Chemistry: Halogens

Organic Compounds Containing Halogens can be divided into two groups:


Alkyl Halides: Aliphatic carbon chain with halogen atom(s) as substitution.. Example: Chlorobutane. 


Aryl Halides: Aromatic carbon ring with halogen atom(s) as substitution on ring. Example: Chlorobenzene.



Methods of Preparation of Alkyl Halides:

Grove’s process: Replacement of “OH” group in primary and secondary alcohols with an “X’ atom in presence of Zinc chloride.




The reaction follows SN2 mechanism when the concentration of zinc chloride is low.

(b) Darzen Process: Reaction of thionyl chloride with straight-chain primary alcohols without  presence or absence of pyridine.

In presence of pyridine:

ROH + SOCl2 → HCl + ROSOCl

HCl+C5H5N →C5H5NH++Cl-

ROSOCl + Cl– →RCl + SO2    (SN2)


Action of a phosphorus halide on the alcohol: ROH + PCl5 →  RCl + HCl + POCl3.


By addition of Halogen to an alefins: R-CH=CH2 +Br2+CCl4  →R-CH(Br)CH2Br 


Photohalogenation: CH4 + Cl2 +hv →CH3Cl + HCl


Displacement of one halogen atom by another:RCl + NaI →RI + NaCl


Bonodine – Hünsdiecker Reaction:RCO2Ag + Br2→RBr + CO2 + AgBr


Hydrohalogenation of unsaturated hydrocarbons:


In absence of peroxide: RCH=CH2 +HBr→RCH(Br)CH3


In presence of peroxide: RCH=CH2 +HBr + Peroxide →RCH2CH2Br







Methods of Preparation of aryl halides


Halogenation:  Ar-H + X2 +Lewis Base → Ar-x + HX


From diazonium salts:


C6H5N2Cl + HBF4 →C6H5F               (Schiemann Reaction)


C6H5N2Cl + CuCl →C6H5Cl              (Sandmeyer Reaction)


C6H5N2Cl + Cu powder →C6H5Cl      (Gatterman Reaction)





SN1 and SN2 mechanism:




 

SN1

SN2





Steps

Two :  
(1) R:Xl → R+ + X-

 (2) R+ + Nu- l →RNu

One :
R:X + Nu- l → RNu + X-





Rate

            =K [RX] (1st order)

=K[RX] [:Nu-] (2nd order)


TS of slow step






Stereochemistry

Inversion and racemization

Inversion (backside attack)


Molecularity

Unimolecular

Bimolecular


Reactivity
structure of R
Determining Factor

Nature of X
Solvent effect on rate

3o> 2o> 1o> CH3

Stability of R+

RI> RBr> RCl> RF

Rate increases in polar solvent

CH3> 1o> 2o> 3o

Steric hindrance in R group

RI> RBr> RCl> RF

with Nu- there is a large rate increase in polar aprotic solvents.


Effect of nucleophile

No effect as it does not appear in the rate expression.

Rate depends on nucleophilicity

I-  > Br-  > Cl- ; RS-  > RO-


Catalysis

Lewis acid, eg. Ag+, AlCl3, ZnCl2

None


Competitive reaction

Elimination, rearrangement

Elimination




Reactions of Alkyl Halides:


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 



Substitution Versus Elimination:




CH3X

RCH2X

R2CHX

R3CX


Methyl

1°

2°

3°


Bimolecular reactions only

SN1/E1 or E2


Gives SN2 reactions

Gives mainly SN2 except with a hindered strong base [e.g., (CH3)3CO–] and then gives mainly E2.

Gives mainly SN2 with weak bases (e.g., I–, CN–, RCO2–) and mainly E2 with strong bases (e.g., RO–)

No SN2 reaction. In solvolysis gives SN1/E1, and at lower temperature S­N1 is favoured. When a strong base (e.g., RO–) is used. E2 predominates.





Haloform(Tri halide):


Preparation: It can be prepared from any alcohol having –CH(OH)CH3 group or from the aldehydes and ketones formed from above type of alcohols i.e, from a carbonyl compound having three a - hydrogen atoms by the action of X2 and an alkali or Na2CO3.


Laboratory Preparation of CHCl3:




Physical properties of CHCl3: colourless liquid with sweet smell and test. It is heavier  than water and insoluble in it but soluble in alcohol and ether.







Chemical Reactions of CHCl3:


Oxidation: CHCl3 + 1/2 O2 → HCl + COCl2(phosgene)


Hydrolysis: CHCl3 + 4NaOH → HCOONa + 3NaCl + 2H2O


Carbyl amine reactions: CHCl3 + CH3NH2+ 3NaOH →CH3N≡C +3NaCl +3H2O






ROSOCl + Cl–RCl