Double bond additions carbon atom-alkene reactivity

The more reactive alkenes are characterized by addition reactions to the double bond, many of which occur easily at room temperature. The carbon-carbon double bond is a reaction site and is classified as a functional group. The n portion of the double bond can be utilized to accommodate two incoming atoms, converting the double bond into one single o bond between the carbon atoms and the n portion into two single a bonds between each carbon and one of the two incoming atoms. 

The delocalization of the Jt-electrons is energetically favorable, and this affects the reactivity of aromatic compounds There is a tendency towards restoring aromaticity. This is why aromatic compounds, in contrast to regular alkenes (linear chains of carbon atoms containing at least one double bond), do not easily undergo addition reactions, whereby a double bond is replaced by two single bonds. Aromatic compounds show a preference for substitution reactions, which means that atoms are replaced. 

Kinetics is used to investigate mechanisms of radical additions to alkenes. Outside the solvent cage, the initiator-derived radicals may undergo the desired bimolecular reaction with the substrate, or side reactions. When the substrate is an alkene, the exothermic intermolecular addition of the reactive radical (R ) to the double bond results in the formation of two new single carbon-carbon bonds in place of the double bond. This reaction represents conversion of an initiator into a propagating radical in radical polymerisations, and is becoming increasingly important in a number of synthetically useful intermolecular small molecule reactions. The addition of R to monosubstituted and 1,1-disubstituted alkenes is nearly always at the unsubstituted carbon atom (tail addition), and thus is normally not affected by the individual steric demand of the alkene substituents. Equation 10.4 is the expression for the rate of addition (R ) of R to an alkene where [M] is the monomeric alkene concentration ... 

The alkenes are more reactive than the alkanes. For example, alkene molecules undergo addition reactions in which the atoms from halogen molecules or from hydrogen halides bond to the carbon atoms involved in the double bond ... 

Addition is the characteristic reaction of alkenes and alkynes. Since the carbons of a double or triple bond do not have the maximum number of attached atoms, they can add additional groups or atoms. Double bonds undergo addition once and triple bonds can undergo addition twice. The reactivity of alkenes and alkynes is due to the presence of pi-bonds. Unlike sigma bonds, pi-bonds are directed away from the carbons the electrons are loosely held, very accessible, and quite attractive electron-deficient species (electrophiles) seeking an electron source. 

Halogenated boranes react with the usual regioselectivity, with addition of the boron atom to the less-substituted end of the carbon-carbon double bond, although they do not always show the same reactivity as other borane derivatives. For example, the far higher reactivity of dibromoborane towards more-substituted alkenes, in contrast to disiamyborane, makes possible the selective hydroboration of the double bonds in 2-methyl-1,5-hexadiene (5.11). 

Although the alkenes will readily undergo combustion, their primary reaction is addition reactions, A double bond is very reactive. One of the bonds can easily be broken, and the two carbons can then form new single bonds to other atoms. One of the most economically important addition reactions is... 

Synthetic polymers can be classified into two main types, depending on how they are made. Chain-growth polymers (also called addition polymers) are made by the addition of one monomer unit to another in a repetitive manner. Alkenes serve as monomers for the preparation of many important chain-growth polymers a catalyst is required to initiate their polymerization. The catalyst adds to a carbon-carbon double bond to form a reactive intermediate and this intermediate then adds to the double bond of a second monomer unit to yield a new intermediate. The process continues until the polymer chain is built. Eventually the process is terminated in some way. Chain-growth polymers retain all the atoms of the monomer units in the polymer. A good example of a chain-growth polymer is polyethylene (Sec. 3.16). 

The presence of an electron withdrawing group increases the SH bond dissociation energy (BDE) relative to that of a simple alkanethiol. For this reason, MTG is a good candidate to serve as a polarity reversal catalyst (PRC) that promotes the overall hydrogen atom transfer from a substrate R-H to a carbon centered radical. This reactivity has been applied to the addition of aldehydes to alkenes (eq A)] to the alkylation of electron-rich alkenes in the presence of silane (eq 5), to the preparation of j8-lactams via aminoacyl radical generation (eq 6), and to hydroamination of double bonds (eq 7). ... 

The carbon-carbon double bond is the functional group of the alkenes and is the site of chemical reactivity in the structure. The pi (jt) bond is weaker than the sigma (a) bond and is relatively easily broken without the molecule falling apart. Thus addition reactions can take place in which various molecules add to the carbon atoms originally participating in the double bond. The products of such reactions are all saturated molecules. 

Ordinary alkenes are more reactive toward electrophilic reagents than alkynes But the aUcenes obtained from the addition of an electrophilic reagent to an alkyne have at least one electronegative atom (Cl, Br, etc.) attached to a carbon atom of the double bond. 

Alkenes and alkynes are used by chemists to make other compounds. The reactive part of these compounds are the tt bonds and the characteristic reaction is an addition reaction, in which atoms or grouping of atoms add to the carbon atoms on either side of a double or triple bond. We have already seen one example of an addition reaction, reaction (26.1), in which hydrogen atoms add across a carbon-carbon bond of an alkene to give an alkane. Although we will examine reactions of alkenes in more detail in Chapter 27, it is worth mentioning now that certain addition reactions form the basis of simple qualitative tests that can be used to determine whether a compound is an alkene or an alkyne. For example, H2C=CH2 will absorb H2 in the presence of a metal catalyst, reaction (26.1), and it will decolorize a solution of bromine water, Br2(aq), because of the following reaction ... 

The presence of carbon-carbon double or triple bonds in hydrocarbons markedly increases their chemical reactivity. The most characteristic reactions of alkenes and alkynes are addition reactions, in which a reactant is added to the two atoms that form the multiple bond. A simple example is the addition of a halogen to ethylene ... 

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