CBSE Class 12 Chemistry Haloalkanes Haloarenes Summary

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HALOALKANES

INTORDUCTION:

HALOALKANES AND HALOARENES (SUMMARY NOTES)

HALOALKANES AND HALOARENES

INTRODUCTION

The replacement of hydrogen atom(s) in a hydrocarbon, aliphatic or aromatic, by halogen atom(s) results in the formation of alkyl halides (haloalkanes) and aryl halides (haloarenes), respectively.

Haloalkanes contain halogen atom(s) attached to the sp³ hybridised carbon atom of an alkyl group, whereas haloarenes contain halogen atom(s) attached to sp² hybridised carbon atom(s) of an aryl group.

Many halogen-containing organic compounds are clinically useful and find wide applications in industry as well as in day-to-day life. Examples:

1. The chlorine-containing antibiotic chloramphenicol, produced by soil microorganisms, is effective for the treatment of typhoid fever.
2. Our body produces the iodine-containing hormone thyroxine, the deficiency of which causes a disease called goitre.
3. Chloroquine is used for the treatment of malaria.
4. Halothane is used as an anaesthetic during surgery.
5. Certain fully fluorinated compounds are being considered as potential blood substitutes in surgery.
6. Haloalkanes are used as solvents for relatively non-polar compounds and as starting materials for the synthesis of a wide range of organic compounds.

CLASSIFICATION

1. On the basis of the number of halogen atoms — mono, di or polyhalogen (tri-, tetra-, etc.) compounds, depending on whether they contain 1, 2 or more halogen atoms in their structures. Examples: monohaloalkane C₂H₅X; dihaloalkane CH₂X-CH₂X; trihaloalkane CH₂X-CHX-CH₂X; and the corresponding mono-, di- and trihaloarenes (one, two or three halogen atoms on a benzene ring).

2. Classification of monohalo compounds according to the hybridisation of the C atom to which the halogen is bonded:

NOMENCLATURE

1. The common name of an alkyl halide = the name of the alkyl group followed by the halide (i.e. alkyl halide).
2. The IUPAC name of an alkyl halide = halo-substituted hydrocarbon (i.e. haloalkane).
3. Haloarenes are the common as well as IUPAC names of aryl halides. For dihalogen derivatives, the prefixes o-, m-, p- are used in the common system, but in the IUPAC system the numerals 1,2; 1,3 and 1,4 are used.
4. Dihalogen derivatives having the same type of halogen atoms on the same carbon are known as geminal (gem) dihalides and are given the common name alkylidene halides or alkylidene dihalides. Dihalogen derivatives having the same type of halogen atoms on adjacent carbon atoms are known as vicinal (vic) dihalides and are given the common name alkylene halides or alkylene dihalides.

ISOMERISM IN HALOALKANES

NATURE OF THE C–X BOND

• Halogen atoms are more electronegative than carbon, so the C–X bond of an alkyl halide is polarised: the C atom bears a +δ charge whereas the halogen atom bears a −δ charge.
• Since the size of the halogen atom increases as we go down the group in the periodic table, the C–X bond length increases from C–F to C–I and the bond energy decreases from C–F to C–I.

REMEMBER

1. Polarity affects physical properties like boiling point, physical state, etc. More polar compounds have higher boiling points. That is why the boiling points of haloalkanes are higher than those of hydrocarbons of comparable molecular mass.
2. Since the size of the halogen atom increases as we go down the group, for the same alkyl group the boiling points of alkyl halides decrease in the order RI > RBr > RCl > RF.
3. The C atom bears a +δ charge whereas the halogen atom bears a −δ charge. Therefore haloalkanes undergo nucleophilic substitution reactions, elimination reactions, reactions with metals and reduction.

METHODS OF PREPARATION

1. From alcohols

The –OH group of an alcohol (ROH) can be replaced by –X on reaction with concentrated HX, PX₃, PX₅ or SOCl₂ (thionyl chloride).

(a) By reaction with halogen acids (HX)

1. Chloroalkanes are obtained by treating alcohols with HCl in the presence of anhydrous ZnCl₂. The mixture of HCl and anhydrous ZnCl₂ is known as Lucas reagent. Anhydrous ZnCl₂ acts as a dehydrating agent and thus prevents the reverse reaction. 1° and 2° alcohols are converted into their chlorides by HCl and anhydrous ZnCl₂ (Groves process); tertiary alcohols react readily with concentrated HCl even in the absence of ZnCl₂.
2. Bromoalkanes are obtained by heating KBr or NaBr with concentrated H₂SO₄ (HBr is generated in situ).
3. Iodoalkanes are obtained by heating the alcohol with KI and 95% phosphoric acid (H₃PO₄).

