I. Intrductin t Alkenes Classifying Alkenes Nuggets f Knwledge fr Chapter 10 Alkenes (I) Chem 2310 There are several categries that can be used t describe cmpunds cntaining carbn-carbn duble bnds. alkenes hydrcarbns cntaining a C=C (nt in a benzene ring) armatic cmpunds cntaining a benzene ring unsaturated cmpunds cntaining a C=C (nt in a benzene ring); may have atms besides carbn and hydrgen Alkenes and ther unsaturated cmpunds underg certain reactins because f the C=C, which we will study in this chapter and the next. We will fcus n alkenes t avid cmplicatins with ther functinal grups. Armatic rings underg a different set f reactins (we will study these in chapter 16). When a cmpund cntains mre than ne carbn-carbn duble bnd, it is classified by the relatinship between the tw duble bnds. islated C=C s are separated by at least ne sp 3 carbn the pi bnds cannt verlap each ther, s they behave just like a single C=C cnjugated C=C s are separated nly by a single bnd the pi bnds verlap with each ther, creating a single pi system they are mre stable than regular C=C s they can absrb UV light if there are at least 3, with a sufficient number they absrb visible light and cause the cmpund t be clred their reactins will be cvered in chapter 15 cumulated C=C s share a carbn atm in rder fr the middle carbn t have tw pi bnds, it is sp hybridized, and the pi bnds are at right angles t each ther (similar t a triple bnd) they are less stable than regular C=C s they ften ismerize t frm triple bnds their reactins are beynd the scpe f this curse, but we will discuss their sterechemistry briefly in chapter 15
Bnd structure A duble bnd cnsists f ne sigma bnd and ne pi bnd. The pi bnd is frmed by the verlap f tw p rbitals. The carbn atms must therefre have sp 2 hybridizatin in rder t have an unhybridized p rbital with which t make the pi bnd. The sigma bnd is frmed by the verlap f tw sp 2 rbitals. The gemetry f the carbns in the duble bnd is trignal planar. Because f this, they cannt have atms cming frward r backward frm them. When frming C=C frm tetrahedral atms (such as in an E2 r E1 reactin), it is imprtant t rearrange the atms cming ut f the C=C s that they are trignal planar. Reactivity f Alkenes The pi bnd is mre reactive than the sigma bnd because it is farther frm the carbn nuclei and is therefre higher in energy. The pi bnd can act as a weak nuclephile r base, r it can be attacked by a radical. If the pi bnd acts as a base, it will attack a strng acid, leaving a carbcatin that will cntinue t react. If the pi bnd acts as a nuclephile, it will attack a strng electrphile, als leaving a carbcatin that will cntinue t react. If the pi bnd is attacked by a radical, the bnd will break, and ne electrn will jin with the radical t frm a new bnd, leaving a radical n the ther carbn, which will cntinue t react. Physical Prperties The physical prperties f alkenes are very similar t thse f alkanes. Alkenes are nnplar, having nly van der Waals frces. Alkenes are insluble in water, since they can t break int the hydrgen bnding netwrk f the water mlecules. Alkenes are highly flammable. They react with xygen t frm carbn dixide and water (r carbn mnxide and st if there isn t enugh xygen r heat).
