Organic Chemistry Part - I Page 136

Day-1

Halogen Compounds

Introduction The replacement of hydrogen atom(s) in a hydrocarbon, aliphatic or aromatic, by halogen

atom(s) results in the formation of alkyl halide (haloalkane) and aryl halide (haloarene),

respectively. These classes of compounds find wide applications in industry as well as in day-to-

day life. They are used as solvents for relatively non-polar compounds and as starting materials

for the synthesis of wide range of organic compounds. Chlorine containing antibiotic,

chloramphenicol, produced by soil microorganisms is very effective for the treatment of typhoid

fever.Our body produces iodine containing hormone, thyroxine, the deficiency of which causes a

disease called goiter. Synthetic halogen compounds, viz. chloroquine is used for the treatment of

malaria; halothane is used as an anaesthetic during surgery.

Classification

Halogen Compounds

Chapter

4

Organic Chemistry Part - I Page 137

Some Examples:-

Nomenclature

Organic Chemistry Part - I Page 138

General Usage → pesticide

→ Refrigerant

→ Solvents

→ Anesthetics

Preparation

(I):- From alcohols → only (for halo-alkane) {Mechanism Sub – Nucleophillic}.

(a) ROH + HX → RX + H2O (in presence of ZnCl2). This is used in test of Alcohol, we

will do in this in detail Alcohol Chapter).

(b) 3ROH + PX3 → 3RX + H3PO3 X = Cl or Br

(c) ROH +PCl5 → RCl + POCl3 +HCl

(d) ROH Red P+X2 → RX X2 = Br2 or I2

(e) ROH + NaBr + H2SO4 → RBr + H2O + NaHSO4

(f) ROH + SOCl2 → RCl + SO2 + HCl – (DARZENS Process – Best Method for

Chlorination).

For The reaction ROH + HX → RX + H2O

Mechanism Substitution – Nucleophilic → Refer reaction Mechanism.

Important Points

1. Reactivity of HX → HI > HBr > HCl > HF

2. Reactivity of Alcohol → Benzylic > Allylic > 3o > 2o > 1o

3. SN1 for Allylic, Benzylie; 3o

4. SN2 for 1o ; CH3OH

5. 2O can be Both depending on strength of Nucleophile.

Example:-

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(II):- From Hydrocarbons (a) Alkanes

(a)Free Radical Halogenation CH4 + Cl2 ℎ𝜈 → CH3Cl + HX

(b) electrophilic substitution

(c) Sand Mayer reaction X is Cl or Br

Important Notes

(1) Reactivity of Halogens F2 > Cl2 > Br2 > I2

(2) Iodine will Alkane is endothermic reaction therefore Alkane Iodide Not formed

easily. Use Finkelstein Rxn → RX + NaI → RI + NaX

(3) Ease of abstraction is in order Benzylic > Aliylic > 3o > 2o > 1o > CH3

(4) Allylic given two products because resonance stabilization of Intermediate free

radical.

(5) Benzylic will not do the same as it leads to breaking of Benzene Ring

(6) NBS is used for Benzylic or Allylic substitution

(III):- From Hydrocarbons (b) from Alkene (A) Add of HX (Mechanism – Add - Electrophilic)

H2C = CH2 +HX → H2C − CH2. X (1):- Intermediate is carbocation

(2):- 1 – 2 shifts are possible

(3):- 1-2 & 1-4 addition in case of dienes

(4):- NBS used for Allylic/Benzylic Bromination

(5):- Follows Markoni koff’s rule

(B) Add of HX in presence of peroxide (Mechanism:- addition free radical)

(1):- Addition – free radical

(2):- Anti Markoni koffs’s Rule

(3):- Only happens for Br (Not Cl & I)

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Ex – (1)

(2)

(3)

(C) – Addition of Halogens

(1) Both Add Electrophilic & Free radical Mech.Mechanism-Addition Electrophillic

(1) Anti Addition

Cis will give → d & l. mixture (Racimic mixture)

Trans will give → Meso compound

(2) SYN Addition when Alkene are reacted with dil alkaline KMnO4. Diols will be product.

(3) Br2 + CCl4 is used for testing of unsaturation.

(4) Dil alkaline KmnO4 is called Baeyer’s reagent is also used to test for unsaturation.

(5) Bromine water is solution of bromine in water. It is also used in test of unsaturation but will

give bromohydrin as product. OH will attach to more substituted carbon & Br to less (Markoni

Koff’s rule).

1, 2 addition

1, 4 addition

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Physical Properties

1. Dipole moment µ = e × d. e ∝ Electronegativity .d ∝ size of halogen Dipole moment of methyl

halogens

The trend of µ changes for CHX2 in Cl, Br etc because being bigger molecule the replsion is

higher, bond angle increase, decreasing μ.

Boiling Point

1. Bp of Alkyl Halide is greater than Alkane, Alkenes, ethers

2. Bp of Alkyl Halide is less than Alcohol, acid and corbonyls

3. RI > RBr > R Cl > RF

4. For same Carbon number same Halogen, more the branching lesser will be boiling

point.

Ex.:-

N – butyl bromide (102oC) > isobutyl bromide (91oC) > tert butyl bromide 73o.

Density → Alkyl fluroid Alkyl chlorides (with one Cl) are less denser the water. Alkyl

chlorides having two or more ‘Cl’ atoms are denser than water. All alkyl Bromide and

iodides heavier the water.

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b.p. 181oC 173oC 170oC

m.p. –17oC –25oC + 54oC

→ Aromatic Halogens are denser than water.

Solubility – Halo alkenes are very slightly soluble in water. In Halo alkanes landon forces

dominate due to large size of Halogens and not dipole. H – bend of water is very strong so

solubility is less.

→ Haloalkane are soluble is organic solvents.

→ Haloarenes are insoluble in water.