附件2 粒大浮 教 案 2003~~2004学年第Ⅰ学期 院(系、所、部)化学与环境学院有机化学研究所 教研室有机化学 课程名称有机化学(双语教学 授课对象化学教育 授课教师杨定乔 职称职务教授 教材名称 Organic Chemistry 2003年09月01日
附件 2 教 案 2003~~ 2004 学年 第 I 学期 院（系、所、部）化学与环境学院有机化学研究所 教 研 室 有机化学 课 程 名 称 有机化学(双语教学) 授 课 对 象 化学教育 授 课 教 师 杨定乔 职 称 职 务 教授 教 材 名 称 Organic Chemistry 2003 年 09 月 01 日
有机化学(双语教学)课程教案 授课题目(教学章节或主题):第二章.单烯烃授课类型|理论课 ( Alkenes 授课时间|第3周第6-12节 教学目标或要求∶了解单烯烃的结构理论以及烯烃的化学性质。掌握马氏规则和亲电 加成理论。 Alkene nomenclature Alkenes represent one of the most common functional groups in organic chemistry. An alkene contains only carbon and hydrogen (a hydrocarbon) and contains at least double bond (termed an unsaturated hydrocarbon. Alkenes have the general formula C,H, thus, an alkene with 10 carbons (n=10) will have 2(10)=20 hydrogens, or the molecular formula C,Ha each double bond therefore contributes one The root, or parent name for an unbranched alkene is taken directly from the number of carbons in the chain according to a scheme of nomenclature established by the International Union of Pure and Applied Chemistry(IUPAc), as described previously for alkanes To name alkenes 1. Find the longest chain containing the alkene The IUPAC name for an alkene is constructed of two parts: 1)a prefix(meth. prop., etc.) which indicates the number of carb parent, chain of the molecule, and 2)the suffix.. ene to indicate that the molecule is an alkane For branched-chain alkanes, the name of the parent hydrocarbon is taken from the longest continuous chain of carbon atoms containing the double bond
有机化学(双语教学) 课程教案 授课题目（教学章节或主题）：第二章．单烯烃 （Alkenes） 授课类型 理论课 授课时间 第 3 周第 6-12 节 教学目标或要求：了解单烯烃的结构理论以及烯烃的化学性质。掌握马氏规则和亲电 加成理论。 Alkene Nomenclature Alkenes represent one of the most common functional groups in organic chemistry. An alkene contains only carbon and hydrogen (a hydrocarbon) and contains at least double bond (termed an unsaturated hydrocarbon. Alkenes have the general formula CnH2 n, thus, an alkene with 10 carbons (n = 10) will have 2(10) = 20 hydrogens, or the molecular formula C1 0H2 0; each double bond therefore contributes one degree of unsaturation. The root, or parent name for an unbranched alkene is taken directly from the number of carbons in the chain according to a scheme of nomenclature established by the International Union of Pure and Applied Chemistry (IUPAC), as described previously for alkanes. To name alkenes: 1. Find the longest chain containing the alkene The IUPAC name for an alkene is constructed of two parts: 1) a prefix (meth... eth... prop..., etc.) which indicates the number of carbons in the main, or parent, chain of the molecule, and 2) the suffix ...ene to indicate that the molecule is an alkane. For branched-chain alkanes, the name of the parent hydrocarbon is taken from the longest continuous chain of carbon atoms containing the double bond
2-ethyl-l-pentene 2. Number the chain, giving the double bond the lowest possible number. Numbering of the carbons in the parent chain is always done in the direction that gives the lowest number to the double bond, or, the lowest number at the first point of difference. If there are different substituents at equivalent positions on the chain, the substituent of lower alphabetical order is given the lowest number If the same substi tuent occurs more than once in a molecule, the number of each carbon of the parent chain where the substituent occurs is given and a multiplier is used to indicate the total number of identical substi tuents: i. e dimethyl.. trimethyl.. tetraethy l. the substituents are arranged in alphabetical order, without regard for multipliers 6, 6-dimethyl-3-heptene 3. For cycloalkenes, begin numbering at the double bo nd and proceed through the double bond in the direction to generate the lowest number at the first point of difference. One of the most common mistakes in naming cycloalkenes is to generate the lowest number sequence around the ring, disregarding this rule. Once again, the numbering must begin at the double bond and proceed through the bond in the direction to generate the lowest number sequence. 1 4-cyclohexadiene 3,3, 6-trimethylcyclohexene 4, 6, 6-trimethylcyclooctene 4. Assign stereochemistry using the E-z designation Historically, alkenes have been named using cis- and trans- to represent stereochemistry around the double bond; cis- for compounds where the main substituents are on the same side of the double bond, and trans- when they are on opposite sides. This system clearly breaks down, however, in more complex molecules where decisions concerning the main substituents" are not easily made, and the e-z system provides a set of rules to aid in these decisions
