Chapter 6 - ALKYL HALIDES IN THIS CHAPTER t Nomenclature u...

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Unformatted text preview: ALKYL HALIDES IN THIS CHAPTER: t/ Nomenclature u/ Synthesis of Alky/ Halides V Chemical Reactions of Alky/ Halldes 6’ Solved Problems Nomenclature Alkyl halides have the general formula RX, where R is an alkyl or sub- stituted alkyl group and X is any halogen atom (F, Cl. Br, or I). These compounds are named as the parent hydrocarbon, using fluoro—, chloro-, bromo-, or iodo- as the substituem names. For example. CH\ /CH2\ /CH3 CH3/ CH CH2 is named 3-bromo-2—chlorohexane. Br Classification is based on the structural features: RCHZBr is 1°, RECHBI‘ is 2°, and R3CBr is 3“. 50 CHAPTER 6: Alkyl Halides 51 Synthesis of Alkyl Halides Alkyl halides are usually prepared from alcohols or from alkenes, although they can be prepared directly from alkanes. 1. Halogenation of alkanes with C12 or Brz. These reactions are usu~ ally initiated by light (by) and involve radical intermediates. Several steps are involved, and summarized as follows: Initiation (produces 2 halogen radicals): hv X7 ——> 2 X- Propagation (produces product and continues the chain): R-H+X~-—> R-+HX R-+X2 ——>RX+Xo Termination (reactions that destroy radicals): 2 R- ——> R-R R- + X' —> R-X 2. From alcohols (ROH) with HX, PX3, or SOClz. These reactions are similar to the nucleophilic substitution reactions discussed later in this chapter. OH Br I PBI'3 —_> CH\ CH\ CH3/ CH3 CH3/ CH3 3. Addition of HX t0 alkenes. These reactions. which tend to follow Markovnikov’s rule, are discussed more fully in Chapter 5. Br H HBI CH CH2 C -—> 2 CH / CH3/ \CH2 \CH2 CH3/ \CH2/ \CH3 52 ORGANIC CHEMlSTRY 4. By reaction of alkenes with X2 (X = Br, C1) to give 1,2-dihalides. These electrophilic addition reactions are discussed more fully in Chapter 5. Br Bl‘a l —~> CH Br /\/ H C CH3/ \CH2 CH3 CH2 Chemical Reactions of Alkyl Halides Alkyl halides react with nucleophiles and with strong bases. Reactions with nucleophiles result in substitution, while elimination reactions result from reactions with bases. Nucleophilic Substitution. Substitution reactions are reactions in which one group (a halogen, for example) is replaced by another group. Alkyl halides are electrophiles, so they react with nucleophiles to give substitution products. The halide ion that is displaced by the incoming nucleophile is called the leaving group. Nucleophile + Substrate —-:~ Product + Leaving Group Sulfonates are excellent leaving groups—much better than the halides. One of the best leaving groups (better than Br) is CF3SO3‘, called triflate. A Brief Table of Nuclei," phIIes More nucleophIlIc _ , Iodide (l) "BromIde (B 90h,l0ridé:(C|: Hydroxide“ T 7 Water (H2 0 Less nucleophIlIc *indlcates a strong base CHAPTER 6: Alkyl Halides 53 Snl and SHZ Mechanisms The two major mechanisms of nucleophilic substitution are Sn] and Sn2. The “S” means substitution, the “n” refers to the fact that a nucleophile is involved. and the “1’ and “2" indicate the order of the reaction. Snl is a first-order reaction mechanism meaning that only a single molecule is involved in the transition state for the rate-determining step. Sn2 is a second—order reaction in which 2 molecules (alkyl halide and nucle- ophile) come together in the transition state for the slow (rate determin- ing) step. The mechanism of the Snl reaction is shown below. Upon heating the alkyl halide, the C—X bond breaks, creating a carbocation and the halide ion leaves. The carbocation then reacts with a nucleophile to pro- duce the substitution product. CH3 CH3 CH3 I A(lteat) I Nuc' I CH5—-—C—Br _. CHa—C e CHa—C—Nuc CH3 CH3 CH3 Since the carbocation is so reactive. the strength of the nucleophile has no effect on the rate of Snl substitution. Also since a cation must be formed, this reaction is