Chapter 11 Halogen Derivatives Part 2 – Chemistry free study material by TEACHING CARE online tuition and coaching classes

Chapter 11 Halogen Derivatives Part 2 – Chemistry free study material by TEACHING CARE online tuition and coaching classes

File name : Chapter-11-Halogen-Derivatives-Part-2.pdf

 

 

  • Reimer-Tiemann reaction : Chloroform reacts with phenol when heated in presence of sodium hydroxide or potassium The product formed is salicylaldehyde.

 

C H OH + CHCl

  • 3NaOH ¾¾65¾°C ® C H           OH     + 3NaCl + 2H  O

 

6    5                         3

6     4       CHO                                2

 

Hydroxy benzaldehyde

  • Carbylamine reaction (Isocyanide test) : This reaction is actually the test of primary amines. Chloroform, when heated with primary amine in presence of alcoholic potassium hydroxide forms a derivative called isocyanide which has a very offensive

 

RNH2  + CHCl3  + 3KOH(alc.) ¾¾D ®

RNC

Carbylaminoalkane

(Alkyl isonitrile)

  • 3KCl + 3H2O

 

This reaction is also used for the test of chloroform.

(4)  Uses

  • It is used as a solvent for fats, waxes, rubber, resins, iodine,
  • It is used for the preparation of chloretone (a drug) and chloropicrin (Insecticide).
  • It is used in laboratory for the test of primary amines, iodides and
  • It can be used as anaesthetic but due to harmful effects it is not used these days for this
  • It may be used to prevent putrefaction of organic materials, e., in the preservation of anatomical species.

(5)  Tests of chloroform

  • It gives isocyanide test (Carbylamine test).
  • It forms silver mirror with Tollen’s
  • Pure Chloroform does not give white precipitate with silver

Iodoform or tri-iodomethane, CHI3

Iodoform resembles chloroform in the methods of preparation and properties.

(1)  Preparation

  • Laboratory preparation : Iodoform is prepared in the laboratory by heating ethanol or acetone with iodine and

Ethanol : CH3CH2OH + I2 ¾¾® CH3CHO+ 2HI

Acetaldehyde

CH3CHO + 3I2 ¾¾® CI3CHO+ 3HI

Iodal

CI3CHO         + KOH ¾¾® CHI3 + HCOOK

 

Triiodoacetaldehyde

Iodoform

Pot. formate

 

Acetone :

CH3COCH3 + 3I2 ¾¾® CI3COCH3 + 3HI

Tri- iodoacetone

CI3COCH3 + KOH ¾¾® CHI3 + CH3COOK

 

Iodoform        Pot. acetate

Sodium carbonate can be used in place of KOH or NaOH. These reactions are called iodoform reactions.

  • Industrial preparation : Iodoform is prepared on large scale by electrolysis of a solution containing ethanol, sodium carbonate and potassium The iodine set free, combine with ethanol in presence of alkali to

 

form iodoform. The electrolysis carried out in presence of CO2

and the temperature is maintained at 60-70°C.

 

KI

Cathode

K + + I

 

 

 

K + + e ® K

2I   ® I 2 + 2e

 

K + H O ¾¾® KOH + 1 H

2                                  2   2

 

 

 

KOH is neutralised by CO2 : C2 H5OH + 4 I2 + 3Na2CO3 ¾¾® CHI3 + HCOONa + 5 NaI + 3CO2 + 2H2O

(2)  Properties

  • It is a yellow crystalline
  • It has a pungent characteristic
  • It is insoluble in water but soluble in organic solvents such as alcohol, ether,
  • It has melting point 119°C. It is steam

 

(3)  Reactions of iodoform

KOH

HCOOK

 

Hydrolysis             Potassium formate

 

 

 

 

CHI3

Reduction Red P/HI

 

Heating

Ag powder

CH2I2

Methylene iodide

 

CH º CH

Acetylene

 

Carbylamine reaction

C H  NC

 

C6H5NH2+KOH (alc.)               6  5

Phenol isocyanide

 

 

Heating alone

 

 

 

With AgNO3

Iodine vapours, 4CHI3+3O2 ® 4CO + 6I2 + 2H2O

(Less stable than CHCl3)

 

Yellow precipitate of AgI

(This reaction is not given by chloroform)

 

 

  • Uses : Iodoform is extensively used as an antiseptic for dressing of wounds; but the antiseptic action is due to the liberation of free iodine and not due to iodoform When it comes in contact with organic matter, iodine is liberated which is responsible for antiseptic properties.

