Haloalkanes and Haloarenes

Classification and Nomenclature

Classification

• Based on number of halogen atoms

  • Monohalogen
  • Dihalogen
  • Polyhalogen (tri-, tetra-, penta- etc.)
  • Examples of haloalkanes:

• Examples of haloarenes:

• Compounds containing sp³ C−X bond (X = F, Cl, Br, I)

(i) Alkyl halides or haloalkanes (R−X) → They form homologous series of general formula C_nH_2n+1X. They are further classified into primary, secondary, and tertiary.

(ii) Allylic halides → Compounds containing halogen atom bonded to an allylic carbon

(iii) Benzylic halides → Compounds containing halogen atom bonded to an sp³ hybridised carbon atom next to an aromatic ring

To test your knowledge of this concept, solve the following puzzle.

• Compounds containing sp² C−X bond (X = F, Cl, Br, I)

(i) Vinylic halides → Compounds containing halogen atom bonded to a vinylic carbon

(ii) Aryl halides → Compounds containing halogen atom bonded to an sp² hybridised carbon atom of an aromatic ring

Nomenclature

• For haloalkanes

• Common name → Name of alkyl group followed by name of the halide

• IUPAC name → Named as halo-substituted hydrocarbon in IUPAC

• Examples:

Structure	Common Name	IUPAC Name
CH3CH2CH2CH2Br	n−Butyl bromide	1-Bromobutane
	Isobutyl chloride	1-Chloro-2-methylpropane
	sec-butyl bromide	2-Bromobutane
CH3CH2CH(Cl)CH3	sec-Butyl chloride	2-Chlorobutane
(CH3)3CCH2Br	neo-Pentyl bromide	1-Bromo-2,2-dimethylpropane
(CH3)3CBr	tert-Butyl bromide	2-Bromo-2-methylpropane
CH2Cl2	Methylene chloride	Dichloromethane

• For haloarenes

• Named as halo-substituted hydrocarbon (for both common and IUPAC name)

• For dihalogen derivatives:

In common names → prefixes o-, m-, p- are used

In IUPAC names → numerals 1, 2; 1, 3; 1, 4 are used

• Examples:

Structure	Common name	IUPAC name
	Chlorobenzene	Chlorobenzene
	o-Dibromobenzene	1,2-Dibromobenzene
	sym-Trichlorobenzene	1,3,5-Trichlorobenzene
	o-Chlorotoluene	1-Chloro-2-methylbenzene or 2-Chlorotoluene
	Benzyl chloride	Chlorophenylmethane

Nature of C−X bond

• C-atom bears partial positive charge and X-atom bears partial negative charge.

• C−X bond length increases down the group.

• Reason − Size of halogen atom increases down the group.
  
  Stability of carbocation:
  
  Carbocations are the chemical species which have positive charge on carbon atom and carry six electrons in the valence shell. Carbocation has a planar geometry (sp²-hybridisation). The carbocation has flat structure having all the three covalent bonds in one plane with bond angle of 120⁰ between them. Carbocations are highly reactive intermediate species, they have an inherent positive charge and thus always in the search for electrons, which tells us about their high reactivity. Carbocations are electron-deficient intermediates figure out prominently in many reactions of organic chemistry, such as

• Nucleophilic substitution  (SN1) and Elimination  (E1) reactions
• Additions of electrophiles to double and triple bonds
• Electrophilic aromatic substitution
• Additions to carbonyl compounds 

Carbocation can be stabilised by following ways:

1) Increasing substiuents: The stability of carbocations increases as we go from primary to secondary to tertiary carbons. 

There are two ways by which carbocation can be stabilised:

a) Inductive effect:  The permanent displacement of electrons towards the more electronegative atom or group of atoms in a covalent bond is called an inductive effect.
Inductive effect is classified into two types as follows.

-I effect: Electron withdrawing substituents tend to attract electron pair, due to which carbon to which substituent is attached carries positive charge. This effect is called as negative inductive effect and the groups which cause this effect are called as -I groups. For example- NO₂, hologens.
+I effect:—Electron donating substituents tend to donate shared electron pair towards the carbon to which that group is connected. Hence, carbon bears partial negative charge, this is known as positive inductive effect and the groups which cause such effect are called as +I groups. For example- alkyl groups.

Due to +I effect of methyl group the positive charge on carbon atom decreases and more the number of alkyl groups, the greater the dispersal of positive charge and therefore, more the stability of carbocation.

b)Hyperconjugation : It involves stabilization through donation of the electrons in C-H sigma bonds to the empty p-orbital of the carbocation. Carbocations are stabilized by adjacent groups that can donate electron density. The positive charge is spread out or delocalised and this stabilising effect is called no-bond resonance. The most common example is alkyl groups. Alkyl groups are electron donating group, since, they have C-H bonds that can be end up donating electrons to the adjacent carbocation. Due to which carbocation stability increases as we go from primary to secondary to tertiary carbocation.
Decreasing order of stab…