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HSC CHEMISTRY

Halogen Derivatives of Alkanes

Introduction

Halogen Derivatives of alkanes compounds in which one or more hydrogen atoms in an alkane are replaced by one or more halogen atoms. They are very important in organic synthesis since many other types of organic compounds can be prepared from them.

Classification of Haloalkanes

They are classified into mono, di, tri etc. halogen derivatives depending on the number of halogen atoms present.

Examples :

Monohalogen derivatives CH₃-CH₂-Cl ethyl chloride (chloro ethane)

Di halogen drivatives

ethylene dichloride (1,2-dichloro ethane)

Tri halogen derivatives CHCl3 chloroform (trichloro methane)

Monohalogen Derivatives or Alkyl Halides

In these one hydrogen atom in an alkane is repalced by one halogen atom.

Its general formula is R-X R = alkyl group

Classification: Alkyl halides are classified into primary, secondary and tertiary alkyl halides depending on the nature of carbon atom to which the halogen atom is attached.

Examples :

i) Primary alkyl halide CH₃-CH₂-Cl ethyl chloride (chloro ethane)

ii) Secondary alkyl halide		iso-propyhl bromide
		(2-bromo propane)

iii) Tertiary alkyl halide		t-butyl bromide
		(2-bromo 2-methyl propane)

Nomenclature

1. Common (Trivial) System : In this system monohalogen derivatives are called "Alkyl halides"

2. IUPAC System : In this system monohalogen derviatives are called "Haloalkanes"

The usual rules are followed while naming them:

(a) The longest continuous chain of carbon containing the halogen is selected as the parent chain.

(b) The parent carbon-chain is then numbered beginning from that end which gives the lowest possible number to the carbon bearing the halogen.

(c) The halogen is indicated by the prefix halo together with its position. If there are other substituents (alkyl groups), they are represented in alphabetical order with their appropriate numbers.

Methods of Preparation of Alkyl Halides

1.From Alkanes (R-H)

chlorination :

bromination :

iodination:

Since it is reversible iodination is carried out in presence of HgO or HIO₃

2R-H + 2I₂ + HgO ® 2R-I + HgI₂ + H₂O

5R-H 2I₂+HIO®5R-I+3H₂O

2. From Alkenes by Addition of HX

CH₂=CH₂ + HCl ® CH₃-CH₂Cl

ethene

ethyl chloride

CH3-CH=CH₂ + HBr		CH₃-CH-CH₃
		½
		Br
		iso-propyl bromide

CH3-CH=CH2 + HBr		CH₃-CH₂-CH₂-Br
		n-propyl bromide

3. From Alcohols (R-OH)

Chlorination :`OH' is substituted by 'Cl' by using PCl₃ (Phosphorus trichloride) PCl₅ (phosphorus penta chloride) or SOCl₂ (thionyl chloride)

3R-OH + PCl₃		3R-Cl + H₃PO₃
		_{Phosphorous acid}

R-OH + PCl₅		R-Cl + HCl _ POCl₃
		_{Phosphoryl chloride}

R-OH + SOCl₂

R-Cl + SO₂ HCl

Bromination and iodination :

Alcohol (R-OH) is heated with red phosphorus and bromine/iodine respectively

2P + 3Br

2PBr₃

3R-OH + PBr₃

3R-Br + H₃PO₃

2P + 3I₂

2PI₃

3R-OH + PI₃

3R-I + H₃PO₃

Chemical Properties of Alkyl Halides

1. Substitution reactions

a) R-X + KOH (aqueous)	® R-OH +KX
	alcohol

b) R-X + KCN (alcohol)	® R-C=N + KX
	alkyl cyanide

c) R-X + NH₃ (alcohol)		R-NH₂ +KX
		P. Amine

d) R-X + R¹-COOR₂

R'-COOR + AgX
ester

e) Williamson's Synthesis :

R-X	+ R'-ONa	® R-O-R' + NaX
	Sodium alkoxide	ethe

2. Wurtz reaction :


R-X + 2Na + X-R	®	2NaX + R-R

		higher alkane

3. Grignard reaction :

R-X+ Mg		R-MgX
		(alkyl magriesium halide)

