In the IUPAC system, the mono halogen-substituted alkanes are named as haloalkanes. Their names are derived by prefixing the words fluoro, chloro, bromo or iodo (according to the halogen atom present) to the name of the longest straight chain present in the compound. In naming haloalkanes, the numbering of carbon atoms in the longest chain is done from the end from which the halogen atom is nearer. A suitable Arabic numeral indicates the halogen atom position. If side chains are present, their names are also mentioned in alphabetical orders, and suitable Arabic numerals also mention their positions. Bromoethane, chloroethane, 2-bromobutane are some of the examples.
Hydrocarbons are regarded as the parent compounds of all other organic compounds. The replacement of hydrogen atoms in saturated aliphatic hydrocarbons by a halogen atom gives rise to haloalkanes. The halogen derivatives of alkanes are usually termed as haloalkanes or alkyl halides. They are obtained by replacing a hydrogen atom present in an alkane by a halogen atom. Depending upon the number of hydrogen atoms replaced by halogen atoms, they are termed as mono-, di-, tri-, tetra- or poly haloalkanes.
R-H --> R-X
The general formula of haloalkanes is CnH2n+1 X, where X is a halogen atom. They can also be represented as R-X, where R is an alkyl group. Some typical haloalkanes are CH3Cl, CH3Br, CH3CH2Br, etc.
The nomenclature of haloalkanes can be done in the following ways:
The trivial or common system: In the trivial system, haloalkanes are termed as alkyl halides. The trivial name is obtained by adding the word halide to the name of the corresponding alkyl group. The derived name is always written as two separate individual words.
For example, CH3-Cl is methyl chloride, CH3-CH2Br is ethyl bromide.
In the trivial system, the prefixes n-, iso-, and neo-, are generally used to describe different types of alkyl groups.
The prefix n- stands for normal. It is used to describe an alkyl group which contains straight chains of carbon atoms.
For example, CH3-CH2-CH2- is named as an n-propyl group.
The prefix iso- is used to describe an alkyl group which contains a methyl (CH3-) branch at the end of the chain.
The prefix neo- is used to describe those alkyl groups which contain two methyl (CH3-) branches at the same carbon atom at the end of the chain.
IUPAC system: It is to be noted that while naming haloalkanes, all the common rules for IUPAC nomenclature of organic compounds described earlier are strictly followed. The IUPAC naming of haloalkanes is always written as a single word.
Depending upon the nature of carbon atom to which halogen atom is attached, the alkyl halides are further classified as follows:
Primary (1o) alkyl halides: These are the haloalkanes in which the halogen atom is attached to a primary carbon. For instance, ethyl bromide.
Secondary (2o) alkyl halides: These are the haloalkanes in which the halogen atom is attached to a secondary carbon. For instance, 2-bromopropane.
Tertiary (3o) alkyl halides: These are the haloalkanes in which the halogen atom is attached to a tertiary carbon. For instance, tert. butyl chloride
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The systematic names of halarenes or aryl halides are derived by adding prefixes fluoro, chloro, bromo, iodo (according to the halogen present ) before the name of the aromatic hydrocarbon. For instance, chlorobenzene, bromobenzene.
In case of disubstituted or trisubstituted compounds, the relative positions of the substituent groups are indicated by the Arabic numerals. The numbering of the ring is done in such a way that the sequence gives the lowest sequence of numbers. In the case of disubstituted derivatives, the relative positions 1,2; 1,3 and 1,4 can also be indicated by prefixes ortho (o-), meta (m-) and para (p-) respectively.
1. Write the IUPAC nomenclature and the structural formula of two such compounds containing five carbon atoms each in their molecule and which are optically active.
The structural formula and the IUPAC names of such compounds are 2-chloropentane and 2-chloro-2-methylbutane, respectively.
2. Which one among the following pairs does have higher boiling points in these nomenclatures of haloalkanes?
1-bromopropane or 1-iodopropane
1-bromopropane or 1-bromobutane
isobutyl bromide or t-butyl bromide
1-iodopropane has a higher boiling point than 1-bromopropane.
1-bromobutane has a higher boiling point than 1-bromopropane.
Isobutyl bromide has a higher boiling point than t-butyl bromide.
1. How Do Haloalkanes Differ Haloarenes?
Solution: The silver nitrate test can distinguish haloalkanes and haloarenes. In this test, the given halide is warmed with aqueous or alcoholic KOH. The solution is cooled and acidified with dilute HNO3. Now, AgNO3 is added to the acidified solution and formation of a precipitate is observed. If a precipitate is observed, it is a haloalkane, and if no precipitate is formed, the given halide is haloarene.
The given test is based on the fact that the halogen present in haloalkanes is labile and reacts with AgNO3 to give a residue. In contrast, the halogen present in haloarenes is inert and does not combine with silver nitrate, thus forming no precipitate.
2. Why are Haloalkanes More Reactive than Haloarenes?
Solution: Haloalkanes are more reactive than haloarenes because of their stability with their respective halogen atoms. The resonance and sp2 hybridization of haloarenes stabilize them. The carbon atom in haloarene has a C=C double bond which is why it is sp2 hybridized. The stability of hybridization are as follows:
sp >sp2 >sp3 >sp3d >sp3d2
The chlorine attached to the ring has three lone pairs by which one of them is delocalized in the ring due to which the chlorine atom develops +R or +M effect, and the chlorine atom gets more tightly bonded with the carbon atom. That is why alkyl halides are more reactive than haloarenes.