R-OH + HCl → R-X + H₂O (anhydrous ZnCl₂)

CH₃CH₂OH + HCl → CH₃CH₂Cl + H₂O (ZnCl₂; ethanol to ethyl chloride)

(CH₃)₂CHOH + HCl → (CH₃)₂CHCl + H₂O (ZnCl₂; isopropanol to isopropyl chloride)

(CH₃)₃COH + HCl → (CH₃)₃CCl + H₂O (tert-butyl alcohol to tert-butyl chloride)

KBr (or NaBr) + H₂SO₄ → KHSO₄ (or NaHSO₄) + HBr

CH₃CH₂OH + HBr → CH₃CH₂Br + H₂O (ethanol to ethyl bromide)

KI + H₃PO₄ → KH₂PO₄ + HI

CH₃CH₂OH + HI → CH₃CH₂I + H₂O (ethanol to ethyl iodide)

Understand these points:

• These are nucleophilic substitutions of –OH by –X.
• The order of reactivity of alcohols with a given halo acid is 3° > 2° > 1°. Reason: the greater the number of electron-releasing groups on the α-carbon atom of the alcohol, the more polar is the C–OH bond, and the greater the ease with which it cleaves.
• The reactivity of the halogen acids towards this reaction is HI > HBr > HCl. Reason: the bond dissociation energy of H–I is less than that of H–Br, which in turn is less than that of H–Cl.
• This method is not suitable for the preparation of aryl halides (because of the partial double-bond character of the C–O bond).

(b) By reaction with phosphorus halides

1. Chloroalkanes are obtained by the reaction of PCl₃ or PCl₅ with alcohols.
2. Bromo and iodoalkanes are obtained by the reaction of alcohols with a mixture of red phosphorus and Br₂ or I₂. The phosphorus first reacts with Br₂ or I₂ to give the phosphorus(III) halide, which then reacts with the alcohol to give the corresponding haloalkane, which can be distilled off.

3CH₃CH₂CH₂OH + PCl₃ → 3CH₃CH₂CH₂Cl + H₃PO₃ (propan-1-ol to 1-chloropropane)

CH₃CH₂CH₂CH₂OH + PCl₅ → CH₃CH₂CH₂CH₂Cl + POCl₃ + HCl (butan-1-ol to 1-chlorobutane)

2P + 3Br₂ → 2PBr₃

2P + 3I₂ → 2PI₃

3CH₃CH₂OH + PBr₃ → 3CH₃CH₂Br + H₃PO₃

3CH₃CH₂OH + PI₃ → 3CH₃CH₂I + H₃PO₃

(c) By reaction with thionyl chloride (SOCl₂): alcohols are converted into the corresponding chlorides by refluxing with thionyl chloride in the presence of a small amount of pyridine. The reaction of a straight-chain primary alcohol with thionyl chloride in the presence or absence of pyridine is called Darzens’ procedure.

CH₃CH₂CH₂OH + SOCl₂ → CH₃CH₂CH₂Cl + SO₂ + HCl (propan-1-ol to 1-chloropropane)

Note: the big advantage this reaction has over the use of the phosphorus chlorides is that the two other products of the reaction (sulphur dioxide and HCl) are both gases. They separate themselves from the reaction mixture, so the product formed is highly pure.

2. From hydrocarbons

Alkyl halides can be prepared from alkanes through substitution and from alkenes through addition of halogen acids (HX) or through allylic substitution reactions.

(a) From alkanes: alkanes react with halogens, chlorine or bromine, in the presence of light or heat, by free radical substitution and produce a mixture of mono-, di- and polyhaloalkanes. (The extent of halogenation depends upon the amount of halogen used.)

CH₄ + X₂ → CH₃X → CH₂X₂ → CHX₃ → CX₄ (X₂, light or heat, successively)

Note the following:

• These reactions follow a free radical mechanism.
• Replacement of an H atom of an alkane by a halogen atom is known as halogenation.
• The reactivity order for halogens is F₂ > Cl₂ > Br₂ > I₂.
• F₂ reacts violently even in the dark, and the reaction may be controlled by diluting fluorine with N₂, whereas iodination is very slow and reversible. Therefore iodination is carried out in the presence of HgO or HIO₃ (oxidants which decompose HI):

[CH₄ + I₂ ⇌ CH₃I + HI] x 5

5HI + HIO₃ → 3I₂ + 3H₂O

4CH₄ + 2I₂ + HIO₃ → 5CH₃I + 3H₂O

• The ease of substitution of the different types of hydrogen atoms is: benzylic, allylic > tertiary > secondary > primary > vinylic, aryl.

