Alkyl Halides

• Physical Properties of Alkyl Halides

Because of greater molecular weight, haloalkanes have considerably higher boiling points than alkanes of the same number of carbons. For a given alkyl group, the boiling point increases with increasing atomic weight of the halogen, so that fluoride has the lowest boiling, and iodide the highest boiling point. In spite of polarity alkyl halides are insoluble in water, probably because of their inability to form hydrogen bonds. They are soluble in typical organic solvents.

• Preparation of Alkyl Halides

1. From Alcohols (Replacement of OH by X)

Examples :

2.Halogenation of  Hydrocarbons 

Examples

3.Side Chain Halogenation of Alkylbenzenes

4. Addition of Hydrogen Halides to Alkenes 

5. Addition of  Halogens to Alkenes and Alkynes 

6. Halide Exchange

An alkyl iodide is prepared often from the corresponding bromide or chloride by treatment with a solution of sodium iodide in acetone ; the less soluble bromide or sodium chloride precipitates from the solution and can be removed by filtration.A halide ion is an extremely weak base. Its reluctance to share its electrons is shown by its great tendency to release a hydrogen ion, that is, by the high acidity of the hydrogen halides.

When attached to carbon, halogen can be readily displaced as halide ion by other, stronger bases. These bases have an unshared pair of electrons and are seeking a relatively positive site, i.e., are seeking a nucleus with which to share their electrons.Alkyl halides are nearly always prepared from alcohols, which are available commercially or are readily synthesized. Although certain alcohol tend to undergo rearrangement during replacement of -OH by -X, this tendency can be minimized by use of phosphorus halides.Certain halides are best prepared by direct halogenation. The most important of these preparations involve substitution of -X for the unusually reactive allylic or benzylic hydrogens.

Basic, electron rich reagents are called nucleophilic reagents. The typical reaction of alkyl halides is nucleophilic substitution.

• Reactions of Alkyl Halides

1.   Nucleophilic Substitution 

Hydrolysis:RX  + OH– → ROH  +  X–

Williamson Synthasis: R-ONa +R'X → R-R' + NaX

Reaction with dry silver oxide: 2R-X + Ag2O → R-O-R

Reaction with sodio-Alkynides: R-C≡C-Na +X-R→ R-C=C-R +NaX

Reaction with potassium-cyanide: KCN+X-R→ RCN +KX

Reaction with silver-cyanide: AgCN+X-R→ RNC +AgX

Reaction with silver-nitrite: AgNO2+X-R→ RNO2 +AgX

Reaction with potassium-nitrite: KNO2+X-R→ R-O-N=O +KX

Fridal Craft Reaction: R-X + C6H6 + AlCl3→C6H5-R

Malonic Ester Synthasis: R-X + -CH(CO2C2H5)2 →R-CH(CO2C2H5)2  +HX

Acetoacetic Ester Synthasis: R-X + -CH(CO2CH3)2 →R-CH(CO2CH3)2 +HX

Reaction with Ammonia: R-X +NH3→ R-NH2 +HX

Wurtz Reaction: 2R-I+ 2Na →R—R + 2NaI

Dehydrohalogenation: CH3.CH2.CH2Br + alco.KOH → CH3–CH = CH2 + KBr + H2O 
Reaction with alcoholic AgNO3:  R-X +AgNO3 → R+ + AgX↓+HNO3 

2.   Dehydrohalogenation

Elimination

3.   Preparation of Grignard reagent

4. Reduction

 RX + M + H⁺ → RH + M⁺ + X^-

Examples :

CH₃)₃C Cl  (CH₃)₃C MgCl (CH₃)₃CD

Characteristic Reaction Chart For Alkyl Halides

• Dihalides?

Two H atoms of alkanes are replaced by two halogen atoms to form dihalides. The General formula of dihalides is C_nH_2nX₂.

Dihalides are classified as :

(a)Gem dihalides :

The term gem has derived from geminal meaning for same position.The two similar halogen atoms are attached to same carbon atom e.g.

(b) vic dihalides :  

The term vic has been derived from vicinal meaning for adjacent carbon atoms.

The two halogens are attached one each on adjacent carbon atoms e.g.

 (c) α-ω diahlides :

In these compounds halogen atoms are attached to terminal carbon atoms and are separated by three or more carbon atoms. They are also known as polymethylene halides e.g.

CH₂X-CH₂-CH₂-CH₂X      tetra methylene dichloride or 1,  4-dihalobutane

(iv) vic and gem diahalides are position isomers to each other.

