Ch08 - Reactions of Alkenes Since bonds are stronger than...

Info iconThis preview shows pages 1–5. Sign up to view the full content.

View Full Document Right Arrow Icon
Reactions of Alkenes Since σ bonds are stronger than π bonds, double bonds tend to react to convert the double bond into σ bonds + X-Y Y X This is an addition reaction. (Other types of reaction have been substitution and elimination). Addition reactions are typically exothermic. Electrophilic Addition The π bond is localized above and below the C-C σ bond. The π electrons are relatively far away from the nuclei and are therefore loosely bound. An electrophile will attract those electrons, and can pull them away to form a new bond. This leaves one carbon with only 3 bonds and a +ve charge (carbocation). The double bond acts as a nucleophile (attacks the electrophile). In most cases, the cation produced will react with another nucleophile to produce the final overall electrophilic addition product. Ch08 Reacns of Alkenes.doc Page1
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
+ E + E + Nuc: - Nuc E Electrophilic addition is probably the most common reaction of alkenes. Consider the electrophilic addition of H-Br to 2-butene: H 3 CC H 3 HH H 3 C CH 3 H + H 3 C H Br H H CH 3 HB r Br: - The alkene abstracts a proton from the HBr, and a carbocation and bromide ion are generated. The bromide ion quickly attacks the cationic center and yields the final product. In the final product, H-Br has been added across the double bond. Ch08 Reacns of Alkenes.doc Page2
Background image of page 2
Orientation of Addition Consider the addition of H-Br to 2-methyl-2-butene: H 3 CC H 3 H 3 CH H 3 C CH 3 H CH 3 H + H 3 C CH 3 Br H H CH 3 HB r Br: - CH 3 H CH 3 H 3 C H + CH 3 H Br CH 3 H H 3 C Br: - Observed Product Not Observed There are two possible products arising from the two different ways of adding H-Br across the double bond. But only one is observed. The observed product is the one resulting from the more stable carbocation intermediate. H 3 C CH 3 H CH 3 + CH 3 H CH 3 H 3 C + > H H Tertiary carbocations are more stable than secondary. The protonation occurs at the least highly substituted end so that the cation produced is at the most highly substituted end (and therefore more stable). Markovnikov’s Rule : The addition of a proton acid to the double bond of an alkene results in a product with the acid hydrogen bound to the carbon atom that already has the greater number of hydrogens attached. Ch08 Reacns of Alkenes.doc Page3
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
(More generally: In an electrophilic addition to an alkene, the electrophile adds in such a way that it generates the most stable intermediate). Reactions that give such products are said to have Markovnikov orientation, and are Markovnikov products. The electrophilic addition of HBr is said to be regiospecific , since it only gives one orientation of addition. HCl and HI undergo analogous electrophilic addition reactions. Free Radical addition to Alkenes It is possible to obtain anti-Markovnikov products when HBr is added to alkenes in the presence of free radical initiators.
Background image of page 4
Image of page 5
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 11/17/2011 for the course CHEM 355 taught by Professor Staff during the Fall '11 term at Rutgers.

Page1 / 22

Ch08 - Reactions of Alkenes Since bonds are stronger than...

This preview shows document pages 1 - 5. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online