(5)  Tests of iodoform

 

  • With AgNO3 : CHI3

gives a yellow precipitate of AgI .

 

  • Carbylamine reaction :

CHI3

on heating with primary amine and alcoholic KOH solution, gives an

 

offensive smell of isocyanide (Carbylamine).

 

  • Iodoform reaction : With I 2

and NaOH or

O

||

I 2 and

Na2CO3 , the iodoform test is mainly given by ethyl

O

||

 

alcohol

(CH3CH2OH), acetaldehyde

(CH3CH),

a-methyl ketone or 2-one

(- CCH3),

secondary alcohols or

 

  • ol

(-CHOH × CH3)

O

||

and secondary alkyl halide at

C2 (-CHCICH3 ) . Also lactic acid ( CH3 – CHOH COOH) ,

O

||

 

Pyruvic acid (CH3 – CCOOH) and methyl phenyl ketone (C6 H5 – CCH3 ) give this test.

It is the most important tetrahalogen derivative of methane.

(1)  Manufacture

  • From methane : Chlorination of methane with excess of chlorine at 400°C yields impure carbon

CH4  + 4Cl2 ¾¾400¾°¾C ® CCl4  + 4 HCl

 

 

 

Methane used in this process is obtained from natural gas.

  • From carbon disulphide : Chlorine reacts with carbon disulphide in presence of catalysts like iron, iodine, aluminium chloride or antimony

 

CS2 + 3Cl2 ¾¾® CCl4 +

S2Cl2

Sulphur

monochloride

 

S2Cl2

further reacts with CS2 to form more of carbon tetrachloride.

CS2 + 2S2Cl2 ¾¾® CCl4 + 6S

 

Carbon tetrachloride is obtained by fractional distillation. It is washed with sodium hydroxide and then distilled to get a pure sample.

  • From propane : Propane is reacted with chlorine at about 400°C and at a pressure of 70-100

 

C3 H8  + 9Cl2  ¾¾He¾at ®

Pressure

 

 

(2)  Physical properties

CCl4          +

Carbon tetrachloride

(Liquid)

C2Cl6

Hexachloroethane (Solid)

  • 8HCl

 

  • It is a colourless liquid having characteristic
  • It is non-inflammable and It has boiling point 77°C.
  • It is insoluble in water but soluble in organic
  • It is an excellent solvent for oils, fats, waxes and
  • Chemical properties : Carbon tetrachloride is less reactive and inert to most organic However, the following reactions are observed.
    • Reaction with steam (Oxidation) : Carbon tetrachloride vapours react with steam above 500°C to form phosgene, a poisonous

 

CCl4  + H2O ¾¾500¾°¾C ®

COCl2

Phosgene (Carbonyl chloride)

  • 2HCl

 

  • Reduction : It is reduced by moist iron filling into

CCl4  + 2H ¾¾Fe /¾HO ® CHCl3  + HCl

  • Hydrolysis : On heating with aqueous potassium hydroxide it forms carbon dioxide which combines with potassium hydroxide to give KCl and potassium carbonate (Inorganic salts).