2. Number the chain, giving the double bond the lowest possible number. Numbering of the carbons in the parent chain is always done in the direction that gives the lowest number to the double bond, or, the lowest number at the first point of difference. If there are different substituents at equivalent positions on the chain, the substituent of lower alphabetical order is given the lowest number. If the same substituent occurs more than once in a molecule, the number of each carbon of the parent chain where the substituent occurs is given and a multiplier is used to indicate the total number of identical substituents; i.e., dimethyl... trimethyl... tetraethyl..., etc. In constructing the name, substituents are arranged in alphabetical order, without regard for multipliers. 3. For cycloalkenes, begin numbering at the double bond and proceed through the double bond in the direction to generate the lowest number at the first point of difference. One of the most common mistakes in naming cycloalkenes is to generate the lowest number sequence around the ring, disregarding this rule. Once again, the numbering must begin at the double bond and proceed through the bond in the direction to generate the lowest number sequence. 4. Assign stereochemistry using the E-Z designation Historically, alkenes have been named using cis- and trans- to represent stereochemistry around the double bond; cis- for compounds where the "main substituents" are on the same side of the double bond, and trans- when they are on opposite sides. This system clearly breaks down, however, in more complex molecules where decisions concerning the "main substituents" are not easily made, and the E-Z system provides a set of rules to aid in these decisions
You will see later that these same rules are used in assigning priorities for determining absolute configuration in chiral molecules H "A-2-butene The rules for assigning e-z designations are as follows: 1. rank atoms directly attached to the double bond according to their atomic 2. if there is a tie at any substituent, look at the second, third, etc until a difference is found 3. multiple bonds count as multiples of that same atom 4. if the highest priority groups are on the same side of the double bond, the molecule is Z; if the highest priority groups are on opposite sides the molecule is E (atomic numbers shown in red) H Z-2-b1000-2-butene Z-3-bromomethyl-4-methyl-2-pentene E- 3-methyl-1, 3-pentadiene H H2BrC, cH 6C 6 CHa CCM HHH HHE ,,,M, (these indicate the atoms attached to each of the atoms directly attached to the alkene) Alkenes: Addition oxidation reactions HBr
You will see later that these same rules are used in assigning priorities for determining absolute configuration in chiral molecules. The rules for assigning E-Z designations are as follows: 1. rank atoms directly attached to the double bond according to their atomic number 2. if there is a "tie" at any substituent, look at the second, third, etc., until a difference is found 3. multiple bonds count as multiples of that same atom 4. if the highest priority groups are on the same side of the double bond, the molecule is Z; if the highest priority groups are on opposite sides, the molecule is E Alkenes: Addition & Oxidation Reactions
H"H。O AC)2,H NaBH4 1. BH. THF 2, H2O2, Ho 1.0804 2. NaHSO3, H2o KMno4, HO /H2O CHCL KOH CH2I2, Zn(Cu), ether H H
2. Zn/H3o KMnOAh,o =0O=c General Mechanisms of Alkene Addition Reactions: The majority of the reactions of alkenes which will be described in this section fall into three basic categories ionic additions, which are init iated by an electrophilic agent interacting with the alkene Tt-cloud, activating the alkene carbons to nucleophilic additions, syn addition reactions occurring on one side of the alkene Tt-cloud, by e ither radical or concerted mechanisms and oxidative cleava ge reactions in wh ich the carbon-carbon double bond is cleaved to form di-carbonyl derivatives In the first of these, the alkene T-cloud, functioning as a Lewis base(an electron donor), donates electron density to a Lewis acid(in these examples, a proton, a halogen cation(halonium ion), or mercuric ion The complex, bearing a positive charge, is now highly reactive towards nucleophiles in the system(anions or water) and undergoes attack anti to the activating Lewis acid, to form the final addition product When the nucleophile is of low reactivity, the initial complex may rearrange to form a sigma bond to the Lewis acid, leaving a full carbocation on the adjacent carbon As with all carbocations, this can react with nucleophiles from either face(being planar) and can undergo rearrangement reactions. Reactions involving these carbocation species are most common in the acid-catalyzed addition of water to alkenes and with halogen acids(HCI and HBr) Syn additions to the alkene T-cloud which are covered in this section include hydrogenation, gem-diol formation from MnO,, organoborane formation from BH, and carbene-dependent cyclopropanation reactions. In each of these, an electrophilic agent reacts with the -system to(more-or-less)simultaneously form bonds to both carbons. In hydrogenation, the hydrogen gas is rendered electrophilic by adsorption to a metal surface (i. e, Pt). The metal surface also binds the alkene, activating the addition, and a variety of carbon-metal intermediate species are probably involved. Trivalent boron is a powerful Lewis acid and reaction with bh, is probably initiated by the formation of a T-complex, as described above this complex, however, seems to decompose in a concerted fashion to form the syn borane Oxidative cleavage of alkene generally involves the intermediate formation of a bridged oxygen species of some sort, followed by spontaneous or reductive cleavage. The examples covered in this section include ozonolysis, where the intermediate ozonide is decomposed by a dissolving-metal reduction, and