limited to 3" and 2° halides. The success of Snl reactions is sensitive to the nature of the leaving group: only the better leaving groups will permit this reaction to proceed. The mechanism of the Sn2 reaction is quite different. In this mechanism, the nucleophile attacks the alkyl halide at the same time that the bond to the leaving group is breaking. There is a requirement that the nucle— ophile must attack the carbon that bears the halogen directly behind the C—X bond, also known as “backside attack." As the new bond forms and the C—X bond breaks, the carbon undergoes “inversion." as shown in the example on the next page. in which the nucleophile is iodide and the leaving group is bromide. 54 ORGANIC CHEMISTRY H3 i CH3 C 1\| s—i 5— | H.\|‘yC\/\‘—> l'”;§\""3‘ —, l/an’fii F? c HQCHa CH3CH2 Notice that the starting material has the R configuration, but the product has the S configuration. This inversion of stereochernistry is a hallmark of the Sn2 reaction. In contrast, racemization occurs in Snl reactions since the sterochemistry of the starting material is destroyed as the pla- nar cation intermediate is formed. ABnefTabIEOf Lea Good leavmg group " " Iodide 't( In Sn2 subsitution. the strength of the nucleophile as well as the nature of the leaving group and the substrate are all important. Only the more pow- erful nucleophiles will successfully react with alkyl halides. The alkyl halide substrate must not be sterically crowded. or the nucleophile will not be able to approach closely enough to displace the leaving group. As a result. SDZ reactions are most favorable with methyl halides (such as CH3I) and 1° halides (like CH3CH2Br), and never occur at 3° centers. Elimination Reactions. Elimination reactions. which produce alkenes, can occur in competition with substitution reactions. As in substitution reactions. there are 2 common mechanisms for elimination reactions. the El and E2 mechanisms. In a B—elimination (dehydrohalogenation) reaction, a halogen and a hydrogen atom are removed from adjacent CHAPTER 6: Alkyl Halides 55 carbon atoms to form a double bond between the two C’s. The reagent commonly used to remove HX is the strong base KOH in ethanol. The E1 Mechanism. Like the Snl mechanism, the E1 mechanism is a 2—step process that proceeds via a cation intermediate. CH3 CH3 CH3 l A l base \ CHg—C—Br ——> CHa—C C9 —> C=CH2 l | / CH3 CH3 CH3 The E2 Mechanism. The E2 reaction is a single-step, bimolecular reac— tion in which no intermediate is formed. This reaction proceeds via a transition state that has an antiperiplanar arrangement of the leaving group and the proton that is being removed. In this arrangement, the hydrogen and the leaving group lie in the same plane, pointing in oppo- site directions. As a result. the reaction is stereospecific—only one of the possible cis/trans stereoisomers is formed. CH3 CHacHz H/C\CH3 52 , \C/ \C/ \c CH3 ll NOT ll Be l /C\ /C\ \» H CchH2 CH3 CH3 CHZCHa (H and Br are antiperiplanar) Substitution versus Elimination. SnZ reactions are preferred when the halide is a good leaving group (1‘. Br) and when the substrate is unhin— dered (methyl, 1°, or 2°) and the nucleophile is a weak base (such as Cl‘, Br‘, or 1'). E2 is preferred when a strong base is used (KOH. NaOCl—lS). Snl and E] can also compete. with the major products usually resulting from substitution. 56 ORGANIC CHEMISTRY Solved Problems Problem 6.1 Give the organic product in the following substitution reaction. Br 'SH ———> \A CH3OH Br SH ‘SH —> \A CH30H Vk Problem 6.2 Indicate the products of the following reactions and point out the mechanism as SN], 3N2, E1, or E2. (a) (CH3)3CB1‘ + CH3CH30H, heat (b) CHSCHBrCH3 + NaOCH3 in CH30H (a) (CH3)3COCH2CH3 (major, via 5N1) (CH3)2C=CH2 (minor, El) (b) CH3CH=CH2 (via E2) cwgcmocngcn3 (via 3N2) ...
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