Electrophilic Addition Reactions of alkenes and alkynes

Transcription

1 Chemistry 2600 Lecture Notes Chapter 9 Electrophilic Addition Page 1 of 16 Electrophilic Addition Reactions of alkenes and alkynes In Chapter 8 we learned about elimination reactions, which produce unsaturated compounds from suitably-substituted saturated compounds. There were two mechanisms of elimination reaction, E 1 and E 2. What was the difference? E 1 reactions are known to be equilibria processes. ow do we know this? Now consider what happens if we reverse the E 1 mechanistic steps:

2 Chemistry 2600 Lecture Notes Chapter 9 Electrophilic Addition Page 2 of 16 The reverse process (usually written left-to-right, of course) is called electrophilic addition. Why? Step 1 of such reactions involves the interaction of π-electron density of alkene or alkyne (i.e. acting as nucleophile) with an electron-deficient component of the addend molecule (i.e. the electrophile) Note this is a confusing terminology, because the addition of, for example, 2 across a double bond involves the addition of both + (electrophile) and (nucleophile) to the alkene. The name comes only from the first step of the mechanism. Cyclohexene is used to illustrate 5 key types of electrophilic addition reactions

3 Chemistry 2600 Lecture Notes Chapter 9 Electrophilic Addition Page 3 of 16 #1 ydration of alkenes is a useful preparation of alcohols Exercise 9.4 Propose a reasonable mechanism for the following transformation: 3 C C 3 3 C C P 4 3 C C C 3 3 C C 3 Write one transformation per step and include all lone pairs and charges Watch the charge flow carefully (use the arrow formalism) 2 is a better nucleophile than the dihydrogenphosphate ion because of the diffuse charge and large size of the latter

4 Chemistry 2600 Lecture Notes Chapter 9 Electrophilic Addition Page 4 of 16 #2 Addition of X to alkenes Addition of X to alkenes is highly regioselective, and therefore useful. 1 δ + δ - X + + X X - X owever, this reaction is extremely slow for terminal and di-substituted alkenes. (The rate is reasonable for tri- and tetrasubstituted alkenes, which are quite rare). The reaction must be done in a non-nucleophilic solvent such as C 2 Cl 2 or pure (glacial) acetic acid, else the water will act as the nucleophile. But, on a silica gel suspension, the reaction works well. Effectively the silica (or alumina) surface acts as a catalyst to speed up this reaction. Typical solvents for such reactions are C 2 Cl 2. X-derivatized silica or alumina surfaces can be prepared by reactions such as: SCl + Si ///{ } Si ///{ Cl} + S C + Al ///{ } Al ///{ } + C PI + Si ///{ } Si ///{ I} + P ( )

5 Chemistry 2600 Lecture Notes Chapter 9 Electrophilic Addition Page 5 of 16 #3 Addition of Cl 2 or 2 are reactions where + is not the electrophile Cannot operate through a conventional carbocation intermediate: The correct stereochemistry is exclusively trans addition: The mechanism for this reaction recognizes that elemental halogens are very polarizable (no less than 6 lone pairs for two atoms. Nucleophiles can therefore induce the presence of a positive, electrophilic, end to the molecule. It is the addition of the unusual electrophiles + or Cl + that is unique to this reaction, and that is used in a mechanism that explains the observed stereochemistry of addition. Long controversial, this mechanism has been substantiated by the crystallization of the following bromonium ion and its structure determination by X-ray crystallography (own, UofA).

6 Chemistry 2600 Lecture Notes Chapter 9 Electrophilic Addition Page 6 of 16 #4 alohydrin addition occurs when 2 and Cl 2 react in aqueous solution In aqueous solution, the halogens are in equilibrium with X and X. This is known as a disproportionation reaction, leading to compounds of halogen in the +1 formal oxidation state. 2 2 pcontrol X + X + + X + This equilibrium is strongly p dependent, with basic conditions favouring products. Why? In neutral solution, these redox equilibria result in the following: Cl 2 2 Total solubility M 0.21 M Concentration of X M ~0.21 M Concentration of X M M The most electrophilic species present is therefore the halogen in X which are in a formal positive oxidation state. The result is stereospecific trans addition of X across the double bond. For Cl 2 : A mechanism for halohydrin addition is the following. It is not important whether the reagent is written as X 2 / 2 or as X. Why is the that is formed in the halogen/water equilibrium not important in this reaction?

7 Chemistry 2600 Lecture Notes Chapter 9 Electrophilic Addition Page 7 of 16 Iodine can react under X conditions It is most commonly used with internal nucleophiles. A classic example is the iodolactonization reaction: I I I I + I #5 xymercuration has a metal ion as the primary electrophile The primary reaction is usually followed by a reduction using sodium borohydride, which we learn about in subsequent chapters. In most cases the organomercurial is undesired as a final product because of the toxicity of mercury. The net reaction is that of the addition of water or alcohol across the double bond. Why would we bother to do this reaction when #1, the acid catalyzed addition of water, will do the same thing more cheaply and with far less waste of reagents (mercury is expensive to dispose of)?

