Alkyl and Aryl Halides

Alkyl halides or haloalkanes are the class of organic compounds in which a halogen is bonded to an alkyl group. The general formula of RX (where R is an alkyl group, and X is a halogen atom) is C_nH_2n+1, e.g. CH₃Cl, C₂H₅Cl. Unsaturated hydrocarbons also form halogen derivatives. For example:

• CH₂=CH-Cl - Chloroethene (Vinyl chloride)
• CH₂=CH-CH₂Cl - 3-chloroprop-1-ene (Allyl chloride)
• H₃C-CH=CH-CH₂Cl - 1-chlorobut-2-ene (Crotyl chloride)
• 3-chloro-1-phenylprop-1-ene (Cinnamyl chloride)

Aromatic halogen compounds or haloarenes are the halogen compounds containing at least one benzene ring. Halogen derivatives of aromatic compounds are of two types:

Aryl halides

In this type of compounds, the halogen atom is directly linked to the carbon of benzene nucleus. They are also called nuclear substitution derivatives.

Aralkyl halides

In this type of compounds, halogen is linked to the carbon atom of the side chain attached to benzene ring. They are also called side chain substitution derivatives.

Preparation of Haloalkenes

By Direct Halogenation of Alkanes

When alkanes are treated with halogen, chlorine or bromine, in the presence of light or heat, a mixture of mono and poly substituted products is obtained.

RH + X₂ → R-X + HX (UV light or heat)

CH₄ + Cl₂ → CH₃Cl + HCl CH₃Cl + Cl₂ → CH₂Cl₂ + HCl CH₂Cl₂ + Cl₂ → CHCl₃ + HCl → CHCl₃ + Cl₂ → CCl₄ + HCl

In the above reaction, the substitution beyond monohalogenation can be suppressed by using alkane in excess. But the method is not practical because of the difficulties arise during separation of the mixture. In case of higher alkanes, different isomeric products are formed even when monosubstitution is carried out.

In general, the ease of substitution of different types of hydrogen atoms follows the order: benzylic > allylic > tertiary > secondary > primary > vinylic, aryl

The iodination of alkanes is reversible and is done by heating the alkane with iodine in the presence of oxidizing agents like conc. HNO₃, HIO₄ or HIO₃. The function of using such agents is to oxidize HI, formed during the reaction, to iodine, and hence shift the equilibrium in the forward direction.

CH₄ + I₂ ⇌ CH₃I + HI (heat)

5HI + HIO₃ → 3I₂ + 3H₂O

By Addition of HX on Alkenes (HX = HCl, HBr, HI)

RCH=CHR + HX → RCH₂-CHXR

CH₂=CH₂ + HX → CH₃CH₂X

In unsymmetrical alkenes the addition of halogen acids takes place according to the Markonikov's rule. Anti-Markonikov's addition is observed during addition of HBr in presence of peroxide (H₂O₂), known as peroxide effect or Kharasch effect.

From Alcohols

Action of halogen acids

Primary alcohols need ZnCl₂ (anhydrous) for preparation of alkyl halides. The mixture of HCl and anhydrous ZnCl₂ is known as Lucas reagent.

R-OH + HCl(g) or HCl(conc.) → RCl + H₂O (ZnCl₂)

ROH + HBr → RBr (48%)

Bromoalkanes can be obtained by heating alcohols with KBr or NaBr and conc. H₂SO₄. HBr is generated in situ.

KBr + H₂SO₄ → KHSO₄ + HBr

C₂H₅OH + HBr → C₂H₅Br + H₂O

Iodoalkanes are obtained by heating alcohols with KI and 95% H₃PO₄. The reactivity order of alcohols is: tertiary > secondary > primary The reactivity order of halogen is: HI > HBr > HCl

By reaction with phosphorus halides

Chloroalkanes are obtained by reaction of alcohols with PCl₃ or PCl₅.

3ROH + PCl₃ → 3RCl + H₃PO₃ ROH + PCl₅ → RCl + POCl₃ + HCl

Note: PBr₃ and PI₃ being less stable, hence for bromides and iodides, P + Br₂ or P + I₂ mixture is used.

Action of Thionyl Chloride or Darzen's Method

R-OH + SOCl₂ → RCl + SO₂ + HCl (Pyridine)

Note: SOBr₂ is less stable and SOI₂ does not exist hence bromides and iodides are not prepared by this method.

By Halide Exchange

(Finkelstein reaction) Iodides are usually prepared by this method.

