Polar reaction electrophiles

The reaction is an example of a polar reaction type known as an electrophilic addition reaction and can be understood using the general ideas discussed in the previous section. Let s begin by looking at the two reactants. 

What about the second reactant, HBr As a strong acid, HBr is a powerful proton (H+) donor and electrophile. Thus, the reaction between HBr and ethylene is a typical electrophile-nucleophile combination, characteristic of all polar reactions. 

The electrophilic addition of HBr to ethylene is only one example of a polar process there are many others that vve ll study in detail in later chapters. But regardless of the details of individual reactions, all polar reactions take place between an electron-poor site and an electron-rich site and involve the donation of an electron pair from a nucleophile to an electrophile. 

A full description of how a reaction occurs is called its mechanism. There are two general kinds of mechanisms by which reactions take place radical mechanisms and polar mechanisms. Polar reactions, the more common type, occur because of an attractive interaction between a nucleophilic (electron-rich) site in one molecule and an electrophilic (electron-poor) site in another molecule. A bond is formed in a polar reaction when the nucleophile donates an electron pair to the electrophile. This movement of electrons is indicated by a curved arrow showing the direction of electron travel from the nucleophile to... 

Before beginning a detailed discussion of alkene reactions, let s review briefly some conclusions from the previous chapter. We said in Section 5.5 that alkenes behave as nucleophiles (Lewis bases) in polar reactions. The carbon-carbon double bond is electron-rich and can donate a pair of electrons to an electrophile (Lewis acid), for example, reaction of 2-methylpropene with HBr yields 2-bromo-2-methylpropane. A careful study of this and similar reactions by Christopher Ingold and others in the 1930s led to the generally accepted mechanism shown in Figure 6.7 for electrophilic addition reactions. 

Historically, ethylene potymerization was carried out at high pressure (1000-3000 atm) and high temperature (100-250 °C) in the presence of a catalyst such as benzoyl peroxide, although other catalysts and reaction conditions are now more often used. The key step is the addition of a radical to the ethylene double bond, a reaction similar in many respects to what takes place in the addition of an electrophile. In writing the mechanism, recall that a curved half-arrow, or "fishhook" A, is used to show the movement of a single electron, as opposed to the full curved arrow used to show the movement of an electron pair in a polar reaction. 

We saw in the preceding chapter that the carbon-ha]ogen bond in an alkyl halide is polar and that the carbon atom is electron-poor. Thus, alkyl halides are electrophiles, and much of their chemistry involves polar reactions with nucleophiles and bases. Alkyl halides do one of two things when they react with a nucleophile/base, such as hydroxide ion either they undergo substitution of the X group by the nucleophile, or they undergo elimination of HX to yield an alkene. 

The chemistry of amines ts dominated by the lone pair of electrons on nitrogen, which makes amines both basic and nucleophilic. They react with acids to form acid-base salts, and they react with electrophiles in many of the polar reactions seen in past chapters. Note in the following electrostatic potential map of trimethylamine how the negative (red) region corresponds to the lone-pair of electrons on nitrogen. 

Polar reaction (Section 5.2) A reaction in which bonds are made when a nucleophile donates two electrons to an electrophile and in which bonds are broken when one fragment leaves with both electrons from the bond. 

The Lead-Off Reaction Addition of HBr to Alkenes Students usually attach great-importance to a text s lead-off reaction because it is the first reaction they see and is discussed in such detail. 1 use the addition of HBr to an alkene as the lead-off to illustrate general principles of organic chemistry for several reasons the reaction is relatively straightforward it involves a common but important functional group no prior knowledge of stereochemistry or kinetics in needed to understand it and, most important, it is a polar reaction. As such, 1 believe that electrophilic addition reactions represent a much more useful and realistic introduction to functional-group chemistry than a lead-off such as radical alkane chlorination. 

Bowman has surveyed the reactions of cx-substituted aliphatic nitro compounds with nucleophiles, which undergo either S l substitution or polar reaction (Scheme 5.16).118 The reactions between a wide variety of nucleophiles and BrCH2N02 are shown in Scheme 5.17.119a b All the thiolates, PhS02 and I attack Br to liberate the anion of nitromethane. The hard nucleophiles, MeO , OH, and BH4 attack the hard H+ electrophilic center. Phosphorous nucleophiles attackthe oxygen electrophilic center, and only Me2S attacks the carbon electrophilic center. 

Molecules in heterolytic (polar) reactions form and break bonds by "coordination" and molecules in homolytic (nonpolar or free radical) reactions form and break bonds by "colligation."75 (Two more new terms ) Heterolytic reactions occur mostly in solutions, usually involving ion formation and electrophilic or nucleophilic reactions homolytic reactions occur mostly in gases and do not involve ions because less energy is required to distance the atoms into neutral radicals.76... 

A. Polarization and Electrophilic Svhstitidion From the various accounts that have been given of the role of 77-complexes and <7-complexes as possible intermediates in reaction mechanisms, that described by Olah e al.(1961)is selected for special attention, since it... 

These simple charge or orbital interactions may be enough to explain simple inorganic reactions but we shall also be concerned with nucleophiles that supply electrons out of bonds and electrophiles that accept electrons into antibonding orbitals. For the moment accept that polar reactions usually involve electrons flowing from a nucleophile and towards an electrophile. 

In Chapter 30, you came across the idea of umpolung, the inversion of the usual reactivity pattern of a molecule. You may have already noticed that radicals often have an umpolung reactivity pattern. Alkyl halides are electrophiles in polar reactions yet they generate nucleophilic radicals that react with electrophilic alkenes. 

Alkyl halides behave as electrophiles in polar reactions. 

Polar reactions take piace between electron rich reagents (mideophtles Lewis bases) snd electron pctor reagents felectropliilesdjewis acids). These reactions are betcrolytic processes and involve species with an even num> her of electrons. Bonds are made when a nucleojAile donates an elecuon pair to an electrophile bonds are broken when one product leaves with an elecmm patr. 

Both carbocations and carbanions are unstable reactive intermediates. A carbocation contains a carbon atom surrounded by only six electrons, and a carbanion has a negative charge on carbon, which is not a very electronegative atom. Carbocations (electrophiles) and carbanions (nucleophiles) can be intermediates in polar reactions— reactions in which a nucleophile reacts with an electrophile. 

Fullerenes such as Ceo are easy to reduce [1], but difficult to oxidize electrochemi-cally [2], and thus are generally regarded as electrophiles, or electron acceptors, rather than nucleophiles or electron donors. Tbe derivatization of fullerenes has therefore been achieved by polar reactions with a variety of nucleophiles [99 104], e.g. electron-rich olefins [105-114], carbenes [115-122], carbanions [123-129], alk-oxides [130, 131], and organometallic reagents [132 138]. When, on the other hand, electrons are chemically or electrochemically added to Ceo, the resulting anions are expected to behave as strong nucleophiles or electron donors. 

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