Nomenclature and Nature of C-X Bond

Do you know initially there was no proper system of naming a compound even though nomenclature and nature of C-X bond is an essential part of organic chemistry? There were just trivial names given on the basis of country and region. Sometimes the naming of a compound was done on the basis of the discoverer, nature of compound or even the place of the discovery.

However, soon scientists realized that the naming system was not standard and it can lead to confusion. Therefore, there was the need for a standard naming system of compounds including organic compounds. This led IUPAC to develop a certain set of rules that can be used universally for the naming of organic compounds.

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Two Names for Every Compound

• Common name- This is separate from the trivial name because nomenclature of the common name also requires a set of rules.
• IUPAC name

Nomenclature

• Naming of compounds by following a certain set of rules is called nomenclature. We have to find three primary things for naming a compound in IUPAC format.
• Find the parent chain of the compounds (longest hydrocarbon).
• Find the functional group. It will form the suffix.
• Find the substituents and its position. It will form the prefix.

The functional group will form suffix or prefix depending upon the priority series.

Step by Step Rules of Nomenclature

• Initially, we have to find the longest carbon chain of the compound.
• The carbon chain should be numbered in such a way that the carbon atom(s) containing the halogen(s) or the X group is/are having the least possible number. Furthermore, the carbon-containing double or triple bond is given the lowest number.
• If multiple halogen atoms are present in the compound, they are named with Greek numerical prefixes like di, tri, tetra, etc to represent the total number of same halogen atoms forms attached to the carbon atom. If more than one halogen atom is joined with the carbon atom, the numeral is repeated that many times.
• If different halogen groups are present in the same compound they are named according to the alphabets in ascending order.
• The position of the halogen atom in the compound is labelled by writing their respective positions and names of the halogen group members just before the name of the original (parent) hydrocarbon.

Nomenclature of Haloalkanes

The common name of haloalkane is alkyl halide. IUPAC name of haloalkane is written in the format-“numerical position-alphabetically placed halo- root-word-ane”

Nomenclature of Haloarenes

The common name of haloarene- Aryl Halide. IUPAC Name- In this nomenclature system, the naming of numerical prefixes depends on the basis of the position of the halogen atom in the aromatic ring.

If more than one halogen atoms are present, the aromatic ring is numbered and the halogen group is named on the basis of the numerical position of the aromatic ring. Numerical prefixes are written as (1, 2) ; (1,3) ; (1,4). The halogen atom is always given the lowest number.    

Nomenclature of Some Compounds

• Common name of the compound: neo − Pentylbromide. IUPAC Name of the compound: 1 − Bromo − 2,2 − dimethylpropane

CH₂ = CH – CH₂Br

• Common Name of the compound: Allyl Bromide. IUPAC Name of the compound: − 3 – Bromoprop – 1 – ene

Alkylidene/ Alkylene Dihalides

The dihaloalkanes having identical halogen atoms are termed as alkylidene or alkylene dihalides. They can be divided into two types:

• Geminal dihalides- When the halogen atoms are placed on the same carbon atom.
• Vicinal dihalides- When the halogen atoms are placed on the adjacent carbon atoms.

Examples of Geminal Hihalides and Vicinal Dihalides

CH3 – CHCl2	Ethylidene chloride	gem-dihalide	1,1 − Dichloroethane
CH2Cl – CH2Cl	Ethylene dichloride	vic-dihalide	1,2 – Dichloroethane

Nature of C-X Bond

It is essential to understand the nature of C-X bond because it determines the reactivity of compound having this kind of bond. The C-X bond is highly polar in nature because halogen atoms are electronegative and the carbon atom is electropositive. The difference in electronegativity results in withdrawal of electron density from sigma bond pair towards the halogen atom.

This result in polarization of C-X bond is polarized in a manner that the carbon atom develops partial positive charge whereas halogen atom in the bond develops a partial negative charge. Therefore, the carbon-halogen bond of any alkyl halide is polarized. This is represented as

The electronegativity of the halogen group varies from one another. The size increases as we go down the group so fluorine atom is the smallest one in the group and iodine atom is largest. Thus, fluorine has the highest electronegativity followed by chlorine then bromine and finally iodine.

Electronegativity of X: F(3.98) > Cl(3.16) > Br(2.96) > I(2.66). However, the electronegativity of carbon atom is 2.55. The electronegativity difference between C-F is maximum. Therefore, C-F is the most polar among all of them.

Parameters Related to Nature of C-X Bond

1) Bond Length (A⁰)

Nature of C-X bond depends upon the bond length between the carbon atom and halogen group. We have previously mentioned that size of the halogen group increases as we move down the group (F < Cl < Br < I). Consequently, the difference in the C-F bond will be the smallest and C-I bond will be the largest. It can be represented as

2) Bond Enthalpy Order

Nature of C-X bond depends upon bond enthalpy order. The size of the carbon and fluorine atom is very similar so the orbitals overlap (2p-2p overlap) into one another. This leads to the formation of a very strong bond. In C-I the atomic size of iodine is very large in comparison to carbon atom so the orbital interaction is very weak.

This results in the formation of weak bond strength. We can conclude that less the bond length stronger will be the bond. Hence, the bond length of C-F is 1.39 A⁰. The stronger the bond, the amount of energy required increases to break that bond. Therefore, C-F has highest bond enthalpy. The bond enthalpy order is

C − F > C − Cl > C − Br > C – I

3) Dipole Moment

Dipole moment helps to calculate the polarity of a chemical bond within a molecule. It occurs due to the separation of positive and negative charges. It is the product of both charge and the distance between them. Bond dipole (μ) is given by the formula

μ = q × d

The order of dipole moment in C-X is CH₃Cl > CH₃F > CH₃Br > CH₃I. Dipole moments of haloalkanes are − CH₃F − 1.847D, CH₃Cl − 1.860 D, CH₃Br − 1.830 D, CH₃I − 1.636 D

Can you notice the abnormal order of the dipole moment in the case where CH₃Cl > CH₃F? Even though fluorine is more electronegative than Chlorine but the C-F bond (139 pm) is shorter than C-Cl bond C − Cl (178 pm). Thus, the dipole moment will be lower in case of CH₃F in comparison to CH₃Cl.

Solved Example for You

Q1: Give the IUPAC name of the compound given below.

Solution: IUPAC Name − 2 − Bromo − 2 – methylpropane