Electrophilic Additions to Alkenes

Bromine (Br2) is brown, and one of the classic tests for alkenes is that they turn a brown aqueous solution of bromine colourless. Alkenes decolourize bromine water alkenes react with bromine. The product of the reaction is a dibromoalkane, and the reaction on the right shows what happens with the simplest alkene, ethylene (ethene). 

In order to understand this reaction, and the other similar ones you will meet in this chapter, you need to think back to Chapter 5, where we started talking about reactivity in terms of nucleophiles and electrophiles. As soon as you see a new reaction, you should immediately think to yourself, Which reagent is the nucleophile which reagent is the electrophile Evidently, neither the alkene nor bromine is charged, but Brj has a low-energy empty orbital (the Br-Br a ), and is therefore an electrophile. The Br-Br bond is exceptionally weak, and bromine reacts with many nucleophiles like this. 

In the reaction with ethylene, the alkene must be the nucleophile, and its HOMO is the C=C n bond. Other simple alkenes are similarly electron-rich and they typically act as nucleophiles and attack electrophiles. 

When it reacts with Bij, the alkene s filled jc orbital (the HOMO) will interact with the bromine s empty a orbital to give a product. But what will that product be Look at the orbitals involved. 

Very hindered alkenes form bromonium ions that are resistant to nucleophilic attack. In this very hindered case, the bromonium ion is sufficiently stable to be characterized by X-ray crystallography. 

How does H initially add to the alkene Does it behave as an electrophile A nucleophile A neutral atom (radical) Step through the sequence of frames depicting addition of the H end of HCl to 2-methylpropene (CIH+ 2-methylpropene). Plot both the charge on H and on Cl (vertical axis) vs. frame number (horizontal axis). Do the charges change significantly during addition or remain constant Why  

At which point in the initial addition is there the greatest separation of charge Draw Lewis structures for C4H9 and Cl that show all nonbonding electrons and formal charges. 

Considering the overall sequence, H addition followed by Cl addition, where is there the greatest separation of charge Would you expect this addition to occur more rapidly in a nonpolar medium like hexane, or a polar medium like water Explain. 

The animations in this problem have been artificially constrained to reveal electronic changes. They do not show realistic geometries for these reaction pathways, and should not be used to estimate reaction barriers. 

The electrons in the pi bond are spread farther from the caibon nuclei and are more loosely held than the sigma electrons. 

In principle, many different reagents could add to a double bond to form more stable products that is, the reactions are energetically favorable. Not all of these reactions have convenient rates, however. For example, the reaction of ethylene with hydrogen (to give ethane) is strongly exothermic, but the rate is very slow. A mixture of ethylene and hydrogen can remain for years without appreciable reaction. Adding a catalyst such as platinum, palladium, or nickel allows the reaction to take place at a rapid rate. 

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When formulating a mechanism for the reaction of alkynes with hydrogen halides we could propose a process analogous to that of electrophilic addition to alkenes m which the first step is formation of a carbocation and is rate determining The second step according to such a mechanism would be nucleophilic capture of the carbocation by a halide ion... 

Recall from Chapter 6 the general mechanism for electrophilic addition to alkenes... 

Aikene chemistry is dominated by electrophilic addition reactions. When HX reacts with an unsymmetrically substituted aikene, Markovnikov s rule predicts that the H will add to the carbon having fewer alky) substituents and the X group will add to the carbon having more alkyl substituents. Electrophilic additions to alkenes take place through carbocation intermediates formed by reaction of the nucleophilic aikene tt bond with electrophilic H+. Carbocation stability follows the order... 

Problem 13,23 How could you use lH NMR to determine the regiochemistry of electrophilic addition to alkenes For example, does addition of HC) to 1-methylcvclohexene yield 1-chloro-l-methylcyclohexane or l-chloro-2-methylcyclohexane ... 

For a summary and detailed discussion of the stereochemistry of electrophilic additions to alkenes and alkynes see R. C. Fahey, in Topics in Stereochemistry, Vol. 3,... 

Although the reaction of ketones and other carbonyl compounds with electrophiles such as bromine leads to substitution rather than addition, the mechanism of the reaction is closely related to electrophilic additions to alkenes. An enol, enolate, or enolate equivalent derived from the carbonyl compound is the nucleophile, and the electrophilic attack by the halogen is analogous to that on alkenes. The reaction is completed by restoration of the carbonyl bond, rather than by addition of a nucleophile. The acid- and base-catalyzed halogenation of ketones, which is discussed briefly in Section 6.4 of Part A, provide the most-studied examples of the reaction from a mechanistic perspective. 

Once the tertiary cations have been formed, they can undergo electrophilic addition to alkene molecules (Reaction (4)). The addition is exothermic and contributes most of all the reaction steps to the overall heat of reaction. It has been proposed (24) that instead of the alkenes, the corresponding esters are added to the carbenium ions, restoring the acid in this way (Reaction (5)). The products of both potential steps are the same. 

The electrophilic addition to alkenes is one of the occasions when the direction of the curl matters and can convey formation of different products. Although the product shown is correct, the curly arrow is wrong. 

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