Nuggets of Knowledge for Chapter 8 Chemical Reactions II Chem 2310

Transcription

1 Nuggets f Knwledge fr Chapter 8 Chemical Reactins II Chem 2310 I. Substitutin, Additin, and Eliminatin Reactins The terms dissciatin, assciatin, and displacement are useful fr describing what happens t the electrns in an individual step in a mechanism. There are ther terms which are mst useful fr describing the verall result f a reactin. In an additin reactin, new atms are added t a mlecule, with lss f a duble bnd. The new atms are always next t each ther, ne n each f the atms where the duble bnd was lst. In a substitutin reactin, a bnd t ne atm is substituted fr a bnd t anther atm. Substitutin reactins always invlve a single carbn atm n the starting material a bnd is bth lst and gained frm that atm. In an eliminatin reactin, atms are lst frm a mlecule, and a duble bnd is left in their place. The atms are always next t each ther, s that the duble bnd can be frmed between them. II. Oxidatin and Reductin Reactins In general chemistry, xidatin and reductin are mst ften defined in terms f lss r gain f electrns (and crrespnding increase r decrease in charge). Hwever, this wn't wrk with rganic mlecules because they can be xidized r reduced withut changing charge. This makes xidatin states very difficult t calculate. In rganic chemistry, xidatin and reductin are defined by lking at the bnds gained r lst t xygen and hydrgen. If a cmpund is xidized, it gains bnd t xygen and/r lses bnds t hydrgen. Examples include an alkene becming an epxide, an alchl becming a carbxylic acid, r a dil breaking apart t becme tw aldehydes (this is als called an xidative cleavage). If a cmpund is reduced, it lses bnds t xygen and/r gains bnds t hydrgen. Examples include an aldehyde becming an alchl, r an alkyne becming an alkene. The highest xidatin state f carbn is carbn dixide, in which it has 4 bnds t xygen; the lwest xidatin state f carbn is methane, in which it has 4 bnds t hydrgen.

2 Atms ther than carbn can smetimes be xidized r reduced in rganic mlecules. Fr example, if a nitr grup becmes an amine, it has been reduced. Althugh xygen is mst ften used, any ther atm which is mre electrnegative than carbn can als cunt when deciding if a cmpund was xidized r reduced. Fr example, a alkyl halide becming an alkane is a reductin. Nt all reactins are xidatins r reductins. The fllwing are examples where n xidatin r reductin has ccurred: the number f bnds t xygen and hydrgen remain the same ne carbn is xidized but anther is reduced at the same time exchange f xygen fr anther electrnegative atm any acid base reactin (althugh this fits the definitin fr reductin with a lss f a bnd t hydrgen, there is als a change f charge, which cunteracts it) It is imprtant fr yu t learn t recgnize xidizing and reducing agents. These are cmpunds which are cause ther mlecules t be xidized r reduced (xidizing agents are easily reduced, while reducing agents are easily xidized. Oxidizing agents usually fit ne f these tw descriptins: a transitin metal in a high xidatin state and lts f xygen atms; fr example, Na 2 CrO 4 (sdium dichrmate) and KMnO 4 (ptassium permanganate) xygen in a high xidatin state (that is, bnded t itself); fr example, H 2 O 2 (hydrgen perxide) r O 3 (zne) Reducing agents usually fit ne f these three descriptins: hydrgen gas (H 2 ) with sme kind f catalyst hydride reagents in which hydrgen is bnded t a grup III atm; fr example, NaBH 4 (sdium brhydride) r LiAlH 4 (lithium aluminum hydride) metals in an elemental state; fr example Na (sdium metal), r Sn (tin) III. Radical Reactins A radical (ften called a free radical) is an atm that has an unpaired electrn, r a mlecule with an atm that has an unpaired electrn.

3 Radicals are very reactive, since they d nt have a cmplete ctet. Radical are als very dangerus when they are frmed in the bdy, because they react with prteins and DNA, and can cause damage t cells. Radicals are thught t be invlved in the aging prcess. Radicals are frmed by an initiatin step. This ccurs when a mlecule with a weak bnd absrbs energy frm light r heat and the bnd breaks hmlytically (ne electrn ges with each atm). Mlecules which typically underg hmlytic cleavage have highly electrnegative atms bnded t each ther fr example, xygen-xygen bnds, and halgen-halgen bnds. Mlecules which can underg hmlytic cleavage must be stred in dark bttles t keep them away frm light and in cld places t keep them frm heat. Carbn radicals d nt have high energy bnds that can break hmlytically, s they are frmed by reacting with radicals made by initiatin reactins. Whenever a radical reacts with a stable mlecule t frm a new radical, this is called a prpagatin step. Carbn radicals can becme stable again by a prpagatin step r a terminatin step. In a prpagatin step, the carbn radical reacts with an ther stable mlecule, taking an atm frm it (usually a H). In a terminatin step, the carbn radical cmbines with anther radical, and a bnd is frmed between them. Terminatin steps can ccur when any f the radicals in the reactin cllide and frm a bnd. Since radicals are s reactive, they are very shrt lived, and there are nt ever many f them in the reactin at the same time. Therefre, a radical is mre likely t cllide with a stable mlecule and underg prpagatin than with anther radical t underg terminatin. Terminatin steps usually happen at the end f the reactin, when mst f the starting materials that the radicals culd react with are used up. Radical mechanisms cnsist f three types f steps. An initiatin step is needed t get the reactin started. The prpagatin steps are where the rganic starting material reacts and frms prducts.

