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Chapter 10

Ch.10 Summary.docx

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CHM 247 CH.10 Organohalides • Organohalide: A compound that contains one or more halogen atoms bonded to carbon. • Alarge variety of organohalides are known. The halogen might be bonded to an alkynyl group (C≡C-X), a vinylic group (C=C-X), an aromatic ring (Ar-X), or an alkyl group. 3 • Alkyl halides: A compound with a halogen atom bonded to a saturated, sp -hybridized carbon atom. 10.1 Names and Properties of Alkyl Halides • Named as haloalkanes. • Step 1: Find longest chain, and name it as the parent. Step 2: Number the carbons of the parent chain beginning at the end nearer the first substituent, whether alkyl or halo. Step 3: If the parent chain can be properly numbered from either end by step 2, begin at the end nearer the substituent that has alphabetical precedence. • Many alkyl halides are also named by identifying the first alkyl group and then the halogen. • Halogens increase in size going down the periodic table, as do the lengths of their bonds. Therefore bond strength weakens as you go down the periodic table. • Halogens are often more electronegative than carbon. This implies that the alkyl halide should behave as an electrophile in polar reactions. 10.2 PreparingAlkyl Halides from Alkanes: Radical Halogenation • Simple alkyl halides can sometimes be prepared from a reaction of an alkane with Cl or 2 Br 2n the presence of light through a radical chain-reaction pathway. • • Radical substitution requires 3 kinds of steps: initiation, propagation, and termination. Once the initiation step has started, the reaction is a self-sustaining cycle. The propagation step is 2 steps. • Chlorination of an alkane does not simply stop at monochlorination, but continues to give a mixture of dichloro, trichloro, and even tetrachloro products. Therefore it is not used often. • • Tertiary hydrogen is more reactive than a secondary, which is more reactive than a primary. 10.3 PreparingAlkyl Halides from Alkenes:Allylic Bromination • The hydrogen halides HI, HBr, and HCl react with alkenes by a polar mechanism to give the product of Markovnikov addition. Bromine and chlorine undergo anti addition through halonium ion intermediates. • Another method of preparing alkyl halides from alkenes is by reaction with NMS in the presence of light to give products resulting from substitution by bromine at the position next to the double bond. • Allylic: the position next to a double bond. For example, H2C=CHCH Br 2s an allylic bromide. • • • As in alkane halogenation, a Br· radical abstracts an allylic hydrogen atom, forming an allylic radical plus HBr. The HBr then reacts with NBS to form Br 2 which in turn reacts
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