Alkenes are less dense than water, s they flat n tp f the water as a separate layer when placed in cntact with water. Alkene biling pints are similar t alkanes. They increase with mlecular weight, and decrease slightly with branching because f the reduced surface area. Occurrence and uses f alkenes Alkenes themselves are nt very cmmn in nature, thugh unsaturated cmpunds are mre cmmn. Tw exceptins are alpha-pinene, a cmpund fund in the ils f cniferus trees such as pine trees, and muscalure, the sex attractant f the cmmn husefly. Alkenes are used in industry as starting materials and intermediates. The mst cmmnly used is ethylene 49 billin punds are prduced each year, which is mre than any ther rganic cmpund. Ethylene is used t make a variety f ther cmpunds. Examples include: plyethylene, a cmmn plastic in things like plastic bags and milk jugs ethanl, used industrially as a slvent and as a gasline additive acetic acid, used as an acid catalyst and as a slvent ethylene glycl, used as antifreeze Spectrscpy IR spectra f alkenes can be recgnized by their characteristic bands. Please learn these frequencies. C-H n C=C is at 3100-3000 cm -1 C=C is at 1680-1620 cm -1 NMR spectra f alkenes can be recgnized by the chemical shift f the vinyl hydrgen atms, and the cmplex splitting that ccurs. Hydrgen atms directly attached t the C=C appear at 4.5-6.5. Please learn this range. Peaks caused by hydrgen atms attached t the C=C ften shw cmplex splitting. This ccurs because the cupling cnstants between different hydrgens are nt the same, making it s that a hydrgen split by tw neighbrs is nt a triplet, but instead a dublet f dublets, and s n. Stability f alkenes The stability f alkenes affects which nes are favred in reactins that frm them, such as E2 and E1.
Alkenes are stabilized by verlap f the pi rbital with nearby C-H bnds (similar t carbcatins and radicals). Because f this, the mst substituted C=C are the mst stable. With C=C that have the same number f carbn substituents, steric hindrance will affect their stability. Trans C=C s are mre stable than cis C=C s, and geminal C=C s are in between. (Remember that this is a smaller affect than substitutin.) Cyclic alkenes can als be less stable because f strained angles. Cyclbutene and cyclprpene are unusually reactive because f their highly strained C=C s. Trans C=C s can exist in rings with 8 carbns r mre. Smaller rings cannt have trans C=C s because there aren t enugh carbns t make the chain. When the ring is large enugh, the ring is mre stable in the trans gemetry. II. Unsaturatin Number The frmula f a cmpund can be used t determine the number f duble bnds r rings that the cmpund must cntain. This is called the unsaturatin number. The presence f either a duble bnd r a ring will cause tw less H s t be needed. We can use the frmula (2C + 2) H / 2 t determine the unsaturatin number fr hydrcarbns. The presence f ther atms can als affect the number f H s, and this must be taken int accunt in rder t determine the unsaturatin number. Oxygen atms d nt change the number f H s needed. Halgen atms take the place f a H, and the number f H s needed is ne less fr every halgen atm present. Nitrgen atms require ne mre H, and the number f H s needed is ne mre fr every nitrgen atm present. The frmula that can be used fr all cmpunds t determine the unsaturatin number is: (2C + 2 X + N) H / 2 III. Nmenclature f Alkenes Naming alkenes is very similar t naming alkanes and alkyl halides. The difference is that the C=C takes pririty in bth the chsing f the principle chain r ring, and the numbering f the chain. Pririties in chsing the principle chain r ring:
It must cntain the C=C. If there is mre than ne C=C, it must cntain all f them, if pssible. If there is mre than ne chain r that cntains the C=C, chse the ne that is the lngest. If there is mre than ne chain r ring that cntains the C=C and are the same length, chse the ne that has the mst substituents. Pririties fr numbering the principle chain r ring: Give the C=C the lwest number. The numbers must g acrss the C=C, with the lwer number being used t label it s lcatin. If there are tw ways t number that bth give the lwest number t the C=C, then give the lwest number t the first substituent, then the secnd, etc. If there are tw ways t number that give the same numbers, give the lwest number t the substituent that cmes first in the alphabet. A ring with nly ne C=C and n substituents des nt need t be numbered. If there is ne C=C in the chain r ring, the ending f the name is changed frm ane t ene. Fr example, a chain f 5 carbns with ne C=C is a pentene. The number giving the lcatin f the C=C is added at the beginning f the name fr example, 2-pentene. If there is mre than ne C=C, a di r tri is inserted befre the ene, and an a is added t make the name easier t say. Fr example, a chain f 5 carbns with tw C=C s is a pentadiene. The numbers giving the lcatins f the C=C s are added befre the name fr example, 1,3-pentadiene. The tw smallest substituents cntaining a C=C have cmmn names. vinyl a C=C attached directly t the chain r ring allyl a C-C=C attached s that the C=C is ne carbn away frm the chain r ring All ther substituents cntaining C=C s must be named by adding en just befre the yl n the end f the alkyl name fr example, butyl becmes butenyl.
