Why are halogenoalkanes reactive?

Halogenoalkanes are reactive due to the polar nature of the carbon-halogen bond, which makes them susceptible to nucleophilic attack.

Halogenoalkanes, also known as alkyl halides, are organic compounds in which one or more hydrogen atoms in an alkane have been replaced by halogen atoms (fluorine, chlorine, bromine or iodine). The carbon-halogen bond in these compounds is polar because halogens are more electronegative than carbon. This means that the halogens pull the shared pair of electrons towards themselves, creating a partial negative charge on the halogen and a partial positive charge on the carbon. This polar nature of the carbon-halogen bond makes halogenoalkanes reactive.

The partial positive charge on the carbon atom makes it susceptible to attack by nucleophiles. A nucleophile is a species that is attracted to positive centres, and it donates a pair of electrons to form a new covalent bond. In the case of halogenoalkanes, the nucleophile will attack the carbon atom, displacing the halogen in a reaction known as nucleophilic substitution. This is the primary reaction of halogenoalkanes.

The reactivity of halogenoalkanes also depends on the type of halogen atom involved. The carbon-fluorine bond is the strongest due to the small size and high electronegativity of fluorine, making fluoroalkanes the least reactive. On the other hand, the carbon-iodine bond is the weakest, making iodoalkanes the most reactive. This trend is known as the 'bond strength effect'.

In addition to nucleophilic substitution, halogenoalkanes can also undergo elimination reactions, where the halogen and a hydrogen atom are removed to form an alkene. This usually occurs under conditions of heat or when a strong base is present. The ability of halogenoalkanes to undergo these different types of reactions makes them very useful in organic synthesis.