IUPAC rules for naming haloalkanes with stepwise examples
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 Explained Clearly
1. What is the nomenclature of haloalkanes?
The nomenclature of haloalkanes is the IUPAC system used to name alkyl halides by treating the halogen as a substituent on the parent alkane chain. In this system:
- The longest carbon chain is selected as the parent alkane.
- The halogen (F, Cl, Br, I) is named as a prefix: fluoro-, chloro-, bromo-, iodo-.
- The position of the halogen is indicated by a number (lowest locant rule).
- Example: CH3CH2Cl is named chloroethane.
2. How do you name haloalkanes using IUPAC rules?
Haloalkanes are named by selecting the longest carbon chain, numbering it to give the halogen the lowest possible number, and adding the halogen prefix before the alkane name. Steps:
- Identify the longest carbon chain as the parent.
- Number the chain from the end nearest the halogen.
- Write the position number + halogen prefix + alkane name.
- Example: CH3CHBrCH3 is named 2-bromopropane.
3. What are the prefixes used for halogens in haloalkane nomenclature?
The prefixes used in haloalkane nomenclature are fluoro-, chloro-, bromo-, and iodo- for F, Cl, Br, and I respectively. These prefixes are added before the parent alkane name:
- F → fluoro
- Cl → chloro
- Br → bromo
- I → iodo
- Example: CH3I is named iodomethane.
4. How do you name haloalkanes with more than one halogen atom?
Haloalkanes with more than one halogen are named by using multiplicative prefixes such as di-, tri-, tetra- along with position numbers. Key rules:
- Number the chain to give the lowest set of locants.
- Use di-, tri-, etc., for identical halogens.
- Example: CH2ClCH2Cl is 1,2-dichloroethane.
- If different halogens are present, list them in alphabetical order.
5. How are haloalkanes named when other functional groups are present?
When other functional groups are present, the halogen is treated as a substituent and the principal functional group determines the suffix. Rules:
- Select the functional group with highest priority as the suffix.
- Name the halogen as a prefix with its position.
- Example: CH3CH(Cl)COOH is named 2-chloropropanoic acid.
6. What is the difference between common names and IUPAC names of haloalkanes?
The common name of a haloalkane is based on the alkyl group + halide, while the IUPAC name follows systematic substitution rules. For example:
- CH3CH2Br
- Common name: ethyl bromide
- IUPAC name: bromoethane
Common names are simpler but less systematic, whereas IUPAC names follow standard nomenclature rules.
7. How do you apply the lowest locant rule in haloalkane nomenclature?
The lowest locant rule states that the carbon chain must be numbered so the halogen gets the smallest possible position number. For example:
- In CH3CH2CHClCH3, numbering from the right gives position 2.
- The correct name is 2-chlorobutane, not 3-chlorobutane.
This rule ensures consistent and unique naming in IUPAC nomenclature of haloalkanes.
8. How are haloalkanes named when both alkyl and halogen substituents are present?
When both alkyl and halogen substituents are present, they are listed in alphabetical order with their position numbers. Steps:
- Number the chain for lowest set of locants.
- Arrange substituents alphabetically (ignore di-, tri-).
- Example: CH3CH(Cl)CH(CH3)CH3 is 2-chloro-3-methylbutane.
9. Can you give an example of naming a branched haloalkane?
A branched haloalkane is named by identifying the longest chain and numbering to give substituents the lowest numbers. Example:
- Structure: (CH3)2CHCH2Br
- Longest chain: propane backbone.
- Numbering from Br end gives position 1 for bromo.
- Correct name: 1-bromo-2-methylpropane.
10. What are the common mistakes in naming haloalkanes?
Common mistakes in naming haloalkanes include incorrect numbering, ignoring alphabetical order, and confusing common and IUPAC names. Important points:
- Always apply the lowest locant rule.
- Arrange substituents in alphabetical order.
- Do not count di-, tri- for alphabetizing.
- Ensure the correct parent chain is chosen.
- Example error: Writing 3-chlorobutane instead of correct 2-chlorobutane.
