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Structural (constitutional) Isomers of Haloalkanes (alkyl halides)

Key Concepts

Haloalkanes are also known by the common term alkyl halides.

Haloalkanes are alkanes in which one or more of the hydrogen atoms has been replaced by a halogen atom.

A halogen is an element that belongs to Group 17 in the Periodic Table.
Halogen atoms are fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At).

Structural isomers, or constitutional isomers, are molecules with:

  • same molecular formula

  • different structural formulae (different constitutional formuale)

Structural isomers, or constitutional isomers, of haloalkanes have:

  • same number of atoms of carbon, hydrogen and halogens

  • different arrangement of carbon, hydrogen and halogen atoms

The structural isomers of a particular haloalkane are different compounds so they have different chemical and physical properties.

Identifying Structural Isomers of Haloalkanes

Structural isomers of halogenated methane, halomethane compounds, are not possible.

Structural isomers of halogenated alkanes only become possible where there are 2 or more carbon atoms present.

Consider ethane, C2H6
H
|
H
|
H-C-C-H
|
H
|
H
if we replaced just 1 of the hydrogen atoms in ethane with a halogen atom, for example chlorine (Cl), we could produce the following molecules:

structure astructure bstructure cstructure dstructure estructure f
Cl
|
H
|
H-C-C-H
|
H
|
H
H
|
H
|
Cl-C-C-H
|
H
|
H
H
|
H
|
H-C-C-H
|
Cl
|
H
H
|
Cl
|
H-C-C-H
|
H
|
H
H
|
H
|
H-C-C-Cl
|
H
|
H
H
|
H
|
H-C-C-H
|
H
|
Cl

Are these structures really different?
No. In each of these structures 3 H atoms are bonded to one of the C atoms and 2 H atoms and 1 Cl atom are bonded to the other carbon atom.
Since the structural formulae we have drawn above are really all the same we can say that structural isomers of C2H5Cl do not exist.

Consider what would happen if we replaced 2 of the hydrogen atoms in ethane with chlorine atoms.

1,1-dichloroethane, C2H4Cl2
All these structures are the same.
3 H atoms are bonded to one C atom and 1 H atom and 2 Cl atoms are bonded to the other C atom.
Cl
|
H
|
Cl-C-C-H
|
H
|
H
Cl
|
H
|
H-C-C-H
|
Cl
|
H
Cl
|
H
|
Cl-C-C-H
|
H
|
H
Cl
|
H
|
H-C-C-H
|
Cl
|
H
H
|
H
|
Cl-C-C-H
|
Cl
|
H
H
|
H
|
Cl-C-C-H
|
Cl
|
H

1,2-dichloroethane, C2H4Cl2
All these structures are the same.
Each carbon atom is bonded to 2 H atoms and 1 Cl atom.
Cl
|
Cl
|
H-C-C-H
|
H
|
H
Cl
|
H
|
H-C-C-Cl
|
H
|
H
Cl
|
H
|
H-C-C-H
|
H
|
Cl
H
|
Cl
|
Cl-C-C-H
|
H
|
H
H
|
H
|
Cl-C-C-Cl
|
H
|
H
H
|
H
|
Cl-C-C-H
|
H
|
Cl

Both 1,1-dichlorethane and 1,2-dichlorethane have the same molecular formula, C2H4Cl2.
That is, one molecule of 1,1-dichloroethane contains 2 C atoms, 4 H atoms and 2 Cl atoms and one molecule of 1,2-dichloroethane also contains 2 C atoms, 4 H atoms and 2 Cl atoms.
However the structural formula of 1,1-dichloroethane is different to the structural formula of 1,2-dichloroethane.
In 1,1-dichloroethane both Cl atoms are attached to the same C atom, but in 1,2-dichloroethane the Cl atoms are attached to different C atoms.
1,1-dichloroethane and 1,2-dichloroethane are said to be structural isomers, or constitutional isomers. These are molecules that have the same molecular formula but a different structural formula (or different constitutional formula).

It is possible to have structural isomers of haloalkanes when only 1 halogen atom is present, but you need 3 or more carbon atoms in the carbon chain (or carbon backbone or carbon skeleton).

Consider molecules with the molecular formula C4H9Cl

First we could just draw the structure for the straight-chain butane molecule and substitute a Cl atom for one of the H atoms in the structure to produce 1-chlorobutane and 2-chlorobutane:
Cl
|
H
|
H
|
H
|
H-C-C-C-C-H
|
H
|
H
|
H
|
H

1-chlorobutane
H
|
Cl
|
H
|
H
|
H-C-C-C-C-H
|
H
|
H
|
H
|
H

2-chlorobutane
Second, we could draw a 3 member carbon chain with a methyl side chain and then substitute a Cl atom for one of the H atoms, producing 1-chloro-2-methylpropane and 2-chloro-2-methylpropane:
Cl
|
CH3
|
H
|
H-C-C-C-H
|
H
|
H
|
H

1-chloro-2-methylpropane
H
|
CH3
|
H
|
H-C-C-C-H
|
H
|
Cl
|
H

2-chloro-2-methylpropane

There are four structural isomers with the molecular formula C4H9Cl.
These structural isomers are 1-chlorobutane, 2-chlorobutane, 1-chloro-2-methylpropane and 2-chloro-2-methylpropane.

Molecules are structural isomers only if they have:

  1. the same molecular formula

  2. different structural formula

Examples

1. Are these two molecules structural isomers?
Cl
|
H
|
H
|
H
|
H-C-C-C-C-H
|
H
|
H
|
H
|
H
H
|
Br
|
H
|
H
|
H-C-C-C-C-H
|
H
|
H
|
H
|
H
Step 1: write the molecular formula of each molecule: C4H9Cl C4H9Br
The molecular formulae of the molecules is NOT the same.
The two molecules are NOT structural isomers.
2. Are these two molecules structural isomers?
Cl
|
H
|
H
|
F
|
H-C-C-C-C-H
|
H
|
H
|
H
|
H
H
|
F
|
Cl
|
H
|
H-C-C-C-C-H
|
H
|
H
|
H
|
H
Step 1: write the molecular formula of each molecule: C4H8ClF C4H8ClF
Both molecules have the same molecular formula.
Step 2: compare the structure of each molecule: First C : 2 H, 1 Cl
Second C : 2 H
Third C : 2 H
Fourth C : 2 H, 1 F
First C : 3 H
Second C : 1 H, 1 F
Third C : 1 H, 1 Cl
Fourth C : 3 H
The arrangement of atoms in each molecule is different, that is, the structural formulae are different.
These two molecules ARE structural isomers.
3. Are these two molecules structural isomers?
Cl
|
H
|
H
|
H
|
H-C-C-C-C-H
|
F
|
H
|
H
|
H
H
|
H
|
H
|
Cl
|
H-C-C-C-C-H
|
H
|
H
|
H
|
F
Step 1: write the molecular formula of each molecule: C4H8ClF C4H8ClF
Both molecules have the same molecular formula.
Step 2: compare the structure of each molecule: First C : 1 H, 1 Cl, 1 F
Second C : 2 H
Third C : 2 H
Fourth C : 3 H
First C : 3 H
Second C : 2 H
Third C : 2 H
Fourth C : 1 H, 1 Cl, 1 F
In both molecules the two central carbon atoms are bonded to 2 H atoms, one of the terminal (end) carbon atoms is bonded to 3 H atoms, and the other terminal (end) carbon atom is bonded to one atom of H, one atom of Cl and one atom of F.
These two molecules are NOT structural isomers, they have the same arrangement of atoms so they have the same structural formula.
(You could rotate the first molecule 180o in space to make the second molecule)


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