4 Terminatin steps are side reactins that slw the reactin dwn. They prduce small amunts f side prducts that are usually ignred in writing the verall reactin. The halgenatin f alkanes is an example f a radical reactin. The initiatin step is hmlytic cleavage f a halgen mlecule t frm tw halgen radicals. There are tw prpagatin steps. In the first, the halgen radical takes a H frm the alkane, leaving it as a radical. In the secnd, the carbn radical takes a halgen atm frm a halgen mlecule, leaving a halgen radical behind. Pssible terminatin steps invlve cmbining f tw halgen radicals, tw carbn radicals, r a carbn and a halgen radical. This reactin ften des nt stp at ne substitutin reactin. The alkyl halide prduct can still react with a halgen radical, s usually a mixture f prducts results. This makes alkane halgenatin useful nly n an industrial scale, where all f the prducts can be distilled frm each ther. Many radical reactins (including alkane halgenatin) are chain reactins. This is because the radical that is frmed by initiatin is ften refrmed by the prpagatin steps, s that initiatin desn't have t keep happening t keep the reactin running, s the reactin ges n withut any mre heat r light. Anther way t say this is that radical reactins have a high quantum yield that is, they give a lt f prduct with nly a small amunt f light r heat added. Radical reactins are much different frm reactins f nuclephiles and electrphiles (r acids and bases) because they invlve mvement f single, unpaired electrns rather than pairs f electrns tgether. The mechanisms f radical reactins are shwn using single-headed arrws t indicate that nly a single electrn is mving. Useful radical reactins are nt as cmmn as reactins f nuclephile and electrphiles in rganic chemistry we will study nly a few during the semester. They can als be ften unwanted reactins that create undesired prducts in ther reactins. IV. Reactive Intermediates In multistep reactins, there are ften high energy intermediates. These are mlecules which are nt stable, but are frmed, then quickly react. They are nly present fr a brief perid f time. They can strngly influence the utcme f the reactin because things which stabilize

5 r destabilize them have a big affect n the pathway the reactin will take. There are fur types f carbn intermediates that we will encunter: carbcatins, carbanins, carbn radicals, and carbenes. Carbcatins Carbcatins have three bnds and ne empty rbital. They have an sp 2 hybridizatin, and a psitive charge. Because f the lack f a cmplete ctet f electrns, carbcatins are very unstable and reactive. Carbcatins are frmed by dissciatin and by the reactin f alkenes with acid. Carbcatins are stabilized by resnance, and by the presence f ther carbns bnded t them because f the verlap f the C-H bnds with the empty p rbital (this is called hypercnjugatin). Carbcatins with three carbns attached are called tertiary r 3 carbns, and are the mst stable. Carbcatins with tw carbns attached are called secndary r 2 carbcatins, and are nt as stable, but can frm. Carbcatins with ne carbn attached are called primary r 1 carbcatins, and d nt usually frm. Carbcatins with n carbns attached are called methyl carbcatins, and d nt usually frm. Carbcatins react as electrphiles in assciatin reactins, r as acids by dnating a H frm a carbn next t the carbcatin. In either case, a new bnd is frmed which fills the ctet f electrns. Carbanins Carbanins have three bnds and a pair f electrns. They may have sp 3 r sp 2 hybridizatin, and are negatively charged. Because carbn is nt a very electrnegative atm, carbanins are very reactive. Carbanins are frmed when a hydrgen is remved frm a carbn atm. Since alkanes are the strngest bases, this cannt be accmplished unless the cnjugate base is stabilized in sme way. A carbanin may als be frmed by a dissciatin reactin if there is smething t stabilize it. Carbanins are mst cmmnly stabilized by resnance, but may als be stabilized by nearby electrnegative atms.

6 Carbanins react as nuclephiles r as bases, frming a new bnd with the electrn pair t restre the neutral charge. Carbn radicals Carbn radicals have three bnds and an unpaired electrn. This electrn ccupies a p rbital, s the radical has an sp 2 hybridizatin. They d nt have a charge. They are highly reactive and unstable because they d nt have an ctet f electrns. Carbn radicals are frmed by reactins with ther radicals, typically during prpagatin steps. Carbn radicals are stabilized by having ther carbns attached, and by resnance (the same as carbcatins). Unlike carbcatins, all carbn radicals f all substitutins can frm. Radical stability: 3 > 2 > 1 > methyl Carbn radicals react t frm ther radicals in prpagatin steps, r by cmbining with ther radicals in terminatin steps. Carbenes Carbenes have tw bnds, an empty rbital, and a lne pair f electrns. They have sp 2 hybridizatin, and they d nt have a charge. They are highly reactive because the carbn des nt have an ctet f electrns. Carbenes are frmed by remval f a H by a base, fllwed by dissciatin f a leaving grup. Only a few types f mlecules are capable f frming carbenes. Carbenes typically have n ther carbn grups attached, and are nt stabilized by substituents. Carbenes react in electrcyclic reactins in which they are bth a nuclephile and an electrphile, mst ften with carbn-carbn duble bnds. Rearrangements f Carbcatins Under certain cnditins, a carbcatin can rearrange t becme mre stable. Rearrangements will nt ccur if the new carbcatin is equally as stable r less stable than the riginal. They will nly ccur if the new carbcatin is mre stable than the riginal. Only 2 carbcatins cmmnly underg rearrangements. Primary carbcatins rarely underg rearrangements because they are t unstable t