CH₃CH₂CH₃ + Cl₂ → CH₃CH₂CH₂Cl (45%, primary hydrogen substituted) + CH₃CHClCH₃ (55%, secondary hydrogen substituted) (hv)

CH₂=CH-CH₃ + Cl₂ → CH₂=CH-CH₂Cl + HCl (400 °C; propene to allyl chloride)

(b) From alkenes: haloalkanes can be prepared from alkenes by the addition of halogen acids (HCl, HBr, HI). The order of reactivity of the halogen acids is HI > HBr > HCl.

CH₂=CH₂ + HI → CH₃CH₂I (ethene to iodoethane)

Markovnikov’s rule: “when an unsymmetrical alkene reacts with a hydrogen halide to give an alkyl halide, the hydrogen adds to the carbon of the alkene that has the greater number of hydrogen substituents, and the halogen to the carbon of the alkene with the fewer number of hydrogen substituents.”

CH₃CH=CH₂ + HI → CH₃CHICH₃ (major) + CH₃CH₂CH₂I (minor)

Reason: the reaction occurs via protonation to give the more stable carbocation. Propene after protonation gives two different carbocations, one 2° and the other 1°. Formation of the more stable 2° carbocation is preferred. The carbocation then reacts with the nucleophile to give the alkyl halide, and hence 2-bromopropane (with HBr) is the major product.

Peroxide effect or Kharasch effect is a special case of a specific halogen acid, namely HBr, adding to an unsymmetrical alkene in such a way that the negative part of the addendum attaches itself to that carbon of the multiple bond which has the least number of hydrogen atoms (anti-Markovnikov addition).

CH₃CH=CH₂ + HBr → CH₃CH₂CH₂Br (major, 1-bromopropane) + CH₃CHBrCH₃ (minor, 2-bromopropane) (benzoyl peroxide)

Mechanism of the peroxide effect (free radical chain):

1. The peroxide splits into radicals, which abstract H from HBr: RO^. + H-Br → RO-H + Br^.
2. The bromine radical adds to the alkene to give the more stable (secondary) radical: CH₃-CH=CH₂ + Br^. → CH₃-CH^.-CH₂Br
3. The radical abstracts H from another HBr molecule: CH₃-CH^.-CH₂Br + H-Br → CH₃-CH₂-CH₂Br + Br^. (the bromine radical is regenerated and the chain continues).

Important: HCl and HI cannot exhibit this effect, because the bond dissociation enthalpy of HCl is too high for its bond to break under the normal conditions, and although HI breaks, the iodine free radicals are unstable and combine to form iodine molecules instead of adding to the alkene.

3. By halide exchange

1. Finkelstein reaction: alkyl iodides can be prepared by the reaction of alkyl chlorides/bromides with NaI in dry acetone.
2. Swarts reaction: alkyl fluorides can be synthesised by heating an alkyl chloride/bromide in the presence of a metallic fluoride such as AgF, Hg₂F₂, CoF₂ or SbF₃.

R-X + NaI → R-I + NaX (X = Cl, Br; dry acetone)

H₃C-Br + AgF → H₃C-F + AgBr

4. From silver salts of fatty acids

Borodin–Hunsdiecker reaction: a bromoalkane can be prepared by refluxing the silver salt of a fatty acid with bromine in CCl₄. (Note: iodoalkanes cannot be prepared by this reaction.)

R-COOAg + Br₂ → R-Br + AgBr + CO₂ (CCl₄)

PREPARATION OF HALOARENES

1. From arenes (by electrophilic substitution)

ArCl and ArBr can be prepared by electrophilic substitution of arenes with Cl₂ and Br₂ respectively in the presence of Lewis acid catalysts like Fe or FeCl₃. For example, toluene with X₂ and Fe in the dark gives o-halotoluene and p-halotoluene.

Note: reactions with I₂ are reversible and require the presence of an oxidising agent (HNO₃, HIO₄) to oxidise the HI formed during iodination. Fluoro compounds are not prepared by this method because of the high reactivity of F₂.

2. From diazonium compounds

Primary aromatic amines react with nitrous acid (NaNO₂ + HCl) at 0–4 °C to produce benzenediazonium chloride; this reaction is known as diazotisation.

