Revision: Std XII >> Halogen Derivatives MAH-MHT CET (PCM/PCB) Halogen Derivatives

When one or more hydrogen atoms of aliphatic or aromatic hydrocarbons are replaced by a corresponding number of halogen atoms (fluorine, chlorine, bromine or iodine), the resulting compounds are called halogen derivatives of alkanes or arenes.

The relationship between a chiral molecule and its mirror image is similar to the relationship between left and right hands. Therefore it is called handedness or chirality

A light having oscillations only in one plane perpendicular to direction of propagation of light is known as plane polarized light.

Dehydrohalogenation of alkyl halide is a β-elimination reaction in which the halogen atom is lost from the α-carbon atom and the hydrogen from the β-carbon (according to the Saytzeff rule).

Active metals like sodium, magnesium cadmium readily combine with alkyl chlorides, bromides and iodides to form compounds containing carbon-metal bonds. These are known as organometallic compounds.

1. On the Basis of Hydrocarbon Skeleton

2. On the Basis of the Number of Halogen Atoms

3. On the Basis of the Nature of the C–X Bond

(A) Compounds containing sp³ C–X bond

(B) Compounds containing a sp² C–X bond

4. Classification of Alkyl Halides

Basic rules

• Alkyl halides → haloalkanes (IUPAC)
• Aryl halides → haloarenes
• Longest chain selection
• Numbering for the lowest position
• Substituents named alphabetically

Formula	Common name	IUPAC name
CH₃CH₂Br	Ethyl bromide	Bromoethane
CH₃CH(Cl)CH₃	Isopropyl chloride	2-Chloropropane
(CH₃)₂CH–CH₂Br	Isobutyl bromide	1-Bromo-2-methylpropane
(CH₃)₃CBr	Tert-butyl bromide	2-Bromo-2-methylpropane
(CH₃)₃CCH₂Cl	Neopentyl chloride	1-Chloro-2,2-dimethylpropane
CH≡C–Cl	Chloroacetylene	Chloroethyne
	Benzyl iodide	Iodophenylmethane
	p-Iodotoluene	1-Iodo-4-methylbenzene or 4-Iodotoluene

1. From alkanes
   Free radical halogenation:
   \[CH_{3}CH_{2}CH_{2}CH_{3}\xrightarrow{Cl_{2}UV-light}CH_{3}CH_{2}CH_{2}CH_{2}Cl+CH_{3}CH_{2}CH(Cl)CH_{3}\]
2. From alkenes
   Addition of hydrogen halide:
   C=C + HX → R–X
3. From alcohols
   \[3ROH+PCl_{3}\longrightarrow3R-Cl+H_{3}PO_{3}\]
   \[ROH+PCl_{5}\longrightarrow R\longrightarrow Cl+POCl_{3}+HCl\]
   \[ROH+HCl\xrightarrow{ZnCl_{2}}R-Cl+H_{2}O\]
   \(ROH\xrightarrow{RedP/Cl_{2}}R-Cl\)
   \[R—OH+SOCl_2\xrightarrow{\Delta}R—Cl+SO_2\uparrow+HCl\]
4. Halogen exchange reactions
   Finkelstein reaction:
   \[R—X+NaI\xrightarrow{Acetone}R—I+NaX\]
   Swart's reaction:
   \[RBr+AgF\longrightarrow R—F+AgBr\]
5. From aromatic amines
   Sandmeyer reaction:
   \[\mathrm{C_{6}H_{5}NH_{2}}\xrightarrow{\mathrm{NaNO_{2}+HX}}\mathrm{C_{6}H_{5}N_{2}^{+}\bar{X}}\xrightarrow{\mathrm{Cu_{2}X_{2}}}\mathrm{C_{6}H_{5}X+N_{2}}\]

• Alkyl halides are colourless when pure but develop colour on exposure to light; they have a sweet smell
• Polarity: Moderately polar
• Solubility: Insoluble in water, soluble in organic solvents

Bond Length

• Increases in order: R–F < R–Cl < R–Br < R–I

Bond Strength

• Decreases in order: R–F > R–Cl > R–Br > R–I

Boiling Point

• Order: R–I > R–Br > R–Cl > R–F
• Increases with an increase in molecular mass
• Decreases with branching

Dipole Moment

• Decreases as the electronegativity of halogen decreases

Isomeric Dihalobenzenes

• Boiling point: nearly the same
• Melting point: para > ortho/meta

• Optical activity: Ability of a compound to rotate plane-polarised light
• Dextrorotatory (+): rotates light to the right
• Laevorotatory (–): rotates light to the left
• (+) and (–) forms are called optical isomers, and the phenomenon is optical isomerism
• Number of optical isomers = 2ⁿ
  (n = number of asymmetric carbon atoms)
• Asymmetric carbon/stereocentre: Carbon attached to four different groups
• Chirality: Property of a molecule whose mirror image is non-superimposable
• Enantiomers: Stereoisomers which are non-superimposable mirror images of each other
• Retention: Preservation of spatial arrangement around the asymmetric carbon during reaction
• Inversion of configuration: In the S_N2 reaction, backside attack gives a product with the opposite configuration. This complete inversion is called Walden inversion
• Racemic mixture: Equimolar mixture of dextro and laevo isomers. It is optically inactive due to external compensation. This phenomenon is called racemisation.

Representation of configuration:

Fischer projection:

• Horizontal bonds → above plane
• Vertical bonds → below plane
  

Wedge formula:

• Solid wedge → above plane
• Dashed wedge → below plane
• Straight line → in plane
  

Alkyl halides mainly undergo substitution and elimination reactions.

1. Nucleophilic Substitution Reactions

S_N1 (Unimolecular Nucleophilic Substitution):

• Occurs in two steps:
  • Formation of carbocation
  • Attack by nucleophile
• First-order reaction (depends only on alkyl halide)
• Reactivity order:
  Benzylic > 3° > 2° > 1° > methyl
• Leads to racemisation

S_N2 (Bimolecular Nucleophilic Substitution):

• Occurs in one step (concerted mechanism)
• Second-order reaction (depends on alkyl halide and nucleophile)
• Reactivity order:
  Methyl > 1° > 2° > 3°
• Strong nucleophiles increase the rate
• Leads to inversion of configuration

2. Reactions of Alkyl Halides

• With aqueous KOH → Alcohol
• With alcoholic KOH → Alkene (elimination)
• With NaOR → Ether
• With NH₃ → Amine
• With KCN → Nitrile
• With AgCN → Isocyanide
• With KNO₂ → Alkyl nitrite
• With AgNO₂ → Nitro compound
• With Mg (dry ether) → Grignard reagent
• With Na (dry ether) → Alkane (Wurtz reaction)

3. Aryl Halides

• Undergo electrophilic substitution
  • Nitration
  • Halogenation
  • Sulphonation
  • Friedel–Crafts reactions
• Show ortho & para directing effect