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Alkenes

Alkenes are unsaturated hydrocarbons with double bond. This double bond is the olefinic or the ethylenic bond. The general formula of alkenes is CnH2n where n is the number of carbon atoms and has an integral value.

Hence the lowest alkene contains two carbon atoms bonded with double bond. Like alkanes, the Alkenes also form homologous series. The homologous series of alkene starts with ethene, because the double bond requires the presence of at least two carbon atoms.

Alkene Root Word Structure IUPAC
C2H4 Eth CH2 = CH2 Ethene
C3H6 Prop CH3-CH = CH2 Propene
C4H8 But CH3-CH = CHCH3 But-2-ene
C4H8 But CH2 = CHCH2CH3 But-1-ene
C5H10 Pent CH2 = CHCH2CH2CH3 1-Pentene
C5H10 Pent CH3CH = CHCH2CH3 2-Pentene
C6H12 Hex CH2 = CHCH2CH2CH2CH3 1-Hexene
C6H12 Hex CH3CH = CHCH2CH2CH3 2-Hexene
C6H12 Hex CH3CH2CH=CHCH2CH3 3-Hexene

 

Nomenclature of Alkenes

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There are certain key steps in the IUPAC nomenclature of alkenes according to the IUPAC rules.

  • First find the longest continuous carbon chain which includes the double bond. Name this longest root chain by using standard naming rules. For example in given molecule there are five carbon atoms in parent chain
2-Pentene

  • Take the unbranched alkane with the same number of carbons and replace ‘-ane’ with -ene. The root word for five carbon atoms is “Pent” added with suffix “ene”, hence “Pentene”.
CH3-CH = CH-CH2-CH3 Pentene
1 2 3 4 5

  • Number the chain in such a way that the doubly bonded carbon gets lowest number in parent chain.
CH3-CH=CH-CH2-CH3 2-Pentene or Pent-2-ene
1 2 3 4 5
  • If there is any substituent attached in parent chain, identify its position by a number. Here methyl group located at 2nd position, hence written as prefix with position number
2-Methyl-2-Pentene
  • It depends on the presence of other functional groups, that which group will take precedence.
  • Like the double bond will take precedence over alkyl groups and halogens during the numbering of parent chain.
  • For example in 2-Methylbut-3-en-1-ol, hydroxy group take precedence over the double bond and methyl will be side chain written as prefix with position number.
2-Methylbut-3-en-1-ol
  • If in an alkene there are more than one double bond is present, it named as a –diene or –triene. The numbers are used to indicate the positions of the multiple bonds. For example; 2,5-Dimethyl-2,4-hexadiene, here double bond located at 2 and 4 position with two substituent (methyl group) at 2 and 5 positions.
2,5-Dimethyl-2,4-hexadiene
  • Cyclic unsaturated hydrocarbons also named in the same manner by the replacement of the -ane ending of the cyclo alkane containing the same number of carbons with -ene.
Cyclohexene
  • The substituent get number through the double bond in the direction that gives the lower number to the first-appearing substituent. Here methyl group located at 3r position with respect to double bond, hence named as 3-Methylcyclobutene.
Methyl Cyclobutene

Preparation of Alkenes

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Alkene can be prepared by using different hydrocarbons like alkane and alkyne as well as from halo alkanes.

Let's discuss some of these methods.

1. Reduction of alkynes


Reduction of alkyne in the presence of sodium or lithium in liquid ammonia yields trans-alkene. For the preparation of cis-alkene Lindlar catalyst is a suitable catalyst which consists of palladium spread over carbon and poisoned with barium sulphate. Both reactions are an good example of heterolytic catalysis.
Alkenes

2. Dehydrohalogenation of haloalkanes


When a haloalkane is heated with an alcoholic solution of potassium hydroxide, a molecule of hydrogen halide is eliminated to form an alkene.

>CH-C(X)< + :B ⇒ >C=C< + H:B + :X-
Haloalkane Base Alkene Protonated base Halide ion

Reaction is also termed as 1,2-elimination or β-elimination as the hydrogen atom eliminated from the β-position with respect to hydrogen.


Reaction proceeds through the attack of base like hydroxy ion on alkene to remove hydrogen in the form of water followed by the removal of halide ion.

