Properties of Alkyl Halides (Halo Alkanes) & Aryl Halides (Halo Arenes)

PHYSICAL PROPERTIES

Halo Alkanes

1. Boiling Points

The boiling points of haloalkanes are in the order RCl < RBr < RI. It is because with increase in size and mass of halogen atom the magnitude of Vander Waal's forces of attraction increases.

Among isomeric alkyl halides, the boiling point decreases with increase in branching in alkyl group.

e.g the decreasing order of boiling point among the isomers of butane is

For same halogen, the boiling point increases with increase in molecular mass.

e.g. CH₃Cl has lower boiling point than CH₃CH₂Cl

The boiling points of various halogen compounds increase with increase in number of halogen atoms.

For e.g. boiling point of CCl₄ is more than boiling point of CHCl₃ which is further more than CH₂Cl₂

Halo Arenes

1. Boiling point

The boiling points of mono halogen derivatives of benzene follows the order:

Iodo > Bromo > Chloro

The boiling points of isomeric dihalo benzenes are nearly the same. However their melting points are quite different. The melting point of para isomer is generally 70 – 100 degrees more than the melting points of ortho & meta isomers.

B.P 453 K 446 K 448 K

M.P. 256 K 249 K 325 K

The higher melting point of p – isomer is due to its symmetry which leads to more close packing of its molecules in the crystal lattice.

2. Solubility

Haloarenes are insoluble in water, acids or base but are soluble in organic solvents.

Haloarenes are insoluble in water because they can not form hydrogen bonds with water molecules.

3. Density

They are all heavier than water. Their densities follow the order:

Iodo > Bromo > Chloro

Illustration 1. Explain why chloroform (CHCl₃) is not soluble in water although it is polar.

Solution: Chloroform is insoluble is water inspite of its polar nature because its molecules can not form hydrogen bonds with water.

Illustration 2. Arrange the following in increasing order of density

CHCl₃, CH₂Cl₂, CCl₄, CH₃Cl

Solution: CH₃Cl < CH₂Cl₂ < CHCl₃ < CCl₄

Illustration 3. Melting and boiling points of alkyl halides are higher then their corresponding alkanes. Why?

Solution: The higher molecular mass and polar nature of alkyl halides is the reason of high melting and boiling points which are in the order

$RH<R-F<R-Cl<R-Br<RI$

CHEMICAL PROPERTIES

Haloalkanes are highly reactive class of aliphatic compounds. Their reactivity is due to the presence of polar carbon – halogen bond in their molecule. In general for a given alkyl group, the order of reactivity decreases as:

Iodides > Bromides > Chlorides

The explanation of above order is that reaction of alkyl halides involve cleavage of C – X bond. So higher the bond dissociation energy smaller reactivity & the bond dissociation energy decrease with increase in size of halogen atom.

The chemical reactions of halo alkanes are of 4 types:

1. Nucleophilic substitution reactions

2. Dehydro halogenation reactions

3. Reactions with metals

4. Reduction reactions

NUCLEOPHILIC SUBSTITUTION REACTION

In halo alkanes, the halogen atom is attached to the carbon atom. As the halogen atom is more electronegative than carbon, the bond between carbon & halogen is polar in character.

Due to the presence of partial positive charge on the carbon atom, the nucleophiles can attack on electron deficient carbon.

The order of reactivity of various alkyl halides towards nucleophilic substitution is in the order:

RI > RBr > RCl > RF

Mechanism

Nucleophilic substitution reactions in halides containing $C-X$ bond may take place through either of the two different mechanism – SN¹& SN².

SN¹ mechanism (Unimolecular Nucleophilic Substitution)

In this type the rate of reaction is dependent only on the concentration of alkyl halide i.e.

Rate = k[RX]

Step1:

In this step the alkyl halide slowly dissociates into halide ion & carbocation.

Step 2:

In the 2^nd step carbocation at once combines with the nucleophile to form the final substituted product.

The order of reactivity of various alkyl halides through SN¹ mechanism is

3⁰ > 2⁰ > 1⁰

Allylic & benzylic halides show greatest reactivity through SN¹ mechanism due to stability of allylic & benzylic carbocations.

SN² Mechanism (Bimolecular Nucleophilic Substitution)

In this type the rate of reaction is dependent on the concentration of alkyl halide as well as nucleophile i.e.

Rate = K[RX] [Z^-]

Primary alkyl halides react by SN² mechanism via formation of transition state.