RX + NaI → RI + NaX (X = Cl, Br) (Acetone)

Fluoroalkanes (which is difficult to prepare directly by action of alkanes) can be obtained by treating alkyl halides with salts like AgF, Hg₂F₂, CoF₃ or SbF₃. This reaction is known as Swarts reaction.

2CH₃CH₂Cl + Hg₂F₂ → 2CH₃CH₂F + Hg₂Cl₂

From Silver Salts of Fatty Acids (Borodin–Hunsdiecker reaction)

Alkyl chlorides and alkyl bromides are obtained by the action of Cl₂ or Br₂ in CCl₄ on silver salt of fatty acids. The reaction proceeds through free radical mechanism.

RCOOAg + X₂ → RX + CO₂ + AgX (Reflux)

Iodoalkanes cannot be obtained by this method because I₂ reacts with silver salts of fatty acids to form ester.

2RCOOAg + I₂ → RCOOR + CO₂ + 2AgI

This reaction is called Birnbaurn–Simoni reaction.

Illustration 1:

The best method to prepare fluoroethane is

1. C₂H₅OH + HF/H₂SO₄
2. C₂H₅OH + HF/SbF₅
3. C₂H₅Cl + Hg₂F₂
4. C₂H₆ + F₂, hν

Solution: (C) Fluoroethane is prepared by halogen exchange method, i.e. Swarts reaction.
2C₂H₅Cl + Hg₂F₂ → 2C₂H₅F + Hg₂Cl₂

Exercise 1

Consider the following reaction sequence:

CH₃C≡CH → A → B (aq. H₂SO₄/HgSO₄, heat) → (PCl₅)

The products (A) and (B) are:

1. CH₃COCH₃ and CH₃CCl₂CH₃
2. CH₃CH₂CHO and CH₃CH₂CHCl₂
3. CH₃CHOHCH₃ and CH₃CHClCH₃
4. CH₃CH₂CH₂OH and CH₃CH₂CH₂Cl

Preparation of Haloarenes

By Direct Halogenation

The alkyl benzene when heated with halogens in the presence of sunlight and in the absence of Lewis acids (halogen carrier) undergoes substitution at the alkyl chain resulting in the formation of aralkyl halides.

For Example

From Diazonium Compounds

The reaction is known as diazotization.

The diazonium compound is treated with CuCl and HCl or CuBr and HBr to give the corresponding haloarene. This reaction is known as Sandmeyer reaction.

If instead of CuCl and CuBr, Cu powder and HCl or HBr is used, the reaction is called Gattermann's reaction. Iodoarenes are obtained by warming benzenediazonium salts with KI.

Fluoroarenes are obtained by the reaction of corresponding diazonium salts with fluoroboric acid to produce diazonium fluoroborate which on heating produces fluorobenzene. This reaction is called Balz–Schiemann reaction.

From Benzene (Commercial Method)

This process is known as Raschig process.

By Hunsdiecker Reaction

C₆H₅COOAg + X₂ → C₆H₅X + CO₂ + AgX (Cl₂ or Br₂, CCl₄/Xylene)

Exercise 2

CH₃-benzene + Br₂/FeBr₃ → A

Product A is:

1. o-bromotoluene
2. m-bromotoluene
3. p-bromotoluene
4. 50% o-bromotoluene + 50% p-bromotoluene

Physical Properties of Haloalkanes

1. Physical state: CH₃Cl, C₂H₅Cl, CH₃Br are gases at room temperature. The alkyl halide up to C₁₈ are liquids, while higher are colourless solids.
2. Boiling point: The boiling point of haloalkanes are in the order RCl < RBr < RI. It is because with the increase in size and mass of the halogen atom, the magnitude of van der Waals forces of attraction increases. Among isomeric alkyl halides, the boiling point decreases with increase in branching in the alkyl group. This is due to the reason that with increase in branching the molecule acquire less surface area due to attainment of spherical shape. As a result, interparticle forces become weaker, resulting in lower boiling point.The boiling point of various halogen compounds increases with the increase in number of halogen atoms. The boiling point of alkyl halides shows following order:
   • Alkyl iodide > Alkyl bromide > Alkyl chloride (for a given alkyl group)
   • Methyl halide < Ethyl halide < Propyl halide (for a given halide)
   • 1° halide > 2° halide > 3° halide (for a given halide and alkyl group)
3. Solubility: The alkyl halides are polar in nature but still they are insoluble in water as they do not form H-bonds with water molecules.

Physical Properties of Haloarenes

1. Boiling point: The boiling point of monohalogen derivatives of benzene, which are all liquids, follow the order: iodo > bromo > chloro. The boiling point of isomeric dihalobenzenes are nearly the same. However, their melting points are quite different.
2. Solubility: They are soluble in organic solvents like alcohol and ether but insoluble in water as they cannot form H-bonding with water molecules.
3. Density: These compounds are heavier than water. Their densities follow the order: Iodo > Bromo > Chloro.

Chemical Properties of Haloalkanes

The alkyl halides are highly reactive due to high electronegativity difference between carbon and halogen atom, which provides polarity in C-X bond. Thus, carbon atom of C-X bond is easily attacked by a nucleophile and shows nucleophilic substitution.

R-X + :Nu^- → R-Nu + X^-

The nucleophilic substitution may follow S_N1 or S_N2 mechanism. In addition to nucleophilic substitution alkyl halides also show elimination reactions.

S_N2 mechanism

The rate of reaction is dependent on the concentration of alkyl halide as well as the nucleophile, i.e. Rate = K[RX][Nu^-]. Hence, the reaction is bimolecular nucleophilic substitution. There is complete inversion of configuration as it involves attack of nucleophile from backside.

S_N1 Mechanism

The rate of reaction is dependent only on the concentration of alkyl halide, i.e. Rate = K[RX]. Hence, the reaction is unimolecular nucleophilic substitution. The mechanism involves two steps:

Step 1: In the first step, the alkyl halide slowly dissociates into halide ion and carbocation.

Step 2: In the second step, carbocation formed immediately combines with the nucleophile to form the final substituted product.

The order of reactivity of various alkyl halides towards nucleophilic substitution reaction by S_N1 mechanism is: 3° > 2° > 1°

A racemised product is obtained by S_N1 mechanism. This is due to the fact that the carbocation formed has planar structure. So, the nucleophile can attack this carbocation from both sides.

Nucleophilic Substitution on Alkyl Halides

The halogen atom of alkyl halide is easily replaced by a nucleophile to give a large variety of nucleophilic substitution reactions.

Replacement by hydroxyl group (formation of alcohols):

R-X + HOH (boil) → R-OH + X^-

R-X + KOH(aq.) → R-OH + KBr

R-X + Ag₂O(moist) → R-OH + AgBr

Replacement by alkoxy group (formation of ethers): In this reaction, alkyl halide is treated with alcoholic sodium or potassium alkoxide to form ether. This reaction is known as Williamson's synthesis.

RX + Na-OR' → R-O-R' + X^-

C₂H₅Br + NaOC₂H₅ → C₂H₅OC₂H₅ + NaBr (Bromoethane → Diethyl ether)

Replacement by cyano group (formation of cyanides or nitriles):

RX + KCN(alc.) → R-C≡N + X^- + RNC (Alkane nitrile - major)

CH₃CH₂I + KCN(alc.) → CH₃CH₂C≡N + KI (Iodoethane → Propane nitrile)

Replacement with Isocyanide (formation of isocyanide):

RX + AgCN(alc.) → R-N≡C + AgX + RCN (major)

C₂H₅Br + AgCN(alc.) → C₂H₅-N≡C + AgBr (Ethyl isocyanide - major)

Replacement by amino group (formation of amines):This reaction is known as Hofmann's ammonolysis.

RX + NH₃(alc.) → R-NH₂ + HX

If haloalkane is present in excess, then other two hydrogen atoms of amino group are also replaced by alkyl groups leading to the formation of secondary and tertiary amines. Tertiary amine so formed further combines with alkyl halide to give quaternary ammonium salt.

Replacement by nitro group

Treatment of alkyl halide with AgNO₂ gives nitro alkane as a major product.

R-X + AgNO₂ → R-NO₂ + AgX

Replacement with nitrite group

Treatment of alkyl halide with potassium nitrite gives alkyl nitrite as a major product.

R-X + KNO₂ → R-O-N=O + KX

Replacement with -SH (hydrosulphide) group

Alkyl halides on reaction with hydrosulphide group forms thiols or mercaptans.

R-X + NaSH → R-SH + NaX

Replacement by mercaptide (SR^-)

Alkyl halides on reaction with mercaptide ion gives thio ethers.

R-X + R'S^-Na⁺ → R-S-R' + NaX

Thioethers can also be prepared by reaction of alkyl halides with sodium or potassium sulphide.

2R-X + Na₂S → R-S-R + 2NaX

Replacement by alkynyl group

Higher alkynes can be prepared on reacting alkyl halides with alkynyl group.

R-C≡C^-Na⁺ + R'-X → R-C≡C-R' + NaX

Note: The reaction is of limited practical use and can be used only in case of primary halides because the secondary and tertiary halides undergo elimination rather than substitution.

Replacement by carboxylate group

Alkyl halides on reaction with silver salts of carboxylic acid gives esters.

R-X + R'COOAg → R'COOR + AgX

Replacement by hydride ion

Alkanes can be prepared from alkyl halides by this substitution reaction.

R-X + H^- → R-H + X^-

Dehydrohalogenation Reactions

When haloalkanes are heated with alcoholic KOH, they undergo dehydrohalogenation to form alkenes. This is also known as β-elimination reaction.

R-CH₂-CH₂-X + KOH (alc.) → R-CH=CH₂ + KX + H₂O

The reactivity of haloalkanes towards elimination reaction follows the order:

Tertiary > Secondary > Primary

Haloalkanes undergo elimination in two different ways to produce two different alkenes. The preferred alkene is one in which more number of alkyl groups are attached to the double bonded carbon atoms. This generalization is known as Saytzeff rule.

Illustration 2: Arrange the following halides in the decreasing order of S_N1 reactivity

CH₃CH₂CH₂Cl      CH₂=CHCH(Cl)CH₃     CH₃CH₂CH(Cl)CH₃

I                                       II                                    III

(A) I > II > III
(B) II > I > III
(C) II > III > I
(D) III > II > I

Solution: (C) Allyl carbocation is more stable than 2° carbocation which in turn is more stable than primary.

Illustration 3: During debromination of meso-dibromobutane, the major compound formed is

(A) n-butane
(B) 1-butene
(C) cis 2-butene
(D) trans 2-butene

Solution: (D) Debromination is a trans elimination reaction and hence meso-dibromobutane on debromination gives trans 2-butene.

Exercise 3

For the reaction, R-Br → R-O-N=O, the suitable reagent is

(A) NaNO₂ + HCl
(B) HNO₂
(C) AgNO₂
(D) KNO₂

Reactions with Metals

1. Reaction with sodium (Wurtz Reaction)

This reaction is known as Wurtz reaction and generally used to prepare symmetrical alkanes with more number of carbon atoms.

2R-X + 2Na → R-R + 2NaX

2. Reaction with magnesium

R-X + Mg → R-Mg-X (Grignard Reagent)

3. Reaction with zinc powder

2R-X + Zn → R-R + ZnX₂

4. Reaction with lithium

R-X + 2Li → R-Li + LiX

5. Reaction with sodium amalgam

R-X + Na(Hg) → R-H + NaX + Hg

6. Reaction with Pb-Na alloy

4R-X + 4Pb-Na → 4R-H + 4NaX + 4Pb

Reduction

1. Reaction with H₂/Ni

R-X + H₂ → R-H + HX

2. Reaction with zinc-copper couple (in presence of alcohol)

R-X + Zn-Cu + C₂H₅OH → R-H + Products

Similarly, Zn/HCl, Zn/NaOH, Na/C₂H₅OH, Sn/HCl or HI/red P at 430 K can also be used to reduce haloalkanes to alkanes.

Action of Heat

Alkenes can be prepared by heating alkyl halides at a very high temperature.

R-CH₂-CH₂-X → R-CH=CH₂ + HX

Illustration 4: The end product of the following reaction is

Solution: (A)

Chemical Properties of Aryl Halides

Aryl halides are much less reactive towards nucleophilic substitution reaction than haloalkanes. However, they can be made to react under drastic conditions, such as high pressure and high temperature.

Nucleophilic Substitution Reactions

1. Replacement by hydroxy group - Dow's process

The reactivity of aryl halides towards nucleophilic substitution increases if some electron withdrawing group such as nitro, cyano or carbonyl group is attached to the ring.

2. Replacement by cyano group

3. Replacement by amino group

4. Replacement by alkoxide group

5. Replacement by -SH group

Reduction

Reaction With Metals

1. Reaction with magnesium

Aryl chlorides do not form Grignard reagent in presence of ether as solvent but it is possible if THF (Tetrahydrofuran) is used.

2. Reaction with sodium

Aryl halides react with sodium in presence of ether.

When aryl halides are treated with haloalkane and sodium in presence of dry ether, undergo Wurtz Fittig reaction.

3. Reaction with lithium

Aryl halides reacts with lithium in presence of dry ether to form organometallic compound.

