Alkyl Halide (Haloalkane) - Structure and Nomenclature

In this article we will discuss about Alkyl halide or Haloalkane. Alkyl halide is an organic compound which has a halogen atom bonded to the sp3 hybrid carbon atom of alkyl group. Here we will learn about structure and nomenclature of Alkyl halide.

Key words: Alkyl halides, Halogen, Types of alkyl halides, Common names and IUPAC names of alkyl halides. 

Introduction

Alkyl halide is an organic compound which has a halogen atom bonded to the sp3 hybrid carbon atom of alkyl group. The important halogens found in nature are: Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), and Astatine (At). The halogen atom is more electronegative than carbon atom due to this the carbon-halogen bond becomes polar. The Reactivity of the alkyl halides is because of these polar bonds. Alkyl halides are known to undergo nucleophilic substitution and elimination reactions. Hence the alkyl halides are used as important intermediates in organic synthesis.

Figure 1: Alkyl halide is a derivative of Alkane

    

1.    Primary Alkyl Halides:

If the halogen atom bonded to a carbon which is bonded to only hydrogen atoms or only one carbon then the alkyl halide is known as primary alkyl halide. For example, chloroform and ethyl bromide are called as primary alkyl halides.

Figure 2: Primary alkyl halide

2.    Secondary alkyl halides:

In this type of alkyl halides the halogen atom is bonded to the carbon atom which is bonded to two carbon atoms. For example, isopropyl iodide has iodine attached carbon which is bonded to two more carbon atoms therefore it is called as secondary alkyl halide.

Figure 3: Secondary alkyl halide

3.    Tertiary alkyl halides:

If the halogen atom is bonded to the carbon atom which is bonded to three carbon atoms then that alkyl halide is called as tertiary alkyl halide. For example, tertiary butyl bromide has carbon atom which is bonded to three carbon atoms therefore it is known as tertiary alkyl halide. 

Figure 4: Tertiary alkyl halide

Common Names of Alkyl Halides

Common names of alkyl halides are formed by naming alkyl groups followed by the word “halide”. The common names are used only for simple molecules. 

Compound	Alkyl Group	Halide	Common Name
	methyl	bromide	Methyl bromide
	ethyl	bromide	Ethyl bromide
	isopropyl	fluoride	Isopropyl fluoride
	tert-butyl	iodide	tert-butyl iodide
	cyclohexyl	chloride	Cyclohexyl chloride
	cyclopentyl	bromide	Cyclopentyl bromide

Table 1: Common names of alkyl halides

IUPAC Names of Alkyl Halides

According to IUPAC naming system, alkyl halides are considered as halogen derivatives of alkanes. Therefore the alkyl halides are named as “halo alkanes” along with the carbon number to which halogen atom is bonded. For example, ethyl bromide is named as “bromoethane” because two carbon atoms alkane is ethane. 

Figure 5: Ethyl bromide

Let’s see another example; n-propyl chloride has the halogen atom bonded to carbon number 1 and three carbons alkane is known as propane, therefore IUPAC name of the compound is “1-chloropropane”. Similarly isopropyl chloride has chlorine atom at carbon number 2 therefore it is named as “2-chloropropane”.

Figure 6: IUPAC names of chloropropane

Now consider the following molecule where more than one halogen atoms are bonded to the carbon chain. The given molecule has four carbon atoms in chain so root name of the alkane is “butane”. The bromide and chloride atoms bonded at carbon number 2. Here the substituents should be written in alphabetical order. Hence name of the compound is “2-bromo-2-chlorobutane”. 

Figure 7: IUPAC name of complex alkyl halide

Consider the following example where bromide atom is bonded to cyclohexyl ring. In cyclic molecules numbering must be given from the carbon to which halogen atom is bonded. Therefore, name of the following compound is “1-bromocyclohexane”.

Figure 8: IUPAC name of cyclohexyl bromide

Let’s see the following example,

To assign name for the compound, first select longest carbon chain. Here the given molecule has seven carbon atoms chain. Therefore root name of the compound is “heptane”. The numbering for the carbons should be given from the end which is closest to a branch. In this molecule fluorine atom bonded to carbon number 2, and chlorine atom bonded to carbon number 3. Also there is “methyl” group is at carbon number 5. Write the substituents in alphabetical order along with carbon number. Therefore name of the compound is “3-chloro-2-fluro-5-methylheptane”.

Figure 9: IUPAC name of branched alkyl halide

Consider the cyclic compound shown below; the given compound has six member cyclic ring so the root name of the compound is “cyclohexane”. Now give numbering to the carbon atoms starting from more branched side. So in the molecule ethyl group present at carbon number 1, chlorine atom at carbon number 2 and bromine atom at carbon number 4. Now arrange the substituents in alphabetical order. Therefore name of the compound is “4-bromo-2-chloro-1-ethylcyclohexane”.

Figure 10: IUPAC name of cyclic alkyl halide

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