8 Chemistry 2600 Lecture Notes Chapter 9 Electrophilic Addition Page 8 of 16 Addition to acyclic alkenes occurs just as for cyclohexene The products depend on the stereochemistry of the alkene that is reacted: Why are two stereoisomers formed in this reaction? The opposite geometric isomer leads to a meso form. When would this not occur? Because there is no ring for reference, we cannot speaks directly of cis or trans addition, but the stereochemistry of the products is consistent only with trans addition. The stereochemistry of the products is described as either threo or erythro Definition of threo and erythro (based on two four-carbon sugars) in Fischer diagram form: C C C 2 C 2 erythro has all three groups of equal type mirrored D-Erythrose D-Threose threo has only one equivalent group mirrored Like groups here are e.g. C of a given oxidation level any heteroatom For example: C 3 and C 2 4 C and C()C 3 and and

9 Chemistry 2600 Lecture Notes Chapter 9 Electrophilic Addition Page 9 of 16 Addition of omine to disubstituted alkenes (both symmetric and asymmetric cases) cis alkene C 3 C 2 C 3 2 C 3 C 2 + C 3 Step 1: bromonium ion C 3 C 2 + C 3 C 3 C 2 S S Step 2: equal chance of adding to the left-hand carbon... C 3 C 3 C 2 + C 3 C 3 C 2 R R C 3 or to the right-hand carbon, resulting in the creation of two enantiomers trans alkene C 3 C 3 C C 3 C 3 C 2 Step 1: bromonium ion + C 3 C 3 C 2 C 3 C 2 C S R 3 Step 2: equal chance of adding to the left-hand carbon... + C 3 C 3 C 2 C 3 C 2 R S C 3 or to the right-hand carbon, resulting in the creation of two enantiomers Stereochemical consequences The products of addition to cis and trans alkenes are diastereomers of one another (any combination of the red-boxed and blue-boxed isomers, look back at Figure 5.5, p.183 of Sorrell). For the trans alkene, if the alkene were symmetric (i.e. two C 3 groups rather than one C 3 and one C 2 5 ) then the two "enantiomers" would be identical, i.e. we would form the meso form. The two enantiomers of addition to the cis isomer are said to be the threo isomers, while... The two enantiomers (or else single meso form) of addition to the trans form are said to be the erythro isomer(s).

10 Chemistry 2600 Lecture Notes Chapter 9 Electrophilic Addition Page 10 of 16 Addition to unsymmetrical alkenes: regiochemistry Regiochemistry refers to the direction of addition. Regioisomers are constitutional isomers. Consider addition of X to illustrate the regiochemical selection:

11 Chemistry 2600 Lecture Notes Chapter 9 Electrophilic Addition Page 11 of 16 An energy level diagram of the two competing pathways for the addition of to methylcyclohexane The regiochemistry of addition to a double bond is given by Markovnikov s Rule The rule states that whenever a polar reagent Y adds to a double bond, the proton + becomes attached to the alkene carbon atom that bore more hydrogen atoms before addition. The rule is better understood via the mechanistic picture developed above. The electrophilic part of a polar reagent adds to a π-bond in such a way that the more stable carbocation intermediate is generated. This last formulation also applies in a slightly modified form when cyclic π- bond/electrophile intermediates form (#3, #4 and #5 type reactions).

12 Chemistry 2600 Lecture Notes Chapter 9 Electrophilic Addition Page 12 of 16 Electrophilic addition to alkynes: #1 alogen alides Electrophilic addition to alkynes: #2 Acid-catalyzed addition of water Keto-enol tautomerism occurs for vinyl alcohols (adjacent double bond and group) 2 C 3 C 3 C 3 Why does this occur? The answer can be traced to the bond energies of the initial and the final product. The gas-phase BDE of a C=C is +602 kj/mol, while that of a C= is +732 kj/mol.

13 Chemistry 2600 Lecture Notes Chapter 9 Electrophilic Addition Page 13 of 16 Electrophilic addition to alkynes: #3 Addition of halogens Addition to conjugated dienes: 1,2 and 1,4 addition under kinetic vs. thermodynamic control A conjugated diene is any hydrocarbon in which two double bonds are separated by one single bond. The simplest example, which we use here for illustration, is 1,3-butadiene in the addition of. Two different regioisomers are formed, in quantities that differ with reaction temperature

14 Chemistry 2600 Lecture Notes Chapter 9 Electrophilic Addition Page 14 of 16 The difference between thermodynamic and kinetic control is illustrated using a reaction profile diagram: Another example of kinetic vs. thermodynamic control occurs with halogen addition to this diene C 2 Cl 2 1,2-addition (kinetic) 1,4-addition (thermodynamic) Note that the cyclic bromonium ion collapses because the resonance-stabilized allylic carbocation is formed instead Note also that once this occurs, free rotation about the C C single bonds leads to loss of stereochemistry. Neither product is stereospecific.

15 Chemistry 2600 Lecture Notes Chapter 9 Electrophilic Addition Page 15 of 16 Addition Reactions of Carbocation Electrophiles with Alkenes Dimerization of isobutylene (2-methylpropene) Polymerization of isobutylene

16 Chemistry 2600 Lecture Notes Chapter 9 Electrophilic Addition Page 16 of 16 Intramolecular addition of a carbocation to a π-bond produces a ring two examples: