Mechanisms: The Basics

Transcription

1 Mechnisms: The Bsics A) The orrect Use of Arrows to Indicte Electron Movement The bility to write n orgnic rection mechnism properly is key to success in orgnic chemistry clsses. rgnic chemists use technique clled rrow pushing to depict the flow or movement of electrons during chemicl rections. Arrow pushing helps chemists keep trck of the wy in which electrons nd their ssocited toms redistribute s bonds re mde nd broken. The first essentil rule to keep in mind is the following: First rule: Arrows re used to indicte movement of electrons A regulr rrow (double-sided rrowhed) is used to indicte the movement of two electrons, while line with single-sided rrowhed (sometimes clled fish hook rrow ) is used for single electron movement involved with rdicl rections tht re first described in hpter 8. Double-sided rrowhed two electron movement Single-sided rrowhed one electron movement The gret mjority of rections tht will be discussed in this book involve movement of pirs of electrons, so they re represented by double-sided rrowheds. Arrow pushing ws first introduced in Section 1.8A in the discussion of resonnce contributing structures. Recll tht when compring two or more contributing structures, n rrow ws used to show how two electrons (lines representing bonds or pirs of dots representing lone pirs) could be redistributed within single chemicl structure to crete n lterntive Lewis line structure representtion of the bonding. By convention, rrows re used to keep trck of ll pirs of electrons tht re in different loctions in the two different contributing Lewis line structures, shown here for the cette nion nd benzene molecule Acette nion contributing structures - Benzene contributing structures

2 Keep in mind tht in the cse of resonnce, 1) the toms do not move between contributing structures, nd 2) the electrons re not ctully moving. The true chemicl structure should be thought of s hybrid of the contributing Lewis line structures. It is worth pointing out tht when used with contributing structures, rrows generlly indicte only the interconversion of π bonds nd lone pirs (cette ions) or just π bonds (benzene), not the formtion or breking of σ bonds. In chemicl rections, both electrons nd toms chnge positions s both π nd σ bonds re formed nd broken. Arrow pushing is used to keep trck of the movement of ll electrons involved with ech step of the overll trnsformtion. Becuse electrons re locted in orbitls surrounding toms, when bonds re formed or broken, the movement of electrons between orbitls is necessrily ccompnied by the movement of the ssocited toms, which leds to the second rule of rrow pushing when depicting chemicl rection mechnisms: Second Rule: Arrows re never used to indicte the movement of toms directly. The rrows only show tom movement indirectly s consequence of electron movement when covlent bonds re mde nd broken. We hve lredy used rrow pushing to show proton trnsfer severl times in hpter 4. The exmple below shows the trnsfer of proton from the reltively cidic cetic cid molecule to the reltively bsic hydroxide nion. We show this process with one rrow (lbeled in the digrm) tht strts t lone pir of electrons on the bsic oxygen tom of the hydroxide nion, then points to the cidic tom of cetic cid to indicte formtion of the new bond being mde. A second rrow origintes t the line representing the breking - bond nd points to the tom to denote cretion of lone pir (rrow b ). In this rection, the proton is being trnsferred between molecules, nd the rrows indicte movement of the electrons involved. 3 b + + orrect use of rrows to indicte electron movement during rection 3 A common mistke beginning students mke is tht they will erroneously write n rrow pointing from the of the cetic cid to the tom of the hydroxide nion. This is wrong, becuse such n rrow would be indicting the tom movement directly, not electron movement! ther common mistkes in rrow pushing re given t the end.

3 3 b X Incorrect rrow becuse it is pointing in the wrong direction! Never use rrows to indicte tom movement directly B) Electron Sources nd Sinks: ow to Predict Wht Will ccur in n rgnic Rection Mechnism ombined with the rrows shown for the contributing structures shown previously, we hve now seen ll three of the situtions illustrted by rrows with double-sided rrowheds, nmely the redistribution of π bonds nd/or lone pirs, formtion of new σ bond (generlly from lone pir or sometimes new π bond), nd breking of σ bond (generlly to form new lone pir or sometimes new π bond). ften, s in the cse of the cette-hydroxide ion rection, more thn one rrow is used in given mechnism step. Now tht you hve seen ll of the importnt types of rrows, we cn point out the most importnt common feture between them: Third Rule: Arrows lwys strt t n electron source nd end t n electron sink. electron source electron sink An electron source is bond or lone pir of electrons. It is either π bond or lone pir on n tom of reltively high electron density in molecule or ion, or bond tht must brek during rection. An electron sink is n tom on molecule or ion tht cn ccept new bond or lone pir of electrons. Lerning to identify the chrcteristic sources nd sinks in different functionl groups is the key to lerning orgnic chemistry rection mechnisms. For exmple, for rrows tht depict the formtion of new σ bonds, the electron source is often redily identified s being lone pir on the most electron rich tom of molecule or ion, nd the electron sink is redily identified s the most electron poor tom of molecule or ion. Thus, the prediction of mny of the most importnt electron sources nd sinks comes down to lessons concerning the differences in electronegtivity between toms tht were presented in Section 1.2, which llow you to identify prtil nd forml negtive nd positive chrges in molecules. As n id to your nlysis, the red nd blue colors of the vrious electrosttic surfce mps given throughout this book indicte the negtive nd positive regions of molecules. We will hve more to sy bout this rectivity pttern little bit lter.

