Skip to content
Related Articles
Open in App
Not now

Related Articles

Nomenclature and Nature of C-X bond

Improve Article
Save Article
  • Last Updated : 08 Mar, 2022
Improve Article
Save Article

The branch of chemistry deals with the study of carbon compounds. For example Methane, Ethane, Urea, DNA etc. It is not necessary that all carbon compounds are organic. Examples: Nitrogen, Halogen, Cyanide (CN-), Carbon dioxide, Amine (NH3) etc. Some naturally occurring organic compounds are present in plants and animals and in synthetic organic compounds from fossil fuels and plant materials.

The replacement of one or more hydrogen atoms of a hydrocarbon, aliphatic or aromatic, by an equal number of halogen atoms, results in the formation of alkyl halides (haloalkanes) and aryl halides (haloarenes) respectively. In haloalkanes, the halogen atom(s) is(are) attached to the sp3-hybridised carbon atom(s) of an alkyl group whereas, in haloarenes, the halogen atom(s) is(are) attached to sp2 -hybridised carbon atom(s) of an aryl group.

Uses of Halogen compounds are,

  • The chlorine-containing antibiotic is very effective for the treatment of typhoid fever.
  • Synthetic organic halogen compounds are extensively used in agriculture and industry.
  • For the treatment of malaria chloroquine (halogen compound) is used.
  • During surgery, halomethane is used as an anaesthetic.

Nomenclature of Haloalkanes (Alkyl Halides) 

In general, the mono halogen derivatives of alkanes are called alkyl halides. The complete name of alkyl halide is written in two separate words. In the IUPAC system, the name of mono halogen derivatives of alkanes is given as haloalkanes. Their names are derived by prefixing the word ‘halo’ to the name of the longest straight chain alkane. The numbering of the parent straight chain of an alkane is done in such a manner that the carbon atom having the halogen atom gets the lowest possible number. The IUPAC name of any mono halogen derivative of an alkane is always written as one word.

  • Dihalogen derivatives: They are of three different types as-
    • Alkylidene dihalides- The di-haloalkanes are in which two halogen atoms of the same type are present on the same carbon atom and are called alkylidene dihalides. The positions on the same carbon atom are called geminal positions. For example, Dichloromethane or Methylene dichloride (CH2Cl2), 1,1-Dibromoethane or Methylene dichloride (CH3CHBr3), etc.
    • Alkylene dihalides- The di-halogen derivatives of alkanes in which the two halogen atoms of the same type are present on the adjacent carbon atoms of the carbon chain and are called alkylene dihalides. In the IUPAC system, the two halogen atoms are prefixed to the name of dihaloalkane. For example, 1,2-Dichloroethane or Ethylene dichloride (ClCH2CH2Cl), 1,2-Dibromoethane or Ethylene dibromide (BrCH2CH2Br), etc.
    • Polymethylene dihalides- In a common system, dihalogen derivatives of alkanes in which the same two halogen atoms are present on the terminal carbon atoms, i.e., α, ω-positions of the carbon chain are called polymethylene dihalides. In the IUPAC system, the halogen atoms are prefixed to the name of the dihaloalkane. For example, 1,3-Dibromopropane or Trimethylene dibromide (BrCH2CH2CH2Br), 1,4-dichlorobutane or Tetramethylene dichloride ( ClCH2CH2CH2CH2Cl), etc.
  • Polyhaloalkanes: In the common system, trihalomethanes and tetrahalomethanes are called haloforms and carbon tetrahalides respectively. For example, Trichloromethane or Chloroform (CHCl3), Tribromomethane or Bromoform (CHBr3), etc.

Nature of the C-X Bond

The nature of the C-X bond is different for Haloalkanes and Haloarenes, therefore both are discussed separately as:

Haloalkanes: Halogen atoms are more electronegative than carbon. Due to this, they share pair of electrons that lies closer to the halogen atom. 

As a result, the halogen carries a small negative charge i.e. δ- while the carbon carries a small positive charge i.e. δ+. As we go down the periodic table, the size of the halogen atom increases. The fluorine atom is the smallest and the iodine atom is the largest. so the carbon-halogen bond length also increases C-F to C-I. Further, as we move from F to me, the electronegativity of the halogen decreases, therefore, the polarity of the C-X bond and hence dipole moment of the haloalkane should also decrease.

Haloarenes: In the case of haloarenes carbon of the benzene ring attached to a halogen is sp2 hybridised which have a short length and can hold more electron pair and the C-X bond is polar in nature. so the lone pair of electrons undergo the resonance with the benzene ring due to this delocalisation of the lone pair of electrons leading to the formation of double character.

