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Chemistry · Class 11 Science

Ch 14Haloalkanes and Haloarenes — Class 11 Chemistry, concept-first.

The previous unit built up the chemistry of hydrocarbons -- compounds of carbon and hydrogen alone. This unit asks what happens when one or more of those hydrogens is swapped out for a halogen atom instead, and why that single substitution opens up an entire new branch of organic synthesis.

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Nomenclature of Haloalkanes

Haloalkanes carry two naming systems side by side. The common system names the alkyl group first, then adds 'halide' -- methyl iodide, ethyl bromide, tert-butyl chloride, neo-pentyl bromide -- and is still widely used fo…

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Chapter contents

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Introduction

The previous unit built up the chemistry of hydrocarbons -- compounds of carbon and hydrogen alone. This unit asks what happens when one or more of those hydrogens is swapped out for a halogen atom in…

14.1

Introduction

Opens the shift from hydrocarbons to compounds bearing one or more halogen atoms (F, Cl, Br, I) in place of hydrogen on an aliphatic or aromatic skeleton -- haloalkanes (aliphatic) and haloarenes (aro…

14.2

Classification of Organic Halogen Compounds

Organic halogen compounds are classified two ways. By the number of halogens present: mono-halo compounds carry a single halogen (e.g.

14.3

Haloalkanes

Mono-halogen derivatives of alkanes are called haloalkanes, general formula R-X, where R is an alkyl group (CnH2n+1) and X is F, Cl, Br or I.

14.3.1

Nomenclature

Haloalkanes can be named by the common system -- naming the alkyl group followed by 'halide' (e.g. methyl iodide, ethyl bromide, tert-butyl chloride) -- or by the IUPAC system, applying the general no…

14.3.2

Nature of C-X Bond in Haloalkane

The C-X bond is polar because halogens are more electronegative than carbon: carbon carries a partial positive charge (delta+) and the halogen a partial negative charge (delta-).

14.3.3

Methods of Preparation

Haloalkanes are prepared industrially and in the laboratory by five broad routes, each starting from a different, readily available class of organic material: from alcohols (the most versatile route,…

From Alcohols

Alcohols convert to haloalkanes with three reagent classes. (a) Hydrogen halide: ethanol + HCl over anhydrous ZnCl2 gives chloroethane + water; the mixture of concentrated HCl and anhydrous ZnCl2 is c…

From Alkenes and From Alkanes

From alkenes: alkenes react with the halogen acids HCl, HBr or HI to give a haloalkane, the addition following Markovnikov's rule (the halogen ends up on the more substituted carbon).

Halogen Exchange Reactions

Two named reactions convert one haloalkane into another by swapping the halogen. (a) Finkelstein reaction: a chloro- or bromoalkane heated with a concentrated solution of sodium iodide in dry acetone…

From Silver Salts of Fatty Acids (Hunsdiecker Reaction)

Silver salts of fatty acids, refluxed with bromine in CCl4, give the bromoalkane with loss of one carbon as CO2: silver propionate + Br2 (CCl4, reflux) gives bromoethane + CO2 + AgBr.

14.3.4

Physical Properties

(1) Pure haloalkanes are colourless; bromo- and iodo-alkanes develop colour on standing in light. (2) Haloalkanes with one to three carbons are gases at room temperature; beyond three carbons they are…

14.3.5

Chemical Properties

Haloalkanes are among the most reactive classes of organic compound, precisely because of the polar C-X bond -- the electron-poor carbon readily attracts nucleophiles, and the halide ion is a reasonab…

Nucleophilic Substitution Reactions

Because the carbon of C(delta+)-X(delta-) is electron-poor, nucleophiles are drawn to it and displace X-, giving substitution.

SN2 Mechanism (Bimolecular Nucleophilic Substitution)

SN2 rate depends on the concentration of both the alkyl halide and the nucleophile: Rate = k2[alkyl halide][nucleophile]; it is second order overall and proceeds in a single step.

SN1 Mechanism (Unimolecular Nucleophilic Substitution)

SN1 ('S' substitution, 'N' nucleophilic, '1' one species in the rate-determining step) has a rate that depends only on the alkyl halide's concentration and is independent of the nucleophile's concentr…

Elimination Reactions and Saytzeff's Rule

When a haloalkane carrying a hydrogen on its beta-carbon is treated with ethanolic KOH, an alkene forms: the halogen leaves the alpha-carbon and a hydrogen leaves the adjacent beta-carbon, and a new d…

E2 Reaction Mechanism

E2 (bimolecular elimination) rate depends on the concentration of both the alkyl halide and the base: Rate = k[alkyl halide][base]; second order overall.