• Preparation of Dihalides

(i)   By alkenes and alkynes :

CH₂=CH₂ + X₂          →        XCH₂-CH₂X

                                           ^{vicinal dihalide}

CH≡CH + 2HX          →        CH₃-CHX₂

                                          ^{geminal dihalide}

(ii)   By the action of PCI₅ :

• Chemical Nature of Dihalides

Lower members are colourless, oily liquids with sweat smell. Higher members are solid.The reactivity of gem dihalides is lesser than alkyl halides, which might be due to the fact that in presence of one halogen atom (with a strong attracting effect i.e. 

-I effect), the other cannot be so easily replaced. However vic dihalides have same order of reactivity as alkyl halides.

Thus reactivity order : vic dihalides > gem dihalides. These are heavier than water. The relative density of CH₂I₂ is 3.325 g/ml which is the 2^nd heaviest liquid known after Hg.Some of the important reactions of dihalides are given below.

(a) Action of KOH_alc :  Both vic and gem dihalides give same products

CH₂X - CH₂X or CH₃-CHX₂  + alcoho. KOH →   CH≡CH   +2HX

b)   Acton of Zn dust :  Both vic and gem dihalides give same products.

CH₂CHX₂ or XH₂C-CH₂X  + Zn dust →  CH₂=CH₂                                                                              

(c)   Action of KOHaq

It is a distinction test for gem & vic dihalides.

A gem dihalide gives either aldeyde or ketone on hydrolysis by KOH_aq ; A vicinal dihalide gives 1, 2-diol.

(d)  Action of KCN followed with hydrolysis and heating the product formed.

A distinction test for geminal and vicinal dihalides.

A geminal dihalide gives acid whereas vicinal dihalide gives anhydride if subjected to action of KCN followed with hydrolysis & heating the product formed.

Note :

-CN gp on acid hydrolysis always converts to -COOH.

Two -COOH gp on one carbon atom on heating always lose CO₂ to form mono carboxylic acid.

Two -COOH gp on vicinal carbon atom, on heating always lose H₂O to form anhydride of acid.

• General Methods of Preparation of Trihalides

(i) The trihalogen derivative of alkanes are prepared by replacement of three hydrogen atoms by three halogen atoms.

(ii) Their general formula is C_nH_2n-1 X₃ :

(iii) The trihalogen derivative of methane are also known as haloform

• Haloform or trihalo methane CHX₃ vis a vis chloroform (CHCI₃)

Preparation :

(i) Lab method and Industrial method : By the distillation of ethyl alcohol or acetone with bleaching Powder :

Bleaching powder on hydrolysis gives slaked lime and CI₂ which acts as oxidizing agent as well as chlorinating agent.

CaOCI₂ + H₂O → Ca(OH)₂ + CI₂

By ethanol :

CI₂ + CH₃CH₂OH → CH₃CHO + 2HCI (CI₂ acts as oxidant)

CH₃CHO + 3CI₂ → CCI₃CHO + 3HCI (CI₂ acts as substituent)

2CCI₃CHO + Ca(OH)₂ → 2CHCI₃ + (HCOO)₂Ca (hydrolysis)

^{chloroform      Ca formate} 

By acetone :

CH₃COCH₃ + 3CI₂ → CCI₃COCH₃ + 3HCI (CI₂ acts as substituent)

2 CCI₃COCH₃ + Ca(OH)₂ → 2 CHCI₃ + (CH₃COO)₂Ca (hydrolysis)

Chloroform is collected with water in lower layer. It is washed with dilute alkali and dried over CaCI₂ and then redistilled at 60-65^oC.The yield is better if acetone is used.

(ii)  By haloform reaction :

Acetaldehyde and all methyl ketones (2-ones) or carbonyl compounds having CH₃CO^-  units as well as alcohols [primary (only ethanol) and secondary (only 2-ol) which produce this unit on oxidation by halogen undergo haloform reaction on heating with halogen and NaOH to give haloform.

If I₂ + NaOH is used, the haloform reaction yields yellow solid as precipitate confirming the presence of CH₃CO^- unit or CH₃CH^-OH in the molecule attached to C or H.

This is known as iodoform test for confirming the presence of acetaldehyde, methyl ketones and alcohols (which produce  of CH₃CO^-unit  on oxidation) e.g.

Reactants which give iodoform test because of  CH₃CO^- unit or producing this unit during oxidation :

In general, acetaldehyde, 2-one, ethanol & sec alcohols (2-ol) give the idoform test. Also pyruvic acid CH₃COCOOH, lactic acid CH₃CHOHCOOH and acetophenone C₆H₅COCH₃ give this test.The reactions are :

By ethanol

CH₃CH₂OH + X₂      →        CH₃CHO + 2HX

CH₃CHO + 3X₂        →        CX₃CHO + 3HX

CX₃CHO + NaOH      →       CHX₃ + HCOONa

5HX + 5NaOH          →        5NaX + H₂O

CH₃CH₂OH + 4X₂ + 6           →        CHX₃ + HCOONa + 5NaX + 5H₂O

By ethanal :

CH₃CHO + 3X₂ →  CX₃CHO + 3HX

CX₃CHO + NaOH  →  CHX₃ + HCOONa

3HX + 3NaOH  → 3NaX + 3H₂O

CH₃CHO + 3X₂ + 4NaOH  → CHX₃ + HCOONa + 3NaX + 3H₂O

Note :

(a)  Ethyl acetoacetate  (CH₃COCH₂COOC₂H₅) does not give iodoform test, although it contains (CH₃CO-gp) attached to carbon (methylene gp). This is due to active nature of methylene group at which iodination occurs and not on the methyl group of  (CH₃CO^-) unit

(b)  Certain quinines, quinols and m-dihydric phenols also give positive iodoform test.

(iii) Pure chloroform is obtained by heating chloral hydrate with concentrate sodium hydroxide solution.

CCI₃CH(OH)₂ + NaOH  CHCI₃ + HCOONa + H₂O

(iv)  By chlorination of methane :

CH₄ + CI₂  CHCI₃ + 3HCI

(v) By CCI₄ a commercial method : Commercially it is obtained by the partial reduction of CCI₄ with iron fillings and water (steam). This chloroform is not pure and is used only as solvent.

CCI₄ + 2H  CHCI₃ + HCI

Note : Iodoform is commercially obtained by electrolysis of a solution containing ethanol, Na₂CO₃ and KI. The liberated I₂ during electrolysis brings in iodoform reaction with C₂H₅OH in presence of Na₂CO₃.

• Properties of Trihalides

Pure chloroform and bromoform are colourless liquids, and iodoform is yellow solids.

All are heavier than water and soluble in organic solvents, but insoluble in water.

CHCI₃ brings temporary unconsciousness when vapours are inhaled for sufficient time and thus used as anaesthetic agent.

CHCI₃ is non inflammable but like other halides its vapours when ignited on Cu wire burn with green edge flame. (Beilstein test).

• Chemical Nature of Trihalides

1.   Oxidation :

On exposure to air and sunlight, chloroform is slowly oxidized to a poisonous gas carbonyl chloride i.e. phoszene

Therefore purity of chloroform should be checked before its use as anaesthetic agent. Pure CHCI₃ does not give white ppt. of AgCI with AgNO₃. Also pure chloroform neither turns blue litmus to red nor give black colour on shaking with H₂SO₄ conc.

Also following precautions are necessary to store CHCI₃ to be used as anaesthetic agent -

It is kept in brown or blue coloured bottles which are filled upto the brim in order to protect the action of light & air.

1% ethanol is also added which acts as negative catalyst for oxidation of CHCI₃as well as converts carbonyl chloride into harmless ethyl carbonate.          

2.  Reduction :

CHCI₃ + 2H  CH₂CI₂ + HCI

CHCI₃ + 4H  CH₂CI₂ + 2HCI

CHCI₃ + 6H  CH₄ + 3HCI

3.  Hydrolysis or action of KOH_aq :

CHCI₃ + 4KOH → HCOOK + 3KCI + 2H₂O

4.  Chlorination :

CHCI₃ + CI₂    CCI₄ + HCI

5.   Action of Ag powder :

6.  Action of HNO₃ : The H atom of CHCI₃ is replaced by -NO₂ gp if heated with conc. HNO₃.

7.  Condensation with acetone :

8.  Reaction with sodium ethoxide

9.  Riemer - Tiemann reacton : (see phenol)

10.  Carbylamine reaction :

All primary amines (may be aliphatic R-NH₂ or aromatic Ar-NH₂) on warming with CHCI₃ and KOH_alc. undergo carbylamne reaction to give an offensive or unpleasant odour of isonitrile or carbylamines.

Uses :

1. CHCI₃, an anaesthetic agent; CHI₃ as antiseptic due to liberation of I₂.
2. CHCI₃ as solvent for fat, waxes rubbers, resins etc.
3. In preparation of chlorotone (drug, a hypnotic agent) and nitrochloroform (an insecticide).
4. CHCI₃ as preservative for anatomical specimens
5. As laboratory reagent to identify primary amines & other analytical tests.

Note :

1. Iodoform on heating with AgNO₃ gives yellow ppt. of AgI whereas   chloroform does not give this test because of stable nature.
2. Iodoform has antiseptic properties because on coming in contact with organic matter of skin it decomposes to give free iodine which acts as an antiseptic.
3. Halothane, CF₃-CHCIBr, is used as a general anaesthetic which has replaced diethyl ether.