The substituent must be numbered frm the pint f attachment, and the number added t the beginning f the substituent name fr example, 3-butenyl. Sterechemistry must be added t the beginning f the name if there are tw pssible ismers fr the cmpund. This will ccur when the substituents n bth sides f the C=C are different. See chapter 6 fr a review f determining sterechemistry f C=C s. trans and cis may be used when there are nly tw substituents n each side f the C=C, and when there is nly ne C=C in the mlecule E and Z may always be used. They must be used when there are three r mre substituents n the C=C, r when there is mre than ne C=C in the mlecule. A number must be added befre E r Z the when there is mre than ne C=C with sterechemistry in the mlecule. Rings with 8 r mre carbns can have tw stereismers, and the sterechemistry must be labeled. Rings with 7 r fewer carbns shuld nt be labeled, as they can nly be in the cis cnfiguratin. IV. Intrductin t Reactins f Alkenes There are three basic types f reactins: substitutin, eliminatin, and additin. In a substitutin reactin, a nuclephile replaces a leaving grup. In an eliminatin reactin, a hydrgen and a leaving grup are remved, leaving a C=C. In an additin reactin, smething is added t bth sides f a duble bnd, giving a saturated cmpund with tw new substituents. Eliminatin and additin reactins are ppsites f each ther. Nearly all reactins f alkenes are additin reactins. We will see different mechanisms and different atms added t the duble bnd, but all f them give the same type f result. The C=C in benzene rings d NOT underg additin reactins. Benzene rings are nt affected by the reagents that cause additin t C=C t ccur.
V. Additin f HX t Alkenes Carbcatin Mechanism When alkenes react with hydrgen halides such as HCl, HBr, r HI, an alkyl halide is frmed. The mechanism invlves tw steps: prtnatin, in which the alkene attacks the hydrgen f the hydrgen halide, adding a hydrgen t the alkene and frming a carbcatin and a halide in attack, in which the halide attacks the carbcatin, frming the alkyl halide This reactin is regiselective. The halgen ends up nly n the mre substituted side f the C=C, because this is where the mre stable carbcatin will frm and then be attacked by the halide. If neither side is mre substituted, tw carbcatins will frm, and tw prducts will be made. Since a carbcatin is frmed, rearrangements may ccur. Prducts will be frmed frm bth the riginal carbcatin as well as the rearranged carbcatin. This reactin is useful fr cnverting alkenes t alkyl halides. When using this reactin fr synthesis, make sure t cnsider the fllwing: Lk at all f the alkenes that culd have been used t make the alkyl halide. They must have the same carbn skeletn as the prduct, and the C=C must cntain the carbn that will have the halide n it. Next, make sure that the halgen will g t the side that will give the alkyl halide yu are trying t make. If the halgen is n the less substituted side, the wrng prduct will be frmed. If the right prduct will be frmed, next lk at whether any ther prducts wuld als be frmed. A gd synthesis gives nly the desired prduct. Things t watch ut fr include: If the C=C is equally substituted, tw carbcatins will frm leading t tw prducts. If the initially frmed carbcatin can rearrange, anther carbcatin will frm leading t additinal prducts. This reactin is mre effective at synthesizing alkyl halides than halgenatin f alkanes because it des nt frm multiple prducts as ften.
Radical mechanism The presence f rganic perxides such as CH 3 OOCH 3 can cause the reactin f alkenes and HBr t fllw a radical mechanism instead f ne invlving a carbcatin. The xygen-xygen bnd in the perxide is very weak because bth electrnegative xygen atms are pulling n the electrns. Expsure t heat r light can cause this bnd t break hmlytically, creating tw xygen radicals. The xygen radical reacts with an HBr mlecule, frming an alchl and a brmine radical. The brmine radical attacks the alkene, frming a carbn radical n the mre substituted side, with the brmine n the less substituted side. The carbn radical then reacts with an HBr mlecule, taking a hydrgen and frming an alkyl halide and a brmine radical. The creatin f a new brmine radical during the reactin causes the perxides t be catalytic. Only a small amunt f perxide is necessary t initiate the reactin; nce it has started it can cntinue withut any mre perxide breaking. The regiselectivity f the reactin is reversed when a radical reactin ccurs. In bth cases, the reactive intermediate, a carbcatin r carbn radical, is stabilized by hypercnjugatin, and frms n the mre substituted side f the C=C. In the carbcatin mechanism, the hydrgen is added first as the alkene attacks the acid, sending the hydrgen t the less substituted side and leaving the mre substituted side fr the carbcatin, which is then attacked by the halide. In the radical mechanism, the brmine is added first as the alkene is attacked by the brmine radical, sending it t the less substituted side and leaving the mre substituted side fr the carbn radical, which then attacks a hydrgen. The radical mechanism is nly effective with HBr, nt with HCl r HI. Even in the presence f perxide, alkenes react with HCl and HI by a carbcatin mechanism, giving alkyl halides with the chlride r idide n the mre substituted carbn. N rearrangements ccur with the radical mechanism. Radicals cannt rearrange because the rbital is nt empty, s a pair f electrns cannt slide int it. This reactin can be used t make alkenes int alkyl brmides. It is a useful cmplement t the regular additin f HX t alkyl halides because it makes it pssible t make alkyl halides in which the halide is attached t a carbn less
substituted than thse arund it. Als, it desn t underg rearrangements. Hwever, it nly wrks with brmine. VI. Acid-catalyzed Hydratin f Alkenes When sulfuric acid is used instead f a hydrgen halide, and the reactin takes place in the presence f water, an alchl is frmed instead f an alkyl halide. The mechanism f the reactin has three steps: prtnatin the pi bnd f the alkene attacks the acid, adding a hydrgen and frming a carbcatin and the cnjugate base f sulfuric acid attack a water mlecule attacks the carbcatin, frming a prtnated alchl deprtnate anther water mlecule takes the hydrgen atm frm the prtnated alchl, frming an alchl and a hydrnium in (H 3 O + ) The difference between the reactin f alkenes with HX and H 2 SO 4 is caused by the cnjugate bases. Bth reactins begin with prtnatin f the alkene t frm a carbcatin, but the halide can act as a nuclephile, while the HSO 4 - can t. The halide reacts as a nuclephile, giving an alkyl halide prduct; if water is present, it can act as the nuclephile, giving an alchl prduct. Phsphric acid als has a nn-nuclephilic cnjugate base, and can be used in this reactin in place f sulfuric acid. This reactin is said t be acid-catalyzed because the hydrnium in frmed in the last step can serve as the acid in the first step. Only a small amunt f acid is necessary fr all f the alkene t be cnverted t alchl. Hwever, a full equivalent f water is needed. Since sulfuric acid reacts rapidly with water, the acid in the first step is actually hydrnium in (althugh it is frequently written as sulfuric acid itself). The sulfuric acid frms hydrnium in and then its cnjugate base remains a spectatr in fr the rest f the reactin. This reactin is called a hydratin because the elements f water are added t the duble bnd a H n ne side and an OH n the ther. This reactin is regiselective. The OH always ges t the mre substituted side, because that is where the mre stable carbcatin will be frmed, then attacked by a water mlecule. If bth sides are equally substituted, tw carbcatins will be frmed, giving tw prducts.
Since a carbcatin is frmed, rearrangements can ccur. Prducts will be frmed frm bth the riginal carbcatin and the rearranged ne. This reactin can be used t make alkenes int alchls. When using this reactin fr synthesis, make sure t cnsider the fllwing: Lk at the alkenes that culd be used t make the alchl. They must have the same carbn skeletn, and the C=C must include the carbn that will have the alchl n it. Check t see if the alkene will give the crrect prduct the OH will g t the mre substituted side. Check t see if any ther additinal prducts will frm because f equally substituted C=C s, r carbcatins that can rearrange. We will see tw mre reactins that frm alchls frm alkenes in the next chapter. We will als discuss several mre additin reactins.