1⁰ > 2⁰ > 3⁰

Factors affecting SN1 & SN2 Mechanism

The reaction mechanism, SN1 or SN2, followed by nucleophilic substitution depends upon a number of factors. These factors are

1. Nature of alkyl halides: Primary alkyl halides react through SN2 & tertiary alkyl halides through SN1 mechanism.

2. Nature of Nucleophile: Strong nuclophile favour SN2 mechanism whereas weak nucleophile favours SN1 mechanism.

3. Concentration of Nucleophile: High concentration of nucleophile favours SN2 while low concentration favours SN1 mechanism.

4. Nature of Solvent: Polar solvents favour SN1 mechanism.

Illustration 4. Why are alkyl halides very reactive?

Solution: C$\delta$⁺ - X$\delta$^- bond in alkyl halides is polar and the negative charge on halogen atom is intensified by the positive inductive effect of alkyl groups.

Illustration 5. Amongst the following the most reactive alkyl halide is

(A) C₂H₅F (B) C₂H₅Cl

(C) C₂H₅Br (D) C₂H₅I

Solution: (D)

Some nucleophilic reactions are as follows:

1. Replacement by hydroxyl group (formation of alcohols)

Haloalkanes on treatment with aqueous solution of KOH or moist silver oxide give alcohol.

$RX+KOH\left(aq\right)\to ROH+KX$

$RX+A{g}_{2}O\underset{oxide}{\overset{moistsilver}{\to }}ROH+AgX$

2. Replacement by Alkoxy (Formation of ethers) (Williamson's synthesis)

Haloalkanes on treatment with alcoholic sodium or potassium hydroxide form ethers. This reaction is known as Williamson's synthesis

3. Replacement by cyano group

Haloalkanes on treatment with alcoholic KCN give alkyl nitriles or alkyl cyanides as major product.

4. Replacement with Isocyanide Group (Formation of isocyanides)

On reaction with alcoholic silver cyanide solution, haloalkanes give alkyl carbylamines or alkyl isocyanides as the major product along with or small amount of alkyl cyanide.

\begin{array}{*{20}{l}} {RX{\text{ }} + {\text{ }}alc.{\text{ }}AgCN\xrightarrow{{}}R - NC + AgX} \\ {Isocyanide} \end{array}

5. Replacement by Amino Group (Formation of amines)

On heating haloalkanes with alcoholic ammonia solution in a sealed tube, halogen is replaced by $-$NH₂ group to form primary amine.

R $-$ X + NH₃ (alc.) $\to$ R $-$ NH₂ + HX

Primary amine

In case haloalkanes is in excess, the other 2 hydrogen atoms of amino group are also replaced by alkyl groups leading to the formation of secondary & tertiary amines.

C₂H₅ $-$ Br + HNHC₂H₅ $\to$ (C₂H₅)₂NH + HBr

Diethylamine

6. Replacement by Nitro group (Formation of nitro alkanes)

On treating ethanolic solution of haloalkanes with silver nitrite (Ag $-$ O $-$ N $=$ O), nitro alkane is formed.

It is because the bond between Ag $-$ O being covalent, the lone pair on nitrogen act as attacking site for nucleophilic substitution.

7. Replacement by Nitrite group (Formation of alkyl nitrites)

On treatment of haloalkanes with potassium nitrite alkyl nitrite is formed.

R $-$ X + KNO₂ $\to$ R $-$ O $-$ N $=$ O + KX

Alkyl nitrite

8. Replacement of halogens by Mercaptide (:SR Group)

On treating haloalkanes with sodium mercaptide, thio ethers are formed.

9. Replacement by $-$SH (Hydrosulphide) group (Formation of thiols or mercaptals)

On treating haloalkanes with KSH or Na SH thioalcohols are formed

10. Replacement by Alkynyl Group (Formation of higher alknes)

On treating halo alkanes with sodium alkynide $R\mathrm{C}\equiv \stackrel{-}{C}\stackrel{+}{N}\mathrm{a}$, higher alkynes are formed.

11. Replacement by Carboxylate Group (Formation of esters)

Haloalkanes on treatment with silver salt of carboxylic acids in ethanol give esters.

12. Replacement by Hydride Ion

Alkyl halides on reaction with lithium aluminium hydride in the presence of dry ether as solvent yield corresponding hydrocarbon.

Dehydrohalogenation Reactions or $\beta$ - elimination Reactions

When halo alkanes are heated with alcoholic KOH, they undergo dehydrohalogenation to form alkanes. These reactions are called $\beta$ - elimination because the hydrogen atom present at

$\beta$ - position of halo alkanes is removed.

CH₃ $-$ CH₂ $-$ Br + KOH (alc.) $\to$ H_­2C = CH­₂ + KBr + H₂O

Ethene

The reactivity of haloalkanes towards elimination reaction follows the order

Tertiary > secondary > Primary

This is because tertiary alkyl halides on dehydrohalogenation form most substituted alkenes which are more stable & are formed at faster rate.

Among various halides with same alkyl group the order of reactivity is

RI > RBr > RCl

In case the haloalkanes can eliminate hydrogen halide in 2 – different ways, the preferred alkane is the one which is maximum alkylated (most substituted).

for e.g.

Illustration 6. What is the function of anhyd. ZnCl₂ in the reaction of alcohols with conc. HCl (or Lucas reagent)?

Solution: The function of anhydrous zinc chloride is to help in the cleavage of C – O bond. Being a lewis acid anhydrous ZnCl₂ co-ordinates to the O-atoms of R – OH and thus weakens the C – O bond which then breaks to give carbocation. Moreover anhyd. ZnCl₂ acts as a dehydrating agent and helps the reaction to go in the forward direction.

Reactions with Metals

1. Reaction with sodium (Wurtz reaction)

Haloalkanes react with sodium in the presene of ether to form alkanes.

$2RX+2Na\stackrel{ether}{\to }R-R+2NaX$

2. Reaction with Magnesium

Haloalkanes react with magnesium in the presence of dry ether to form alkyl magnesium halide (Grignard reagents)

$RX+Mg\stackrel{dryether}{\to }RMgX$

Grignard reagents are organometallic compounds, i.e. compounds having metal carbon bond. Grignard regents are highly reactive. They react with proton donors (acids) to give hydrocarbons.

$RMgX+HZ\to R-H+MgX\left(Z\right)$

3. Reaction with Lithium

Haloalkanes react with lithium in the presence of dry ether to form alkyl lithium. These salts serve as strong bases.

$C{H}_{3}C{H}_{2}Br+2Li\to \underset{Ethyllithium}{C{H}_{3}C{H}_2}Li+LiBr$

4. Reaction with lead sodium Alloy

Ethyl bromide react with lead sodium alloy in the presence of dry ether to form tetra ethyl lead (TEL).

$4C{H}_{3}C{H}_{2}Br+4Pb\left(Na\right)\to {\left(C{H}_{3}C{H}_2\right)}_{4}Pb+4NaBr+3Pb$

Reduction

Haloalkanes can be reduced to alkanes by any of following methods

1. Reaction with H₂/Ni

$R-X+H_2\underset{525K}{\overset{Ni}{\to }}R-H+HX$

2. Reaction with zinc copper couple

In the presence of alcohols, Zn – Cu couple reduces haloalkanes to alkanes.

$\underset{\left(Zn-Cucouple\right)}{\mathrm{Zn}}+2C_2{H}_{5}OH\to Zn{\left(O{C}_2{H}_5\right)}_2+2H$

$RX+2H\to \underset{Alkane}{R-H}+HX$

Illustration 7. The order of reactivity of alkyl halides towards elimination reaction is :

Solution: (A)

Illustration 8. The end product "Z" in the following reaction, ethylamine $\stackrel{HN{O}_2}{\to }X\stackrel{PC{l}_5}{\to }Y\stackrel{N{H}_3}{\to }Z$ is

(A) Methyl amine (B) Acetamide

(C) Ethylamine (D) Propylamine

Solution: (C)

Illustration 9. Why Grignard reagents should be prepared under anhydrous conditions?

Solution: Grignard reagents react with water & get decomposed (hydrolysed) hence they should be prepared under anhydrous conditions.

Difference in Reactivity of C – X bond in Alkyl halides & Aryl halides

Aryl halides are much less reactive towards nucleophilic substitution reaction than haloalkanes. The less reactivity of aryl halides can be explained as follows:

1. Withdrawal of Electrons by benzene & stabilization by resonance

In aryl halide, the electron pair of halogen atom is in conjugation with p electrons of benzene ring. Thus halobenzene is a resonance hybrid of following structures:

The contributing structures II, III & IV indicate that C $-$ X bond has partial double bond characters.