Dehydrohalogenation of haloalkanes

  1. If there is a possibility to get more than two alkenes, more substituted alkene is the major product. For example, dehydrohalogenation of 2-Bromo-2-methylbutane forms two alkenes; 2-Methyl-2-Butene and 2-Methyl-1-Butene. Out of these two, 2-Methyl-2-Butene (71%) would be major product compare to less substituted 2-Methyl-1-Butene (29%). This generalisation is called as Saytzeff rule.
  2. If a haloalkane can eliminate the hydrogen halide in two different ways, that alkene will be formed in excess in which carbon atoms joined by double bond are more alkylated. This is because more substituted alkene is more stable due to hyperconjugation.
  3. Rate of elimination depends on carbon-halogen bond; reaction becomes faster with weaker carbon-halogen bond because the rate determining step involves the cleavage of carbon-halogen bond

3. Dehydration of alcohols


The dehydration of alcohols in the presence of an acid like phosphoric acid gives alkenes. Other reagent for dehydration is alumina at around 623K. When alcohol vapours are passed over alumina, it formed alkenes.
Secondary and tertiary alcohols are best dehydrated by acid catalysts such as concentrated sulfuric acid.
For example: On dehydration propan-2-ol gives propene.

CH3CH(OH)CH3 → CH3CH=CH2 +H2O
Propan-2-ol propene

The dehydration of secondary and tertiary alcohol follows E1 path in elimination through the formation of transition state as an intermediate.

Dehydration of alcohols
  1. In place of concentrated acid, anhydrous zinc chloride may also be used for dehydration of alcohols.
  2. A series of steps is involved in the mechanism of dehydration of alcohols.
  3. The acid reacts with alcohol to make it protonated.
  4. This process is usually reversible. In the second step, the protonated alcohol loses water to give a carbocation.
  5. Finally the carbocation loses a proton to give the alkene. Generally primary alcohols follow this E2 path in elimination.

Primary Alcohols Elimination

4. Dehalogenation of vicinal dihalides


Vicinal dihalide gets dehalogenated to form an alkene in the presence of zinc in alcoholic solution at high temperature.


Dehydration Reaction

5. Kolbe's Electrolysis


Electrolysis of sodium or potassium salts of dibasic acid forms alkenes. When an aqueous solution of a sodium or potassium salt of a dibasic acid is electrolyzed, an alkene is produced. For example; during electrolysis of potassium succinate, reaction at anode carbon dioxide released with ethene.

-OOC-CH2-CH2-COO- CH2=CH2 + 2CO2(g) +2e-

While at cathode hydrogen gas releases with hydroxyl ion which further react with
potassium to form potassium hydroxide.

2H2O +2e- 2OH- +H2(g)
2K+ +2OH- 2KOH

6. Cracking


The cleavage of large hydrocarbon molecules into more useful smaller hydrocarbon molecules by using high temperature is called as cracking. Cracking of C15H32 gives a mixture of ethene, propene and octane.


C15H32 2C2H4 + C3H6 + C8H18
n-Pentadecane Ethene Propene Octane

Instead of high temperature, now a day Zeolites are used as a catalyst for cracking process. Zeolites are complex alumino silicates consist of silicon, aluminium and oxygen.

7. From vinyl halide


When vinyl halides react with Grignard reagents, they form higher alkenes. For example; Methyl magnesium bromide forms propene with vinyl bromide by substitution reaction.

CH3MgBr + BrCH=CH2 CH3CH=CH2 + MgBr2

Vinyl halides can also alkylated with dialkyl copper and form higher alkenes. For example; vinyl chloride forms propene with dimethyl copper.

R2Cu
2CH2 = CHCl →
2CH2 = CHR + CuCl2
Alkene

2CH2 = CHCl → 2CH3-CH = CH2 + CuCl2
vinyl chloride Propene


Reactions of Alkenes

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Any alkene must contain at least one double bond which forms through one sigma and one pi bond which are loosely held and readily react with electrophiles or free radical. Hence alkene generally gives addition reactions.
Reactions of Alkenes
Due to the presence of pi bond alkenes are much more reactive compare to alkanes and usually undergo addition reactions apart from substitution reaction of alkanes. An electrophile can easily attack on double bonded carbon atom and form carbocation which further convert in additional product. Hence there is an overall conversion of one pi bond to two new sigma bonds with new groups.
Substitution Reaction

Some common examples of electrophilic addition reaction of alkene are like hydrogenation, halogenation, hydrohalogenation and ozonolysis etc.