4 This leds us to nother commonly encountered type of process tht deserves mention. As you will see in this nd mny lter chpters, mking new bond to n electron sink often requires the simultneous breking of one of the bonds present t the sink tom to void overfilling its vlence orbitls, sitution referred to s hypervlence. Fourth rule: Breking bond will occur to void overfilling vlence (hypervlence) on n tom serving s n electron sink. In these cses, the electron source for the rrow is the bond being broken, nd the sink is n tom ble to ccommodte the electrons s lone pir, generlly n electronegtive tom such s n tom or hlogen. If n ion is creted, tht ion is often stbilized by resonnce delocliztion or other stbilizing interctions. Returning to the proton trnsfer rection between cetic cid nd hydroxide, we cn now summrize our nlysis of this simple one-step mechnism. Bond being broken on sink tom 3 b δ δ + electron sink (reltively low electron density) electron source (reltively high electron density) Acette ion Reltively stble ion creted during the bond breking process (stbilized by resonnce delocliztion) Viewed in the context of the third rule, when considering the rrow used to mke new σ bond (rrow ), the hydroxide tom is the electron source (most negtively chrged tom) nd the cetic cid tom is the electron sink (tom with highest prtil positive chrge). This is illustrted using the electrosttic moleculr surfces shown below the rection eqution. The tom of hydroxide ion hs the gretest loclized negtive chrge s indicted by the most intense red color nd the cetic cid proton being trnsferred hs the most intense positive chrge chrcter indicted by the most intense blue color. In order to void overfilling the vlence of the tom during the

5 rection (fourth rule), the - bond of cetic cid must be broken (rrow b ). In so doing, the cette ion is formed. Note tht the cette ion is stbilized by resonnce delocliztion. Bsed on our nlysis of the rection between cetic cid nd the hydroxide nion, you should now pprecite tht the trnsfer of proton ( so-clled Brønsted cidbse rection) is relly just specil cse of the common pttern of rectivity between n electron source (the bse) nd the proton s n electron sink, combined with breking bond to stisfy vlence nd crete reltively stble ion. The ddition or removl of protons during chemicl rections is so common tht proton trnsfer steps re referred to by nme directly, nd we will use phrses such s dd proton or tke proton wy when referring to them. owever, proton trnsfer rections re not the only cse in which we use specil nmes to describe prticulr type of common rection tht involves rrows between electron sources nd electron sinks. As briefly mentioned in Section 4.7, broder terminology is pplied to the very common cse of rections in which new σ bonds form between electron rich nd electron poor regions of molecules. Nucleophiles (mening nucleus seeking) re molecules tht hve reltively electron rich π bonds or lone pirs tht ct s electron sources for rrows mking new bonds. Electrophiles (mening electron seeking) re molecules with reltively electron poor toms tht serve s sinks for these rrows. Anlogously, molecule, or region of molecule, tht is source for such n rrow is clled nucleophilic, while molecule or region of molecule tht is sink for these rrows is referred to s being electrophilic. Bsed on this description, it should be cler tht nucleophiles re nlogous to Lewis bses nd electrophiles re nlogous to Lewis cids. hemists use these terms interchngebly, lthough nucleophile nd electrophile re more commonly used in kinetics discussions while Lewis cid nd Lewis bse re more commonly used in discussions bout rection thermodynmics. We will use ll of these terms throughout the rest of the book. It is helpful to summrize the pproprite use of key terms ssocited with rrow pushing nd rection mechnisms. The terms source nd sink re used to identify the strt nd end of ech rection mechnism rrow, which is indicting the chnge in loction of electron pirs. The terms nucleophile nd electrophile (s well s Lewis bse nd Lewis cid ) re used to describe molecules bsed on their chemicl rectivity nd propensity to either donte or receive electrons when they interct. Protons cn be thought of s specific type of electrophile, nd for rections in which proton is trnsferred, the nucleophile is clled bse.