4. Elimination reaction (Dehydrohalogenation)

CH₃-CH₂-Cl + KOH		CH₂=CH₂ + KCl + H₂O
ethyl chloride	alcohol	ethene

Dihalogen Derivatives

Preparation of ethylene dichloride

1. CH2=CH2 +Cl2	®CH2	-CH2
	½	½
ethene	Cl	Cl
	1,2-dichloro ethane

2.CH₂	-CH₂	+ 2PCl₅	CH₂	-CH₂	+ 2POCl₃ + 2HCl
½	½		½	½
OH	OH		Cl	Cl
			1,2-dichloro ethane

Preparation of ethylidene dichloride

1. CHºCH + HCl

® CH₂=CH-Cl (vinyl chloride)

CH₂=CHCl + HCl		CH₃-CHCl₂
		1,1-dichloro ethane

2. CH₃-CHO + PCl₅	® CH₃-CHCl₂ + POCl₃
Acetaldehyde	1,1-dichloro ethane

Chloroform/Trichloro Methane (CHCl₃)

Preparation :

i) CaOCl₂ + H₂O ® Ca(OH)₂ + Cl₂

ii) CH₃-CH₂OH + Cl₂ ® CH₃-CHO + 2HCl

iii) CH₃-CHO + 3Cl₂	® CCl₃-CHO + 3HCl
	Chloral

iv) 2CCl₃-CHO + Ca(OH)₂	® 2CHCl₃	+ (H-COO)₂Ca
		Calcium formate

Reactions of chloroform

i) CHCl₃ + 2[H]		CH₂Cl₂ + 2HCl
		methylene dichloride
		(Partial reduction)

ii) CHCl₃ + 6[H]		CH₄ + 3HCl
		methane
		(complete reduction)

iii) CHCl₃ + 3KOH	® CH(OH)₃	+3KCl
	¯
Alcohol	H-COOH + H₂O
	formic acid

H-COOH + KOH	® H-COOK + H₂O
	Potassium formate

iv) CHCl₃	+ HNO₃	® CCl₃NO₂ + H₂O
	(con)	chloropierin

v) 2CHCl₃	+ O₂		2COCl₂	+ 2HCl
	Air		phosgene

vi) R-NH2 + CHCl3 + 3KOH		R-NC +3KCl + H2O
P. Amine		alkyl
(carbylamine reaction)		iso-cyanide

Physical properties of chloroform :
Colourless sweet smelling liquid with boiling point 334K. It is immiscible with water and heavier than water.

Iodoform (tri-iodo methane) CHI₃

Preparation :

i) 2NaOH + I₂ ® NaI + NaOI + H₂O

ii) CH₃-CH₂-OH + NaCl ® CH₃-CHO + H₂O + Nal

iii) CH₃-CHO + 3NaOI	® CI₃-CHO + 3NaOH
	iodal

iv) CI₃-CHO + NaOH	® CHI₃ + H-COONa
	Sodium format

Physical properties of idodoform :
Pale yellow, crystalline solid, smells of iodine insoluble in water. Melting point 392 K.

Reactions of iodoform :

i) CHI₃ + 2[H]		CH₂I₂ + HI
(partial reduction)		methylene di iodide

ii) CHI₃ + 6[H]		CH₄ + 3HI
(complete reduction)		methane

iii) CHI₃ + 3KOH ®	CH(OH)₃	+ 3KI
	¯
Alcohol	H-COOH + H₂O
	formic acid

H-COOH + KOH	® H-COOK + H₂O

iv) Carbylamine reaction

R-NH₂ + 3KOH = CHI₃		R-NC+ 3KI + 3H₂O
alcohol		alkyl isocyanide

Uses :

1. Chloroform is widely used as solvent. It si also used as anaesthetic and for preparation of important compounds like chloropicrin, chloretone etc.

2. Iodoform : It is used as antiseptic. It is also used for preparation of compounds like acetylene, methane etc. Both chloroform and iodoform are used in carbylamine test.


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