CCl4  + 4 KOH ¾¾-4K¾¾Cl ®[C(OH)4 ] ¾¾-2H¾2¾O ® CO2 ¾¾2KO¾H ® K2CO3  + H2O

Unstable

  • Reaction with phenol (Reimer-tiemann reaction) : It combines with phenol in presence of sodium hydroxide to form salicylic

 

 

C H OH + CCl

¾¾+ 4 N¾aO¾H ® C  H              OH       + 4 NaCl + 2H  O

 

6    5                     4

6   4    COOH                                 2

 

(4)  Uses

Salicylic acid

 

  • It is used as a fire extinguisher under the name pyrene. The dense vapours form a protective layer on the burning objects and prevent the oxygen or air to come in contact with the burning
  • It is used as a solvent for fats, oils, waxes and greases, resins, iodine

 

  • It finds use in medicine as helmenthicide for elimination of hook

Vinyl chloride or chloroethene, CH2=CHCl

  • Synthesis : Vinyl chloride can be synthesised by a number of methods described below:
    • From ethylene chloride : It is easily prepared in the laboratory by the action of dilute alcoholic solution of potassium hydroxide on ethylene

 

CHCl

|

CH2Cl

+ Alc. KOH

CHCl

||

CH2

  • KCl + H 2O

 

Ethylene chloride                                         Vinyl chloride

Vinyl chloride can also be obtained from ethylene chloride by thermal decomposition at 600-650°C.

 

CHCl

|

CH2Cl

CHCl + HCl

||

CH2

 

  • From ethylene : Free radical chlorination of ethylene at 500°C yields vinyl

CH2  = CH2  + Cl2 ¾¾500¾°¾C ® CH2  = CHCl

Vinyl chloride

  • From acetylene : Vinyl chloride is obtained by controlled addition of HCl on acetylene. Acetylene is

 

passed through dilute hydrochloric acid at about 70°C in presence of This method is also used for its manufacture.

CH  º CH + HCl ¾¾HgC¾l2  ® CH2  = CHCl

HgCl2

as a catalyst to form vinyl chloride.

 

70°C

Vinyl chloride

 

  • Properties : It is a colourless gas at room Its boiling point is –13°C. The halogen atom in

 

vinyl chloride is not reactive as in other alkyl halides. However, addition reactions.

C = C

bond of vinyl chloride gives the usual

 

The non-reactivity of chlorine atom is due to resonance stabilization. The lone pair on chlorine can participate in delocalization (Resonance) to give two structures.

. .                                   +

CH2 = CH Cl ¬¾® C H2 – CH = Cl

  • (ii)

The following two effects are observed due to resonance stabilization.

  • Carbon-chlorine bond in vinyl chloride has some double bond character and is, therefore, stronger than a pure single

 

  • Carbon atom is

sp2

hybridized and

CCl

bond length is shorter (1.69Å) and stronger than in alkyl

 

halides (1.80Å) due to

sp3

hybridization of the carbon atom.

 

 

 

 

Addition reactions

Br2 CCl4

CH2Br CHBrCl

1,2-Dibromo-1-Chloroethane

 

 

HBr

CH3CHBrCl

 

1-Bromo-1-Chloroethane

 

CH2 = CHCl

 

Polymerisation Peroxide

é

ê
2

ê- CH

Cl

|

  • C H
  • CH2

Cl         ù

|          ú

  • C H  ú

 

ê          Polyvinyl chloride (PVC)              ú

ë                                           ûn

 

NaOH

No reaction

 

 

 

  • Uses : The main use of vinyl chloride is in the manufacture of polyvinyl chloride (PVC) plastic which is employed these days for making synthetic leather goods, rain coats, pipes, floor tiles, gramophone records, packaging materials,

Allyl iodide or 3-iodopropene-1, ICH2CH = CH2

  • Synthesis : It is obtained,
    • By heating allyl chloride with sodium iodide in Allyl chloride required in the reactions is prepared

 

either by chlorination of propene at 500°C or by action of

CH3CH  = CH2 + Cl2 ¾¾500¾°¾C ® CH2 – CH  = CH2

PCl3

on allyl alcohol.