C₆H₅NH₂ + NaNO₂ + 2HX → C₆H₅N₂⁺X⁻ + NaX + 2H₂O (273–278 K)

1. Sandmeyer’s reaction: mixing the solution of the diazonium salt with cuprous chloride (Cu₂Cl₂) or cuprous bromide (Cu₂Br₂) results in the replacement of the diazonium group by –Cl or –Br.
2. Gattermann’s reaction: a modification of the Sandmeyer reaction. Mixing the solution of the diazonium salt with HCl/HBr in the presence of copper powder results in the replacement of the diazonium group by –Cl or –Br.
3. Balz–Schiemann reaction: fluorobenzene can be prepared by treating benzenediazonium chloride with fluoroboric acid. The reaction produces diazonium fluoroborate, which on heating produces fluorobenzene.
4. Preparation of iodobenzene by warming the benzenediazonium salt with KI.

C₆H₅N₂⁺X⁻ → C₆H₅X + N₂ (Cu₂X₂, Sandmeyer; Cu/HX, Gattermann; X = Cl, Br)

C₆H₅N₂⁺Cl⁻ + HBF₄ → C₆H₅N₂⁺BF₄⁻ + HCl

C₆H₅N₂⁺BF₄⁻ → C₆H₅F + BF₃ + N₂ (heat)

C₆H₅N₂⁺Cl⁻ + KI → C₆H₅I + KCl + N₂

3. From benzene (commercial methods)

1. Raschig process: chlorobenzene is prepared by passing a mixture of benzene, hydrogen chloride and oxygen over CuCl₂ as catalyst at about 525 °C.
2. Friedel–Crafts halogenation: benzene reacts with a halogen in the presence of a Lewis catalyst like FeCl₃/AlCl₃ to give a halobenzene.

C₆H₆ + HCl + 1/2 O₂ → C₆H₅Cl + H₂O (CuCl₂, about 525 °C)

C₆H₆ + X₂ → C₆H₅X + HX (Lewis acid)

Note: aryl fluorides cannot be prepared by this method because of the high affinity of fluorine for hydrogen.

Example: C₆H₆ + Cl₂ → C₆H₅Cl + HCl (anhydrous AlCl₃; benzene to chlorobenzene)

PHYSICAL PROPERTIES

1. Colour: RCl is colourless when pure. RBr and RI develop colour when exposed to light (the colour develops due to atmospheric oxidation).
2. Physical state: CH₃Cl, CH₃Br, C₂H₅Cl and some chlorofluoromethanes are gases at room temperature. Higher members are liquids or solids.
3. Boiling points:

1. The boiling points of haloalkanes are higher than those of hydrocarbons of comparable molecular mass. Reason: due to the greater polarity and higher molecular mass as compared to the parent hydrocarbon, the intermolecular dipole–dipole and van der Waals forces of attraction are stronger in the halogen derivatives.
2. For the same alkyl group, the boiling points of alkyl halides decrease in the order RI > RBr > RCl > RF. Example: b.p. CH₄ < CH₃F < CH₃Cl < CH₃Br < CH₃I (with the increase in the size of the halogen atom, the magnitude of the van der Waals forces increases).
3. For the same halogen atom, the boiling points increase with the increase in the size of the alkyl group. Example: b.p. CH₄ < CH₃CH₂Cl < CH₃(CH₂)₂Cl < CH₃(CH₂)₃Cl.
4. The boiling points of chloro, bromo and iodo compounds increase with the increase in the number of halogen atoms. Example: CH₃Cl < CH₂Cl₂ < CHCl₃ < CCl₄.
5. The boiling points of isomeric haloalkanes decrease with increase in branching. Reason: with increase in branching the surface area decreases, causing a decrease in the van der Waals forces of attraction.
6. The boiling points of isomeric dihalobenzenes are very nearly the same; however, the p-isomers are high melting as compared to their o- and m-isomers, because the symmetry of the para isomer lets it fit in the crystal lattice better.

Boiling points (K) of the isomeric butyl bromides: CH₃CH₂CH₂CH₂Br 375; CH₃CH₂CH(Br)CH₃ 364; (CH₃)₃CBr 346. Dichlorobenzenes:

NUCLEOPHILIC SUBSTITUTION

Nucleophile: a negatively charged or neutral but electron-rich species which attacks an electron-deficient or positively charged site of the substrate. Neutral electron-rich nucleophiles: H₂O, ROH, ROR, NH₃, RNH₂, RSH. Negatively charged nucleophiles: X⁻ (halide), CN⁻, OH⁻, RCOO⁻, NO₂⁻.

Nucleophilic substitution reaction: a substitution reaction which starts with the initial attack of a nucleophile. The stronger nucleophile displaces a weaker nucleophile; the atom or group which departs with its bond pair of electrons is called the leaving group.