General Mechanisms of Alkene Addition Reactions: The majority of the reactions of alkenes which will be described in this section fall into three basic categories: • ionic additions, which are initiated by an electrophilic agent interacting with the alkene -cloud, activating the alkene carbons to nucleophilic additions, • syn addition reactions occurring on one side of the alkene -cloud, by either radical or concerted mechanisms, and, • oxidative cleavage reactions in which the carbon-carbon double bond is c leaved to form di-carbonyl derivatives. In the first of these, the alkene -cloud, functioning as a Lewis base (an electron donor), donates electron density to a Lewis acid (in these examples, a proton, a halogen cation (halonium ion), or mercuric ion). The complex, bearing a positive charge, is now highly reactive towards nucleophiles in the system (anions or water) and undergoes attack anti to the activating Lewis acid, to form the final addition product. When the nucleophile is of low reactivity, the initial complex may rearrange to form a sigma bond to the Lewis acid, leaving a full carbocation on the adjacent carbon. As with all carbocations, this can react with nucleophiles from either face (being planar) and can undergo rearrangement reactions. Reactions involving these carbocation species are most common in the acid-catalyzed addition of water to alkenes, and with halogen acids (HCl and HBr). Syn additions to the alkene -cloud which are covered in this section include hydrogenation, gem-diol formation from MnO4 -, organoborane formation from BH3 and carbene-dependent cyclopropanation reactions. In each of these, an electrophilic agent reacts with the -system to (more-or-less) simultaneously form bonds to both carbons. In hydrogenation, the hydrogen gas is rendered electrophilic by adsorption to a metal surface (i.e., Pt). The metal surface also binds the alkene, activating the addition, and a variety of carbon-metal intermediate species are probably involved. Trivalent boron is a powerful Lewis acid and reaction with BH3 is probably initiated by the formation of a -complex, as described above; this complex, however, seems to decompose in a concerted fashion to form the syn borane. Oxidative cleavage of alkene generally involves the intermediate formation of a bridged oxygen species of some sort, followed by spontaneous or reductive cleavage. The examples covered in this section include ozonolysis, where the intermediate ozonide is decomposed by a dissolving-metal reduction, and
acid-catalyzed cleavage of the intermediate metal di-ester formed during mno oxidation As you work through each of these sets of reactions, you should pay particular attention to the common features of the mechanisms in each group An understanding of a few basic chemical generalities will help you view reactions as broad classes, and help you avoid overt memorization The addition of halogen acids to alkenes is a stepwise process which generally involves a solvent-equilibrated carbocation intermediate. The formation of this intermediate is initiated through a simple acid-base equilibrium in which the halogen acid donates a proton to the alkene T-system, which is functioning as a Lewis base. The protonated T-system has a short lifetime and can rapidly revert to starting materials, or can rearrange from a(cationic) protonated T-bond, to an sp sigma bond adjacent to an sp carbocation center. If the alkene is unsymmetrical, the protonated -cloud intermediate can brea down by two pathways, as shown below, to potentially form carbocations having differing ground-state energies. The reaction pathways leading from this intermediate to the two carbocations will differ in energy, and, in general, the pathway leading to the more stable intermediate will be of lower energy, and will be the preferred pathway. Transition State for Formation of Primary Carbocation Transition State for Formation of More Stable Secondary Carbocation Ptotonated T-cloud The resulting carbocation is formed on the carbon of the alkene which is best able to stabilize the cationic center In simple unstrained non-conjugated systems, without adjacent heteroatoms, the order of stability of carbocations will be tertiary secondary primary. Since tertiary centers have no attached hydrogens, secondary centers have one and primary centers have two, there is an apparent inverse relationship between the number of attached hydrogens and the likelihood that the carbocation will form at that center This is the
