Alkene-Addition Reaction

Alkenes react in many addition reaction, which occur by opening up the double-bond.

• Catalytic hydrogenation: Hydrogenation of alkenes produces the corresponding alkaness. The reaction is carried out under pressure in the presence of a metallic catalyst. Common industrial catalysts are based on platinumm, nickel or palladium. For laboratory syntheses, Raney nickel (an alloy of nickel and aluminium)is often employed. The simplest example of this reaction is the catalytic hydrogenation of ethylene to yield ethane:

CH₂=CH₂ + H₂ → CH₃-CH₃

• Electrophilic halogenation: Addition of elemental bromine or chlorine to alkenes yields vicinal dibromo- and dichloroalkanes, respectively. The decoloration of a solution of bromine in water is an analytical test for the presence of alkenes:

CH₂=CH₂ + Br₂ → BrCH₂-CH₂Br

It is also used as a quantitive test of unsaturation, expressed as the bromine number of a single compound or mixture. The reaction works because the high electron density at the double bond causes a temporary shift of electrons in the Br-Br bond causing a temporary induced dipole. This makes the Br closest to the double bond slightly positive and therefore an electrophile.

• Hydrohalogenation: Addition of hydrohalic acids such as HCl and HBr :CH₃-CH=CH₂ + HBr → CH₃-CHBr-CH₂–H

If the two carbon atoms at the double bond are linked to a different number of hydrogen atoms, the halogen is found preferentially at the carbon with fewer hydrogen substituents (Markovnikov’s rule).

To read more, go to http://en.wikipedia.org/wiki/Alkene#Addition_reactions

Uses of alkenes

Alkenes are produced from the alkanes in crude oil by a process called cracking. Cracking uses heat and a catalyst to decompose alkanes. The cracking process is important because it turns less useful alkanes into more useful smaller ones and produces very useful alkenes like ethene, the starting material for many plastics.

catalyst: A substance which speeds up a chemical reaction without being altered or consumed in the process itself.

Alkenes are unsaturated hydrocarbons with one double bond (R-C=C-R). They are from a homologous series with the general formula C_nH_2n. The 1st member to possess this functional group is ethane, C₂H₄. Ethene is an important raw material for the petrochemical industry. In fact, more commercial chemicals are produced from Ethene than from any other hydrocarbon intermediate. This unique position of ethene is due to favorable inherent properties as summarized below :

i. Simple but reactive structure
ii. Inexpensive
iii. Easily available through cracking of hydrocarbons

Alkenes are much more reactive than alkanes because alkenes are unsaturated (they have a reactive double bond). An alkene may be distinguished from an alkane by shaking the hydrocarbon with bromine water. Bromine water is reddish-brown, and will decolourised with an alkene but not with an alkane. Bromine adds across the double bond of an alkene to form a colourless dibromo alkane. This is an example of an addition reaction. An addition reaction occurs when two or more reactants join together to form a single product.

Uses of alkenes

Alkenes are extremely important in the manufacture of plastics. All plastics are in some way related to alkenes. The names of some plastics (Polythene or Poly Ethene, Polypropene), relate to their alkene partners. Plastics are used for all kinds of tasks, from packaging and wrapping, to clothing and outdoor apparel.

Lower alkenes are used as fuel and illuminant. These may be obtained by the cracking of kerosene or petrol.

For the manufacture of a wide variety of polymers, e.g., polyethene, polyvinylchloride (PVC) and teflon etc.

As raw materials for the manufacture of industrial Chemicals such as alcohols, aldehydes, and etc.

Besides, alkenes also used for artificial ripening of fruits, as a general anesthetic, for making poisonous mustard gas (War gas) and ethylene-oxygen flame.

References :

1. GCSE Science

2. Chemistry Guide

3. Carbon Chemistry

Alkene

Alkenes are unsaturated hydrocarbons with general formula; CnH2n and isomeric with cycloalkenes. Alkenes can be prepared from dehydration of alcohols and dehydrohalogenation of alkyl halide. To form the root of the IUPAC (International Union and Applied Chemistry ) names for alkenes, simply change the -an- infix of the parent to -en-. For example, CH3-CH3 is the alkane therefore ethene. The name of CH2=CH2 is therefore ethene. Alkenes are hydrocarbons with carbon-carbon double bonds. In contrast to the easy rotation of sigma bonds, pi bonds cannot be rotated. This rigidity gives rise to cis-trans isomerism, also called geometric isomerism. Heats of hydrogenation can be used to compare the stabilities of alkenes. In general, more highly substituted double bonds are more stable, and trans isomers are slightly more stable than cis. Therefore, most eliminations give the most highly substituted alkene as their major product (Saytzeff’s rule). Alkenes undergo mainly electrophilic addition. The major products determined using Markovnikov’s Rule and stability of carbocation.

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.

IUPAC NOMENCLATURE

Alkenes: Nomenclature

Alkenes are normally named using the IUPAC system. The rules for alkenes are similar to those used for alkanes. The following rules summarize alkene nomenclature.

1. Identify the longest continuous chain of carbon atoms that contains the carbon-carbon double bond. The parent name of the alkene comes from the IUPAC name for the alkane with the same number of carbon atoms, except the -ane ending is changed to -ene to signify the presence of a double bond. For example, if the longest continuous chain of carbon atoms containing a double bond has five carbon atoms, the compound is a pentene.
2. Number the carbon atoms of the longest continuous chain, starting at the end closest to the double bond. Thus,
   

   is numbered from right to left, placing the double bond between the second and third carbon atoms of the chain. (Numbering the chain from left to right incorrectly places the double bond between the third and fourth carbons of the chain.)

3. The position of the double bond is indicated by placing the lower of the pair of numbers assigned to the double-bonded carbon atoms in front of the name of the alkene. Thus, the compound shown in rule 2 is 2-pentene.
4. The location and name of any substituent molecule or group is indicated. For example,
   

   is 5-chloro-2-hexene.

5. Finally, if the correct three-dimensional relationship is known about the groups attached to the double-bonded carbons, the cis or trans conformation label may be assigned. Thus, the complete name of the compound in rule 4 (shown differently here)
   

   is cis-5-chloro-2-hexene.

Read more: http://www.cliffsnotes.com/study_guide/Alkenes-Nomenclature.topicArticleId-22667,articleId-22613.html#ixzz0drILjOkb

Definition Of Alkenes

al·kene (al′kēn′)

noun

any of a series of unsaturated open-chain hydrocarbons containing a double bond and having the general formula CH: these compounds are sometimes said to be in the ethylene or olefin series

Etymology: alk(yl) + -ene

Alkene

In organic chemistry, an alkene is an unsaturated chemical compound .

To form the root of the IUPAC names for alkenes, simply change the -an- infix of the parent to -en-. The simplest alkenes, with only one double bond and no other functional groups. The  general formula of alkene is  C_nH_{2n.The physical properties of alkenes are comparable with those of alkanes and depends on molecular mass . Alkenes are relatively stable compounds, but are more reactive than alkanes due to the presence of a carbon-carbon pi-bond. The reaction of alkene are  hydrogenation halogenation and hydrohalogenation.}

Alkene..ITS INTERESTING!!

Alkene is actually a type of chemical that we are familiar with. Most of us do not even notice how familiar we are with alkenes. For example, benzene. It is a type of alkene, to be exact, cyclic alkene. Benzene is widely use in everyday life. Thus, this proves that we are actually familiar with alkene, but most of us just dont even bother to know about it. So, if you want to know more about it, click here to fill up your minds with some interesting facts of alkene.

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.

Alkenes in everyday life!

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