1.1 Aliphatic halogen derivatives:
Compounds obtained by the replacement of one or more hydrogen atom(s) from hydrocarbons are known as halogen derivatives. The halogen derivatives of alkanes, alkenes, alkynes and arenes are known as alkyl halide (haloalkene), alkenyl halide (haloalkenes), alkynyl halides (haloalkynes) and aryl halides (halobenzenes) respectively.
Alkyl halides: Monohalogen derivatives of alkanes are known as alkyl halides.
Structure of alkyl halides:
Classification of alkyl halides:
(i) Primary halide: If the halogen bearing carbon is bonded to one carbon atom or with no carbon atom.
CH3 - X, R - CH2 - X
(ii) Secondary halide: If two carbon atoms are bonded to the halogen bearing carbon atom.
(ii) Tertiary halide: Three other carbon atoms bonded to the halogen bearing carbon atom.
Halolkanes can be classified into following three categories.
(i) Monohaloalkanes (ii) Dihaloalkanes (iii) Polyhaloalkanes
1.2 IUPAC Nomenclature of alkyl halides
2 – Chloro-2-methylpropane
1.3 Isomerism in Haloalkanes
1. Structural Isomerism:
IUPAC name -
(a) Optical isomerism
1.4 Bonding in alkyl halide:
Table : 1 Carbon halogen bond lengths
Ex.1 Classify the compound as a primary, secondary and tertiary halide.
(e) Isopentyl bromide
(f) Neopentyl iodide
Sol. (a) Primary
1.5 Physical properties of alkyl halides:
1.5.1 Dipole moment of the halogen derivatives:
m = 4.8 × δ × d
Where δ is the charge and d is the bond length
These two effects e.g. charge and distance oppose each other, with the larger halogens having longer bond but weaker electronegativity. The overall result is that the bond dipole moment increase in the order.
C - I < C - Br < C - F < C - Cl
μ : 1.29 D 1.48 D 1.51 D 1.56 D
The electronegativities of the halogen increase in the order:
I < Br < Cl < F
Table: 2 Molecular dipole moments of methylhalides
1.5.2 Boiling point:
(a) With respect to the halogen in a group of alkyl halides, the boiling point increases as one descends the periodic table. Alkyl fluorides have the lowest boiling points and alkyl iodides have the highest boiling point. This trend matches the order of increasing polarizability of the halogens. (Polarizability is the ease with which the electrons distribution around an atom is distorted by a nearby electric field and is a significant factor in determining the strength of induced-dipole/induced-dipole and dipole/induced-dipole attractions). Forces that depend on induced dipoles are strongest when the halogen is a highly polarizable iodine, and weakest when the halogen is a nonpolarizable fluorine.
Table: 3 Boiling points of some alkyl halide in ºC (1 atm)
Fluorine is unique among the halogens is that increasing the number of fluorine does not lead to higher and higher boiling point.
(b) The boiling points of the chlorinated derivatives of methane increase with the number of chlorine atoms because of an increase in the induced-dipole/dipole attractive forces.
Alkyl fluorides and chlorides are less dense and alkyl bromides and iodides more dense than water.
Because alkyl halides are insoluble in water, mixture of an alkyl halide and water separates into two layers. When the alkyl halides have a fluoride or chloride, then alkyl halides is on the upper layer and water is the lower. The situation is reversed when the alkyl halide is a bromide or an iodide. In these cases the alkyl halide is in the lower layer. Polyhalogenation increases the density. The compounds CH2Cl2 CHCl3 and CCl4, for example, are all more dense than water.
1.6 Preparation of alkyl halide:
1.6.1 From alkane :
R - H R - X HX
1.6.2 From alkenes and alkynes (Detail in alkene and alkyne)
1.6.3 From alcohol (Detail in the alcohol)
R - OH R - X
1.6.4 From other halides
R - X I- R - I X-
R - Cl KF R - F
1. CH3 - CH2 - Cl CH3 - CH2 - I
Nucleophility - in Polar Protic solvent - F- < Cl- <Br- <I-
Polar Aprotic solvent - F- > Cl- >Br- > I-
Covalent Nature : NaF < NaCl < NaBr < NaI
Acetone → acetone is soluble.
NaF < NaCl < NaBr < NaI
2. C - C - Br C - C - Br
3. C - C - Cl C - C - Cl
4. C - C - Cl C - C - F
5. C - C - Cl C - C - F (Swart's reaction)