 

Propene

|

Cl

Allyl chloride

 

Or  3 C H 2 – CH  = CH 2 + PCl3  ¾¾He¾at ® 3 C H 2 – CH  = CH 2  + H3 PO3

|                                                                          |

OH                                                                     Cl

Allyl alcohol

C H 2  – CH  = CH2  + NaI ¾¾Ace¾to¾ne ® C H 2 – CH  = CH 2 + NaCl

 

|

Cl

Allyl chloride

Heat

|

I

Allyl iodide

 

This is halogen- exchange reaction and is called Finkelstein reaction.

  • By heating glycerol with HI.

 

CH2OH

|

CHOH

|

2

CH2OH

+ 3HI ¾¾- 3 H¾¾O ®

CHI

|

CHI

|

CH2 I

¾¾He¾at ®

I2

CHI

|

CH

||

CH2

 

Glycerol

1,2,3-Tri-iodopropane

Allyl iodide

 

  • Properties : It is a colourless It boils at 103.1°C.The halogen atom in allyl iodide is quite reactive. The p-orbital of the halogen atom does not interact with pmolecular orbital of the double bond because these are

separated by a saturated sp3 -hybridized carbon atom. Thus, the halogen atom in allyl halides can be easily

replaced and the reactions of allyl halides are similar to the reaction of alkyl halides.

In terms of valence bond approach, the reactivity of halogen atom is due to ionisation to yield a carbonium ion which can stabilize by resonance as shown below,

 

CH2

= CH CH2

I ¾¾®[CH2

+

= CH CH2

+

¬¾® CH2

  • CH = CH2

] + I

 

Substitution reactions : Nucleophilic substitution reactions occur,

 

 

NaOH

CH2

= CH CH2OH

Allyl alcohol

 

 

 

CH2 = CHCH2I

CH2

 

CH

= CH CH2CN

Allyl cyanide

 

= CH CH NH

 

2                          2       2

Allyl amine

CH = CH CH OCH

2                          2           3

Allyl methyl ether

 

CH = CH CH NO

2                          2       2

3-Nitropropene-1

Addition reactions : Electrophilic addition reactions take place in accordance to Markownikoff’s rule.

 

 

 

 

CH2 = CH CH2 I + Br2 ¾¾® CH2 Br × CHBr × CH2 I   ;

1,2-Dibromo- 3-iodopropane

Allyl iodide is widely used in organic synthesis.

CH2  = CH CH2 I + HBr ¾¾® CH3CHBrCH2 I

2-Bromo-1-iodopropane

 

 

 

In these compounds the halogen is linked directly to the carbon of the benzene nucleus.

  • Nomenclature : Common name is aryl halide IUPAC name is halo-arene.

Example :          Cl                                 Br

 

 

;

Benzylchloride Chlorobenzene

 

(2)  Structure

 

Benzylbromide Bromobenzene

 

 

(3)  Methods of preparation

  • By direct halogination of benzene ring

 

 

 

+ X2

¾¾Lew¾is a¾c¾id ®

X + HX

Lewis acid = FeX3 , AlX3 ,Tl(OAC)3 ;

X 2  = Cl2 , Br2

 

 

 

 

 

  • From diazonium salts

 

CuCl

C6H5Cl

 

C6H5Br C6H5I C6H5F

 

 

 

C H NH

¾¾NaN¾O2¾, H¾Cl  ®              Å            –

 

6     5         2

C

C6 H5 N 2  Cl

 

 

 

 

3       2
  • Hunsdiecker reaction : C6 H5COO Ag + ¾¾B¾r2 ® C6 H5 Br + CO2 + AgBr

 

  • From Aryl thalium compound :

 

(4)  Physical properties

ArH + Tl(OOCCF3 )3 ¾¾-CF¾CO¾¾H  ®

ArTl(OOCF3 )2 ¾¾KI ® ArI

D

Aryl thallium trifluoroacetate

 

  • Physical state : Haloarenes are colourless liquid or crystalline
  • Solubility : They are insoluble in water, but dissolve readily in organic Insolubility is due to inability to break hydrogen bonding in water. Para isomer is less soluble than ortho isomer.
  • Halo-arenes are heavier than
  • P. of halo-arenes follow the tend. Iodo arene > Bromo arene > Chloro arene.