E1 Reaction Mechanism

E1 (unimolecular elimination) generally occurs for tertiary alkyl halides with alcoholic KOH and follows first-order kinetics.

Reaction with Metals

(a) Grignard reaction: a solution of haloalkane in dry ether treated with magnesium gives an alkyl magnesium halide, the Grignard reagent, e.g.

Reduction Reactions

Haloalkanes are reduced to the parent alkane by either of two routes. Catalytic hydrogenation: haloalkane + H2 over a nickel or palladium catalyst (523 K) gives the alkane + HX, e.g.

14.3.6

Uses of Haloalkane

Chloroform is used as a solvent in the pharmaceutical industry and for manufacturing pesticides and drugs, and formerly as an anaesthetic and as a preservative for anatomical specimens.

14.4

Organo Metallic Compounds

Organometallic compounds are organic compounds containing a direct carbon-metal bond, e.g. methyl magnesium iodide (CH3MgI) and ethyl magnesium bromide (CH3CH2MgBr).

14.4.1

Preparation

A solution of an alkyl halide in ether, left standing over pieces of magnesium metal, gradually dissolves the metal and forms the alkyl magnesium halide (Grignard reagent) directly, with no separate c…

14.4.2

Uses of Grignard Reagent

Grignard reagents are exceptionally versatile: the alkyl group, being electron-rich, behaves as a carbanion/nucleophile and attacks the electron-poor centre of a polarised molecule, converting the Gri…

Preparation of Alcohols, Aldehydes and Ketones

(1) Primary alcohol: formaldehyde (HCHO) + RMgX gives an addition product that, on hydrolysis (H2O/H+), gives a primary alcohol + Mg(OH)X, e.g. + CH3MgI gives ethanol.

Preparation of Acids, Esters, Ethers, Nitriles and Alkanes

(6) Carboxylic acid: solid CO2 (dry ice) + RMgX gives an addition product that, on hydrolysis (H+/H2O), gives a carboxylic acid, e.g. + CH3MgI gives acetic acid.

14.5

Haloarenes

Haloarenes are the compounds in which a halogen is attached directly to a benzene ring -- X bonded straight to a ring carbon itself, not to a side chain hanging off the ring (which would instead be a…

14.5.1

Nomenclature of Haloarenes

IUPAC names a haloarene by prefixing 'halo' (fluoro/chloro/bromo/iodo) to the aromatic hydrocarbon's name, exactly as for a substituted benzene -- e.g. C6H5Cl is chlorobenzene.

14.5.2

Nature of C-X Bond in Haloarenes

In a haloarene, the ring carbon bonded to the halogen is sp2 hybridised; sp2 orbitals are shorter and hold the bonding electron pair more tightly than sp3 orbitals do.

14.5.3

Methods of Preparation

(1) Direct halogenation: benzene + Cl2 over a Lewis-acid catalyst (FeCl3) gives chlorobenzene + HCl. (2) From benzene diazonium chloride: (i) Sandmeyer reaction -- aqueous benzenediazonium chloride wa…

14.5.4

Physical Properties

1. Melting and boiling points: among the monohalobenzenes (all liquids at room temperature), boiling point follows iodo bromo chloro.

14.5.5

Chemical Properties

Haloarene chemistry splits cleanly into two groups. (A) Reactions involving the halogen atom itself -- aromatic nucleophilic substitution (only under forcing conditions) and reaction with metals (Wurt…

Aromatic Nucleophilic Substitution Reaction

Haloarenes do not undergo nucleophilic substitution readily, because the C-X bond is short and strong (section 14.5.2) and because the aromatic ring is itself a centre of high electron density that re…

Reaction with Metals

(a) Wurtz-Fittig reaction: a haloarene and a haloalkane heated together with sodium in dry ether give an alkylbenzene, e.g. chlorobenzene + chloroethane + 2 Na (ether) gives ethylbenzene + 2 NaCl.

Electrophilic Substitution Reaction

Haloarenes DO undergo the ring's characteristic aromatic electrophilic substitution, but more slowly than benzene itself, because the halogen's -I (inductive, electron-withdrawing) effect nets out dea…

Reduction and Formation of Grignard Reagent

Reduction: a haloarene reduced with Ni-Al alloy in the presence of NaOH gives the parent arene, e.g. chlorobenzene + 2[H] gives benzene + HCl.

