Haloalkanes and Haloarenes Class 12 Notes Chemistry Chapter 10 - CBSE

Chapter:10

What are Haloalkanes and Haloarenes ?

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In aliphatic or aromatic compounds, the replacement of hydrogen atom(s) by halogen atoms(s) results in the formation of alkyl halide (haloalkane) and aryl halide (haloarene), respectively.
In case of haloalkanes, halogen atom is attached to the sp³ hybridised carbon atom of an alkyl group whereas in haloarenes, halogen atom is attached to sp² hybridised carbon atom of an aryl group.
Haloalkanes or haloarenes are classified as mono, di- tri-, tetra- haloalkanes or haloarenes, etc., according to the one, two, three, four, etc., halogen atoms respectively present in their molecule.

On the basis of the halogen atom attached to primary, secondary and tertiary carbon atoms alkyl halides are further classified as classified as 1°, 2°, 3°, allylic, benzylic, vinylic and aryl derivatives.

Classification of Haloalkane s and Haloar enes

Compounds containing sp³ C–X Bond

➥ Alkyl halides or haloalkanes (R–X)
➥ Allylic halides This have X bonded to the carbon atom adjacent to C–C double bond.
➥ Benezylic halidesThis have X bonded to the carbon atom adjacent to a benzene ring.

Classification of Haloalkane s and Haloar enes

Compounds containing sp² C–X Bond

➥ Vinylic halides This have a halogen atom (X) bonded to a C–C double bond.
➥ Aryl halides This have a halogen atom bonded to a benzene ring.

Nature Of C-x Bond

Since halogen atoms are more electronegative than carbon, the carbon-halogen
bond of alkyl halide is polarised; the carbon atom bears a partial positive
charge whereas the halogen atom bears a partial negative charge.

Methods Of Preparation Of Haloalkanes

Haloalkanes can be prepared from displacement of alcoholic group in alkyl alcohol by halogen acid, PCl₅ and PCl₃. Haloalkanes can also be prepared by addition of halogen acids or halogens on alkene and alkyne. Alkyl halides can aslo be prepared by free radical halogenation of alkene.

Finkelstein Reaction

Finkelstein Reaction

$$R-X+NaI\xrightarrow[X = Cl, Br]{acetone} R—I + NaX$$

Swarts Reaction

$$\\ H_3C- Br + AgF\longrightarrow H_3C -F + AgBr$$
$$\\ Hg_2F_2,\space COF_2 \space and \space SbF_3 \space \text{can also be used as a reagent for Swarts reaction.}$$

Hunsdiecker Reaction

$$\\CH_3COOAg + Br_2\xrightarrow[CCl_4]{}CH_3Br + AgBr + CO_2$$

Free Radical halogenat ion of alkane s

Addition of hydrogen halides on alkene s

Physical Properties Of Haloalkanes

Boiling point orders 1. R-I > R - Br > R-Cl > R -F
CH₃–(CH₂)₂–CH₂Br > (CH₃)₂CHCH₂Br > (CH₃)₃CBr
CH₃CH₂CH₂ > CH₃CH₂X > CH₃X
Bond strength of haloalkanes decreases as the size of the halogen atom increases. Thus, the order of bond strength is CH₃F > CH₃Cl > CH₃Br > CH₃I.
Dipole moment decreases as the electronegativity of the halogen decreases.
Haloalkanes though polar but are insoluble in water as they do not form hydrogen bonding with water.
Density order is RI> RBr > RCl > RF (for the same alkyl group) CH₃I > C₂H₅I > C₃H₇I

Chemical Properties Of Haloalkanes

Relative reactivity of alkyl halides for same alkyl group is RI > RBr > RCI> RF

(I) Nucleophilic Substitution R eactions (S N reactions)

Nucleophilic Substitution R eactions

Characteristics of S_N1 Mechanism	Characteristics of S_N2 Mechanism
Kinetics: First-order kinetics; rate = k[RX]	Kinetics: Second-order kinetics; rate = k[RX][:Nu]
Mechanism: Two steps	Mechanism: One step
Stereochemistry: Trigonal planar carbocation intermediate. Racemization at a single stereogenic center	Stereochemistry: Backside attack of the nucleophile. Inversion of configuration at a stereogenic center
Identity of R: More substituted halides react fastest.	Identity of R: Unhindered halides react fastest.
Rate : R₃CX > R₂CHX > RCH₂X > CH₃X	Rate : CH₃X > RCH₂X > R₂CHX > R₃CX

