Lesson 1 of 13 · 5 min

Classification

NCERT §6.1

The six bottles on Kavya's shelf all hold carbon compounds with a halogen in them. Before she can predict how any of them react, she has to sort them.

The story this chapter follows: Kavya's week at the reagent shelf

Kavya, a Class 12 student, spends a week helping in her college's organic lab. Her job starts at one shelf: dichloromethane, chloroform, carbon tetrachloride, 2-bromobutane, tert-butyl bromide and chlorobenzene, each in a brown or amber bottle. Her opening task is to measure out 25.0 mL of CCl₄, and the lab technician asks her to say what it weighs before she puts it on the balance.

The lesson in notes

In short

Swapping one or more hydrogen atoms of a hydrocarbon for halogen gives a haloalkane (alkyl halide) when the parent is aliphatic and a haloarene (aryl halide) when it is aromatic. In haloalkanes the halogen sits on an sp³ carbon; in haloarenes it sits on an sp² carbon of an aryl ring.

Halogen compounds matter in medicine: the chlorine-containing antibiotic chloramphenicol treats typhoid, the body's iodine hormone thyroxine prevents goiter, chloroquine treats malaria and halothane is a surgical anaesthetic. Some fully fluorinated compounds are being studied as blood substitutes.

By the number of halogen atoms, compounds are mono-, di- or polyhalogen (tri-, tetra- and so on).

Compounds with an sp³ C–X bond: alkyl halides (R–X, general formula CₙH₂ₙ₊₁X), allylic halides (X on the sp³ carbon next to a C=C) and benzylic halides (X on the sp³ carbon joined to an aromatic ring).

Alkyl halides are called primary (1°), secondary (2°) or tertiary (3°) according to whether the carbon holding the halogen is itself primary, secondary or tertiary.

Compounds with an sp² C–X bond: vinylic halides (X on a carbon of a C=C) and aryl halides (X directly on a carbon of an aromatic ring).

Halogenated compounds persist in the environment because soil bacteria break them down poorly.