CHAPTER 11: Unit 7. Haloalkanes

Halogenated Hydrocarbons

Many organic compounds are closely related to the alkanes. As we noted in Section 1.7 “Chemical Properties of Alkanes”, alkanes react with halogens to produce halogenated hydrocarbons, the simplest of which have a single halogen atom substituted for a hydrogen atom of the alkane. Even more closely related are the cycloalkanes, compounds in which the carbon atoms are joined in a ring, or cyclic fashion.

The reactions of alkanes with halogens produce halogenated hydrocarbons, compounds in which one or more hydrogen atoms of a hydrocarbon have been replaced by halogen atoms:

Halogenated alkanes: halogenated alkenes are hydrocarbon derivative in which one or more halogen atoms have replaced hydrogen atoms of the alkane.

The replacement of only one hydrogen atom gives an alkyl halide (or haloalkane). The common names of alkyl halides consist of two parts: the name of the alkyl group plus the stem of the name of the halogen, with the ending -ide. The IUPAC system uses the name of the parent alkane with a prefix indicating the halogen substituents, preceded by number indicating the substituent’s location. The prefixes are fluoro-, chloro-, bromo-, and iodo-. Thus CH3CH2Cl has the common name ethyl chloride and the IUPAC name chloroethane. Alkyl halides with simple alkyl groups (one to four carbon atoms) are often called by common names. Those with a larger number of carbon atoms are usually given IUPAC names.Halogenated alkane nomenclature:Learning ObjectiveName halogenated hydrocarbons given formulas and write formulas for these compounds given names.Halogenated alkanes are named by using the rules that apply to branched chain alkanes with halogens substituents being treated the same as alkyl groups. ExampleGive the common and IUPAC names for each compound.CH3CH2CH2Br(CH3)2CHClSolutionThe alkyl group (CH3CH2CH2–) is a propyl group, and the halogen is bromine (Br). The common name is therefore propyl bromide. For the IUPAC name, the prefix for bromine (bromo) is combined with the name for a three-carbon chain (propane), preceded by a number identifying the carbon atom to which the Br atom is attached, so the IUPAC name is 1-bromopropane.The alkyl group [(CH3)2CH–] has three carbon atoms, with a chlorine (Cl) atom attached to the middle carbon atom. The alkyl group is therefore isopropyl, and the common name of the compound is isopropyl chloride. For the IUPAC name, the Cl atom (prefix chloro-) attached to the middle (second) carbon atom of a propane chain results in 2-chloropropane.

CH3CH(Br)CH2CH2CH2CH3: 2-Bromohexane

PRACTICE PROBLEM:

1.  Give the structure of the following compounds:

a.) 3-chloro-2-methylhexaneb.) 1,1,3-tribromocyclohexanec.) Propyl chloride

2. Give the IUPAC name of the following:

Important haloalkanes: many simple alkyl halides make excellent solvents because they are not flammable and they dissolve a wide variety of organic compounds. Most common solvents are CHCl3( chloroform or trichloromethane), CH2Cl2( methylene dichloride or dichloromethane) and CCl4( carbon tetrachloride). CH3Cl can be found in the emissions of volcanoes and produced by giant kelp and algae.While some alkyl halides are useful, some artificially generated alkyl halides may cause harm to the environment. They are called chlorofluorocarbons(CFC’s, trade name: Freon). Two common examples are CFCl3 and CF2Cl2. They are inert in nature and mostly used as refrigerants, solvents and aerosol propellants. But CFCs get deomposed by sunlight and produces a highly reactive intermediate that can destroy ozone layer.

You may also read about more information here:

https://www.nasa.gov/image-feature/goddard/2015-antarctic-ozone-hole-area-approaches-annual-maximum

Nasal spray fluticasone contains alkyl halide group.

Practice Problem: Can you classify each carbon in the following compound (fluticasone) as primary (10), secondary (20) and tertiary (30)?

HOMEWORK EXERCISE:Write the condensed structural formula for each compound.methyl chloridechloroform2. Write the condensed structural formula for each compound.ethyl bromidecarbon tetrachloride3Write the condensed structural formulas for the two isomers that have the molecular formula C3H7Br. Give the common name and the IUPAC name of each.Write the condensed structural formulas for the four isomers that have the molecular formula C4H9Br. Give the IUPAC name of each.What is a CFC? How are CFCs involved in the destruction of the ozone layer?Explain why each compound is less destructive to the ozone layer than are CFCs.fluorocarbonsHCFCsAnswers1.CH3ClCHCl3 CH3CH2CH2Br, propyl bromide, 1-bromopropane; CH3CHBrCH3, isopropyl bromide, 2-bromopropane compounds containing Cl, F, and C; by releasing Cl atoms in the stratosphere