(5)  Chemical properties

2

Inert nature of chlorobenzene : Aryl halides are unreactive as compared to alkyl halides as the halogen atom in these compounds is firmly attached and cannot be replaced by nucleophiles. Such as OH, NH ,CN etc.

 

ClÅ

ClÅ

 

 

 

Thus delocalization of electrons by resonance in aryl halides, brings extra stability and double bond character

between C X bond. This makes the bond stronger and shorter than pure single bond. However under vigorous

conditions the following nucleophilic substitution reactions are observed,

  • Nucleophilic displacement : C6 H5 Cl ¾¾NaO¾H,¾350¾°¾C ® C6 H5OH

500 atm.

 

  • Electrophilic aromatic substitution

Cl

Cl                                          Cl

 

 

+ HNO3

H2SO4                               NO2     +

 

 

 

Cl                                      Cl

 

+

 

Br

Cl                          Cl

Br ;

NO2

 

CH3Cl

AlCl3

Cl

CH3   +

Cl

 

 

 

CH3

 

  • Wurtz – fittig reaction : C6 H5 Br + CH3 Br ¾¾N¾a ® C6 H5CH3 + 2NaBr

Ether

  • Formation of grignard reagent : C6 H5 Br ¾¾M¾g ® C6 H5 MgBr

Ether

  • Ullmann reaction

 

2            I                    Cu                                                                                            +

CuI2

 

 

 

  • Freons : The chloro fluoro derivatives of methane and ethane are called Some of the derivatives

 

are:

CHF2Cl (monochlorodifluoromethane),

CF2Cl2

(dichlorodifluoromethane),

HCF2CHCl2

(1,1-dichloro-2,2-

 

 

difluoroethane). These derivatives are non-inflammable, colourless, non-toxic, low boiling liquids. These are stable upto 550°C. The most important and useful derivative is CF2Cl2 which is commonly known as freon and freon-12.

 

Freon or freon-12

(CF2Cl2 )

is prepared by treating carbon tetrachloride with antimony trifluoride in the

 

presence of antimony pentachloride as a catalyst. 3CCl4  + 2SbF3  ¾¾SbC¾l5  ® 3CCl2 F2  + 2SbCl3

Catalyst

Or it can be obtained by reacting carbon tetrachloride with hydrofluoric acid in presence of antimony pentafluoride.

CCl4  + 2HF ¾¾Sb¾F5  ® CCl2 F2  + 2HCl

Under ordinary conditions freon is a gas. Its boiling point is –29.8°C. It can easily be liquified. It is chemically inert. It is used in air-conditioning and in domestic refrigerators for cooling purposes (As refrigerant). It causes depletion of ozone layer.

  • Teflon : It is plastic like substance produced by the polymerisation of tetrafluoroethylene (CF2 = CF2) . Tetrafluoroethylene is formed when chloroform is treated with antimony trifluoride and hydrofluoric

CHCl3 ¾¾Sb¾F3  ® CHF2Cl ¾¾800¾°¾C ® CF2  = CF2

 

HF                                               HCl

(b.pt.-76°C)

 

On polymerisation tetrafluoroethylene forms a plastic-like material which is called teflon.

nCF2  = CF2  ¾¾®(-CF2  – CF2 -)n

Tetrafluoroethylene                          Teflon

Teflon is chemically inert substance. It is not affected by strong acids and even by boiling aqua-regia. It is stable at high temperatures. It is, thus, used for electrical insulation, preparation of gasket materials and non-sticking frying pans.

  • Acetylene tetrachloride (Westron), CHCl2CHCl2 : Acetylene tetrachloride is also known as sym. It is prepared by the action of chlorine on acetylene in presence of a catalyst such as ferric chloride, aluminium chloride, iron, quartz or kieselguhr.