14.5.6

Uses of Chlorobenzene

Chlorobenzene's own reactivity (section 14.5.5) makes it a valuable synthetic building block in its own right, beyond simply being an example haloarene: (i) it is used in the manufacture of pesticides…

14.6

Poly Halogen Compounds

Carbon compounds carrying more than one halogen atom are poly halogen compounds. Two important dihalide classes are distinguished by WHERE the two halogens sit: (a) gem-dihalides carry both halogens o…

14.6.1

Preparation of gem- and vic-Dihalides

gem-Dihalides: ethylidene dichloride is made either (i) by treating acetaldehyde with PCl5 (CH3CHO + PCl5 gives CH3CHCl2 + POCl3), or (ii) by adding HCl to acetylene via the intermediate vinyl chlorid…

Properties of gem- and vic-Dihalides

Physical: both classes are sweet-smelling, colourless liquids with relatively high boiling points; ethylidene chloride boils lower than ethylene dichloride.

Methylene Chloride (Dichloromethane)

Preparation: (1) by reduction of chloroform, either with Zn + HCl (CHCl3 + 2[H] gives CH2Cl2 + HCl) or with H2/Ni; (2) by chlorination of methane (CH4 + Cl2/hv gives CH3Cl, which + Cl2/hv gives CH2Cl2…

14.6.2

Trihaloalkane -- Chloroform

Trihaloalkanes replace three hydrogens of a hydrocarbon with three halogens, e.g. CHCl3 (chloroform) and CHI3 (iodoform). Chloroform was named by Dumas because it yields formic acid on hydrolysis.

14.6.3

Tetrahaloalkane -- Carbon Tetrachloride

Carbon tetrachloride is the standard example of a tetrahaloalkane. Preparation: (1) chlorination of methane -- methane + excess Cl2 (sunlight) gives CCl4 + 4 HCl as the major product; (2) carbon disul…

14.6.4

Freons (CFC)

Chlorofluoro derivatives of methane and ethane are called Freons (chlorofluorocarbons, CFCs), named Freon-cba where c = (number of carbon atoms - 1), b = (number of hydrogen atoms + 1), a = total numb…

14.6.5

DDT

DDT (p,p'-dichlorodiphenyltrichloroethane) was first prepared in 1873; Paul Muller discovered its effectiveness as an insecticide in 1939 and was awarded the 1948 Nobel Prize in Medicine and Physiolog…

