NCERT Revision Notes for Chapter 6 Haloalkanes And Haloarenes Class 12 Chemistry

Answer

Haloalkanes = R – X Where R is an alkyl group = alkyl halide
Haloarene = Ar – X where Ar is arene (benzene ring) = aryl halide (C₆H₅ – X)
X = halogen, atom = F, Cl, Br, I[Good leaving group : I > Br > Cl > F]

Answer

(a) 
(b) Primary(1°) – Carbon adjacent to X – atom which is attached to one or no C – side chain
Secondary (2°) – attached to two side chains,
Tertiary (3°) – attached to three side chains,
Vinyl halide – Carbon attached to X – is sp² hybridised or double bonded. CH₂ = CH – X
(c) Allyl halide – Carbon adjacent to the carbon attached to the X – atom is sp² hybridised
CH₂ = CH – CH₂ – X
(d) Trihalides – CHCl₃ = chloroform = Trichloromethane
CHI₃ = iodoform = Triiodomethane

Answer

I. From Hydrocarbons
(a) From alkanes : By Halogenation (By Free radical mechanism)

Higher alkanes give mixture of isomeric products. So, this method is not suitable for preparation of haloalkanes.

(b) From alkenes :
(i) CH₂ = CH₂ + HBr → CH₃CH₂Br

Markownikoff’s rule : During addition across unsymmetrical double bond, negative part of attacking reagent attaches itself to the carbon atom carrying lesser no. of H – atoms.

Anti – Markownikoff rule or peroxide or Kharasch effect – According to it, negative part of attacking reagent will join to the carbon – atom carrying more H – atoms across double bond of alkene.

(iv) From benzene :

II. From alcohols :
(a) Groove’s process

(b) CH₃CH₂OH + PCl₅ → CH₃CH₂Cl + POCl₃ + HCl
3CH₃CH₂OH + PCl₃ → 3CH₃CH₂Cl + H₃PO₃
(c) CH₃CH₂OH + SOCl₂ → CH₃CH₂Cl + SO₂↑ + HCl↑
III. Halogen exchange reactions
(a) Finkelstein reaction – for the preparation of idoalkanes

(b) Swart’s reaction – for the preparation of fluoroalkanes

IV. Sandmeyer’s reaction

V. Gatterman reaction

VI.

Answer

I. Nucleophilic substitution reaction
(a) R – X + KCN(aq) → RCN + K – X
(b) R – X + AgCN → RCN + AgX
(c) R – X + R’ONa → ROR’ + NaX [ Williamson’s synthesis]
(d) R – X + KNO₂ → RONO + KX
(e) R – X + AgNO₂ → R – NO₂ + AgX
Note : (a) Nucleophilic substitution unimolecular reaction (S_n1) – for 3° alkyl halides
Order or reactivity : 3° > 2° > 1°
(b) Nucleophilic substitution bimolecular reaction (S_n2) – for 1° alkyl halides
Order or reactivity : 3° > 2° > 1°
Order or reactivity : 1° > 2° > 3°

II β – elimination reactions or dehydrohalogenation

Saytzeff’s rule : According to it, that alkene will be preferred as major alkene in which carbon atoms joined by the double bond are maximum alkylated i.e. contains maximum no. of substituents or alkyl groups.
Note : R – X + aq KOH → alcohol (substitution reaction dominates)
R – X + alcoholic KOH → alkene (elimination reaction dominates)
In aqueous medium, base (KOH) ionises to give OH^– which can act as nucleophile. In alcoholic medium, these ions abstract protons so that elimination would occur.

III Reaction with Metals : (D.E. = Dry Ether to provide anhydrous conditions)
(a) R – X + Mg(in presence of dry ether) → RMgX (Grignard reagent)
It only attacks at the carbonyl group (−CO−) and is used to increase the number of C – atoms in a compound.
(b) Wurtz raction – for the preparation of symmetrical alkanes
R – X + 2Na + X –R (in presence of dry ether) → R – R + 2NaX
(c) Wurtz – Fitting reaction –

(d) Fitting reaction –

(e) X – atoms present on benzene act as ortho , para – directors as they have lone pairs which are involved in resonance :

As electron density are more at ortho and para positions so the electrophile will attack at ortho and para positions.
• Haloarenes are less reactive towards electrophilic and nucleophilic substitution reaction than haloalkanes because –
(i) Resonance effect – Haloarenes are involved in resonance due to which C – X bond length decreases and its cleavage becomes difficult. More the number of resonating structures, more is the stability.
(ii) Polarity of bond - Due to resonance, C – X bond polarity decreases, dipole moment decreases and reactivity decreases.
(iii) Hybridisation of C – atom – Due to resonance, the C – atom of C – X bond become sp² hybridised by attaining double bond character and has less tendency to release electrons to halogen.