 

CH º CH + 2Cl2 ¾¾®

CHCl2 × CHCl2

(1,1,2,2- Tetrachloroethane)

 

In absence of catalyst, the reaction between chlorine and acetylene is highly explosive producing carbon and

HCl. The reaction is less violent in presence of a catalyst.

It is a heavy, non-inflammable liquid. It boils at 146°C. It is highly toxic in nature. Its smell is similar to chloroform. It is insoluble in water but soluble in organic solvents.

On further chlorination, it forms penta and hexachloroethane. On heating with lime (Calcium hydroxide), it is converted to useful product westrosol (CCl2 = CHCl) .

2CHCl2 – CHCl2 + Ca(OH)2 ¾¾® 2CHCl = CCl2 + CaCl2 + 2H 2 O

 

Westron

Westrosol (Trichloroethene)

 

Both westron and westrosol are used as solvents for oils, fats, waxes, resins, varnishes and paints, etc.

  • p-Dichlorobenzene : It is prepared by chlorination of

It is a white, volatile solid having melting point of 325 K, which readily sublimes. It resembles chlorobenzene in their properties.

It is used as general insecticides, germicide, soil fumigant deodorant. It is used as a larvicide for cloth moth and peach tee borer.

  • DDT; 2, 2-bis (p-Chlorophenyl) –1,1,1-trichloroethane : It is synthesised by heating a mixture of

 

chloral (1mol) with chlorobenzene (2mol) in the presence of concentrated

H2SO4 .

 

 

 

 

 

 

 

 

 

CCl3

H

|

– C =

Cl

 

Conc. H2SO4

H

|

Cl3C – C

Cl

 

+ H2O

 

 

Cl                                                                                               Cl

 

 

Chloral (1mol)

Chlorobenzene (2mol)

D.D.T.

 

 

Properties and uses of D.D.T.

  • D.T. is almost insoluble in water but it is moderately soluble in polar solvents.
  • D.T. is a powerful insecticide. It is widely used as an insecticide for killing mosquitoes and other insects.

Side Effects of D.D.T. : D.D.T. is not biodegradable. Its residues accumulate in environment and its long term effects could be highly dangerous. It has been proved to be toxic to living beings. Therefore, its use has been abandoned in many western countries. However, inspite of its dangerous side effects, D.D.T. is still being widely used in India due to non-availability of other cheaper insecticides.

  • BHC (Benzene hexachloride), C6H6Cl6 : It is prepared by chlorination of benzene in the presence of

 

sunlight.

 

+

 

Benzene

 

3Cl2

 

 

Sunlight

Cl

Cl                 Cl

 

Cl                 Cl

Cl

BHC

 

Uses : It is an important agricultural pesticide mainly used for exterminating white ants, leaf hopper, termite, etc. It is also known by the common name gammaxene or lindane or 666.

 

Note : ® aaaeee conformation of C6 H6Cl6

is most powerful insecticide.

 

  • Perfluorocarbons (PFCs) : Perfluorocarbons

(Cn F2n+ 2 )

are obtained by controlled fluorination of

 

vapourized alkanes diluted with nitrogen gas in the presence of a catalyst.

 

C7 H16  + 16F2  ¾¾Vap¾our¾pha¾se, ¾N2 ,¾573¾K  ®

CoF2 (Catalyst)

C7 F16

Perfluoroheptane

  • 16HF

 

These are colourless, odourless, non-toxic, non-corrosive, non-flammable, non-polar, extremely stable and unreactive gases, liquids and solids. These are stable to ultraviolet radiations and other ionising radiations and therefore, they do not deplete the ozone layer like freons.

These are good electrical insulators. These have many important uses such as :

  • These are used as lubricants, surface coatings and
  • These are used as heat transfer media in high voltage electrical
  • These are used for vapour phase soldering, gross leak detection of sealed microchips in electronic industry.
  • These are also used in health care and medicine such as skin care cosmetics, wound healing, liquid ventilation, carbon monoxide poisoning and many medical

 

 

 

 

Organic compounds in which a metal atom is directly linked to carbon or organic compounds which contain

at least one carbon-metal bond are called organometallic compounds.