Flow Charts -- Haloalkane Chemistry and Grignard Synthesis

Summary

Evaluation

54 Q
+Choose the Best Answer25 questions
  1. Q1The IUPAC name of the compound drawn as CH₃-CH=CH-CH(Br)-CH₃ (a trans-drawn pent-2-ene skeleton, with H₃C and H on the left alkene carbon an…Free
  2. Q2Of the following compounds, which has the highest boiling point? a) n-Butyl chloride b) Isobutyl chloride c) t-Butyl chloride d) n-propyl ch…Free
  3. Q3Arrange the following compounds in increasing order of their density: A) CCl₄ B) CHCl₃ C) CH₂Cl₂ D) CH₃Cl a) D<C<B<A b) C>B>A>D c) A<B<C<D d…Free
  4. Q4With respect to the position of -Cl in the compound CH₃-CH=CH-CH₂-Cl, it is classified as a) Vinyl b) Allyl c) Secondary d) AralkylPreview
  5. Q5What should be the correct IUPAC name of diethyl chloromethane? a) 3-Chloropentane b) 1-Chloropentane c) 1-Chloro-1,1-diethylmethane d) 1-Ch…Preview
  6. Q6C-X bond is strongest in a) Chloromethane b) Iodomethane c) Bromomethane d) FluoromethanePreview
  7. Q7In the reaction (benzenediazonium chloride) + Cu/HCl → X + N₂, X is ________ a) benzene (plain ring) b) chlorobenzene (ring with one Cl) c)…Preview
  8. Q8Which of the following compounds will give a racemic mixture on nucleophilic substitution by OH⁻ ion? i) CH₃-CH(C₂H₅)-CH₂Br ii) (CH₃)₂C(Br)(…Preview
  9. Q9The treatment of ethyl formate with excess of RMgX gives a) R-CO-R b) R-CH(R)-OH c) R-CHO d) R-O-RPreview
  10. Q10Benzene reacts with Cl₂ in the presence of FeCl₃ and in absence of sunlight to form a) Chlorobenzene b) Benzyl chloride c) Benzal chloride d…Preview
  11. Q11The name of C₂F₄Cl₂ is ___________ a) Freon-112 b) Freon-113 c) Freon-114 d) Freon-115Preview
  12. Q12Which of the following reagents is helpful to differentiate ethylene dichloride and ethylidene chloride? a) Zn/methanol b) KOH/ethanol c) aq…Preview
  13. Q13Match the compounds given in Column I with suitable items given in Column II. Column I: A) Iodoform B) Carbon tetrachloride C) CFC D) DDT. C…Preview
  14. Q14Assertion: In mono haloarenes, electrophilic substitution occurs at ortho and para positions. Reason: Halogen atom is a ring deactivator. (i…Preview
  15. Q15Consider the reaction, CH₃CH₂CH₂Br + NaCN → CH₃CH₂CH₂CN + NaBr. This reaction will be the fastest in a) ethanol b) methanol c) DMF (N,N'-dim…Preview
  16. Q16Freon-12 is manufactured from tetrachloromethane by a) Wurtz reaction b) Swarts reaction c) Haloform reaction d) Gattermann reactionPreview
  17. Q17The most easily hydrolysed molecule under SN1 condition is a) allyl chloride b) ethyl chloride c) isopropyl chloride d) benzyl chloridePreview
  18. Q18The carbocation formed in the SN1 reaction of an alkyl halide in the slow step is a) sp³ hybridised b) sp² hybridised c) sp hybridised d) no…Preview
  19. Q19The major products obtained when chlorobenzene is nitrated with HNO₃ and conc. H₂SO₄ are a) 1-chloro-4-nitrobenzene b) 1-chloro-2-nitrobenze…Preview
  20. Q20Which one of the following is most reactive towards nucleophilic substitution reaction? a) an acid chloride, C₆H₅-CO-Cl (benzoyl chloride) b…Preview
  21. Q21Ethylidene chloride on treatment with aqueous KOH gives a) acetaldehyde b) ethylene glycol c) formaldehyde d) glyoxalPreview
  22. Q22The raw material for the Raschig process is a) chlorobenzene b) phenol c) benzene d) anisolePreview
  23. Q23Chloroform reacts with nitric acid to produce a) nitrotoluene b) nitroglycerine c) chloropicrin d) chloropicric acidPreview
  24. Q24acetone --i) CH₃MgI, ii) H₂O/H⁺--> X, X is a) 2-propanol b) 2-methyl-2-propanol c) 1-propanol d) acetonolPreview
  25. Q25Silver propionate when refluxed with bromine in carbon tetrachloride gives a) propionic acid b) chloroethane c) bromoethane d) chloropropanePreview
+Write Brief Answer29 questions
  1. Q1Classify the following compounds in the form of alkyl, allylic, vinyl or benzylic halides: i) CH₃-CH=CH-Cl ii) C₆H₅CH₂I iii) CH₃-CH(Br)-CH₃…Free
  2. Q2Why is chlorination of methane not possible in the dark?Free
  3. Q3How will you prepare n-propyl iodide from n-propyl bromide?Free
  4. Q4Which alkyl halide from the following pair is i) chiral ii) undergoes faster SN2 reaction? Left structure: 2-bromobutane, CH₃-CH(Br)-CH₂-CH₃…Preview
  5. Q5How does chlorobenzene react with sodium in the presence of ether? What is the name of the reaction?Preview
  6. Q6Give reasons for the polarity of the C-X bond in a haloalkane.Preview
  7. Q7Why is it necessary to avoid even traces of moisture during the use of a Grignard reagent?Preview
  8. Q8What happens when acetyl chloride is treated with excess of CH₃MgI?Preview
  9. Q9Arrange the following alkyl halides in increasing order of bond enthalpy of R-X: CH₃Br, CH₃F, CH₃Cl, CH₃IPreview
  10. Q10What happens when chloroform reacts with oxygen in the presence of sunlight?Preview
  11. Q11Write down the possible isomers of C₅H₁₁Br and give their IUPAC and common names.Preview
  12. Q12Mention any three methods of preparation of haloalkanes from alcohols.Preview
  13. Q13Compare SN1 and SN2 reaction mechanisms.Preview
  14. Q14Reagents and the conditions used in the reactions are given below. Complete the table by writing down the product and the name of the reacti…Preview
  15. Q15Discuss the aromatic nucleophilic substitution reaction of chlorobenzene.Preview
  16. Q16Account for the following: (i) t-butyl chloride reacts with aqueous KOH by SN1 mechanism while n-butyl chloride reacts by SN2 mechanism. (ii…Preview
  17. Q17In an experiment, ethyl iodide in ether is allowed to stand over magnesium pieces. Magnesium dissolves and a product is formed. a) Name the…Preview
  18. Q18Write a chemical reaction useful to prepare the following: i) Freon-12 from carbon tetrachloride ii) Carbon tetrachloride from carbon disulp…Preview
  19. Q19What are Freons? Discuss their uses and environmental effects.Preview
  20. Q20Predict the products when bromoethane is treated with the following: i) KNO₂ ii) AgNO₂Preview
  21. Q21Explain the mechanism of the SN1 reaction, highlighting the stereochemistry behind it.Preview
  22. Q22Write short notes on the following: i) Raschig process ii) Dow's process iii) Darzen's processPreview
  23. Q23Starting from CH₃MgI, how will you prepare the following? i) Acetic acid ii) Acetone iii) Ethyl acetate iv) Isopropyl alcohol v) Methyl cyan…Preview
  24. Q24Complete the following reactions: i) CH₃-CH=CH₂ + HBr --Peroxide--> ? ii) CH₃-CH₂-Br + NaSH --alcohol/H₂O--> ? iii) C₆H₅Cl + Mg --THF--> ? i…Preview
  25. Q25Explain the preparation of the following compounds: i) DDT ii) Chloroform iii) Biphenyl iv) Chloropicrin v) Freon-12Preview
  26. Q26An organic compound (A) with molecular formula C₂H₅Cl reacts with KOH to give compound (B), and with alcoholic KOH gives compound (C). Ident…Preview
  27. Q27The simplest alkene (A) reacts with HCl to form compound (B). Compound (B) reacts with ammonia to form compound (C) of molecular formula C₂H…Preview
  28. Q28A hydrocarbon C₃H₆ (A) reacts with HBr to form compound (B). Compound (B) reacts with aqueous potassium hydroxide to give (C) of molecular f…Preview
  29. Q29Two isomers (A) and (B) have the same molecular formula C₂H₄Cl₂. Compound (A) reacts with aqueous KOH to give compound (C) of molecular form…Preview

