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Topic 4

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Chemistry 2223B
Felix Lee

Chemistry 2213a  Fall 2012  Western University Topic 4. Haloalkanes (Alkyl Halides) A. Structure and Nomenclature  Like hydrogen, the halogens have a valence of one. So, a halogen atom can replace a hydrogen atom in a molecule, so the general formula of a saturated mono-halo acyclic compound is C H n 2n+1X.  Therefore, treat X as an H when calculating unsaturation units. e.g. C 3 B4 =2 one unit of unsaturation.  Non-IUPAC, two-word names are commonly used for simple compounds. The first word identifies the alkyl group, the second identifies the halogen. Br Cl IUPAC: 2-chlorobutane bromocyclohexane Common: sec-butyl chloride cyclohexyl bromide Haloalkanes  2  The reactions that alkyl halides undergo strongly depend on the nature of the alkyl position. It’s essential that we recognize the “type” of alkyl halide. 1. Primary halide (1°). The CH 2 carbon bearing the halogen is bonded to one alkyl CH 3H B2 (CH 3 3CH B2 group and has two H. Cl Br 2. Secondary halide (2°). The carbon bearing the halogen is bonded to two alkyl groups and one H. 3. Tertiary halide (3°). The carbon bearing CH the halogen is bonded to three alkyl 3 Br groups and no H. Cl 3  All the above have halogens bonded to sp carbon (alkyl). We need to know that there can be aryl or vinyl halides, which have rather different chemistry because the halogen is bonded to an sp carbon (covered later on). Haloalkanes  3 B. Physical Properties  The halogens are more electronegative than carbon, so an sp 3 + - C–X bond is polar. The polar bond does not cause alkyl halides CH CH Br 3 2 to be water-soluble.  Halogenated organic compounds rarely occur in nature is why many organic halides are highly toxic and are very slowly destroyed. However, we study alkyl halides because they are very useful in laboratory syntheses. As well, they serve as excellent examples of natural reaction mechanisms. C. Characteristic Reactions  Haloalkanes can undergo substitution reactions, where a nucleophile replaces the halogen (the leaving group). Haloalkanes  4  Nucleophiles can be either neutral or negatively charged, but they must have at least one nonbonding pair of electrons. Leaving groups are all atoms/groups that create a  charge on an sp carbon atom before they leave.  The above reaction is a template for a wide range of reactions differing in Nu and LG. If a neutrally charged nucleophile is used, there is usually a deprotonation step after the substitution reaction. Haloalkanes  5  Because nucleophiles have nonbonding electrons, they can also act as bases. Therefore, substitution and -elimination are frequently competing reactions. D. Substitution Mechanisms  How does substitution occur? That is, how does the nucleophile replace the leaving group? Two possibilities: 1.Nu waits until LG departs C on its own, after which Nu comes and takes the empty spot; or 2.Nu kicks out the LG through backside attack. Haloalkanes  6  With mechanism #1, the rate of the reaction is dependent entirely on how fast LG decides to leave on its own. This would entail first-order kinetics. Rate = k [R−LG]  Whereas, the rate of mechanism #2 depends on how willing LG is to allow itself to be kicked out, and also how good Nu is at kicking LG out. This entails second-order kinetics. Rate = k [Nu] [R−LG] S 2 Reaction Mechanism N  This is a one-step mechanism that involves the collision of the reagents to form a high-energy transition state that falls apart to give the products. Everything happens at once (concerted). The nucleophile attacks as LG leaves. Haloalkanes  7  The stereochemical and energetic requirements of the mechanism allow us to predict various aspects.  Useful reactions have H sufficiently exothermic so that products are formed in good yield 1. The mechanism is bimolecular  Both the Nu and R−LG molecules must collide to form the transition state. This is the only step of reaction, so it is the rate-determining step. The rate law is second-order. Rate = k [Nu] [R−LG] Haloalkanes  8 2. Stereochemical inversion at the centre  Due to backside attack, the stereochemical configuration at that carbon is always inverted.  Almost always, an R configuration becomes S, or an S becomes R. However, the label sometimes remains the same even though inversion has occurred. H COH C CH OCH 3 2 S N 2 3 Br NC Br NC H H H3C CH3 S S  The reason for the retention of the S label in this example is due to the change in group priorities. In this case, the LG and the Nu don’t have the same priority. Haloalkanes  9 3. Steric bulk around the centre slows the reaction  Nu needs to attack from the back. If something is in its way, the reaction will be slower.  The reaction works best when groups on the carbon are small. Large groups block the incoming Nu.  In general, the rate of an S N reaction for alkyl halides is: tertiary < secondary < primary < methyl  In fact, tertiary alkyl halides DO NOT react by S 2 Nt all. Haloalkanes  10  In the example below, the only product formed is one where substitution has occurred at the secondary carbon. CH 3 CH 3 CN Br Br Br NC 4. Better the nucleophile, faster reaction  Better nucleophiles are better at kicking out the LG.  In general, less-electronegative atoms are better nucleophiles. And, anions (which are more attracted to the carbon atom of the haloalkane) are better than neutral. Haloalkanes  11 5. Aprotic solvents increase the reaction rate  Aprotic (non-protic) solvents do not have an H atom with a  charge; i.e. they can’t form hydrogen bonds.  In aprotic solvents, nucleophiles are much more reactive due to the lack of stabilization from the solvent. The more reactive the nucleophile, the better the nucleophile, and the faster is the reaction.  Common aprotic solvents include acetone, dimethylsulfoxide, dichloromethane, etc.  Note that this doesn’t mean that an S 2 Nill not occur at all in protic solvents. They just prefer aprotic solvents. Protic solvents include water, alcohols, acids, etc. Haloalkanes  12 6. Better leaving group, faster reaction  Better leaving groups are those that are more stable. We use the same criteria used to assess the relative stabilities of conjugate bases. S 1 Reaction Mechanism N "top" and "bottom" products + -   slow Nu LG Nu Nu  The first, rate-determining step involves the ionization of the R−LG bond to form a carbocation intermediate. i.e. LG must come off on its own. The second 2 step is attack by Nu (neutral or anion) on both sides of the carbocation (sp = planar) to yield the products. Haloalkanes  13  The reaction coordinate diagram is just like the addition of HX to alkenes. An S 1Nreaction also involves a carbocation.  The reaction coordinate diagram and mechanism tell us a lot… 1. The first, RDS is unimolecular  The rate-determining step is the departure of LG on its own. Rate = k [R−LG] 2. Mixtures of stereoisomers are formed  If a new stereocentre is formed, there will be a mixture of stereoisomers. 2 Because the carbocation intermediate is sp (flat), the Nu can attack either face of the carbocation. Haloalkanes  14  If we start with any one enantiomer, we will obtain a racemic mixture. In this case, an optically active reagent will give a mixture that is optically inactive. For example: O O O S N Cl Br Cl Cl pure R
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