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 such as 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.

Synthesis of Halogenoalkanes

Halogenoalkanes can be synthesized from alkanes, alkenes, or alcohols.

From an alkane

Alkanes react with halogens by free radical halogenation. In this reaction a hydrogen atom is removed from the alkane, then replaced by a halogen atom by reaction with a diatomic halogen molecule. Thus:

Step 1. X₂ --> 2 X^. (Initiation Step)

Step 2. X^. + R-H --> R^. + HX (1st Propagation Step)

Step 3. R^. + X₂ --> R-X + X^. (2nd Propagation Step)

Steps 2 and 3 keep repeating, each providing the reactive intermediate needed for the other step. This is called a radical chain reaction.

From an alkene

An alkene reacts with a hydrohalic acid (HX) to form an alkyl halide. The double bond of the alkene is replaced by two new bonds, one to the halogen and one to the hydrogen atom of the hydrohalic acid. Markovnikov's rule states that in this reaction, the halogen becomes attached to the more substituted carbon. Example:

CH₃-CH=CH₂ + HBr --> CH₃-CHBr-CH₃ (not CH₃-CH₂-CH₂Br).

Alkenes also react with halogens to form halogenoalkanes with two neighboring halogen atoms. This is sometimes known as "decolorizing" the halogen since the reagent X₂ is colored and the product is usually colorless. Example:

CH₃-CH=CH₂ + Br₂ --> CH₃-CHBr-CH₂Br

From an alcohol

Certain reagents such as SOCl₂ (thionyl chloride) can be used to convert alcohols to alkyl halides.

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--a reaction in which water breaks a bond--is a good example of the nucleophilic nature of halogenoalkanes. The polar bond attracts a hydroxide ion, OH^-. (NaOH_(aq) being a common source of this ion). 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 covalent bond between the two. Thus C-X is broken by heterolytic fission resulting in a bromide ion, Br^-. As can be seen, the OH is now attached to the alkyl group, creating an alcohol. (Hydrolysis of bromoethane, for example, yields ethanol).

One should note that within the halogen series, the C-X bond weakens as one goes to heavier halogens, and this affects the rate of reaction. Thus, the C-I of an iodoalkane generally reacts faster than the C-F of a fluoroalkane.

Other Substitution Reactions

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

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

(One should note that a nitrile can be further hydrolyzed into a carboxylic acid.)

Elimination Reactions

Rather than creating a molecule with the halogen substituted with something else, one can completely eliminate both the halogen and a nearby hydrogen, thus forming an alkene. For example, with bromoethane and NaOH in ethanol, the hydroxide ion OH^- attracts a hydrogen atom - thus removing a hydrogen and bromine from bromoethane. This results in C₂H₄ (ethylene), H₂O and Br^-.