General reaction: For example, with ethene and hydrogen chloride, you get chloroethane: Figure 7.7.1 Electrophilic addition of HCl to ethene. The world of organic chemistry, or the branch of chemistry that studies carbon-containing compounds, can look pretty scary. The most characteristic type of alkene reaction is addition at the carbon–carbon double bond. Or, looking from the perspective of the X group, we can say that the X atom (or group) adds to the carbon that already has the greater number of carbon atoms: This is the regiochemistry of the reaction as it shows why one regioisomer (constitutional isomer) is formed preferentially over the other … For example, with ethene and hydrogen chloride, you get chloroethane: Weak Brønsted acids such as water (pKa = 15.7) and acetic acid (pKa = 4.75) do not normally add to alkenes. For instance, hydrogen halides can be added to an alkyne by way of a mechanism similar to that of alkenes. Addition reactions to alkynes are similar to additions to alkenes. Let's start by looking at alkenes, or organic compounds that have at least one double bond between carbon atoms. However, the addition of a strong acid serves to catalyze the addition of water, and in this way alcohols may be prepared from alkenes. Don't worry though, this lesson will make it all seem doable! Double bonds can have sigma and pi bonds. Sigma bonds are pretty strong, whereas pi bonds are a little weake…
Alkenes react with halogen acid to form corresponding alkyl halide. The order of reactivity is HI > HBr > HCl. A hydrogen atom joins to one of the carbon atoms originally in the double bond, and a halogen atom to the other. HCl reacts as per Markownikoff’s rule only. In the previous post, we talked about the Markovnikov’s rule and learned that in the addition reaction of HX to an unsymmetrical alkene, the H adds to the carbon that already has the greater number of hydrogen atoms. Addition of Hydrogen Halides to Alkenes All alkenes undergo addition reactions with the hydrogen halides. The double bonds in alkenes are covalentbonds, meaning that electrons are shared between the atoms. A hydrogen atom joins to one of the carbon atoms originally in the double bond, and a halogen atom to the other. Consider the addition of HX (a generic hydrogen halide) to 1-propyne; according to Markovnikov's rule, the halogen adds to the carbon with the fewest hydrogen atoms. Action of Haloacids on Symmetric Alkenes: The addition of halogen acids like HCl, HBr or HI to a compound containing multiple bond is known as hydrohalogenation. For example, if sulfuric acid is dissolved in water it is completely ionized to the hydronium ion, H3O(+), and this strongly acidic (pKa = -1.74) species effects hydration of ethene and other alkenes.
The addition reaction can be represented generally as follows: In an addition reaction, the carbon–carbon p bond of the alkene and the XLY bond of the reagent are broken, and new CLX and CLY bonds are formed. All alkenes undergo addition reactions with the hydrogen halides.
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