• Methods of Preparation of Tetrachloromethane

Tetrachloromethane (CCI₄)

Manufacture :

 (i) From methane : Chlorination of methane with excess of chlorine at 400^oC yields impure carbon tetrachloride.

Methane used in this process is obtained from natural gas.

(ii)From carbon disulphide : Chlorine reacts with carbon disulphide in presence of catalysts like iron, iodine, aluminium chloride or antimony pentachloride.

 CS₂  +  3CI₂   →   CCI₄   +   S₂CI₂                

S₂CI₂ further reacts with CS₂ to form more of carbon tetrachloride.

CS₂ + 2S₂CI₂ → CCI₄ + 6S

 CCI₄ is washed with NaOH solution and then distilled to get pure sample.

(iii)By the action of chlorine on chloroform.

 CHCI₃ + CI₂ → CCI₄ + HCI

• Chemical Nature of Tetrachloromethane

Properties :

It is colourless, non inflammable, poisonous liquid with characteristic smell.Insoluble in water but soluble in ethanol and ether. It is a good solvent for oils, fats and greases.

Chemical nature : Less reactive than other halogen derivatives.

1.Oxidation or reaction with steam :

2. Reduction : most iron fillings reduce CCI₄ to CHCI₃

3.Hydrolysis (action of aqueous KOH) :

CCI₄ + 6KOH_(aq.) → 4KCI + K₂CO₃ + 3H₂O

Freon-12 (dichlorodifluoromethane) is widely used as a refrigerant and propellant in aerosol sprays of all kinds.

4. Action of hydrogen fluoride in the presence of SbF5.

 CCI₄ + 2HF   CCI₃F₂ + 2HCI

5. Reaction with phenol and alkali (Reimer-Tiemann reaction).

Uses : Carbon tetrachloride is used

1. as fire extinguisher under the name pyrene. Then dense, non combustile vapours over the burning substances and prevents oxygen from reaching them. However, since CCI₄ forms phosgene, after the use of pyrene to extinguisher a fire the room should be well ventilated.
2. as a laboratory reagent
3. as a solvent for oils, fats, resins iodine and in dry-cleaning.
4. as a fumigant
5. in medicine for the elimination of hookworms as helmenthicide due to antihelminthic nature.

• Some Useful Halogen Derivatives

1.Freons : The chloro fluoro derivatives of methane and ethane are called freons. Some of the derivatives are : CHF₂CI (monochlorodifluoromethane), CF₂CI₂ (dichloro difluro methane), HCF₂CHCI₂ (1, 1-dichloro 2, 2-difluoroethane). These are non-inflammable, colourless, non-toxic and low boiling liquids. These are stable upto 550^oC. The most important and useful derivative is CF₂CI₂ which is commonly known as Freon on Freon-12.

Freon or freon-12 (CF₂CI₂) is prepared by treating carbon tetrachloride with antimony trifluoride in the presence of antimony penta chloride (a catalyst).

               3CCI₄ + 2SbF₃  3CCI₂F₂ + 2SbCI₃                                                    

or by reacting carbon tetrachloride with hydrofluoric acid in presence of antimony penta fluoride.

               CCI₄ +  2HF  CCI₂F₂ + 2HCI

Under normal conditions Freon is a gas. (b.pt. - 29.8^oC). It can easily be liquefied. It is chemically inert and is used in air-conditioning and in domestic refrigerators.

Note : Freon-14 is CF₄, Freon-13 is CF₃CI, Freon-11 is CFCI₃. All these are used as refrigerant.

2. Teflon : A plastic like substance produced by the polymerization of tetrafluoroethylene (CF₂=CF₂). Tetrafluoroethylene is formed when chloroform is treated with antimony trifluoride and hydrofluoric acid.

On polymerization, tetrafluoro ethylene forms a plastic-like material which is called teflon.

 nCF₂=CF₂     →    (CF₂-CF₂)_n

   ^{tetrafluoro ethylene              Teflon}

Teflon is chemically inert substances. It is not affected by strong acids and even by boiling aquaregia. It is stable at high temperature and thus, used for electrical insulation and preparation of gasket materials.

3.Acetylene tetrachloride (Westron), CHCI₂CHCI₂ :

Acetylene tetrachloride is also known as sym. tetrachloroethane. 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≡CH + 2CI₂ → CHCI₂CHCI₂

In absence of catalyst, the reaction between chlorine and acetylene is highly explosive producing carbon and HCI. The reaction is less violent in presence of catalyst.

It is a heavy, non-inflammable toxic liquid with smell like CHCI₃. 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 a useful product westrosol (CCI₂=CHCI)

2C₂H₂CI₄ + Ca(OH)₂ → 2CHCI₂=CCI₂ + CaCI₂ + 2H₂O

Both westron and westrosol are used as solvent for oils, fats and varnishes.

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