Alkyl halides are colourless when pure but bromides and iodides develop colour when exposed to light because they decompose into halogens.
2R−I → R−R + I2
Melting and Boiling Points
The melting points and boiling points of haloalkanes are higher than those of corresponding hydrocarbons due to higher molecular mass of haloalkanes
The melting and boiling points increase with increase in size due to higher vander waal's forces.
Hence the order is : R−I > R−Br > R−Cl > R−F
The boiling points of isomeric haloalkanes decrease with increase in branching as surface area decreases on branching and Vander
waal's forces decrease.
Boiling point decreases with branching
(CH3)2CHCH2Cl > (CH3)3CCl
The boiling points of isomeric dihalobenzene are nearly the same.
The melting point of p-dihalobenzene is always higher than those of the o- and p- isomers. Because p-isomers are more symmetrical
and hence their molecules pack closely in the crystal lattice. This leads to stronger molecular forces of attraction.
Arrange 'propane, 1-chloropropane and isopropyl chloride' in order of increasing boiling points.
Let's calculate the molecular masses of the given compounds :
M.Mass(CH3CH2CH3) = (12×3 +8×1) = 44.
M.MassCH3CH(Cl)CH3 = (12×3 + 7×1 + 1×35.5) = 78.5
M.MassCH3CH2CH2Cl = (12×3 + 7×1 + 1×35.5) = 78.5
Since boiling point increases with increase in molecular mass, propane has the lowest boiling point among them. Further, boiling point
decreases with branching; therefore, isopropyl chloride has lower boiling point than 1-chloropropane.
Haloalkanes and haloarenes, though polar, are immiscible with water because they neither form H-bond nor break the already existing H-bonds (because new attraction of
water and haloalkanes is weaker).
Haloalkanes are soluble in organic solvents of low polarity like ether, benzene etc., because new intermolecular forces are
similar to those in the low polarity solvents.
Alkyl fluorides and alkyl chlorides are generally lighter than water whereas alkyl bromides and alkyl iodides are heavier. The relative
densities follow the order :RI > RBr > RCl.
The densities of the alkyl halides decreases as the size of the alkyl group increases.
Which alkyl halide has the highest density ?
For a given alkyl group, alkyl iodide has the highest density; therefore the halide must be iodine. Since
density decreases as the size of alkyl group increases; the alkyl must be methyl.
Therefore, methyl iodide has the highest density among the alkyl halides.
The stability of the haloalkanes having the same alkyl group decreases in the order : R−F > R−Cl > R−Br
> R−I since the strength of the C−X bond decreases in the order : C−F > C−Cl > C−Br >
The dipole moment of methyl halides follows the order: CH3Cl > CH3F >
CH3Br > CH3I. Due to the very small size of F, fluorides have lower dipole moments than
Question : Which one of the following has the highest dipole moment?
Answer : The three dimensional structures of the above compounds along with the direction of dipole
moment in each of their bonds are given below:
CCl4 being symmetrical has zero dipole moment. In CHCl3, the two C-Cl dipole moments being
equal and opposite cancel the effect of each other. The other two dipoles (C-H and C-Cl) are opposite but not equal
because the dipole moment of C-H bond is smaller than that of C-Cl bond. As a result, CHCl3 has a finite
dipole moment — 1.03 D to be precise.
In CH2Cl2, the resultant of two C-Cl dipole moments is in the same direction as the resultant
of two C-H dipoles. Therefore, CH2Cl2 has a dipole moment higher than that of CHCl3.
Thus, CH2Cl2 has the highest dipole moment among the three.
Question : The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride. Why? Answer : The chlorine atom in chlorobenzene is attached to an sp2 hybridised carbon whereas
in cyclohexyl choride, it is attached to an sp3 hybridised carbon.
An sp2 hybridised carbon is
more electronegative than an sp3 hybridised carbon due to greater s character. Due to this reason, the
difference of electronegativiy between carbon and chlorine is less in chlorobenzene. Further, due to resonance in chlorobenzene,
the C-Cl bond in chlorobenzene acquires some double bond character while the C-Cl bond in cyclohexyl chloride is a pure
single bond. In other words, the C-Cl bond in chlorobenzene is shorter than in cyclohexyl chloride. Therefore, the
dipole moment — which is defined as the product of charge and distance — of chlorobenzene is lower than
that of cyclohexyl chloride.
Question : p-Dichlorobenzene has higher melting point than those of o- and
m-isomers. Discuss. Answer : The p-isomer being more symmetrical fits closely in the crystal lattice. This leads to
forces of attraction than those of o- and m-isomers. Thus, a larger amount of energy is required to melt the p-isomer
than the corresponding o- and m-isomers.
For the same alkyl group, the order of boiling points of alkyl halides is:
The dipole moment of CH3X, where X is a halogen, follows the order:
Which of the following orders regarding the boiling points of the given alkyl chloride is correct?