As a resultant the C $-$ X bond in halobenzene is shorter & hence stronger as compared to that in alkyl halides. Thus cleavage of C $-$ X bond in halobenzene becomes difficult which makes it less reactive towards nucleophilic substitution.

2. Different hybrid states of carbon atom

In haloalkanes, the carbon atom bearing halogen is sp³ hydridized while halogen bearing carbon atom is sp² hybrized in haloarenes. sp² hybrid orbital is smaller in size due to greater

s – character. As a result bond in haloarenes is smaller & is cleaved with difficulty.

3. Polarity of C $-$ X Bond

The C $-$ X bond in halo alkanes is more polar than the C $-$ X bond in haloarenes. Now greater the polarity of bond more is the reactivity.

Illustration10. Benzyl chloride is more reactive than chlorobenzene towards nuclephilic substitution. Explain.

Solution: In the molecule of benzyl chloride, the electron pairs on the chlorine atom are not in a position to conjugate with the $\pi$-electrons of the ring.

whereas electron pairs on the chlorine atom are in conjugation with the

$\pi$-electrons of the ring in the molecule of chlorobenzene. Consequently the

C-Cl bond in chlorobenzene acquires some double bond character which is not present in benzyl chloride. As a result, the reactivity of chlorobenzene towards nucleophilic substitution is much less than that of benzyl chloride.

Reactions of Aryl Halides

Aryl halides being less reactive, can be made to react under drastic conditions.

Nucleophilic Substitution Reactions

1. Replacement by Hydroxyl group

On heating chlorobenzene with an aq. solution of NaOH at 623 K under 300 atm pressure, sodium phenoxide is formed which on subsequent acidification produces phenol.

2. Replacement by Cyano Group

When heated with anhydrous CuCN in the presence of pyridine or dimethyl formamide at 470 K, bromo benzene gives cyanobenzene.

3. Replacement by Amino Group

Halogen atom of haloarenes is replaced by amino group by reacting it with aq. NH₃; & in presence of catalyst, Cu₂O

Reactions with metals

1. Action with magnesium

Aryl bromides & iodides form Grignard's reagents with Mg in dry ether.

2. Reaction with Sodium

Aryl halides react with sodium in the presence of ether. During reaction two phenyl rings unite. The reaction is called Fittig reaction.

However aryl halides when treated with halo alkane & sodium in dry ether undergo Wurtz fitting reaction.

3. Reaction with Lithium

Aryl halides react with lithium metal to form the corresponding organometallic compound.

These organometallic compounds behave like Grignard reagents.

4. Reaction with Copper Powder

Iodo benzene when heated with copper powder in a sealed tube gives diphenyl

Reduction

An aryl halide is reduced to parent hydrocarbon by the action of nickel aluminium alloy in the presence of an alkali.

Illustration 11.

The above transformation proceed through

(A) Electrophilic addition (B) Benzyne intermediate

(C) Activated nucleophilic substitution (D) Oxirane

Solution: (C)

RING SUBSTITUTION REACTIONS

An aryl halide undergoes electrophilic substitution reactions in benzene ring. The presence of halogen atom in the ring directs the incoming substituent to the ortho & para position.

Aryl halides are less reactive than benzene towards electrophilic substitution reactions.

Some ring substitution reactions of aryl halides are given below:

1. Halogenation

Halogentation takes place in the presence of iron or FeCl₃ or anhydrous AlCl₃ as a catalyst.

2. Nitration

3. Sulphonation

4. Friedel Craft's Alkylation

With alkyl halide in presence of anhydrous AlCl₃, alkylation takes place for e.g.

5. Friedel Craft's Acylation

Acylation of haloarenes can be carried out with the reaction of acyl chlorides in the presence of anhydrous AlCl₃. for example

Illustration 12. Complete the following giving structures of the principal organic products.

Solution:

Illustration 13. How would you prepare 1 – iodopropane from?

(i) Propene (ii) 1 – propanol

Solution: (i) $C{H}_{3}CH=C{H}_2+HBr\stackrel{Peroxide}{\to }C{H}_{3}C{H}_{2}C{H}_{2}Br$

$C{H}_{3}C{H}_{2}C{H}_{2}Br+NaI\underset{reflux}{\overset{Acetone}{\to }}C{H}_{3}C{H}_{2}C{H}_{2}I+NaBr$