Haloalkanes and Haloarenes

Alkyl halides also known as haloalkanes or halogen alkanes are chemical compounds that are derived from alkanes that contain one or more than one halogens. Alkyl halides or haloalkanes are made by substitution or replacement of a hydrogen atom in an open-chain hydrocarbon with halogen atoms (Fluorine, chlorine, bromine, or iodine). Dipole moment depends on the difference between the electronegativity order of carbon and halogen compounds.

Definition of Dipole Moment

Dipole moments occur when there's a separation of charge. When atoms in a molecule share electrons unevenly, a dipole moment is formed. This happens when one atom is more electronegative as compared to a different atom or when one atom includes a lone pair of electrons and also the difference of electronegativity vector points within the same approach. 

They can occur between 2 ions in an ionic bond or between atoms in a chemical covalent bond. The higher the difference in electronegativity, the higher the dipole moment. The gap between the charge separation is also a significant factor in the scale of the dipole moment. The dipole moment may be a measure of the polarity of the molecule.

Dipole Moment of Haloalkanes

Dipole moment decreases with a decrease in the electronegativity of the halogen compound. Although Cl is less electronegative than F, the dipole moment of the C-Cl bond is higher than the C-F bond. This is often due to the smaller C-F bond length that dominates the impact of greater electronegativity.

Dipole Moment in Halogen Compounds

Electronegativity of the alkyl halides :

F>Cl>Br>I

Bond length increases with the increase in the size of the halogen group.

C-F<C-Cl<C-Br<C-I

Order for bond dipoles:

\begin{align}

  & C-Cl>C-F>C-Br>C-I \\

 & 1.56D>1.51D>1.48D>1.29D \\

\end{align}

Molecular dipole depends on the geometry of the molecule.

The Dipole Moment of RX Depends on:

• the sizes of the partial charges.

• the distance between the charges.

• the polarizability of the unshared electrons present on halogen.

Properties of Alkyl Halides

Physical Properties of Haloalkanes

• Haloalkanes are colourless, odourless, and hydrophobic.

• They are heavier than alkanes.

• Density is directly proportional to the mass of the compound, therefore down the homologous series, density will increase, also fluoro derivatives are less dense than chloro derivatives; chloro derivatives are less dense than Bromo derivatives, and so on.

• The boiling point of chlorides, bromides, and iodides is relatively more than those of the hydrocarbons of comparable molecular mass. The boiling point reduces with the increase in branching.

Properties of Haloarenes 

• All halogen compounds are less ignitable than hydrocarbons. The inflammability decreases with a rise in halogen groups.

• In haloarenes halogen groups are connected to carbon atoms, hence dipole moment develops between them. The dipole moment will increase because the number of halogen atoms increases.

• Due to the double character of the C-X bond in aryl halides, the C-X bond is shorter in length and stronger than in the alkyl halides, hence, their boiling points are more than in alkyl halides. Boiling points will increase because the number of halogen atoms increases in rings.

• Density order for  halo arenes increases as follows:  

Ar-I>Ar-Br>Ar-Cl>Ar-F

Interesting Fact

A large variety of halogen-containing compounds are found in nature and plenty of those are utilised in drugs and technology.

Some haloalkanes (those containing Cl or bromine) have negative effects on the surroundings like ozone depletion. The foremost widely renowned family among this group is the chlorofluorocarbons (CFCs).

Important Questions

1. Arrange propane, 1-chloropropane, and isopropyl chloride in order of increasing boiling points.

Ans: First, calculate the molecular masses of the given compounds :

\begin{align}

& MolecularMas{{s}_{\left( C{{H}_{3}}C{{H}_{2}}C{{H}_{3}} \right)}}=\left( 12\times 3+8\times 1 \right)=44 \\ 

& MolecularMas{{s}_{\left( C{{H}_{3}}CH\left( Cl \right)C{{H}_{3}} \right)}}=\left( 12\times 3+7\times 1+1\times 35.5 \right)=78.5 \\ 

& MolecularMas{{s}_{\left( C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}Cl \right)}}=\left( 12\times 3+7\times 1+1\times 35.5 \right)=78.5 \\ 

\end{align}

Since the boiling point will increase with a rise in molecular mass, propane has the lowest boiling point among them. Further, the boiling point decreases with branching; thus, isopropyl chloride contains a lower boiling point than 1-chloropropane.

CH3CH2CH3<CH3CH(Cl)CH3<CH3CH2CH2Cl

2. Para-Dichlorobenzene has a higher melting point than those of o- and m-isomers. Explain.

Ans: The para-isomer is more symmetrical and fits closely within the crystal lattice. This results in stronger intermolecular forces of attraction than those of ortho- and meta-isomers. Thus, a larger value of energy is needed to melt the para-isomer than the corresponding ortho- and meta-isomers.

Summary

Halogens are group 17 components within the periodic table and are electronegative (F, Cl, Br, I, At). Fluorine (F) is the most electronegative element. The group 17 components need only 1 electron to complete their outer shell. The dipole moment of alkyl halides and aryl halides varies according to the electronegativity order of halide compounds. Down the group, the electronegativity of halogen compounds decreases, and therefore the dipole moment also decreases.