Hydrogenation of Alkenes

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  1. Hydrogenation of alkene leads to the formation of alkanes.
  2. The overall reaction is the cleavage of double bond to form two sigma bonds with double bonded carbon atoms.
  3. Hydrogenation of alkene is an exothermic reaction and the energy released during the reaction is called as heat of hydrogenation of that alkene.
  4. Although the reaction is exothermic in nature but some temperature and catalyst is required to initiate the reaction as the high activation energy prevents it from taking place under normal conditions.
Some common catalyst for the hydrogenation of alkenes are finely divided metals, like platinum, palladium and nickel. Catalytic hydrogenation of alkene takes place in two steps.
  • First alkene molecule gets adsorbed on the surface of the catalyst along with some of the hydrogen.
  • Absorbed two hydrogen atom shift from the metal surface to the double bond on carbon atom and form saturated hydrocarbon, alkane.
  • Desorption of alkane from catalyst surface.
Hydrogenation of Alkenes

Heat of hydrogenation is used as a measurement of stability of alkenes. As the value of heat of hydrogenation increases, stability of alkenes decreases. For example; three isomers with the molecular formula C5H10 show different heat of hydrogenation which proves that 2-methyl-2-butene is highly stable due to its symmetrical nature.

Alkene Isomer
(CH3)2CHCH = CH2
3-methyl-1-butene
CH2 = C(CH2)CH2CH3
2-methyl-1-butene
(CH3)2C = CHCH3
2-methyl-2-butene
Heat of reaction (ΔH) -30.3 kcal/mole
-28.5 kcal/mole -26.9 kcal/mole

Halogenation of Alkenes

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Halogens readily add to the double bond to give dihalo saturated product. Bromine cation attacks the pi bond to form a three-member ring containing a positively charged chlorine atom called bromonium ion. This bromonium ion is attacked by bromide ion to form a di bromo alkane. Halogenation is stereoselective and predominantly trans addition takes place.

Halogenation of Alkenes
Hence, the halogenation of alkene proceeds through the anti addition and form trans-product. But because of free rotation of carbon-carbon bond, the trans-product cannot be isolated. If in an alkene the free rotation is restricted due to cyclic ring, the trans-isomer can be isolated. As the cyclic ring would be remain same in product also.

For example, the halogenation of Cyclohexene form trans-substituted product. However halogenation is a stereospecific reaction. In bromination of methylcyclohexene , total four products can be possible but only two products A and B obtained which are enantiomers of each other, due to trans-addition of bromo group.
Stereospecific Reaction

Ozonolysis of Alkenes

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  • The oxidation of alkenes by ozone followed by the decomposition of the formed ozonide with water is termed 'ozonolysis'.
  • The addition of ozone is not like other additional reactions of alkenes.
  • It involves a complex reactive intermediate, known as ozonides which have been isolated from the interaction of ozone with alkenes and extensively studied.
  • This unstable reactive intermediate further converted to stable products by either a reduction with Zn dust in water or alcohol or by oxidation with hydrogen peroxide.
  • The reduction of this intermediate forms carbonyl compounds, aldehyde and ketones. However oxidative workup gives a carboxylic acid or carbon dioxide.
  • The ozonolysis of alkene is an example of syn-addition followed by rearrangement.
  • Hence overall in ozonolysis, ozone reacts with ethene to form primary ozonoide.
  • Primary ozonide rearranges to secondary ozonide, which forms carbonyl compounds with Zn in acetic acid. Ozonolysis of Alkenes
More topics in Alkenes
Reactions of Alkenes Catalytic Hydrogenation
Hydrogenation of Alkenes Dehydration Synthesis
Alkene to Alcohol Dehydration Reaction
Hydration of Alkene Stability of Alkenes
Olefin Elimination Reaction
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