Nucleophilic substitution of alkyl halides (R–X): a nucleophile reacts with the haloalkane (the substrate), which has a partial positive charge on the carbon atom bonded to the halogen. The halogen atom, called the leaving group, departs as the halide ion.

R-X + Nu⁻ → R-Nu + X⁻

Ambident nucleophile: a nucleophile which has more than one site through which reaction can occur. Examples: cyanide (⁻C≡N ↔ :C=N⁻) and nitrite (O=N-O⁻) have two nucleophilic centres. Linkage of CN⁻ through the C atom gives alkyl cyanides, and through the N atom gives isocyanides. Linkage of the nitrite ion through the O atom gives alkyl nitrites, while through the N atom it gives nitroalkanes.

MECHANISM OF SUBSTITUTION REACTIONS OF ALKYL HALIDES (R–X)

(CH₃)₃C-Cl → (CH₃)₃C⁺ + Cl⁻ (step 1, slow)

(CH₃)₃C⁺ + OH⁻ → (CH₃)₃C-OH (step 2, fast)

Overall: (CH₃)₃CBr + OH⁻ → (CH₃)₃COH + Br⁻

HO⁻ + CH₃-Cl → [HO...CH₃...Cl]⁻ → HO-CH₃ + Cl⁻ (SN₂: single step through a transition state)

Relative rates of S_N2 reaction for CH₃Br, CH₃CH₂Br (1°), (CH₃)₂CHBr (2°) and (CH₃)₃CBr (3°) are 30, 1, 0.02 and 0 respectively.

FACTORS AFFECTING S_N1 AND S_N2 MECHANISMS

1. Nature of the alkyl halide: a 1° alkyl halide reacts through S_N2 and a 3° alkyl halide reacts through S_N1. 2° alkyl, 1° allylic and 1° benzylic halides may react either by S_N2 or S_N1 or by both, depending upon reaction conditions. Order for S_N2: CH₃X > 1° > 2° > 3°; for S_N1: 3° > 2° > 1° > CH₃X. Allylic and benzylic halides show high reactivity towards the S_N1 reaction, because the carbocation formed is stabilised by resonance (the positive charge is delocalised over the allyl system, or over the ortho and para positions of the benzene ring).
2. Nature of the nucleophile: a strong nucleophile favours S_N2 and a weak nucleophile favours S_N1.
3. Concentration of the nucleophile: a high concentration of nucleophile favours S_N2 and a low concentration favours S_N1.
4. Nature of the solvent: polar solvents favour S_N2 and solvents of low polarity favour S_N1 mechanism.
5. For a given alkyl group, the reactivity of the halide R–X follows the same order in both mechanisms: R–I > R–Br > R–Cl >> R–F.

Resonance stabilisation of the allylic and benzylic carbocations:

SOME USEFUL TERMS RELATED TO OPTICAL ACTIVITY

1. Monochromatic light: a beam of light of a single wavelength. Ordinary light can be turned into monochromatic light by passing it through a filter.
2. Plane polarised light: light whose propagation takes place along only one plane. Ordinary monochromatic light can be turned into plane polarised light by passing it through a Nicol prism. A Nicol prism is made of calcite (a special crystalline form of CaCO₃).
3. Optically active substance: a substance which rotates the plane of polarisation of plane polarised light, e.g. L(+)- and D(−)-lactic acid.
4. Optical activity: the ability of an optically active substance to cause rotation in the plane of polarisation of plane polarised light.
5. Optical rotation (α_obs): the angle through which the plane of plane polarised light is rotated, either towards the left or towards the right. It depends upon: (i) wavelength λ of the light; (ii) length of the sample tube; (iii) concentration of the solution; (iv) solvent used; (v) temperature of the sample.
6. Polarimeter: the instrument used to measure the extent of optical rotation by a substance.
7. Dextrorotatory substance: a substance which rotates the plane of plane polarised light in the clockwise direction, i.e. towards the right. This is indicated by putting the letter d or (+) before the name, e.g. d-lactic acid or (+)-lactic acid.
8. Laevorotatory substance: a substance which rotates the plane of plane polarised light in the anticlockwise direction, i.e. towards the left. This is indicated by putting the letter l or (−) before the name, e.g. l-lactic acid or (−)-lactic acid.
9. Specific rotation: [α]_λ^t = (observed rotation or angle of rotation) / (length of the sample tube in dm × concentration of the solution in g/mL).
10. Enantiomers: stereoisomers related to each other as non-superimposable mirror images. An enantiomer rotates the plane of plane polarised light through the same angle but in the opposite direction; if one enantiomer is dextrorotatory, the other is laevorotatory.

