Halogenoalkanes - Introduction

This lesson covers: 

1. How to name halogenoalkanes
2. Polarity of the carbon-halogen bond
3. Trend in reactivity of halogenoalkanes

Naming halogenoalkanes

A halogenoalkane is a type of chemical compound where one or more hydrogen atoms in an alkane have been replaced by halogen atoms (like fluorine, chlorine, bromine, or iodine).

To name a halogenoalkane, we use prefixes (like fluoro-, chloro-, bromo-, iodo-) to indicate the type and number of halogen atoms.

Here are some examples of halogenoalkanes:

Polarity of the carbon-halogen bond

In halogenoalkanes, the carbon-halogen bond is polar because halogen atoms have a higher electronegativity than carbon. This causes an uneven distribution of electrons, making the carbon atom partially positively charged (δ⁺) and the halogen atom partially negatively charged (δ^-).

This polarity in the bond makes the carbon atom a target for nucleophiles (electron pair donors). Common nucleophiles include OH^-, CN^-, NH₃, and H₂O.

Influence of carbon-halogen bond enthalpy on reaction rates

Bond enthalpy is a measure of bond strength, defined as the energy required to break one mole of bonds between two atoms in the gaseous state. The bond enthalpy of the carbon-halogen bond determines the reactivity of halogenoalkanes.

Halogenoalkanes with lower carbon-halogen bond enthalpies have weaker bonds that require less energy to break. As a result, these compounds react more quickly than those with higher bond enthalpies.

The carbon-halogen bond enthalpy decreases down group 7 due to several factors:

• Increasing atomic radius of the halogens
• Increasing carbon-halogen bond length
• Decreasing electrostatic attraction between bonding electrons and nuclei

As a consequence, the energy needed to break these longer, weaker bonds decreases, leading to faster reaction rates.

Therefore, iodoalkanes (with the weakest carbon-halogen bonds) react the fastest, while fluoroalkanes (with the strongest bonds) react the slowest.