH+NaX
haloalkane alcohol
CH₃CH₂Cl+ aq.KOH -Δ⟶ CH₃CH₂OH+KCl
chloroethane(1⁰) ethanol (1 ̊)
Ambident Nucleophile
Those nucleophiles that consist of possible two attacking sites on the electron-deficient center are called ambident nucleophiles. For example, NO₂⁻, CN⁻, etc. Cyanide ion is an ambident nucleophile because both carbon and nitrogen can supply a pair of electrons during the nucleophilic attack.
⟶
-C≡N & -N=C
Similarly, NO₂⁻has two attacking sites.
-NO₂ & -ONO
R-X+CN⁻ ⟶ R-CN (Attack by carbon)
R-X+CN⁻ ⟶ R-NC (Attack by nitrogen)
2. Reaction with alcoholic NaCN or KCN
When haloalkane is heated with an alcoholic solution of NaCN or KCN then alkane nitrile (Cyanides) are formed. This reaction is largely used to increase the number of carbon atoms during organic conversion.
R-X+alc.KCN -Δ⟶ R-CN+KX
haloalkane ⟶ alkanenitrile(cyanide)
CH₃CH₂Cl+ alc.KCN -Δ⟶ CH₃CH₂CN+ KCl
chloroethane ⟶ propane nitrile (ethyl cyanide)
(2 carbon atoms) (3 carbon atoms)
Alkane nitrile(Cyanides) is a beneficial chemical that gives various products when treated with different reagents.
(a) Reduction Reaction
CH₃-CN+ 4[H] Ni/Pt/Pd or LiAlH4⟶ CH₃CH₂NH₂
ethane nitrile (C2H5OH/Na, mendius rxn) ethanamine
A reaction in which an organic nitrile is reduced by nascent hydrogen (e.g. from sodium in ethanol) to a primary amine is REDUCTION REACTION.
b) Complete hydrolysis
CH₃-CN+ H₂O + dil.HCl -Δ⟶ CH₃COOH+ NH₄Cl
ethane nitrile ethanoic acid
c) partial hydrolysis
CH₃-CN+Conc.HCl (Δ + alc. H2O2 ) ⟶ CH₃CONH₂
ethane nitrile ethanamide
Q. Convert methane to ethanoic acid
3. Reaction with alcoholic AgCN
When haloalkane is heated with an alcoholic solution of AgCN then alkyl isocyanide is formed.
R-X+alc.AgCN -Δ⟶ R-NC+AgX
haloalkane alkyl isocyanide
CH₃CH₂Cl+ alc.AgCN -Δ⟶ CH₃CH₂-NC+AgCl
chloroethane ethyl isocyanide
Here, AgCN is a covalent compound. So it does not dissociate easily. Therefore the lone pair of electrons in the nitrogen atom attacks haloalkane to form isocyanides.
Similarly, isocyanide forms different compounds as:
(a)Reduction
CH₃-NC+ 4[H] Ni/Pt/Pd or LiAlH4-> CH₃-NH-CH₃
methyl isocyanide (C2H5OH/Na) N-methylmethanamine
(b)Acidic hydrolysis
CH₃-NC + H₂O+ dil.HCl -Δ⟶ CH₃NH₂+ HCOOH
methyl isocyanide methenamine
4. Formation of amines
When haloalkane is heated with alc. ammonia then amines are formed.
R-X + alc.NH3 -Δ⟶ R-NH2+HX
Alkane alc. Ammonia Amine
CH₃CH₂-Cl + alc.NH3 -Δ⟶ CH₃CH₂-NH2+HCl
5. Reaction with sodium alkoxide(R-ONa) (Williamson’s ether synthesis/ Formation of ether)
When haloalkane is heated with sodium alkoxide then ether is formed. This reaction is called Williamson’s etherification reaction.
R-OH+ Na -Δ⟶R-ONa+ ½H₂↑
alcohol sodium alkoxide
R-X+R-ONa -Δ⟶ R-O-R+NaX
haloalkane ether
CH₃CH₂Cl+ CH₃CH₂-ONa -Δ⟶CH₃CH₂-O-CH₂CH₃+NaCl
chloro ethane ethyoxyethane (diethyl ether)
Both symmetrical and unsymmetrical ether can be prepared by this method.
6. Formation of thioether
Thioether is (R-S-R”) analog ofether.