Sample & Board Papers

Sample papers and previous-year board questions for this subject.

+Show 18 questions18 questions
  1. Q1When alkyl halides are treated with alcoholic KOH, the products are ________. (a) alcohols (b) olefins (c) aldehydes (d) alkanesPreview
  2. Q2The SN1 reaction of alkyl halides is not affected by the nature of the ________. (a) medium (b) alkyl group (c) nucleophile (d) the halogenPreview
  3. Q3Match the following: Compound: (1) Chloro picrin (2) Methyl Isocyanide (3) Chloro benzene (4) Methylene chloride Uses: (i) Detection of prim…Preview
  4. Q4Write note on Williamson's Synthesis.Preview
  5. Q5n-propyl bromide on reaction with alcoholic KOH gives: (a) Butyl alcohol (b) Propene (c) Butene (d) Propyl alcoholPreview
  6. Q6Match the following: (1) Iodoform - (i) Fire extinguisher; (2) Carbon tetrachloride - (ii) Insecticide; (3) CFC - (iii) Antiseptic; (4) DDT…Preview
  7. Q7Give the structure and uses of DDT.Preview
  8. Q8Assertion: In monohaloarenes, electrophilic substitution occurs at ortho and para positions. Reason: Halogen atom is a ring deactivator. (a)…Preview
  9. Q9How will you convert ethyl chloride to ethane?Preview
  10. Q10Complete the following reactions: (i) C6H5Cl + 2NH3 --[250 degree C, 50 atm]--> ? (ii) C6H5Cl + 2Na + Cl-C6H5 --[Ether, heat]--> ?Preview
  11. Q11Chloroform reacts with Nitric acid to produce: (a) Chloropicrin (b) Nitro toluene (c) Chloropicric acid (d) Nitro glycerinePreview
  12. Q12An organic compound (A) with molecular formula C2H5Cl reacts with aqueous KOH and gives compound (B) and with alcoholic KOH gives compound (…Preview
  13. Q13The name of C2F2Cl4 is: (a) Freon - 111 (b) Freon - 113 (c) Freon - 112 (d) Freon - 11Preview
  14. Q14Ethylidene chloride on treatment with aqueous KOH gives : (a) formaldehyde (b) acetaldehyde (c) glyoxal (d) ethylene glycolPreview
  15. Q15What are the uses of Chlorobenzene ?Preview
  16. Q16C - X bond is strongest in : (a) Chloromethane (b) Iodomethane (c) Bromo methane (d) FluoromethanePreview
  17. Q17What happens when ethylene is passed through cold dilute alkaline Potassium Permanganate ?Preview
  18. Q18(a) Explain the structure of benzene. (5) **OR** (b) (i) Write short notes on Friedel-Crafts reaction. (2) (ii) What are the various methods…Preview