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Unformatted text preview: Ch. 11 The Unsaturated Hydrocarbons Alkenes, Alkynes, & Aromatics Hydrocarbons with only single bonds (alkanes and cycloalkanes) are saturated , while hydrocarbons with double bonds (alkenes) and those with t riple bonds (alkynes) are unsaturated . Alkene s include at least one carbon-carbon double bond. The double bond atoms are planar, and there is no free rotation about the double bond as seen in an alkane. Each bond angle is 120. The two smallest alkenes are CH 2 =CH 2 (ethene or ethylene) & CH 3 CH=CH 2 (propene or propylene). Ethylene, the most important of all synthetic organic chemicals (25 billion kg produced annually in U.S.), goes into the manufacture of polyethylene (a very common plastic) and ethylene glycol (the major ingredient in antifreeze for car radiators). Propylene is also important in the production of plastics. The general formula for alkenes is C n H 2n (same as for cycloalkanes). The I UPAC system of nomenclature is primarily used for these compounds, given in the following rules. IUPAC Rules For Naming Alkenes (sect. 11.2) The longest chain of carbon atoms (LCC) including the double bond is the parent compound, with the same stem as the corresponding alkane but instead ending in ene . Example: If the LCC is 5, the alkene ia a pentene; if the LCC is 8, it is an octene; etc. The position of the double bond is indicated by the first carbon involved in the double bond, and is given the lowest possible #. Example: CH 3 CH 2 CH=CHCH 2 CH 2 CH 3 , the double bond is between the 3 rd and 4 th C atoms, not the 4 th and 5 th , so this is 3-heptene. Substituent groups are named as we did with alkanes, where its position is indicated by the number of the C atom on the LCC. The parent chain is always numbered such that the double bond is given the lowest possible #, even if the substituent gets a higher #. Remember! The double bond gets priority in the numbering scheme! Alkenes having two double bonds are called alkadi enes and those with three double bonds are called alkat ri enes (examples on p. 344). Properties of Alkenes Physical properties of alkenes are similar to those for alkanes the boiling points increase as the chain lengthens (for the first three straight-chain alkenes, see Table 11.1 on p. 343) and are insoluble in H 2 O, but soluble in organic nonpolar solvents (other hydrocarbons like n-hexane). Physiological properties of alkenes are like those for alkanes. Ethylene has been used as an inhalation anesthetic (can also cause unconsciousness and death) and helps in r ipening fruits and vegetables (plant growth substance). Dienes and polyenes (with 3 or more double bonds) are common in nature (sect. 11.4). Butadiene is found in coffee (CH 2 =CH-CH=CH 2 ). Lycopene and the carotenes are isomeric polyenes (C 40 H 56 ) which produce vivid red, orange, yellow colors in watermelons, tomatoes, carrots, other vegetables & fruits. Isoprenoids, or terpenes, are polyenes discussed on pp. 352-353, and Isoprenoids, or terpenes, are polyenes discussed on pp....
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- Spring '08