JEE Chemistry Haloalkanes and Haloarenes Notes Set 03

Classification and Names

Replacing hydrogen of a hydrocarbon by halogen gives a halogen derivative. Three families are met: haloalkanes (from alkanes), haloalkenes and haloalkynes (from unsaturated hydrocarbons) and haloarenes (from benzene and its relatives).
A halide is 1°, 2° or 3° according to whether the carbon bearing X is attached to one, two or three other carbons. A gem-dihalide has both halogens on the same carbon (ethylidene dichloride, \( \ce{CH3CHCl2} \)); a vicinal dihalide has them on adjacent carbons (ethylene dichloride, \( \ce{ClCH2CH2Cl} \)). A haloform is \( \ce{CHX3} \). Benzylidene chloride is \( \ce{C6H5CHCl2} \) and benzotrichloride is \( \ce{C6H5CCl3} \). Benzene hexachloride is \( \ce{C6H6Cl6} \), 1,2,3,4,5,6-hexachlorocyclohexane.

Methods of Preparation

Physical Properties

The C-X bond is polar, but alkyl halides do not hydrogen bond with water, so they dissolve in organic solvents and not in water. Methyl chloride, methyl bromide, ethyl chloride and methyl fluoride are gases; most others are colourless liquids, higher members being solids. For a given alkyl group the boiling point and the density follow RI > RBr > RCl > RF, while bond strength is in the order C-F > C-Cl > C-Br > C-I and C-X polarity falls from Cl to I. Among isomers, boiling point falls with branching. Iodides slowly turn brown or violet in light because of liberated iodine. Alkyl halides burn with a green-edged flame on copper wire (Beilstein test).

Nucleophilic Substitution

The carbon of C-X is electron poor, so a nucleophile can replace X. Two mechanisms operate.
SN1: slow ionisation to a carbocation, then fast attack of the nucleophile. Rate depends on the halide only. Favoured by tertiary and benzylic or allylic halides, polar protic solvents and good leaving groups; stereochemistry is racemisation.
SN2: one step, backside attack, rate depends on both halide and nucleophile. Favoured by methyl and primary halides and polar aprotic solvents; stereochemistry is inversion (Walden inversion).
Leaving group ability: I- > Br- > Cl- > F-. A tertiary halide hydrolysed by aqueous alkali follows a rate independent of the alkali concentration.

Elimination and Other Reactions

Alcoholic KOH removes HX (dehydrohalogenation, E2) to give an alkene; the more substituted alkene predominates (Saytzeff rule). A gem- or vicinal dihalide with strong base such as \( \ce{NaNH2} \) gives an alkyne. Tertiary halides eliminate even with weakly basic nucleophiles.
Wurtz reaction: \( \ce{2RX + 2Na} \) in dry ether gives R-R (fluorides do not react). Wurtz-Fittig uses an aryl halide and an alkyl halide. Grignard reagent: \( \ce{RX + Mg} \) in dry ether gives \( \ce{RMgX} \), reactivity RI > RBr > RCl, fluorides inert; it is destroyed by water, alcohols, acids and any acidic hydrogen, and it gives an alkane with such compounds. Reduction with \( \ce{LiAlH4} \) or Zn/HCl gives the alkane. Zn with methyl bromide (Frankland) gives ethane. Heating with Na-Pb alloy gives tetraethyl lead. Friedel-Crafts alkylation of benzene with an alkyl halide and \( \ce{AlCl3} \) may rearrange the chain (n-propyl bromide gives isopropylbenzene).

Haloarenes and Vinyl Halides

The C-X bond in chlorobenzene and vinyl chloride is shorter and stronger: the lone pair on Cl overlaps with the pi system and the carbon is sp2. Hence they are inert towards ordinary nucleophilic substitution and give no precipitate with AgNO3 after shaking with water. They are less reactive than alkyl and benzyl halides, and the order of ascending reactivity is aryl and vinyl halides, then alkyl halides, then benzyl and allyl halides.
Aryl halides need drastic conditions or activation. Chlorobenzene heated with NaOH at about 570 K and 300 atm gives phenol (Dow process). Nitro groups at ortho and para positions make the ring electron deficient and stabilise the intermediate, so 2,4-dinitrochlorobenzene reacts readily with aqueous alkali. Halogens deactivate the ring towards electrophilic substitution but direct incoming groups ortho and para. Among dichlorobenzenes the para isomer has the highest melting point because of symmetry.

Polyhalogen Compounds

Chloral, \( \ce{CCl3CHO} \), is made by exhaustive chlorination of ethanol and gives chloroform with aqueous alkali. Phosgene is carbonyl chloride, a poisonous gas. Chloretone, \( \ce{(CH3)2C(OH)CCl3} \), is made from acetone and chloroform and is a hypnotic.

Quick Checks

1. Aqueous KOH substitutes, alcoholic KOH eliminates.
2. Reactivity towards SN2: methyl > 1° > 2° > 3°; towards SN1 the order reverses.
3. Alkyl halide plus AgNO3 (aq/alc): immediate precipitate for iodides and benzyl halides, slower for chlorides, none for aryl and vinyl halides.
4. Number of distinct monohalogen products equals the number of non-equivalent hydrogen sets; check symmetry first.
5. Peroxide effect: only HBr, only with alkenes.