IUPAC Nomenclature of Haloalkanes
In the IUPAC system, the mono halogen-substituted alkanes are named 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.
What Are Haloalkanes?
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 haloalkanes or alkyl halides. They are obtained by replacing a hydrogen atom present in an alkane with a halogen atom. Depending upon the number of hydrogen atoms replaced by halogen atoms, they are termed mono-, di-, tri-, tetra- or poly haloalkanes.
R-H R-X
The general formula of haloalkanes is, where is a halogen atom. They can also be represented as an alkyl group. Some typical haloalkanes are, etc.
Process for the Nomenclature of Haloalkanes
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, methyl chloride 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 that contains straight chains of carbon atoms.
For example, is named as an n-propyl group.
The prefix iso is used to describe an alkyl group that contains a methyl branch at the end of the chain.
The prefix neo- is used to describe those alkyl groups which contain two methyl branches at the same carbon atom at the end of the chain.
Classification of Haloalkanes
Depending upon the nature of the carbon atom to which the halogen atom is attached, the alkyl halides are further classified as follows:
Primary alkyl halides: These are the haloalkanes in which the halogen atom is attached to a primary carbon. For instance, ethyl bromide.
Secondary alkyl halides: These are the haloalkanes in which the halogen atom is attached to a secondary carbon. For instance, 2-bromopropane.
Tertiary 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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Nomenclature of Haloarenes
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 the 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, and 1,4 can also be indicated by prefixes ortho (o-), meta and para respectively.
Solved Examples
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.
Solution:
The structural formula and the IUPAC names of such compounds are 2-chloropentane and 2-chloro-2-methylbutane, respectively.
Which one among the following pairs does have higher boiling points in these nomenclatures of haloalkanes?
a. 1-bromopropane or 1-iodopropane
b. 1-bromopropane or 1-bromobutane
c. isobutyl bromide or t-butyl bromide
The Significance of Organic Compound Nomenclature
Organic chemistry covers the major portion of the syllabus in the chemistry of class 11 and 12 in CBSE. in the introduction chapters of chemistry in class 11 we get to know about different types of organic compounds or hydrocarbons which are formed by the combination of carbon ( C ), Oxygen (O)and hydrogen (H). The valency of a carbon atom is 4 which combines with 4 atoms of hydrogen to form the basic structure of methane (CH4) in organic chemistry. oxygen atoms with the valency of 2 often replace two hydrogen atoms to combine with the carbon atom. There are a vast number of compounds formed by the combinations of atoms of these 3 elements. And also the atoms of other elements like nitrogen (N), sulfur (S), and many more are also found in nature in a state combined with the hydrocarbons.
So it is essential for a definite structure of naming these compounds for easy recognition and remembering them. There was a time when any organic compounds found were given the local name for day-to-day use. Later it was found difficult to use different names in different places. With respect to general concern, it was decided to name all the compounds which would be acceptable all over the world. The body named as International Union of Pure and Applied Chemistry (IUPAC) was bestowed with the responsibility of identifying and naming all hydrocarbons available and developing a particular system of nomenclature for the naming of any hydrocarbons that would come up in the future.
Haloalkanes are such a category of hydrocarbons named by IUPAC. Halogen atoms are the atoms of Fluorine, Chlorine, Bromine, Iodine. These halogen atoms are the deciding factor in the nomenclature of the compounds. The prefix used for the naming of the Organic compound is as follows: 'floro', 'chloro', 'Bromo', and 'iodo'. The presence of such halogens imparts a certain aromatic nature to the substance. That is why such organic compounds are also known as aromatic hydrocarbons.
FAQs on Nomenclature of Haloalkanes
1. What is the IUPAC system for the nomenclature of haloalkanes?
In the IUPAC system, haloalkanes are named by treating the halogen as a substituent on the parent alkane chain. The name is formed by adding a prefix for the halogen (fluoro-, chloro-, bromo-, or iodo-) to the name of the longest continuous carbon chain. A number is used to indicate the position of the halogen on the chain, with numbering starting from the end that gives the halogen the lowest possible locant number.
2. How does the common naming system for haloalkanes differ from the IUPAC system?
The two systems differ in their structure and conventions. The common system names them as alkyl halides, where the name of the alkyl group is followed by the halide name (e.g., Ethyl bromide). It often uses prefixes like n-, iso-, and neo-. In contrast, the IUPAC system names them as haloalkanes, where the halogen is a prefix to the parent alkane name (e.g., Bromoethane). The IUPAC system is more systematic and can name complex molecules unambiguously.
3. What are the rules for naming haloalkanes with multiple substituents?
When a haloalkane has multiple substituents (halogens or alkyl groups), the following rules apply:
- Numbering: The carbon chain is numbered from the end that results in the lowest locant for the first substituent encountered.
- Alphabetical Order: If different substituents are present, they are listed alphabetically in the name. For example, 'bromo' comes before 'chloro', and 'chloro' comes before 'methyl'.
- Multiple Halogens: If the same halogen appears more than once, prefixes like di-, tri-, and tetra- are used (e.g., 1,2-dichloroethane).
4. How are haloalkanes classified, and does this classification affect their IUPAC name?
Haloalkanes are classified as primary (1°), secondary (2°), or tertiary (3°) based on the nature of the carbon atom to which the halogen is attached. A primary haloalkane has the halogen bonded to a primary carbon (a carbon bonded to only one other carbon). While this classification is crucial for predicting chemical reactivity, it does not directly alter the IUPAC name. For example, 2-bromopropane is a secondary haloalkane, but its IUPAC name is determined by substituent position, not its classification.
5. How would you name a haloalkane containing different halogen atoms?
If a haloalkane contains more than one type of halogen, they are all treated as substituents and are named alphabetically. The position of each halogen is indicated by a number. The numbering of the parent chain follows the 'lowest locant' rule, regardless of which halogen gets the lower number. For instance, in 1-Bromo-4-chlorobutane, 'bromo' is listed first because 'b' comes before 'c' in the alphabet, even though it is on carbon-1.
6. Why is IUPAC nomenclature considered superior to the common naming system for haloalkanes?
IUPAC nomenclature is considered superior because it is a systematic and unambiguous method. Every distinct haloalkane can be given a unique name that precisely describes its chemical structure, which is not always possible with common names. The common system, with prefixes like 'iso' and 'neo', is limited to simpler, smaller molecules and can lead to confusion with more complex branched structures. The IUPAC system provides a universal standard for chemists worldwide.
7. What are geminal and vicinal dihalides, and how are they distinguished in nomenclature?
Geminal and vicinal describe the relative positions of two halogen atoms in a dihaloalkane:
- Geminal (gem) Dihalides: Both halogen atoms are attached to the same carbon atom. In the IUPAC system, this is shown by repeating the same number for the locant, for example, 1,1-dichloroethane.
- Vicinal (vic) Dihalides: The two halogen atoms are attached to adjacent carbon atoms. In the IUPAC system, their positions are indicated by consecutive numbers, such as 1,2-dichloroethane.
8. How does the nomenclature change for a haloalkane that is part of a cyclic compound?
For cyclic haloalkanes (or cycloalkyl halides), the halogen is treated as a substituent on the cycloalkane ring. The ring is considered the parent hydrocarbon. The prefix for the halogen is added to the name of the cycloalkane, for example, Chlorocyclohexane. If there are other substituents, numbering starts from the carbon bearing the halogen and proceeds in a direction that gives the other substituents the lowest possible numbers, following alphabetical order for priority if necessary.
