Classification of Halides and the C-X Bond | JEE Notes

Halogen is more electronegative than carbon, so the C-X bond is polar and the carbon is slightly positive. Nearly everything in this chapter follows from that one fact.

Sorting halides

By number of halogens, a compound is mono-, di- or polyhalogenated. For reactions, the useful sort is by the carbon that holds the halogen.

Check the hybridisation of the carbon holding X first.

Alkyl halides are primary, secondary or tertiary according to the carbon holding X.
Allylic halides have X on an $s p^{3}$ carbon next to a double bond, as in $C H_{2} = C H - C H_{2} C l$ .
Benzylic halides have X on an $s p^{3}$ carbon next to a ring, as in $C_{6} H_{5} C H_{2} C l$ .
Vinylic halides have X on a doubly bonded carbon, as in $C H_{2} = C H C l$ .
Aryl halides have X on the ring itself, as in $C_{6} H_{5} C l$ .

Dihalides. Geminal means both halogens on one carbon, and vicinal means on neighbouring carbons. Ethylidene chloride, $C H_{3} C H C l_{2}$ , is 1,1-dichloroethane. Ethylene dichloride, $C l C H_{2} C H_{2} C l$ , is 1,2-dichloroethane.

Worked example: sort these five

Classify (a) $C H_{3} C H = C H C H_{2} C l$ , (b) $C H_{3} C H = C C l C H_{3}$ , (c) $C_{6} H_{5} C H (C l) C H_{3}$ , (d) $C H_{3} C_{6} H_{4} C l$ , (e) $(C H_{3})_{2} C C l C H_{2} C H_{3}$ .

Show the solution

Look only at the carbon holding Cl.

(a) $s p^{3}$ , next to C=C: allylic.
(b) On C=C: vinylic.
(c) $s p^{3}$ , next to the ring: benzylic, and also secondary.
(d) On the ring: aryl.
(e) Three carbons attached: tertiary.

Common and IUPAC names

Formula	Common name	IUPAC name
$C H_{3} C H_{2} C H (C l) C H_{3}$	sec-Butyl chloride	2-Chlorobutane
$(C H_{3})_{3} C C l$	tert-Butyl chloride	2-Chloro-2-methylpropane
$(C H_{3})_{2} C H C H_{2} C l$	Isobutyl chloride	1-Chloro-2-methylpropane
$(C H_{3})_{3} C C H_{2} B r$	Neopentyl bromide	1-Bromo-2,2-dimethylpropane
$C H_{2} = C H C l$	Vinyl chloride	Chloroethene
$C H_{2} = C H C H_{2} B r$	Allyl bromide	3-Bromopropene
$C H C l_{3}$	Chloroform	Trichloromethane

What changes down the group

Longer bond, weaker bond. Data: NCERT Class 12.

The halogen gets bigger, so the bond gets longer and weaker. The bond enthalpy falls from 452 to 234 kJ/mol. That is why iodides are the most reactive and fluorides the least.

Dipole moment trap. The values are $C H_{3} C l$ 1.860 D, $C H_{3} F$ 1.847 D, $C H_{3} B r$ 1.830 D and $C H_{3} I$ 1.636 D. Fluorine has the larger charge separation, but the C-F bond is much shorter. Dipole moment multiplies charge by distance, so $C H_{3} C l$ comes first.

Classifying halides and the C-X bond

In 60 seconds

Sorting halides

Worked example: sort these five

Common and IUPAC names

What changes down the group