Name sulfides like ethers, replacing ”sulfide” for “ether” in the common name, or “alkylthio” for “alkoxy” in the IUPAC system.
CH3-Cl -Δ⟶ C2H5-S-CH3
Chloromethane ethyl methyl sulfide
Thioethers (or sulfides) are prepared by the SN2 reaction of primary or secondary alkyle halides with a thiolate anions a(RS-),. The reaction is similar to the Williamson ether synthesis.
Symmetrical thioethers can be prepared by treating an alkyl halide with KOH and an equivalent of hydrogen sulfide. The reaction produces a thiol which is ionized again by KOH and reacts with another molecule of alkyl halide to give thioether.
7. Carbylanime
When primary animes are heated with alcoholic potassium hydroxide (KOH) and chloroform forms a product which is a foul-smelling substance called carbylanime reaction.
R-NH2 + CHCl3 + 3KOH → RNC (Carbylamine) + 3KCl + 3H2O
Amine, Chloroform, alc. potassium hydroxide -Δ⟶Carbylanimes
CH₃CH₂-NH2 + CHCl3 + 3KOH → CH₃CH₂NC + 3KCl + 3H2O
8. Reaction with aqueous NaNO₂ or KNO₂
When haloalkanes are heated with aqueous NaNO₂ or KNO₂ solution then alkyl nitrite is formed.
R-X+aq.NaNO₂ -Δ⟶R-ONO+NaX
haloalkane alkyl nitrite
CH₃CH₂-Cl +aq.NaNO₂ -Δ⟶ CH₃CH₂-ONO+NaCl
chloroethane ethyl nitrite
Here, NaNO₂ is an ionic compound. Hence Na−O bond breaks easily and negatively charged oxygen attacks haloalkane to form alkyl nitrite.[Na-O-N=O]→[Na⁺+ ONO⁻]
9. Reaction with alcoholic silver nitrite (AgNO₂)
When haloalkanes are heated with alcoholic AgNO₂ solution then nitroalkanes are formed.
R-X+alc.AgNO₂ -Δ⟶R-NO₂+AgX
haloalkane nitroalkane
CH₃-Cl+alc.AgNO₂ -Δ⟶CH₃-NO₂+AgCl
chloromethane nitromethane
Here, AgNO₂ is a covalent compound. Hence the lone pair of electrons on the nitrogen atom attacks an alkyl group of haloalkane to form nitroalkane.[Ag-O-N=O]→[Ag−NO₂]
[B] Elimination reaction (ᵦ- elimination reaction) Dehydrohalogenation reaction
When haloalkane is boiled with an alcoholic solution of NaOH or KOH then alkene is formed. In this reaction, one hydrogen and halogen atom are removed from adjacent carbon. So this reaction is called dehydrohalogenation reaction.
R-CH₂CH₂X+alc.NaOH-Δ⟶R-CH=CH₂+HX +H₂O
haloalkane alkene
CH₃CH₂Br+ alc. KOH-Δ⟶CH₂=CH₂+KBr+ H₂O
bromo ethane ETHENE
This reaction is also called β-elimination or 1,2-elimination.
Saytzeff’s rule
If dehydrohalogenation of haloalkane gives two or more alkenes then alkene containing a greater number of the alkyl group on double bonded carbon is the major product. This rule is called Saytzeff ’s rule.
Q. Convert 1-bromopropane to 2-bromopropane.
[C] Reaction with metal
Reaction with sodium (Wurtz’s reaction)
When alkyl halide is heated with sodium metal in presence of dry ether, then alkane having a double number of carbon atoms is formed. Therefore it is used to increase carbon length.
[D] Reduction reaction of Haloalkane
1. Catalytic reduction
CH₃CH₂Cl+H₂ (Ni/Pt/Pd) ⟶CH₃CH₃+HCl
chloroethane ⟶ETHANE
2. Reduction with metal hydride
CH₃CH₂Br +2[H] ( Lialh4 or NaBH4 )⟶ CH₃CH₃+ HBr
bromoethane ⟶ Ethane
3. Reduction with metallic solution
- Sn-Zn-Fe/HCl
- C₂H₅OH/Na
- Red.P₄/HI
CH₃CH₂Cl+ 2[H] —(Sn/ HCl )or(1/2/3) ⟶ CH₃CH₃+HCl
chloroethane ⟶ ethane
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