6 Exmple The following two sets of rections (A nd B) show possibilities for rrow pushing in individul rection steps. Identify which is wrong nd explin why. Next, using rrow pushing correctly, lbel which molecule is the nucleophile nd which is the electrophile. A) l l l B) 3 3 N 3 3 N l 3 3 N 3 3 N Solution In ech cse the first rrow pushing scenrio is wrong. The rrows shown below with strs over them do not strt t source of electrons, but rther they strt t positions of reltive positive chrge, which is incorrect. Incorrect 3 3 δ * 2 δ l δ 3 δ 3 * N orrect 3 3 b δ δ l electrophile 2 nucleophile b δ 3 δ electrophile 3 N nucleophile In the correct rrow pushing, the rrow lbeled depicts the interction of region of reltive high negtive chrge ( π-bond or lone pir) with n tom of reltively high prtil positive chrge on the other rectnt. Therefore, the molecule cting s the source for rrow the σ bond-forming rrow is the nucleophile while the molecule contining the sink tom is the electrophile. The rrow lbeled b is needed to stisfy vlence, nd is not considered when defining the nucleophile nd electrophile.

7 ) Putting it All Together: It omes Down to Multiple hoice Sitution In the sections nd chpters tht follow, mny different rection mechnisms will be described in stepwise fshion. Ech rrow cn be clssified ccording to one of the three overll situtions we hve lredy encountered (redistribution of π bonds nd/or lone pirs, formtion of new σ bond from lone pir or π bond, breking σ bond to give new lone pir or π bond). When lerning new mechnisms, first focus on the overll trnsformtion tht tkes plce. It might be rection in which toms or groups re dded (n ddition rection), rection in which toms or groups re removed (n elimintion rection), rection in which toms or groups replce n tom or group ( substitution rection), or other processes we will encounter. ften, the overll process is composed of multiple steps. nce you hve the overll process in mind, it is time to think bout the individul steps tht convert strting mteril(s) into product(s). Predicting complete multi-step mechnisms, then, comes down to lerning how to predict the individul steps. Understnding, s opposed to memorizing, mechnisms is criticl to mstering orgnic chemistry. Although the mechnisms you encounter throughout the course my seem entirely different, they re ctully relted in fundmentl wys. In fct, lmost ll of the orgnic rection mechnisms you will lern re composed of only few different individul elements (steps) tht re put together in vrious combintions. Your job is to lern these individul mechnism elements, nd then understnd how to ssemble them into the steps of the correct mechnism for the overll rection. Fortuntely, there re surprisingly smll number of different types of chrcteristic mechnism elements (ptterns of rrows) to be considered when trying to predict individul steps of even complex chemicl rections. For this reson, you should view the prediction of ech step in n orgnic mechnism s essentilly multiple choice sitution in which your most common choices re the following: 1. Mke new bond between nucleophile (source for n rrow) nd n electrophile (sink for n rrow). Use this element when there is nucleophile present in the solution s well s n electrophile suitble for rection to occur. Wter ( nucleophile) Isopropyl ction xomium ion intermedite intermedite (n electrophile)

8 2. Brek bond so tht reltively stble molecules or ions re creted Use this element when there is no suitble nucleophile-electrophile or proton trnsfer rection, but breking bond cn crete neutrl molecules or reltively stble ions, or both. Br Br 3 2-Bromo-2-methylpropne 3 Bromide ion (reltively stble nion) tert-butyl ction intermedite (stbilized by inductive nd hyperconjugtion effects. See Section 6.3A) 3. Add proton Use this element when there is no suitble nucleophile-electrophile rection, but the molecule hs strongly bsic functionl group or there is strong cid present Ethyl cette ( crboxylic ester) ydronium ion ( strong cid) A protonted intermedite Wter 4. Tke proton wy Use this element when there is no suitble nucleophileelectrophile rection, but the molecule hs strongly cidic proton or there is strong bse present. 3 3 xomium ion intermedite (strongly cidic) 3 3 Wter (cn ct s bse) 2-Propnol Wter The sitution is even simpler thn you might expect becuse 1. nd 2. re the functionl reverse of ech other, s re 3. nd 4. in mny cses. Mny times, more thn one of the four choices occurs simultneously in the sme mechnism step nd there re some specil situtions in which unique or different processes such s electrophilic ddition or 1,2 shifts occur. These different processes re described in detil s they re encountered.

9 In this course, you will lern importnt properties of the different functionl groups tht llow you to deduce the pproprite choices for the individul steps in rection mechnisms. To help you ccomplish this, s new mechnisms re introduced throughout the rest of the book, we will lbel ech mechnistic step s one of the four mentioned here when pproprite, emphsizing the common fetures between even complex mechnisms. When you re ble to predict which of the bove choices is(re) the most pproprite for given step in mechnism, you will then be ble to push electrons correctly without relying on memoriztion. At tht point, you will hve tken mjor step towrd mstering orgnic chemistry.