- CFCs (chlorofluorocarbons, like Freon) in refrigerators/air conditioners
- Teflon in non-stick surfaces
- Some are toxic and are banned – the
insecticide DDT (dichlorodiphenyltrichloroethane) and the PCBs (polychlorinated
biphenyls) used in electrical transformers (interesting/horrifying aside: DDT used to be sprayed in neighbourhoods - right on the people - yikes! 😱)
Properties of the Alkyl Halides
The halogen substituent on the carbon chain renders the molecule more polar, since halogens are more electronegative than C or H atoms. The increased polarity of the C-halogen bond creates stronger intermolecular forces, resulting in higher bp than the corresponding hydrocarbons. Also, the increased polarity makes them more soluble in polar solvents.
When organic halides are synthesized, a mix of products forms, with one to several halogens per molecule. As the number of halogens per molecule increases, so does the molecule’s polarity, and thus boiling point. The different molecules can be separated by fractional distillation.
Preparing Organic Halides
Recall from the last lesson that alkyl halides are produced in a substitution (halogenation) reaction with an alkane Repeated substitution may occur until all the hydrogens are replaced by halogen atoms.
Alkenes and alkynes undergo addition (halogenation, hydrohalogenation) reactions and can easily add halogens or hydrogen halides across their multiple bonds.
We can also produce a halide of a benzene ring through a substitution reaction with a halogen.
Preparing Alkenes from Alkyl Halides: Elimination Reactions
Alkyl halides can eliminate a hydrogen and a halide ion from neighbouring C atoms, producing a double bond in their place, becoming an alkene in the process. A hydroxide ion is necessary to facilitate the reaction.
An elimination reaction is the reverse reaction to the addition (halogenation) of a alkene.
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| **Note that typically we would not show the H atom on the 2-bromopropane; it is shown to make the process of the reaction more clear.** The Br and the H are removed by the action of the hydroxide ion, OH-. The two carbons (that were previously bonded to Br and H respectively) form another bond to each other, resulting in a double bond in that position. The H bonds to the OH, producing water. The bromide ion, Br-, is also a product. |
Homework #16-20
