Haloalkane

Haloalkane, Halogenoalkanes or Alkyl Halides are compounds derived from alkanes by substituting one or more hydrogen atoms with halogen atoms. Fluoroalkanes, chloroalkanes, bromoalkanes and iodoalkanes are possible, as are mixed compounds chlorofluorocarbons - the notorious CFCs responsible for ozone depletion) etc.

The general formula for halogenoalkanes is C_nH_2n+1X (where X is the halogen). Thus, an example of a structural formula is, for bromoethane, CH₃CH₂Br. As is noted, the naming convention involves the halogen as a prefix to the alcohol. This is why ethane with bromine becomes bromoethane; as butane with chlorine becomes chlorobutane.

The Nucleophilic Reactivity of Halogenoalkanes

There is a polarity about halogenoalkanes - the carbon to which the halogen is attached is slightly electropositive where the halogen is slightly electronegative. This results in an electron deficient carbon which, inevitably, attracts nucleophiles.

Substitution Reactions

Substitution reactions involve the replacement of the halogen with another molecule - thus leaving saturated hydrocarbons, as well as the halogen product.

Hydrolysis

Hydrolysis where water breaks a bond is a good example of the nucleophilic nature of halogenoalkanes. The polar bond attracts an OH^- molecule (NaOH_(aq) being a common source of this molecule). This OH^- is a nucleophile with a clearly negative charge, as it has excess electrons it donates them to the carbon, which results in a covalant bond between the two. Thus C-X is broken by heterolytic fission resulting in a Br^- molecule. As can be seen the OH creates an alcohol (if starting with bromoethane we now have ethanol).

One should note that as one travels down the halogens the C-X bond weakens, and thus the above reaction occurs more quickly. Thus, C-I (Iodine) reaction occurs faster than C-F (Fluorine).

Other Substitution Reactions

Apart from hydrolysis, there are a few other isolated examples of nucleophilic substitution:

• If one adds ammonia (NH₃) to bromoethane, amine (CH₃CH₂NH₂) will form along with HBr.
• If one adds cyanide (CN^-) to bromoethane, nitrile (CH₃CH₂CN) will form along with Br^-.

[One should note that nitriles will further be hydrolised into carboxylic acid.)

Elimination Reactions

Rather than creating a molecule with the halogen substituted with something else, one can completely eliminate the molecule and hence form an alkene.

This needs occur in water-free conditions: with bromoethane in ethanol along with NaOH, the hydroxide attracts a hydrogen atom - thus removing a hydrogen and bromine from bromoethane. This results in C₂H₄, H₂O and Br^-.

See Also

• Organic chemistry
• Alcohol
• Halogen