11. Chirality centre: a carbon atom bonded to four different atoms/groups in the molecule; it is represented by an asterisk (*). The term “chirality centre” was approved by IUPAC in 1996; earlier such a carbon was called an asymmetric carbon / stereo centre / chiral carbon. A molecule (or an object) is said to be chiral or dissymmetric if it does not possess any element of symmetry and is not superimposable on its mirror image; this property is called chirality. A molecule (or an object) which is superimposable on its mirror image is called achiral (non-dissymmetric or symmetric). For example, the letter ‘P’ is chiral, whereas ‘A’ is achiral.
12. Dissymmetric object/molecule: a molecule (or an object) which does not have a plane of symmetry is called a chiral or dissymmetric molecule.
13. Plane of symmetry: an imaginary plane which divides the molecule (or an object) into two identical halves, each being the mirror image of the other (also known as the mirror plane).
14. Centre of symmetry: a point (atom) within the molecule such that if an imaginary straight line is drawn from any point of the molecule through that point, it meets a similar feature at an equal distance on either side of the point.
15. Alternating axis of symmetry: a molecule is said to possess an alternating axis of symmetry if an orientation indistinguishable from the original is obtained when the molecule is rotated through a certain angle around an axis passing through the molecule and the rotated molecule is reflected in a mirror that is perpendicular to the axis of rotation.
16. Symmetric, asymmetric and dissymmetric molecules: (a) Symmetric molecules: if any symmetry is present in the molecule, the molecule is symmetric. (b) Dissymmetric molecules: a molecule is dissymmetric if it has no plane of symmetry, no centre of symmetry and no alternating axis of symmetry. (c) Asymmetric molecules: a dissymmetric molecule having at least one asymmetric carbon is known as an asymmetric molecule. All asymmetric molecules are also dissymmetric molecules, but the reverse is not necessarily true.
17. Racemic mixture: an equimolar mixture of the two enantiomers. A racemic mixture has zero optical rotation, as the rotation due to one isomer is cancelled by the rotation due to the other. It is indicated by prefixing dl or (±) before the name, e.g. dl-butan-2-ol or (±)-butan-2-ol.
18. Racemisation: the process of conversion of an enantiomer into a racemic mixture. A racemic mixture is optically inactive due to external compensation.
19. Configuration: the spatial arrangement of atoms or groups around an asymmetric carbon that characterises a particular enantiomer.
20. Inversion, retention and racemisation: there are three outcomes for a substitution reaction at an asymmetric carbon atom when a group X is replaced by Y. If only one compound (A), with the same arrangement as the reactant, is obtained, the process is called retention of configuration. If only the compound (B) with the opposite arrangement is obtained, the process is called inversion of configuration. If a 50 : 50 mixture of the two is obtained, the process is called racemisation and the product is optically inactive, as one isomer rotates light in the direction opposite to the other. If during a reaction no bond to the stereocentre is broken, the product has the same general configuration as the reactant (retention). Example of racemisation: hydrolysis of optically active 2-bromobutane by the S_N1 route, in which the planar carbocation is attacked by OH⁻ from either side, giving (+)-butan-2-ol and (−)-butan-2-ol in equal amounts.
21. Diastereomers: stereoisomers which are not mirror images of each other (the phenomenon is called diastereoisomerism). Example: 2,3-dihydroxybutanedioic acid (tartaric acid): the meso form and the (+) and (−) forms.
22. Meso compound: a compound which does not show optical activity in spite of the presence of chiral carbon atoms. A meso compound consists of two parts, the lower half being the mirror image of the upper half; the rotation caused by the upper half is cancelled by the equal and opposite rotation of the lower half. Meso compounds are optically inactive due to internal compensation.

ELIMINATION REACTION

Haloalkanes having a β-hydrogen atom, on heating with alcoholic KOH, undergo dehydrohalogenation to form an alkene. These reactions are called β-elimination reactions because the hydrogen atom present at the β-position of the halogen is removed. A base (B:) removes the β-hydrogen while the leaving group (X) departs: B: + H-C-C-X → C=C + B-H + X⁻.

Saytzeff rule: in case the haloalkane can eliminate hydrogen halide in two different ways, the preferred alkene is the one which carries the larger number of alkyl groups attached to the doubly bonded carbon atoms. Reason: the more substituted alkene is more stable and hence is formed at a faster rate (stability: tetrasubstituted > trisubstituted > disubstituted > monosubstituted > unsubstituted).

