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An alkyl halide, also known as alkyl halogenide, haloalkane, halogenalkane, or halogenoalkane, is a chemical compound derived from an alkane by substituting one or more hydrogen atoms with halogen atoms. Substitution with fluorine, chlorine, bromine and iodine results in fluoroalkanes, chloroalkanes, bromoalkanes and iodoalkanes, respectively. Mixed compounds are also possible, examples are the chlorofluorocarbons (CFCs) which are mainly responsible for ozone depletion. Alkyl halides are used in semiconductor device fabrication, as refrigerants, foam blowing agents , solvents, aerosol propellants, fire extinguishing agents, and chemical reagents.
An example of a structural formula is, for bromoethane, CH3CH2Br. As is noted, the naming convention involves the halogen as a prefix to the alkane. This is why ethane with bromine becomes bromoethane; as butane with chlorine becomes chlorobutane.
The CFC naming system is mainly used for fluorinated and chlorinated refrigerants. The standard is specified in the ANSI/ASHRAE Standard 34-1992 with additional annual supplements . The specified ANSI/ASHRAE prefixes were FC (fluorocarbon), or R (refrigerant), but today most are prefixed by a more specific classification:
- CFC chlorofluorocarbons
- HCFC hydrogenchlorofluorocarbons
- HFC hydrogenfluorocarbons
- FC fluorocarbons
- PFC perfluorocarbons (completely fluorinated)
The CFC name has a structure like XYZ-c0123b4a where the letters and numbers stand for:
|XYZ:||CFC, HCFC, HFC, FC, or PFC, e.g. CFC 12, HCFC 22, HFC 125, FC 23, and PFC 218.|
|c:||cyclic compound, e.g. PFC c318 for octafluorocyclobutane, HFC 338 for octafluorobutane.|
|0:||number of double bonds (omitted if zero).|
|1:||carbon atoms - 1 (omitted if 0), e.g HFC 41 for fluoromethane, HFC 161 for fluoroethane, HFC 281 for fluoropropane.|
|2:||hydrogen atoms + 1.|
|b4:||chlorine atoms replaced by bromine, e.g. CFC 13b1 for CBrF3.|
|a:||letter added to identify isomers, the "normal" isomer in any number has the smallest mass difference on each carbon, and a, b, or c are added as the masses diverge from normal.|
Synthesis of alkyl halides
Alkyl halides can be synthesized from alkanes, alkenes, or alcohols.
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.
An alkene reacts with a hydrogen halogenides (HX) like hydrogen chloride (HCl) or hydrogen bromide (HBr) 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 more likely. Example:
CH3-CH=CH2 + HBr → CH3-CHBr-CH3 (major product) + CH3-CH2-CH2Br (minor product).
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
Tertiary alcohols react with hydrochloric acid directly to produce tertiary alkyl halides, but if primary or secondary alcohols are used, an activator such as zinc chloride is also needed. Alternatively the conversion may be performed directly using thionyl chloride. Alcohols may likewise be converted to alkyl bromides using hydrobromic acid or phosphorus tribromide or alkyl iodides using red phosphorus and iodine. Some typical examples of alkyl chloride preparations are:
The reactivity of alkyl halides towards nucleophiles
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 (electrophilic) carbon which, inevitably, attracts nucleophiles.
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.)
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-.
- Chlorofluorocarbon (CFC)
- Hydrochlorofluorocarbon (HCFC)
- Halocarbon (includes aromatic halides)
- B. S. Furnell et al., Vogel's Textbook of Practical Organic Chemistry, 5th edition, Longman/Wiley, New York, 1989.
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