IV Nucleophilic reactions in haloarenes :
(a) Dow’s Process :

Effect of substitutents in aryl halides or haloarenes -
Presence of electron withdrawing group (EWG) like –NO₂, −CN, −COOH, etc. at ortho and para positions to halogen atoms activates the halogen towards nucleophilic substitution reactions due to stablising effect of resonance.

Note : R – Cl + KCN → R – CN + KCl

Answer

(a) Halogenation :

(b) Nitration :

(c) Sulphonation :

(d) Friedal Craft reaction :

Answer

• Optically active compounds : Compound which can rotate the plane polarised light when it is passed their solutions.
• Optical rotation (?) : The angle by which plane polarised light is rotated is measured by polarimeter and is known as optical rotation. If a compound rotates the plane polarised light to the right, i.e. in clockwise direction, it is called dextrorotatory or d or (+). If a compound rotates the plane polarised light to the left, i.e. in anticlockwise direction, it is called laevorotatory or I or (−). Such isomers are known as optical isomers and phenomenon is termed as optical isomerism.
50%d + 50% l = Racemic mixture (dl) or (±)
• Chiral compounds :  Compounds which have non – superimposable mirror images. This property of a compound is known as chirality. While compounds with superimposable mirror images are known as achiral compounds. The stereoisomers related to each other as non – superimposable mirror images are known as enantiomers, Enantiomers have identical physical properties such as melting point, boiling point, solubility, refractive index. They only differ with respect to rotation of plane polarised light. If one of the enantiomers is dextrorotatory, then other will be laevorotatory.
The stereoisomers which are not mirror images of each others are called diastereomers.

• Racemic mixture or modification : A mixture containing two enantiomers in equal proportions will have zero optical rotation, as the rotation due to one isomer would be cancelled out by the rotation due to another isomer (dl or ±). The process of conversion of enantiomer into a racemic mixture is known as recemisation.
• Retention of configuration : Is the reversing of spatial arrangement of bonds in a chemical reaction. 50% retention +50% inversion = racemic mixture

Answer

(a) CH₂Cl₂ = Dichloromethane – used as a solvent as paint remover, as propellant in aerosols, in drug manufacturing.
• Harms central nervous system, leads to impaired hearing and vision.
(b) CHCl₃ = Trichloromethane = Chloroform = used in production of Freon refrigerant R – 22, earlier used as anaesthetic.
• Depresses central nervous system, headache, dizziness, damage kidney and liver.
• Chloroform should be kept in dark bottles completely filled because chloroform is slowly oxidized by air in the presence of light to form poisonous gas carbonyl chloride.
(c) CHI₃ = Iodoform = Used as antiseptic
(d) CCl₄ = in manufacture of refrigerants and propellants
(e) Freons = extremely stable gases. CCl₂F₂ (Freon 12) is used in industries
• Cause ozone depletion due to which harmful ultraviolet radiations would reach to the surface of the earth and would cause leukaemia, cataract, etc.
(f) DDT = p,p’ – Dichloro – diphenyl – trichloro – ethane
• It is a banned pesticide.

Answer

Alkyl halide when heated with aqueous KOH undergoes hydrolysis to potassium halide. K – X is acidified with dilute HNO₃ and treated with silver nitrate solution.
R–X + aq. KOH(warm) → R–OH + K–X
K–X + AgNO₃ → KNO₃ + Ag–X↓
AgCl = white ppt., AgBr = pale yellow ppt, AgI = Yellow ppt.

Answer

A nucleophile which is capable of attacking through more than one atom (site) is known as ambident nucleophile.
Such as : (a) CN^– and NC^– (b) SCN^– & NCS^– ; (c) NO₂⁻ & ONO^– .
• R–X + KCN → R – CN + K – X
R – X + AgCN → R – NC + Ag – X
Reason : KCN is predominantly ionic and therefore, both C and N atoms are free to donate electron pair. Since C – C bond is relatively stronger than C – N bond, therefore, the attack occurs mostly through the carbon atom of the cyanide group alkyl cyanides as the major product.
AgCN is predominantly covalent. Therefore, in this case only N – atom is free to donate electron pair and the attack mostly occurs through the N – atom of the cyanide group forming alkyl isocyanides as the major product.