CH₃CH₂CHBrCH₃ → CH₃CH₂CH=CH₂ (20%, but-1-ene) + CH₃CH=CHCH₃ (80%, but-2-ene, trans + cis) (2-bromobutane, alcoholic KOH)

(CH₃)₂CH-CBr(CH₃)₂ → (CH₃)₂CH-C(CH₃)=CH₂ (21%, 2,3-dimethylbut-1-ene) + (CH₃)₂C=C(CH₃)₂ (79%, 2,3-dimethylbut-2-ene) (2-bromo-2,3-dimethylbutane, alcoholic KOH)

Cis–trans isomerism of Saytzeff’s product: if the alkene formed during Saytzeff elimination is capable of showing cis–trans isomerism, then the trans alkene, being more stable, is formed as the major product.

Order of reactivity in elimination reactions:

1. Reactivity of haloalkanes towards elimination: tertiary > secondary > primary. Reason: the alkyl halide which forms the more stable alkene is more reactive.
2. Among various halides with the same alkyl group: RI > RBr > RCl. Reason: a bigger halogen is a better leaving group.

Elimination versus substitution: an alkyl halide with α-hydrogen atoms, when reacted with a base or a nucleophile, has two competing routes: substitution (S_N1 and S_N2) and elimination. Which route is taken depends upon the nature of the alkyl halide, the strength and size of the base/nucleophile and the reaction conditions. A bulkier nucleophile will prefer to act as a base and abstract a proton rather than approach a tetravalent carbon atom (steric reasons), and vice versa. Similarly, a primary alkyl halide will prefer S_N2; a secondary halide, S_N2 or elimination depending upon the strength of the base/nucleophile; and a tertiary halide, S_N1 or elimination depending upon the stability of the carbocation or the more substituted alkene.

MISCELLANEOUS REACTIONS

Organometallic compounds: compounds having metal–carbon bonds, e.g. alkylmagnesium halide (RMgX) and arylmagnesium halide (ArMgX), known as Grignard reagents.

Preparation of alkylmagnesium halide (RMgX): by the action of an alkyl halide with magnesium in dry ether.

CH₃CH₂Br + Mg → CH₃CH₂MgBr (dry ether)

Characteristic of the Grignard reagent: because carbon is more electronegative than Mg, the C–Mg bond is polar (C^δ−-Mg^δ+); the bond is pulled towards the more electronegative carbon atom, which becomes slightly negatively charged. Due to this bond polarity the Grignard reagent reacts with any compound having acidic hydrogen (like H₂O, ROH, NH₃, HC≡CH, acids, etc.) to give hydrocarbons. It also reacts with O₂ and CO₂. This is why the Grignard reagent should be prepared and used in the absence of moisture and air. The Grignard reagent is used for the preparation of alcohols, aldehydes, ketones, carboxylic acids, etc.

Wurtz reaction: an alkyl halide on reaction with sodium in dry ether gives a hydrocarbon containing double the number of carbon atoms present in the alkyl halide.

R-X + 2Na + X-R → R-R + 2NaX

CH₃-Br + 2Na + Br-CH₃ → CH₃-CH₃ + 2NaBr

Note: (1) Tertiary halides do not undergo this reaction. (2) When a mixture of two different alkyl halides is used, altogether three possible alkanes are formed. Example:

CH₃-Br + Na + Br-C₂H₅ → CH₃-C₂H₅ (propane) + CH₃-CH₃ (ethane) + C₂H₅-C₂H₅ (butane)

Reduction: alkyl halides can be reduced to the corresponding alkane by the following reagents: (i) LiAlH₄ in ether solution; (ii) H₂/Ni or Pt; (iii) Na in moist ether; (iv) H₂/Zn–Cu; (v) Na/C₂H₅OH.

Isocyanide test (carbylamine reaction): chloroform on reaction with a primary amine in the presence of an ethanolic solution of KOH/NaOH gives a foul-smelling compound called an isocyanide (carbylamine).

R-NH₂ + CHCl₃ + 3KOH → R-N≡C + 3KCl + 3H₂O

Significance of the isocyanide test: the reaction is used to distinguish (i) trihalomethanes from other halogenated methanes; (ii) 1° amines from 2° and 3° amines; and (iii) for the preparation of isocyanides.

Haloform reaction: compounds having the –CO–CH₃ or –CH(OH)–CH₃ structure on reaction with Br₂/Cl₂ in the presence of alkali (like NaOH, KOH, NaHCO₃, etc.) give bromoform (CHBr₃) / chloroform (CHCl₃).

R-CO-CH₃ + 3X₂ + 4OH⁻ → R-COO⁻ + CHX₃ + 3X⁻ + 3H₂O (R = H, alkyl, aryl; X = Cl, Br, I)

Iodoform test: compounds having the –CO–CH₃ or –CH(OH)–CH₃ structure on reaction with I₂ in the presence of NaOH give a yellow precipitate of iodoform (CHI₃).

