Properties of Alkenes in Organic Chemistry
Properties of alkenes and alkanes in organic chemistry are very similar since the hydrocarbon alkene molecule has only weak van der Waals attractive force. The name alkene or olefin coming from ethylene, called oil forming or olefiant gas molecules. Alkenes are the hydrocarbon having the physical properties to form an oily liquid when chemical reaction with chlorine or bromine. Alkenes containing two or four carbon atom (ethylene, propylene, and butylene) are gases, five to seventeen are liquid but eighteen on words are crystalline solid organic molecules. Alkene has properties to burn-in air with a luminous oxidation flame.
Hydrogenation Reaction of Alkene
The heat released for hydrogenation of alkenes to form alkanes known as the heat of hydrogenation reaction.
CH3CH = CH2 → CH3CH2 – CH3 + ΔH
The heat of hydrogenation value decreases when the stability of the alkenes increases due to the stability of the corresponding alkanes. Therefore, the heat of hydrogenation value compares the stability of the alkenes series. But thermodynamics heat of hydrogenation alkenes is an exothermic reaction and a numerically smaller value of enthalpy (∆H) are more stable. The heat of hydrogenation of some common alkenes given below table
|CH2=CH2||-137.0 kJ mol-1|
|MeCH=CH2||-125.9 kJ mol-1|
|MeCH2CH=CH2||-126.8 kJ mol-1|
|cis-MeCH=CHMe||-119.7 kJ mol-1|
|trans-MeCH=CHMe||-119.7 kJ mol-1|
|Me2C=CH2||-188.8 kJ mol-1|
Stability of Hydrocarbon (Butylene)
Enthalpy of formation of alkenes not purely addictive properties. Therefore the stability of alkenes depends on steric effects, hyperconjugation, and stability of the corresponding alkanes. Since three n- butylene isomers give the n-butane on reduction and the order of stability of these butylene isomers, trans-but-2-ene 〉cis-but-2-ene 〉 but-1-ene.
The order of stability explained in terms of steric effect and hyperconjugation. In cis-but-2-ene, the two methyl groups in cis isomer being closer together than in the trans isomer. Therefore, cis-but-2ene experiences greater steric repulsion, and consequently, cis-isomer has greater strain than the trans-isomer. Due to steric destabilizes the stability order butylene isomer, trans-but-2-ene 〉cis-but-2-ene.
On the other hand, hyperconjugation stabilizes the organic molecule. Among these three hydrocarbons, but-1-ene has less number of hyperconjugation structure. Thus but-1-ene less stable among these three alkenes. Since trans-but-2-ene is the most stable isomer, it follows that hyperconjugation has a greater stabilizing effect then steric repulsion a destabilizing effect.
Question: Arrange Me2C=CH2, cis-MeCH=CHMe, trans-MeCH=CHMe, MeCH2CH=CH2, these alkenes in order of increasing stability.
Solution: MeCH2CH=CH2 < cis-MeCH=CHMe < trans-MeCH=CHMe < Me2C=CH2. The order of stability of general alkenes, R2C=CR2 > R2C=CHR > R2C=CH2 ~ RCH=CHR > RCH=CH2 > CH2=CH2, where R = alkyl or methyl group.
Chemical Properties of Alkenes in Organic Chemistry
Owing to the presence of a double bond, the alkenes undergo a large number of addition reactions but under special conditions, they also undergo substitution reactions. The high reactivity of alkenes due to double bond or olefinic chemical bond containing π- electrons. When addition reaction occurs, the trigonal arrangement in the alkenes changed to the tetrahedral arrangement like methane molecule and a saturated compound produced.
Combustion Reaction of Alkenes
Alkenes are flammable substances they burn in air with a luminous smoky flame to produce carbon dioxide and water.
2CnH2n + 3H2O → 2nCO2 + 2nH2O + ΔH
CH2=CH2 + 3O2 → 2CO2 + 2H2O + ΔH
Addition of Hydrogen or Catalytic Hydrogenation Reaction
Alkenes readily addition hydrogen atom (hydrogenation) under pressure in the presence of a chemical catalyst. Finely divided platinum and palladium at room temperature, nickel on between 200°C and 300°C or raney nickel at 200°C use for the conversation of alkenes to alkanes.
CH3CH=CH2 + H2 → CH3CH2CH3
Addition Reaction of Halogen to Alkenes
Alkenes react with chlorine or bromine to form addition products.
CH2=CH2 + Br2 → BrCH2-CH2Br
Halogen addition can take place either by a heterolytic (polar bonding) or a free-radical mechanism. Halogen addition radially occurs in solution, in the absence of light or peroxides (hydrogen peroxide) which catalyzed by inorganic halides. Aluminum chloride or by polar surfaces use as a catalyst. These facts lead to the conclusion that reaction occurs by a polar mechanism.
But the free radical mechanism has generally accepted due to the properties of the halogen addition to an alkene in the absence of light is polar. Stewart showed that the addition of chlorine to ethylene, propylene, and butylene is accelerated by light and this suggested the free radical mechanism.
Addition of Halogen Acid to Alkene
Ethylene adds hydrogen bromide to form ethyl bromide. The order of reactivity of the halogen acid solution HI > HBr > HCl > HF (this is also the order of acid strength).
CH2=CH2 + HBr → CH3⎯CH2Br
The conditions for the addition are similar to those for halogens, only the addition of hydrogen fluoride occurs under pressure. In the case of unsymmetrical alkenes like propylene, possible for the addition of the halogen acid on propylene to take place in two different ways, propylene might add on hydrogen iodide to form propyl iodide or isopropyl iodide.
CH3-CH=CH2 + HI → CH3-CH2– CH2I
CH3-CH=CH2 + HI → CH3-CHI-CH2H
Markovnikov studied properties many halogen addition reactions and formulated the rule for addition reaction of the alkene.
Markovnikov Rule and Properties of Alkenes
The negative part of the addendum adds on to the carbon atom that is joined to the less number of hydrogen atoms. Thus according to Markovnikov rule, when propene reacts with halogen acid it forms isopropyl halide.
Markovnikov’s rule is empirical but may be explained theoretically on the basis that the addition occurs by a polar mechanism. The addition of halogen acid is an electrophilic reaction thus the proton adding fast, followed by halide ion. Also, the addition predominantly in trans position and this may explain in terms of the formation of a bridge carbonium ion.
Since the methyl group has a +I effect, the π electron is displaced towards the terminal carbon atom which, in consequence, acquires a negative charge. Thus, the proton added on to the carbon fastest from the methyl group, and the halide ion then adds to the carbonium ion.
An alternative explanation for Markovnikov’s rule is in terms of the stabilities of carbonium ions. Represent the formation and stability of primary, secondary, and tertiary carbonium ion.
Bonding Properties of Alkene in Organic Chemistry
Many organic compounds including alkenes or olefins contain one double bond that exists in the two forms which differ in most of their physical properties and chemical reaction. Vant Hoff suggested, if we assume there is no free rotation about a double bond, two structural arrangements are possible for this molecule.
If we assume two unsaturated carbon atoms (ethylene, propylene, butylene) are SP3 hybridized, then they are joined by two banana bonds with the maximum overlap of orbitals. Therefore, these decrease for the overlap of the double bond of ethylene, bond dissociation energy, and resistance to rotation properties. On the other hand, if we assume SP2 hybridization, the double-bonded pi-orbitals are maximum overlap which resistance the rotation properties of alkenes (ethylene, propylene, and butylene) in organic chemistry.