7 frm in the first place, while tertiary carbcatins are already as stable as they can be, s they d nt usually underg rearrangements. Secndary carbcatins are stable enugh t frm, but if there is a way fr them t rearrange and becme tertiary, this can ccur. There are tw ways fr a 2 carbcatin t becme tertiary by underging a rearrangement. Bth f them invlve electrns n a neighbring bnd shifting t ccupy the empty p rbital, leaving anther, mre stable ne behind. They are called hydride shifts and alkyl shifts. In a hydride shift, a hydrgen mves with its pair f electrns frm a tertiary carbn t the carbcatin, leaving the psitive charge n the tertiary carbn. In an alkyl shift, a carbn grup mves with its pair f electrns frm a quaternary carbn t the carbcatin, leaving the psitive charge n the nw tertiary carbn. When tw pssible alkyl grups culd shift frm a neighbring carbn, the smallest ne is the mst likely. The mst cmmn alkyl shifts are methyl shifts. In bth cases, it is the electrns that mve, attracted by the psitive charge and the empty rbital, carrying the atms with them. Alkyl and hydride shifts can nly ccur frm a neighbring carbn atm. Electrns cannt leap ver intervening carbns t the carbcatin. V. Sterechemistry f Reactins The sterechemistry f cmpunds is imprtant when an existing sterecenter is invlved in a reactin, r when ne r tw new sterecenters are frmed by a reactin. When a C=C underges a reactin creating tw new asymmetric carbns, the carbns were actually already sterecenters, s we will use the term new asymmetric carbns fr these situatins. When an existing sterecenter is invlved in a reactin (it gains and/r lses bnds), there are three pssible utcmes: it may be cnserved, inverted, r racemized. Which ne will happen depends n the mechanism f the mechanism f the individual reactin. If the sterecenter is cnserved, this means that it has the same rientatin in the prduct as it did in the starting material. (This is nt very cmmn usually it happens when tw inversins result in a net cnservatin.) If the sterecenter is inverted, this means that it has the ppsite rientatin in the prduct as it did in the starting material. (This is quite cmmn, as displacements usually result in inversin.) If the sterecenter is racemized, this means that a racemic mixture has frmed; an equal

8 number f mlecules have the same rientatin and ppsite rientatin in the starting material as in the prduct. (This is als quite cmmn, and usually happens because f an achiral reactive intermediate such as a carbcatin. It results in the lss f ptical purity, which is generally bad.) If the starting material is racemic, it will be impssible t tell which f these has happened. The starting material must be enantimerically pure, r at least ptically active, fr the results t shw what happened. If the starting material is ptically active, but the sterecenter is nt invlved in the reactin, the sterechemistry is unchanged. When ne new asymmetric carbn is frmed, mst ften a racemic mixture will result. This is because the tw prducts are enantimers, and have the same free energy, s the reactins that frm them have the same activatin energy and the same change in enthalpy. Bth enantimers are frmed at the same rate. When ne new asymmetric carbn is frmed, but ne enantimer is favred ver anther, this is called a stereselective reactin. There are tw ways that this can be accmplished. A chiral, enantimerically pure reagent may be used. This will cause the activatin energy f ne reactin t be higher than the ther, s that ne enantimer will be frmed faster than the ther and there will be mre f it. This is a majr area f research in rganic chemistry. Yu can t usually tell by lking at the structures which enantimer will be favred; this is usually discvered by experiment. A chiral starting material may be used in which the sterecenter is clse t the new asymmetric carbn that will be frmed, and affects the reactin. Bth the activatin energy and change in enthalpy will be different, again resulting in ne f the prducts being frmed faster than the ther (in this case, they are diasteremers). Yu can ften tell by lking which diasterimer will be favred it will be the ne that is less sterically hindered. When tw new asymmetric carbns are frmed, each sterecenter may be R r S, creating fur pssible stereismers. Depending n the mechanism f the reactin, all fur may frm, r nly tw may frm: Syn additin the stereismers in which the new substituents are added t the same side are frmed, giving a maximum f 2 prducts. Anti additin the stereismers in which the new substituents are added t the ppsite side are frmed, giving a maximum f 2 prducts. Nnselective additin all pssible stereismers are frmed, giving a maximum f 4 prducts.