(II) Eliminat ion Reactions

Dehydrohalogenation is a -elimination reaction in which halogen is from α carbon atom and the hydrogen from the β carbon according to Saytzeff rule, e.g.,

(III) Reduction

$$\text{C}_2\text{H}_5 — \text{Br} + \text{H}_2\xrightarrow[]{\text{Ni, 575 K}}\text{C}_2\text{H}_6 + \text{HBr}\\\text{C}_2\text{H}_5\text{I} + \text{HI}\xrightarrow[]{\text{Red P, 420 K}}\text{C}_2\text{H}_6 + \text{I}_2$$

(IV) Reaction with Metals


(1) Wurtz reaction:	$$\\ RX + 2Na + XR \xrightarrow[]{Dry ether}R—R (alkane) + 2NaX$$
(2) Wurtz-Fitting reaction:	$$\\C_6H_3—Cl + 2Na + Cl CH_3\xrightarrow[]{Dry ether}C_6H_5—CH_3+ 2NaCl$$
(3) Reaction with Mg:	$$\\C_2H_5Br + Mg\xrightarrow[]{Dry ether}\underset{\mathrm{(Grignard’s reagent)}}{C_2H_5—Mg—Br}$$

Methods Of Preparation Of Haloarenes

(a) By Electrophilic Substitution Reaction:

(b) Sandmeyer’s Reaction

(c) Gattermann Reaction:

(d) In the presence of KI

(e) Balz-Schiemann reaction:

Chemical Properties Of Haloarenes

Aromatic nucleophilic s ubstitution reaction Haloarenes do not undergo nucleophilic substitution reaction readily. This is due to C–X bond in aryl halide is short and strong and also the aromatic ring is a centre of high electron density. The halogen of haloarenes can be substituted by OH-, NH₂ -, or CN- with appropriate nucleophilic reagents at high temperature and pressure. For Example :

$$\\\underset{\mathrm{Chlorobenzene}}{C_6H_5Cl + NaOH}\xrightarrow[300 atm]{350°C}\underset{\mathrm{Chlorobenzene}}{C_6H_5OH + NaCl}$$

$$\\\underset{\mathrm{Chlorobenzene}}{C_6H_5Cl + 2NH_3}\xrightarrow[50 atm]{250°C}\underset{\mathrm{Aniline}}{C_6H_5NH_2 + NH_4Cl}$$

$$\\\underset{\mathrm{Chlorobenzene}}{C_6H_5Cl + CuCN}\xrightarrow[Pyridine]{250°C}\underset{\mathrm{Phenyl cyanide}}{C_6H_5CN + CuCl}$$

Polyhalogen Compounds: Uses And Environmental Effects

(i) Dichloromethane (Methylene chloride)

It is used as solvent, paint remover, propellant in aerosols, process solvent in
the manufacture of drugs and in the metal cleaning and finishing solvent.

(ii) Triiodomethane (Iodoform)

They are used as an antiseptic due to the liberation of free iodine. It is not because of Iodoform itself.

(iii) Trichloromethane (Chloroform)

It is used as anesthetic because when pure chloroform is inhaled it affects
the heart due to which after mixing with ether and other suitable anesthetics
chloroform can be used as anesthetic.

(iv) Tetra Chloromethane (Carbon Tetrachloride)

They are used in manufacturing refrigerants, propellants for aerosol cans and for the synthesis of chlorofluorocarbons, pharmaceutical etc.

(v) Freons

The Chlorofluorocarbon compounds of methane and ethane are jointly called freons.
They are very stable, non-corrosive, non-toxic, and unreactive liquefiable gases.

(vi) P,P'-Dichlorodiphenyltrichloroethane (DDT)

DDT stands to be the first chlorinated organic insecticides. It is highly
poisonous to all living organisms as it does not get metabolized
rapidly by animals and gets deposited and stored in the fatty tissues.