Salient Features of the nature of C-X bond in Haloarenes and Haloalkanes

  • The C-X bond of haloalkanes is more polar than the C-X bond of haloarenes. This is due to the higher electronegativity of halogen over carbon.
  • There is a delocalized lone pair of electrons on the X atom present over the benzene ring which results in the double bond character in the C-X bond of haloarenes.
  • As we move in the periodic table the atomic size down the group increases hence C-X bond length in haloarenes also increases.
  • Also moving down the group the dipole moment decreases as the dipole moment depends on the electronegativity difference. As we know, Cl has less electronegativity than F, but the dipole moment of the C–Cl bond is more than C–F which is an exceptional case.

Parameters related to the Nature of the C-X bond

  • Bond Length – The bond length between the carbon and halogen group affect the nature of the C-X bond. As we move down the group the size of the halogen atom goes on increasing.

F < Cl < Br < I

So, the difference in C-F is the least and largest in the C-I bond.

  • Dipole moment – The general formula of dipole moment is μ = q × d, where ‘d’ is the distance or the charge separation between positive and negative charges. i.e. the dipole moment is the product of charge and distance and is the calculated polarity of chemical bond in the molecule.

So the order is: 

CH3Cl > CH3F > CH3Br > CH3I

  • Bond Enthalpy – If we talk about the size of carbon and fluorine atoms then it is similar and there is an overlapping of orbitals that forms a very strong bond. But in the case of C-I, the size of the Iodine is large compared to that of carbon and can form a weak bond. So a large amount of energy is required to break the C-F bond then the C-I bond.

Here is the order of the bond enthalpy:

C-F > C-Cl > C-Br > C-I

Physical Data of important Halomethanes (CH3-X)

Halomethane

C-X bond length (pm)

C-X bond enthalpy (KJ mol-1)

Dipole moment (Debye)

CH3F

139

452

1.847

CH3Cl

178

351

1.860

CH3Br

193

293

1.830

CH3I

214

234

1.636

Sample Questions

Question 1: Haloarenes are insoluble in water but are soluble in benzene, Explain.

Answer:

Haloarenes are insoluble in water because they cannot form hydrogen with water molecules. how ever, these are soluble in benzene in accordance with the general principle of solubility i.e., like dissolves like. Haloarenes are organic compounds having a large hydrocarbon part and are solutions in hydrocarbon solvents like benzene.

Question 2: The p-isomer of dichlorobenzene has a higher melting point than o- and m-isomer. Why?

Answer:

The melting point of para isomer is quite higher than that of ortho or meta isomers. This is due to that it has a symmetrical structure and therefore, its molecules can easily pack closely in crystal lattices as a result intermolecular forces of attraction are stronger and therefore, greater energy is required to 88 its lattice and its melts at a higher temperature.

Question 3: Iodoform gives a precipitate with silver nitrate on heating while chloroform does not. Explain.

Answer:

The Carbon-iodine bond is quite weak (213.4 KJ mol-1) as compared to the carbon-chlorine bond (326.4 KJ mol-1) Therefore, when the iodoform is heated with AgNO3 solution, C-l bond gets cleaved easily and iodide reacts with AgNO3 solution to give a precipitate of Agl. On the other hand, the C-Cl bond does not get dissolve.

Question 4: Organic halogen compounds used in industry as solvents are chlorides rather than bromides and iodides. Explain.

Answer:

Organic alkyl chlorides are used in industry as solvents because chlorides are more volatile than bromides and iodides.

Question 5: Chloroform contains chlorine but gives no reaction with AgNO3 solution. Why?

Answer:

CHCl2 contains chlorine but it is bonded to carbon by a covalent bond and therefore, it is not ionised. Hence, it does not combine with AgNO2 solution.

CHCl2 + AgNO2  ⇢  No reaction

Question 6: Why is chloroform stored in dark coloured bottles?

Answer:

Chloroform is oxidised to poisonous phosgene in the presence of air as,

CHCl2 + 1/2O2    COCl2 + HCl

Therefore, to protect it from light, chloroform is stored in dark coloured bottles.

Question 7: A small amount of ethyl alcohol is usually added to chloroform bottles. Why?

Answer:

Alcohol retards the oxidation of chloroform to phosgene and it converts phosgene to ethyl carbonate.

2C2H5OH + COHCl2    ⇢    (C2H6)2CO3 + 2 HCl


My Personal Notes arrow_drop_up
Related Articles

Start Your Coding Journey Now!