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· An ion in which a carbon atom has a posit ive charge is known as a carbocation.
· Four types o f carbocations are possible: H H H C C C C H H methyl carbocation C C C C C H o primary (1 ) o secondary (2 ) carbocation carbocation C o tertiary (3 ) carbocation Stability of carbocations: + 3° > 2° > 1° > C H3 · Markovnikov’s Rule states that when an unsymmetrical mo lecule such as HX adds to a carboncarbon double bond, the hydrogen fro m HX goes to the carbon atom that has the greater number of hydrogen atoms.
· Such addit ion produces the more stable carbocation. More stable carbocation 9 Chemistry 52 Chapter 20 AROMATIC HYDROCARBONS
· Unsaturated hydrocarbons that contain benzene rings are called aromatic hydrocarbons.
· Benzene is a sixmembered ring with mo lecular formula C6H6. It has alternating double bonds, and is represented by the fo llowing structures: · Removal o f a H fro m a benzene r ing results in an aryl group with formula C6H5¾ called phenyl.
· Some monosubst itued aromat ic co mpounds and their names are shown below: Br CH CH 2 3 Cl ethylbenzene chlorobenzene 10 bromobenzene Chemistry 52 Chapter 20 NAMING AROMATIC COMPOUNDS
· Simple monosubst ituted aromat ic co mpounds can be named as benzene derivat ives, as previously discussed.
· Some other simple aromat ic co mpounds that have special names are shown below: OH Phenol CH 3 NH 2 Toluene Aniline · Compounds with benzene r ings that cannot be easily named as benzene derivat ives are named with phenyl groups. For example:
1 CH 3
2 CH 3 4 5 CHCH CH 2 3
Cl 3chloro2pheny lpentane · When two subst ituents occur on a benzene r ing, three iso mers are possible. These iso mers are named either by numbering of the ring atoms or the Greek prefixes ortho (o), meta (m) and para (p). 11 Chemistry 52 Chapter 20 Examples: Name the fo llowing aromat ic co mpounds by at least 2 different names: 1) CH 3 CH 3 CH 3 2) HO Cl CH 3 H C 3 H C 3 CH 3 3. Name the following co mpounds: CH 3 H C 3 CH 3 12 CH 3...
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