Summary Haloalkane-_-Haloarene-Notes.pdf

Haloalkanes and Haloarenes


Haloalkanes
Haloalkanes are the halogen deivatives of
hydrocarbons. Concept Ladder
Classification of Haloalkanes
Monohalides (R–X): Monohalides are In haloalkanes, the halogen
monohalogen derivatives of alkanes which have atom is attached to the sp3-
a general formula CnH2n+1X and are known as alkyl hybridised carbon atom of
halides. an alkyl group whereas in
R–X may be of three types: haloarenes, the halogen
1. Primary R–CH2X atom is attached to sp2-
2. Secondary R2CHX hybridized carbon atom of
3. Tertiary R3CX an aryl group.

Dihalides (CnH2nX2): Dihalides are the di-halogen
derivatives of alkanes and are of geminal and
vicinal types.


Definitions

The monohalogen derivatives of
alkanes are called alkyl halides
or monohaloalkanes or simply
haloalkanes.
Example
(a) CH3CHBr2 (Ethylidine dibromide),
(b) BrCH2CH2Br (Ethylene dibromide)

Trihalides (CnH2n–1X3) or haloforms: Trihalides
are tri-halogen derivatives of alkanes and
are represented by a general formula CHX3
(haloforms). Concept Ladder

Tetrahalides: Tetrahalides are tetra-halogen Haloalkynes or Alkynyl
derivatives of alkanes and arerepresented by a halides are the halogen
Haloalkanes and Haloarenes



general formula CX4 or CX2Y2. derivatives of alkynes. The
monohalogen derivatives
Aromatic Halogen Compounds of alkynes have the general
When halogen atom is attached to aromatic formula CnH2n-3X where X=F,
hydrocarbons directly to the ring , the substance Cl Br or I and n = 2, 3, 4.....,
is known aryl halides. Their general formula is etc.
Ar—X.


1.

, (a) Mono Halogen Derivatives
Examples
(i) Chlorobenzene (ii) p-Bromotoluene





(iii) Bromobenzene (iv) m-Bromotoluene





(b) Side chain Halogen Derivatives
Example
(i) Benzyl chloride (ii) b-Phenylethyl bromide





Allylic, Vinylic and Benzylic Halides
(a) Allylic Halides
Example
(i) 3-Haloprop-1-ene (1°) (ii) 3-Halo-3-methylcyclohex-1-ene (3°)





(b) Vinylic Halides
(i) Chloroethene (ii) 1-Chloroprop-1-ene

Haloalkanes and Haloarenes




(c) Benzylic Halides
(i) Benzyl halide (1°) (ii) A benzylic halide (3°)





2.

,Methods of preparation
Halogenation of alkanes
hv
R − H + X2  → R − X + HX
For example
hv
CH4 + Cl 2 
or520 −650K
→ CH3Cl + CH2Cl 2 + CHCl 3 + CCl 4
Cl 2 ,hv
CH3 — CH2 — CH3 
298K
→ CH3 — CH2 — CH2 — Cl + CH3 — CHCl — CH3
Propane 1−Chloropropane (45%) 2-Chloropropane (55%)

Cl 2 ,hv
CH3CH2CH2CH3 
298 K
→ CH3CH2CH2CH2Cl + CH3CH2 — CHCl — CH3
Butane 1−Chlorobu tane(28%) 2 −Chlorobu tane (72%)

In general, the ease of subsitution of various
hydrogens follows the sequence:
Allylic > 3° > 2° > 1° > CH4
Reactivity order in halogens: F2 > Cl2 > Br2 > I2
Iodination is reversible, but it may be carried out Concept Ladder
in presence of an oxidising agent, such as, HIO3,
Vinylic and aryl hydrogen
HNO3, HgO, etc., which oxidises the HI as it is
are so much unreactive
formed.
hv
CH4 + I2 
  CH3I + HI ; 5HI + HIO3 
→ 3I2 + 3H2O that they do not participate
in free radical halogenation.
Flourination: The best way to prepare alkyl
However, allylic and
fluorides is by halogen exchange. An alkyl chloride
benzylic halides can be
or bromide is heated in the presence of a metallic
prepared from alkenes
fluoride, such as AgF, Hg2F2, CoF3 or SbF3 to give
and arenes without any
alkyl fluorides.
complication.
CH3Br + AgF 
→ CH3F + AgBr
Bromomethane Fluoromethane

2CH3CH2Cl + Hg 2F2 
→ 2 CH3CH2 F+ Hg 2 Cl 2

This reaction is called Swarts reaction.
When the organic halide contains two or three
halogen atoms at the same carbon, CoF3or the Definitions
more easily available SbF3 is used. For example,
Swarts' reaction is generally
Haloalkanes and Haloarenes



3 CH3CCl 2CH3 + 2 SbF3 
→ 3 CH3CF2CH3 + 2 SbCl 3 used to get alkyl fluorides from
2, 2 −Dichloropropane 2, 2 −Difluoropropane
alkyl chlorides or alkyl bromides.
from Alcohols : Generally alkyl halides are This is done by heating of the
prepared from alcohols by replacement of —OH alkyl chloride/bromide in the
group by an halogen ion. This is usually by using presence of the fluoride of some
HX or PX5. heavy metals (silver fluoride or
R – OH + X–  → R – X + OH– mercurous fluoride for example).



3.

, (a) By the action of Halogen acids :
Catalyst
R – OH + HX  → R – X + H2O
(i) Using HCl :
CH CH OH + HCl ( g )  anhy. ZnCl 2
→ CH CH Cl + H O

3 2 3 2 2
anhy. ZnCl 2
CH3 — CH — CH3 + HCl  → CH3 — CH — CH3 + H2O

| |
OH Cl

Concept Ladder
Anhydrous ZnCl2 helps in the cleavage Order of reactivity of the
of C–O bond. Being a Lewis acid, it halogen acids HI>HBr>HCl.
co-ordinates with the oxygen atom Order of reactivity of
of the alcohol. As a result, C–O bond Alcohols:
weakens and ultimately breaks to form R3COH > R2CHOH > RCH2OH
carbocations, which form chlorides.

(CH3 ) 3 C— OH + (HCl
conc. )
Room

temp
→ ( CH3 ) 3 C — Cl + H2O

Yield is improved when vapours of alcohol
and HCl are passed over alumina at 350°C.
(ii) Using HBr :
Alkyl bromides are obtained by refluxing
the alcohol with constant boiling in HBr
(40%) in presence of a little conc. H2SO4
H2SO4
CH3CH2OH + HBr 
Re flux
→ CH3CH2 − Br + H2O
C2H5OH + KBr + H2SO4 
→ C2H5Br + KHSO4 + H2O
(iii) Using HI :
Concept Ladder

CH3CH2OH + HI → CH3CHI + H2O
Re flux
In alkyl halide electro
CH3OH + KI + H3PO4 
→ CH3I + KH2PO4 + H2O
∆
-negativity difference
between the carbon and
(b) By the action of Phosphorus Halides:
the halogen, the shared
Phosphorus halides react with alcohols to pair of electron lies closer
Haloalkanes and Haloarenes




form haloalkanes in excellent yield (80% or to the halogen atom. As a
above). result, the halogen carries
ROH + PX5  → R – X + POX3 + HX a small negative charge,
Here X can be either chlorine or bromine or and carbon carries a small
iodine. positive charge. So, C—X
3ROH + PX3  → 3R – X + H3PO3 bond is always a polar
covalent bond.



4.