Step-by-Step Guide to Naming 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 Haloalkanes Nomenclature Made Easy
1. What is the fundamental principle for naming haloalkanes according to the IUPAC system?
The fundamental principle of the IUPAC system is to treat haloalkanes as substituted alkanes. The halogen atom (F, Cl, Br, I) is considered a substituent on the parent carbon chain. The name is constructed by adding a prefix (fluoro-, chloro-, bromo-, iodo-) to the name of the longest continuous carbon chain, with its position indicated by the lowest possible number (locant).
2. How does the common naming system for haloalkanes differ from the IUPAC system?
The two systems have different structures. The common system names them as alkyl halides (e.g., Ethyl bromide), where the alkyl group name is followed by the halide name. In contrast, the IUPAC system provides a more systematic name, haloalkane (e.g., Bromoethane), where the halogen is a prefix. The IUPAC system is universally accepted as it can unambiguously name even highly complex molecules, which is a limitation of the common system.
3. What are the key rules for naming a haloalkane that has multiple different substituents?
When a haloalkane has multiple substituents, such as different halogens or alkyl groups, the following rules are applied:
Lowest Locant Rule: The parent carbon chain is numbered from the end that gives the first substituent encountered the lowest possible number.
Alphabetical Order: All substituents are listed alphabetically in the final name, regardless of their position number. For example, 'bromo' comes before 'chloro', and 'chloro' comes before 'methyl'.
Prefixes: Prefixes like 'di-', 'tri-' are used if the same substituent appears more than once, but these prefixes are ignored when determining alphabetical order.
4. In IUPAC nomenclature, why is alphabetical order used for substituents instead of numerical order of their positions?
Alphabetical order is used to ensure a single, unique, and systematic name for every compound. If substituents were named by position, a molecule like 4-Bromo-2-chloropentane could also be named 2-Chloro-4-bromopentane, leading to ambiguity. By enforcing a strict alphabetical rule (Bromo before Chloro), the IUPAC system guarantees that there is only one correct name, making chemical communication precise and universal.
5. How does the presence of a double or triple bond affect the nomenclature of a haloalkane?
The presence of a double bond (alkene) or triple bond (alkyne) changes the naming priority. The multiple bond gets priority over the halogen for the lowest locant number when numbering the carbon chain. The ending of the parent chain name changes from '-ane' to '-ene' or '-yne'. The halogen is still named as a prefix with its position number. For example, CH₂=CH-CH₂Br is named 3-Bromoprop-1-ene, not 1-Bromoprop-2-ene.
6. What is the importance of distinguishing between geminal and vicinal dihalides in nomenclature?
Distinguishing between geminal and vicinal dihalides is crucial for describing the exact structure. The IUPAC nomenclature clearly shows this difference:
Geminal (gem) dihalides have both halogens on the same carbon atom. Their IUPAC name reflects this with a repeated locant number, like in 1,1-Dibromoethane.
Vicinal (vic) dihalides have halogens on adjacent carbon atoms. Their name uses consecutive locant numbers, as in 1,2-Dibromoethane. This distinction is vital as their chemical properties and synthesis methods differ significantly.
7. How is the nomenclature of a cyclic haloalkane determined?
For cyclic haloalkanes, the cycloalkane ring is considered the parent structure. The halogen is treated as a substituent. For example, a chlorine atom on a cyclohexane ring is named Chlorocyclohexane. If only one substituent is present, no number is needed. If multiple substituents are present, numbering starts from the carbon bearing the substituent that comes first alphabetically, and proceeds in a direction that gives the other substituents the lowest possible numbers.
8. Does classifying a haloalkane as primary (1°), secondary (2°), or tertiary (3°) affect its IUPAC name?
No, this classification does not directly affect the IUPAC name. The classification as primary, secondary, or tertiary depends on the carbon atom to which the halogen is attached. While this is extremely important for predicting chemical reactivity (e.g., in SN1 vs. SN2 reactions), the IUPAC name is determined solely by the structure, chain length, and substituent positions according to established rules. For instance, 2-Bromopropane is a secondary (2°) haloalkane, but its name is based on the position of bromine on a three-carbon chain.
9. What is a common mistake students make when numbering the carbon chain in a complex haloalkane?
A very common mistake is to number the chain to give the halogen the lowest number, even when other rules should take precedence. According to the 'first point of difference' rule, the chain should be numbered to give the lowest locant to the first substituent encountered, regardless of whether it is a halogen or an alkyl group. For example, in 2-Bromo-5-methylhexane, numbering should start from the end closer to the bromine atom, not the methyl group.