 

Example : Methyl lithium

¾¾® CH3Li ; Dialkyl zinc

¾¾® R2Zn ; Alkyl magnesium halide

¾¾® RMgX

 

(1)  Methyl lithium :

CH3 I

Methyl iodide

  • 2Li ¾¾Eth¾er ®

-10°C

CH3 Li

Methyl lithium

  • LiI

 

Note : ® High reactivity of CH3 Li

over grignard reagent is due to greater polar character of C Li bond in

 

comparison to C Mg bond.

Chemical properties

  • CH3 – Li + H × OH ¾¾® CH4 + LiOH
  • CH3 – Li + CH2 – CH2 ¾¾® CH3CH2CH2OLi ¾¾HO ® CH3CH2CH2OH + LiOH O

 

 

  • CH3
  • Li + CO2

¾¾® CH3

O

–                                          H2O

||

COLi ¾¾¾® CH

3COOH + LiOH

 

  • CH3 – Li + HC = O ¾¾® CH3CH2  – OLi ¾¾HO ® CH3CH2OH + LiOH

|

H

Note : ® Unlike grignard reagents, alkyl lithium can add to an alkenic double bond.

  • R Li + CH2 = CH2 ¾¾® R CH2 – CH2 – Li
  • Dialkyl zinc : First organometallic compound discovered by Frankland in

 

2RI + 2Zn ¾¾He¾a¾t ® 2RZnI ¾¾He¾a¾t ®

R2 Zn + ZnI 2

 

CO2

Chemical properties

CO2

Dialkyl zinc

 

Preparation of quaternary hydrocarbon : (CH3 )3 CCl + (CH3 )2 Zn ¾¾®(CH3 )4 C+ CH3 ZnCl

Neopentane

  • Grignard reagent : Grignard reagent are prepared by the action of alkyl halide on dry burn magnesium in presence of alcohol free dry

Dry ether dissolves the grignard reagent through solvolysis.

 

C2H5          R

|                        |

: O :           Mg

|                        |

C2H5               X

C2H5

|

: O :

|

C2H5

 

Grignard reagents are never isolated in free sate on account of their explosive nature.

Note : ® For given alkyl radical the ease of formation of a grignard reagent is, Iodide > Bromide > Chloride

Usually alkyl bromides are used.

  • For a given halogen, the ease of formation of grignard reagent is, CH3 X > C2 H5 X > C3 H7 X……….
  • Since tertiary alkyl iodides eliminate HI to form an alkene, tertiary alkyl chlorides are used in their
  • Grignard reagent cannot be prepared from a compound which consists in addition to halogen, some

reactive group such as – OH because it will react rapidly with the grignard reagent.

 

 

 

The C Mg bond in grignard reagent is some what covalent but highly polar.

 

| d –                                    d +

d –         d +

 

  • C Mg X

|

or   RMg X

 

The alkyl group acts as carbanion. The majority of reaction of grignard reagent fall into two groups:

  • Double decomposition with compound containing active hydrogen atom or reactive halogen atom

 

RMgX + HOH ¾¾® RH + Mg(OH)X

; RMgX + D2O ¾¾® RD + Mg(OD)X

 

RMgX + ROH ¾¾® RH + Mg(OR‘)X ;

RMgX + RNH2 ¾¾® RH + Mg(RNH)X

 

RMgX + RI ¾¾® R R‘+MgIX

; RMgX + ClCH2OR‘ ¾¾® RCH2OR‘+MgClX

 

  • Addition reaction with compounds containing

C = O ;

  • C º N,

C = S

etc.

 

 

C = O + RMgX ¾¾®

H

CO

|

R

H

OH

MgX ¾¾®

 

OH

COH + Mg        OH

|                                 X

R

OH

 

  • C º N + RMgX ¾¾® – C = N MgX ¾¾® – C = O + NH 3 + Mg
R
R

| O H2                                |                                            X

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