R-CH(OH)-CH₃ + I₂ + 2OH⁻ → R-CO-CH₃ + 2I⁻ + 2H₂O

R-CO-CH₃ + 3I₂ + 3OH⁻ → R-CO-CI₃ + 3I⁻ + 3H₂O

R-CO-CI₃ + OH⁻ → R-COO⁻ + CHI₃ (yellow ppt.)

R-CO-CH₃ + 3I₂ + 4NaOH → R-COONa + 3NaI + 3H₂O + CHI₃

Significance of the iodoform reaction: it is used (i) for structural elucidation; (ii) to distinguish pairs of alcohols, aldehydes and ketones; (iii) for the preparation of iodoform.

REACTIONS OF HALOARENES

Electrophilic substitution reactions: those reactions in which a strong electrophile replaces a weaker electrophile. The halogen atom, besides being slightly deactivating, is o,p-directing; therefore further substitution occurs at the ortho and para positions with respect to the halogen atom.

Reason: due to resonance (the lone pair on the halogen is delocalised into the ring), the electron density increases more at the ortho and para positions than at the meta positions. Further, the halogen atom, because of its –I effect, has some tendency to withdraw electrons from the benzene ring. As a result the ring gets somewhat deactivated as compared to benzene, and hence the electrophilic substitution reactions in haloarenes occur slowly and require more drastic conditions than those in benzene.

Mechanism of electrophilic substitution: the electrophile E⁺ attacks the arene to give a resonance-stabilised carbocation (arenium ion) — the slow step, since it results in the loss of aromaticity even though the resulting cation is still resonance stabilised; the arenium ion then loses H⁺ (fast) to restore the aromatic ring: C₆H₆ + E⁺ → C₆H₆E⁺ (slow) → C₆H₅E + H⁺ (fast).

Nucleophilic substitution: aryl halides are extremely less reactive towards nucleophilic substitution reactions due to the following reasons:

1. Resonance effect: due to resonance the C–X bond acquires partial double-bond character, hence cleavage of the C–X bond is difficult.
2. Difference in hybridisation of the carbon atom in the C–X bond: the sp² hybridised carbon with a greater s-character is more electronegative and can hold the electron pair of the C–X bond more tightly than the sp³ hybridised carbon in a haloalkane with less s-character.

Replacement by hydroxyl group: chlorobenzene can be converted into phenol by heating with aqueous sodium hydroxide solution at a temperature of 623 K and a pressure of 300 atmospheres.

C₆H₅Cl + NaOH → C₆H₅ONa → C₆H₅OH (i. NaOH, 623 K, 300 atm; ii. H⁺)

The presence of an electron-withdrawing group (–NO₂) at the ortho and para positions increases the reactivity of haloarenes:

4-O₂N-C₆H₄-Cl → 4-O₂N-C₆H₄-OH (i. NaOH, 443 K; ii. H⁺)

2,4-(O₂N)₂C₆H₃-Cl → 2,4-(O₂N)₂C₆H₃-OH (i. NaOH, 368 K; ii. H⁺)

2,4,6-(O₂N)₃C₆H₂-Cl + H₂O → 2,4,6-(O₂N)₃C₆H₂-OH (warm water; picric acid)

Vinyl halides are less reactive towards nucleophilic substitution reactions: this is due to resonance, as the C–X bond acquires partial double-bond character.

Reaction with metals:

1. Reaction with magnesium: haloarenes react with Mg in dry ether to give arylmagnesium halides: C₆H₅Br + Mg → C₆H₅MgBr.
2. Wurtz–Fittig reaction: a mixture of an alkyl halide and an aryl halide gives an alkylarene when treated with sodium in dry ether.
3. Fittig reaction: aryl halides also give analogous compounds when treated with sodium in dry ether, in which two aryl groups are joined together.
4. Ullmann reaction: iodobenzene when heated with copper in a sealed tube gives diphenyl.

C₆H₅-X + Na + R-X → C₆H₅-R + 2NaX (dry ether; e.g. C₆H₅Br + CH₃I + 2Na → C₆H₅CH₃ + NaBr + NaI)

2C₆H₅-X + 2Na → C₆H₅-C₆H₅ + 2NaX (dry ether; diphenyl)

2C₆H₅I + 2Cu → C₆H₅-C₆H₅ + 2CuI (heat, sealed tube; diphenyl)

POLYHALOGEN COMPOUNDS

 Summary notes on haloalkanes and haloarenes for CBSE Class 12 Chemistry revision.