4. Reaction with copper powder (Ullmann Reaction)

Aryl chloride and aryl bromide usually do not give this reaction, however if o and p-nitro substituted aryl chloride and aryl bromide are used, then it shows Ullmann reaction.

Ring Substitution Reactions

Aryl Halides undergo electrophilic substitution reaction in the benzene ring. The halogen atoms are o and p-directing groups but deactivates the ring at the same time. Therefore, substitution occurs relatively at slower rate in comparison to benzene.

1. Halogenation

2. Nitration

3. Sulphonation

4. Friedel-Crafts alkylation

5. Friedel-Crafts acylation

6. Action with chloral

Two molecules of chlorobenzene reacts with chloral to form DDT.

Illustration 5: Which of the following halogen containing compound will not undergo S_N reactions easily?

Solution: (A) Vinyl halide cannot be substituted by other nucleophile due to double bond character and instability of vinyl cation.

Trichloromethane (Chloroform)

Methods of Preparation

1. From methane

Chloroform is prepared by chlorination of methane in the presence of light.

CH₄ + Cl₂ → CH₃Cl + CH₂Cl₂ + CHCl₃ + CCl₄

The mixture of CH₃Cl, CH₂Cl₂, CHCl₃, and CCl₄ are separated by fractional distillation.

2. From chloral hydrate

Chloroform (in pure form) can be prepared by distillation of chloral or chloral hydrate with concentrated aqueous NaOH solution or KOH solution.

CCl₃CH(OH)₂ + NaOH → CHCl₃ + HCOONa + H₂O

3. From ethanol or acetone (lab method)

Chloroform is obtained from ethanol or acetone by reaction with a paste of bleaching powder and water.

(a) In case of alcohol:

C₂H₅OH + Cl₂ → CH₃CHO + 2HCl

CH₃CHO + 3Cl₂ → CCl₃CHO + 3HCl

2CCl₃CHO + Ca(OH)₂ → 2CHCl₃ + (HCOO)₂Ca

(b) In case of acetone:

4. From carbon tetrachloride

By partial reduction of carbon tetrachloride with iron filings and water.

Physical Properties

Chloroform is a colourless, heavy liquid. It has sweetish, sickly odour and taste. It is heavier than water and slightly soluble in water. Its boiling point is 334 K. Inhaling of chloroform causes unconsciousness and thus used as anaesthetic.

Chemical Properties

1. Action of sunlight and air: It is oxidized by air to produce a highly poisonous compound called phosgene (COCl₂).

To avoid the formation of phosgene, chloroform is kept in dark bottles to cut off active light radiations and filled upto brim to keep out air. Also a little amount of 1% ethanol is added which converts the toxic COCl₂ as non-poisonous diethyl carbonate.

2. Hydrolysis: When boiled with aqueous KOH, chloroform is hydrolysed to potassium formate.

CHCL₃ + 4KOH → HCOOK = 3KCl + 2H₂O

3. Action of Ag powder

 2CHCl3 + 6Ag → HC = CH + 6AgCl

4. Reaction with nitric acid

CHCl₃ + HNO₃ → CCl₃ NO₂ + H₂O

Chloropicrin is used as an insecticide and war gas.

5. Reaction with acetone

Chloretone is used as hypnotic (a sleep inducing drug).

6. Reaction with primary amines (Carbylamine reaction).

RNH₂ + CHCl₃ + 3KOH → RNC + 3KCl + 3H₂O

Both aromatic and aliphatic primary amines give isocyanide with KOH (alcoholic) and chloroform. The isocyanide formed has a characteristic smell and this reaction is known as isocyanide or carbylamine reaction. It is used for the detection of primary amines.

7. Reduction with zinc and HCl

8. Reaction with sodium ethoxide

9. Riemer–Tiemann reaction

10. Reaction with silver nitrate: Chloroform does not give a precipitate when treated with aqueous AgNO₃. It is because the C–Cl bond in chloroform is covalent and does not ionize in aqueous solution to produce chloride ions.

Triiodomethane (Iodoform)

Methods of Preparation

Iodoform is prepared by the action of iodine and alkali on acetaldehyde, methyl ketones and alcohols (which produces unit on oxidation).

1. From alcohol

2. From acetone

Physical Properties

Iodoform is a yellow coloured, solid compound having melting point 382 K. It is insoluble in water but dissolves readily in organic solvents.

Chemical Properties

Chemical reactions of iodoform is similar to that of chloroform.