All About Alkenes

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Reactions Of Alkenes

Addition Reactions Of Alkenes

The most common chemical transformation of a carbon-carbon double bond is the addition reaction. A large number of reagents, both inorganic and organic, have been found to add to this functional group, and in this section we shall review many of these reactions. A majority of these reactions are exothermic, due to the fact that the C-C pi-bond is relatively weak (ca. 63 kcal/mole) relative to the sigma-bonds formed to the atoms or groups of the reagent. Remember, the bond energies of a molecule are the energies required to break (homolytically) all the covalent bonds in the molecule. Consequently, if the bond energies of the product molecules are greater than the bond energies of the reactants, the reaction will be exothermic. The following calculations for the addition of H-Br are typical. Note that by convention exothermic reactions have a negative heat of reaction.

Regioselectivity and the Markovnikov Rule

Only one product is possible from the addition of these strong acids to symmetrical alkenes such as ethene and cyclohexene. However, if the double bond carbon atoms are not structurally equivalent, as in molecules of 1-butene, 2-methyl-2-butene and 1-methylcyclohexene, the reagent conceivably may add in two different ways. This is shown for 2-methyl-2-butene in the following equation.

(CH3)2C=CHCH3 +   H-Cl (CH3)2CH–CHClCH3 or (CH3)2CCl–CHHCH3
2-methyl-2-butene 2-chloro-3-methylbutane 2-chloro-2-methylbutane

When addition reactions to such unsymmetrical alkenes are carried out, we find that one of the two possible constitutionally isomeric products is formed preferentially. Selectivity of this sort is termed regioselectivity. In the above example, 2-chloro-2-methylbutane is nearly the exclusive product. Similarly, 1-butene forms 2-bromobutane as the predominant product on treatment with HBr.

After studying many addition reactions of this kind, the Russian chemist Vladimir Markovnikov noticed a trend in the structure of the favored addition product. He formulated this trend as an empirical rule we now call The Markovnikov Rule: When a Brønsted acid, HX, adds to an unsymmetrically substituted double bond, the acidic hydrogen of the acid bonds to that carbon of the double bond that has the greater number of hydrogen atoms already attached to it.
In more homelier vernacular this rule may be restated as, “Them that has gits.

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February 3, 2010 Posted by | Reactivity Of Alkenes | Leave a comment

THE REACTIVITY OF ALKENES

THE REACTIVITY OF ALKENES

(1) Hydrogenation of alkenes

The reaction of the carbon-carbon double bond in alkenes with hydrogen in the presence of a metal catalyst such as platinum, nickel or either palladium.  This is called hydrogenation. It includes the manufacture of margarine from animal or vegetable fats and oils.

(2) Halogenation of alkenes

The reaction of the carbon-carbon double bond in alkenes such as ethene with halogens such as chlorine, bromine and iodine. This is called halogenation. Alkenes react rapidly with chlorine or bromine in CH2Cl2 at room temperatura to form vicinal dihalides.

Reactions where the chlorine or bromine are in solution (for example, “bromine water”) are slightly more complicated and are treated separately at the end.

Chlorine reacts faster than bromine, but the chemistry is similar. Iodine reacts much, much more slowly, but again the chemistry is similar. You are much more likely to meet the bromine case than either of these.

(3) Hydrohalogenation of Hydrocarbons

Unsaturated hydrocarbons (alkenes and alkynes) react with hydrogen halides (HX). In addition reactions producing halogenated compounds .

Markovnikov’s Rule:

–       In additions of HX to unsymmetrical alkenes, the H+ of HX goes to the double-bonded carbon that already has the greatest number of hydrogens

–       Hydrogen halides are polarized molecules, which easily form ions. Hydrogen halides also add to alkenes by electrophilic addition.

Anti-Markovnikov’s Rule:

–       Hydrogen bromide can also be added to an alkene in an anti-Markovnikov fashion.

–       In anti-Markovnikov additions, the hydrogen atom of the hydrogen halide adds to the carbon of the double bond that is bonded to fewer hydrogen atoms.

–       For this to result, the reaction must proceed by a noncarbocation intermediate; thus in the presence of peroxide, the reaction proceeds via a free-radical mechanism, with the major product being generated from the more stable free radical.

February 1, 2010 Posted by | Reactivity Of Alkenes | Leave a comment

Alkenes:Reactivity of Alkenes

There are several ways of reactivity of alkenes occur.First is hydrogenation.This is a reaction  of alkenes with hydrogen in the presence of catalyst such as platinium,nickel and palladium.

Second is halogenation which is occur in two conditions.First condition is in inert solvent where alkenes reacts with chlorine or bromine in CH2Cl2 at room temperature and forming vicinal dihalides.The second condition is in aqueous solution which the last product is haloalcohol called  halohydrin.

Next is hydrohalogenation which is a reaction when hydrogen halides(HI,HBr,HCl and HF)add to double bonds of alkenes and forming haloalkenes.The addition of the hydrogen halides follow Markovnikov Rule.

January 27, 2010 Posted by | Reactivity Of Alkenes, Uncategorized | Leave a comment

THE MECHANISM REACTION OF ALKENES

1) Hydrogenation
2) Halogenation
3) Hydrohalogenation

THE HYDROGENATION OF ALKENES
Hydrogenation is an addition of hydrogen (H2) to unsaturated organic compounds in the presence of catalyst such as platinum, nickel and palladium to form alkenes. It includes the manufacture of margarine from animal or vegetable fats and oils.

THE HALOGENATION OF ALKENES
Halogenation is the addition of halogen atoms to a π-bond system.
The reaction of a halogen with an alkane in the presence of ultraviolet (UV) light or heat leads to the formation of a haloalkane (alkyl halide).
Experiments have shown that when the alkane and halogen reactants are not exposed to UV light or heat, the reaction does not occur. However, once a reaction is started, the light or heat source can be removed and the reaction will continue. The mechanism of the reaction explains this phenomenon.

THE HYDROHALOGENATION OF ALKENES
A hydrohalogenation reaction is the addition of hydrohalic acids like hydrogen chloride or hydrogen bromide .
Unlike halogens, hydrogen halides are polarized molecules, which easily form ions. Hydrogen halides also add to alkenes by electrophilic addition.
The addition of hydrogen halides to asymmetrically substituted alkenes leads to two products.
The major product is predicted by the Markovnikov rule, which states that when a hydrogen halide is added to an asymmetrically substituted alkene, the major product results from the addition of the hydrogen atom to the double-bonded carbon that is attached to more hydrogen atoms, while the halide ion adds to the other double-bonded carbon. This arrangement creates a more stable carbocation intermediate.

January 27, 2010 Posted by | Reactivity Of Alkenes, Uncategorized | Leave a comment