acid-catalyzed cleavage of the intermediate metal di-ester formed during MnO4 - oxidation. As you work through each of these sets of reactions, you should pay particular attention to the common features of the mechanisms in each group. An understanding of a few basic chemical generalities will help you view reactions as broad classes, and help you avoid overt memorization. The addition of halogen acids to alkenes is a stepwise process which generally involves a solvent-equilibrated carbocation intermediate. The formation of this intermediate is initiated through a simple acid-base equilibrium in which the halogen acid donates a proton to the alkene -system, which is functioning as a Lewis base. The protonated -system has a short lifetime and can rapidly revert to starting materials, or can rearrange from a (cationic) protonated -bond, to an sp3 sigma bond adjacent to an sp2 carbocation center. If the alkene is unsymmetrical, the protonated -cloud intermediate can break down by two pathways, as shown below, to potentially form carbocations having differing ground-state energies. The reaction pathways leading from this intermediate to the two carbocations will differ in energy, and, in general, the pathway leading to the more stable intermediate will be of lower energy, and will be the preferred pathway. The resulting carbocation is formed on the carbon of the alkene which is best able to stabilize the cationic center. In simple unstrained non-conjugated systems, without adjacent heteroatoms, the order of stability of carbocations will be tertiary > secondary > primary. Since tertiary centers have no attached hydrogens, secondary centers have one and primary centers have two, there is an apparent inverse relationship between the "number of attached hydrogens" and the likelihood that the carbocation will form at that center. This is the
origin of Markovnikov's Rule, which states that in the addition ofHX to an alkene, the proton will attach to the center having the greatest number of often restated as them that has, gets. While the rule is a useful guide, you should remember that the selectivity is actually to place the carbocation on the carbon which can best stabilize the charge. Once the carbocation is formed. the most favorable reaction will involve the addition of a nucleophile to form an sp center. In the reaction with halogen acid(HCl and HBr), the most nucleophilic molecules in the system will be the chloride and bromide anions. Attack of these on the planar (sp )carbocation can occur from either above, or below the plane defined by the spa center, and the net addition of HX can therefore occur either syn or (cis, on the same side) or anti (trans; on the opposite side), relative to the hydrogen atom. H H3C + HBr The addition of HBr to an alkene in the presence of peroxides is a stepwise process involving radical intermediates. The radical chain reaction is initiated by the peroxide, and the most energetically favorable initiation reaction is abstraction of a hydrogen atom from HBr by the peroxy radical, to generate the omine ra HBr Roo RoOH+Br· 〔 A secondary radical) Br + HBr The bromine radical is electrophilic and attacks the alkene T-system, which donates an electron, forming a o>-bond to the bromine and leaving an unpaired electron (a radical)on one of the carbons of the alk If the alke unsymmetrical, bond formation can occur by two pathways, as described above for halogen acid addition. Once again, the reaction pathways leading to the two radicals will differ in energy, and, in general, the pathway leading to the more stable intermediate will be of lower energy, and will be the preferred pathway
origin of Markovnikov's Rule, which states that... ...in the addition of HX to an alkene, the proton will attach to the center having the greatest number of hydrogens... often restated as "them that has, gets". While the rule is a useful guide, you should remember that the selectivity is actually to place the carbocation on the carbon which can best stabilize the charge. Once the carbocation is formed, the most favorable reaction will involve the addition of a nucleophile to form an sp3 center. In the reaction with halogen acid (HCl and HBr), the most nucleophilic molecules in the system will be the chloride and bromide anions. Attack of these on the planar (sp2) carbocation can occur from either above, or below the plane defined by the sp2 center, and the net addition of HX can therefore occur either syn or (cis; on the same side) or anti (trans; on the opposite side), relative to the hydrogen atom. The addition of HBr to an alkene in the presence of peroxides is a stepwise process involving radical intermediates. The radical chain reaction is initiated by the peroxide, and the most energetically favorable initiation reaction is abstraction of a hydrogen atom from HBr by the peroxy radical, to generate the bromine radical. The bromine radical is electrophilic and attacks the alkene -system, which donates an electron, forming a >-bond to the bromine and leaving an unpaired electron (a radical) on one of the carbons of the alkene. If the alkene is unsymmetrical, bond formation can occur by two pathways, as described above for halogen acid addition. Once again, the reaction pathways leading to the two radicals will differ in energy, and, in general, the pathway leading to the more stable intermediate will be of lower energy, and will be the preferred pathway
