Dihalogen Derivatives


Dihalogen Derivatives

1. When both halogen atoms are attached to the same carbon atom, they are said to be in the germinal (gem-) position. Since the loss of two hydrogen atoms from the same carbon atom gives the alkylidene group, gem-dihalides are named as the alkylidene same carbon atom gives the alkylidene group, gem-dihalides are named as the alkylidene dihalides, e.g., CH3CHBr2 ethylidene dibromide; CH3CCl2CH3 isopropylidene dichloride.

2. When the two halogen atoms are on adjacent carbon atoms they are said to be in the vicinal (vic-) position, and these dihalides are named as the dihalide of the alkene from which they may be prepared by the addition of halogen, e.g., CH2ClCH2Cl ethylene dichloride; (CH3)CBrCH2Br isobutene dibromide.

3.  When there is a halogen atom on each of the terminal carbon atoms of the chain, i.e., in the α-position, the compound is named as the polymethylene dihalide, e.g., CH2ClCH2CH2CH2Cl tetramethylene dichloride.

4.  When the two halogen atoms occupy position other than those mentioned above, the compounds are named as dihalogen derivatives of the parent hydrocarbon, the positions of the halogen atoms being indicated by numbers (us principle of lowest numbers), e.g., 

CH3CHClCH2CHClCH2CH3  2,4-dichlorohexane

The alkyl halides, gem-, vic- and α-dihalides may also be named by this (I.U.P.A.C.) system, e.g., CH3CH2CHBrCH3 2-bromobutane; CH2ClCH2CH2Cl  1,3-dichloropropane.

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