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Chapter 12 - Organohalides: Nucleophilic Substitutions and Eliminations Chapter Outline I. Alkyl halides (Sections 12.1-12.4). A. Names and structures of alkyl halides (Section 12.1). 1. Rules for naming alkyl halides: a, Find the longest chain and name it as the parent. i. Ifa double or triple bond is present, the parent chain must contain it. b. Number the carbon atoms of the parent chain, beginning at the end nearer the first substituent, whether alkyl or halo. c. Number each substituent. i. If more than one of the same kind of substituent is present, number each, and use the prefixes di-, tri-, tetra- and so on. ii. If different halogens are present, number all and list them in alphabetical order. d. If the parent chain can be numbered from either end, start at the end nearer the substituent that has alphabetical priority. 2. Some alkyl halides are named by first citing the name of the alkyl group and then citing the halogen. 3. Structure of alkyl halides. - a. Alkyl halides have approximately tetrahedral geometry. b. Bond lengths increase with increasing size of the halogen bonded to carbon. c. Bond strengths decrease with increasing size of the halogen bonded to carbon. d. Carbon-halogen bonds are polar, and halomethanes have dipole moments. e. Alkyl halides behave as electrophiles in polar reactions. B. Preparation of alkyl halides (Sections 12.2-12.3). 1. Alkyl halides can be prepared by reacting alkenes with HX or X> (Section 12.2). 2. Allylic bromination of alkenes. a. Reaction of an alkene with NBS (N-bromosuccinimide) causes bromination at the position allylic to the double bond. b. This reaction occurs by a radical chain mechanism. i. Br abstracts an allylic hydrogen. ii. The allylic radical reacts with Br to form an allylic bromide, plus Br. c. Reaction occurs at the allylic position because an allylic C-H bond is weaker than most other C-H bonds, and an allylic radical is more stable. d. Reasons for stability of an allylic radical. i, The carbon with the unpaired electron is sp”-hybridized, and its p orbital can overlap with the p orbitals of the double-bond carbons. ii, The radical intermediate is thus stabilized by resonance. iii. This stability is due to delocalization of the unpaired electron over an extended x network. e. Reaction of the allylic radical with Br) can occur at either end of the x orbital system. . i. A mixture of products may be formed if the alkene is unsymmetrical. ii, These products aren't usually formed in equal quantities; reaction to form the more substituted double bond is favored. . f. Products of allylic bromination can be dehydrohalogenated to form dienes. 