The resulting radical is formed on the carbon of the alkene which is best abl to stabilize the electrophilic site(the unpaired electron). In simple unstrained non-conjugated systems, without adjacent heteroatoms, the order of stability of carbon radicals parallels that of carbocations, with tertiary secondary primary. Since tertiary centers have no attached hydrogens, secondary centers have one and primary centers have two, there is an apparent inverse relationship between the number of attached hydrogens and the likelihood that the radical will form at that center The carbon radical which is formed abstracts a hydrogen atom(most likely from HBr), propagating the chain and giving one mole of product. In this product, the hydrogen attached to the center which formed the radical, that is, the center with the fewest number of hydrogens"(a secondary or tertiary center) and the bromine is attached to the carbon which is adjacent to the most stable radical ("the center with the most hydrogens"). This is opposite to Markovni kov's Rule, as described in the previous example, and the orientation in this reaction is often termed anti-Markovnikov". While the rule is a useful guide, you should remember that the selectivity is actually to place the radical character on the carbon which can best stabilize the unpaired electron (the electrophilic center) As with carbocation intermediates, carbon radicals are planar (sp ),and hydrogen abstraction from the second molecule of HBr can occur from either above, or below the plane defined by the sp center. The net addition of HBr can therefore occur either syn (cis, on the same side) or anti(trans, on the opposite side), as described in the previous example The addition of halogen to alkenes is a stepwise process involving a"halonium"ion intermediate. The ormation of this intermediate is initiated through attack of halogen on the alkene T-system, to form the cyclic halonium ion (i.e, bromonium or chloronium ion) and expel the halogen anion (ie, bromide or chloride). This intermediate is highly electrophilic and reacts rapidly with the best nucleophile in the system;that is, the halide anion expelled in the previous step. Since the halonium ion effectively blocks attack by halide on the same side, attack must be come from the backside(relative to the large halogen atom)to form the trans-1, 2-dihalide. This is demonstrated below for the addition of bromine to 1-propene The large bromine on the intermediate bromonium ion(shown as a space-filling overlay) effectively blocks attack from the top, forcing the addition to be anti(trans: from the opposite side). The attack of Br on the bromonium ion is an example of an SN2 reaction in which a nucleophile attacks at the carbon and displaces the leaving group is a single, smooth, concerted process
The resulting radical is formed on the carbon of the alkene which is best able to stabilize the electrophilic site (the unpaired electron). In simple unstrained non-conjugated systems, without adjacent heteroatoms, the order of stability of carbon radicals parallels that of carbocations, with tertiary > secondary > primary. Since tertiary centers have no attached hydrogens, secondary centers have one and primary centers have two, there is an apparent inverse relationship between the "number of attached hydrogens" and the likelihood that the radical will form at that center. The carbon radical which is formed abstracts a hydrogen atom (most likely from HBr), propagating the chain and giving one mole of product. In this product, the "hydrogen" attached to the center which formed the radical, that is, the center with "the fewest number of hydrogens" (a secondary or tertiary center) and the bromine is attached to the carbon which is adjacent to the most stable radical ("the center with the most hydrogens"). This is opposite to Markovnikov's Rule, as described in the previous example, and the orientation in this reaction is often termed "anti-Markovnikov". While the rule is a useful guide, you should remember that the selectivity is actually to place the radical character on the carbon which can best stabilize the unpaired electron (the electrophilic center). As with carbocation intermediates, carbon radicals are planar (sp2), and hydrogen abstraction from the second molecule of HBr can occur from either above, or below the plane defined by the sp2 center. The net addition of HBr can therefore occur either syn (cis; on the same side) or anti(trans; on the opposite side), as described in the previous example. The addition of halogen to alkenes is a stepwise process involving a "halonium" ion intermediate. The formation of this intermediate is initiated through attack of halogen on the alkene -system, to form the cyclic halonium ion (i.e., bromonium or chloronium ion) and expel the halogen anion (i.e., bromide or chloride). This intermediate is highly electrophilic and reacts rapidly with the best nucleophile in the system; that is, the halide anion expelled in the previous step. Since the halonium ion effectively blocks attack by halide on the same side, attack must be come from the backside (relative to the large halogen atom) to form the trans-1,2-dihalide. This is demonstrated below for the addition of bromine to 1-propene. The large bromine on the intermediate bromonium ion (shown as a space-filling overlay) effectively blocks attack from the top, forcing the addition to be anti (trans; from the opposite side). The attack of Br+ on the bromonium ion is an example of an SN2 reaction in which a nucleophile attacks at the carbon and displaces the leaving group is a single, smooth, concerted process
HOBr H As with the addition of halogen, the addition of hypohalous acid(HOX) to alkenes is a stepwise process which also involves the"halonium"ion intermediate. As described previously for the addition of halogen (X2), the halonium ion intermediate is highly electrophilic and reacts rapidly with the best nucleophile in the system. In the case of HOX addition, the most powerful nucleophile in the system is hydroxide anion Again, because of the steric bulk of the halonium ion, attack by hydroxide must come from the backside (relative to the large halogen atom)to form the trans product. If the alkene is unsymmetrical, hydroxide can potentially attack at either the carbon. The distribution of positive charge, however, will not necessarily be the same on both carbons, and attack will typically occur on the carbon which is best able to stabilize the partial positive charge. In general, this will also be the carbon which would form the most stable carbocation, and, as a general rule, hydroxide anion will attach to the carbon of the alkene which would form the most stable carbocation. another way to view the selectivi Rule, the carbon which would get the halogen in the addition of Hx, will get the hydroxyl group in the ffectively block attack from the top forcing the addition of halogen and hydroxide to be anti (trans: on the opposite side), relative to each other HIH20 heat H The acid-catalyzed addition of water to alkenes is another example of a stepwise process which generally involves a solvent-equilibrated carbocation intermediate. The formation of this intermediate is initiated through a simple acid-base equilibrium in which the acid donates a proton to the alkene T-system, which is functioning as a Lewis base. The protonated T-system to form an sp sigma bond adjacent to an spa carbocation center. If the alkene is unsymmetrical, two carbocations are possible and the addition will proceed to form the most stable carbocation. As before, tertiary centers will be favored over secondary
As with the addition of halogen, the addition of hypohalous acid (HOX) to alkenes is a stepwise process which also involves the "halonium" ion intermediate. As described previously for the addition of halogen (X2), the halonium ion intermediate is highly electrophilic and reacts rapidly with the best nucleophile in the system. In the case of HOX addition, the most powerful nucleophile in the system is hydroxide anion. Again, because of the steric bulk of the halonium ion, attack by hydroxide must come from the backside (relative to the large halogen atom) to form the trans product. If the alkene is unsymmetrical, hydroxide can potentially attack at either the carbon. The distribution of positive charge, however, will not necessarily be the same on both carbons, and attack will typically occur on the carbon which is best able to stabilize the partial positive charge. In general, this will also be the carbon which would form the most stable carbocation, and, as a general rule, hydroxide anion will attach to the carbon of the alkene which would form the most stable carbocation. Another way to view the selectivity is using Markovnikov's Rule; the carbon which would get the halogen in the addition of HX, will get the hydroxyl group in the addition of HOX. As before, the large halogen on the intermediate halonium ion effectively blocks attack from the top, forcing the addition of halogen and hydroxide to be anti (trans; on the opposite side), relative to each other. Return to Top The acid-catalyzed addition of water to alkenes is another example of a stepwise process which generally involves a solvent-equilibrated carbocation intermediate. The formation of this intermediate is initiated through a simple acid-base equilibrium in which the acid donates a proton to the alkene -system, which is functioning as a Lewis base. The protonated -system to form an sp3 sigma bond adjacent to an sp2 carbocation center. If the alkene is unsymmetrical, two carbocations are possible and the addition will proceed to form the most stable carbocation. As before, tertiary centers will be favored over secondary