Physical properties of alkenes
The name alkene or olefin coming from the ethylene was called oil-forming gas or olefiant gas. Alkenes have properties to forms an oily liquid when treated with chlorine or bromine.
Alkenes or olefins containing two to four carbon atoms are gases, five to seventeen are liquid and eighteen on words solid at room temperature and they burn in air with a luminous flame.
Physical properties of alkenes similar to those of alkanes since the alkenes have only weak Van der Waals attractive forces.
Hydrogenation of alkenes
The heat released for hydrogenation of alkenes to form alkane known as the heat of hydrogenation.
CH3CH = CH2 → CH3CH2 – CH3 + ΔH
When the heat of hydrogenation value decreases the stability of the alkenes increases because alkane is most stable than the respective alkenes.
Thus heat of hydrogenation value compares the stability of the alkenes series.
|Alkenes or olefins||ΔH|
|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|
Since the thermodynamic heat of hydrogenation is an exothermic reaction. Thus the numerically smaller value of ∆H, the more stable is the alkene.
Stability of alkenes hyperconjugation
Enthalpy of formation of alkenes not purely addictive properties. Thus the stability of alkenes also depends on steric effects and hyperconjugation.
Since three n- butenes all give the n-butane on reduction and the order of stability of these alkenes
trans-but-2-ene 〉cis-but-2-ene 〉 but-1-ene
Thus this 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.
Thus it experiences greater steric repulsion and consequently, cis-isomer has greater strain than the trans-isomer. Thus steric repulsion destabilizes the cis-isomer.
∴ trans-but-2-ene 〉cis-but-2-ene
On the other hand, hyperconjugation stabilizes the molecule. Among this three hydrocarbon, 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.
Arrange the following alkenes in order of increasing stability.
MeCH2CH=CH2 < cis-MeCH=CHMe < trans-MeCH=CHMe < Me2C=CH2
In general, the order of stability of alkenes
The chemical properties of alkenes
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 this chemical bond due to the presence of two π- electrons.
Thus when addition reaction occurs, the trigonal arrangement in the alkene changed to the tetrahedral arrangement like methane and a saturated compound produced.
The 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 reactions of alkenes
An organic reaction where two or more molecules combine to form the larger one is called addition reaction the product called an additive product.
Catalytic hydrogenation of alkenes
Alkenes readily hydrogenated under pressure in the presence of a catalyst.
Finely divided platinum and palladium at room temperature, nickel on between 2000 C and 3000 C, Raney nickel at 2000 C use for this conversation.
Addition of halogens to alkenes
Alkenes react with chlorine or bromine to form addition products.
Halogen addition can take place either by a heterolytic (polar) or a free-radical mechanism.
Halogen addition radially occurs in solution, in the absence of light or peroxides 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 that the addition of halogen to alkenes in the absence of light is polar. Stewart showed that the addition of chlorine to ethylene is accelerated by light and this suggested the free radical mechanism.
Addition of halogen acids
Ethylene adds hydrogen bromide to form ethyl bromide.
CH2=CH2 + HBr → CH3⎯CH2Br
The order of reactivity of the halogen acids
This is also the order of acid strength.
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, it is possible for the addition of the halogen acid to take place in two different ways,
Propane might add on hydrogen iodide to form propyl iodide or isopropyl iodide.
CH3-CH=CH2 + HI → CH3-CH(H)- CH2(I)
CH3-CH=CH2 + HI → CH3-CH(I)-CH2(H)
Markovnikov studied many reactions of this kind, and as a result of his work, formulated the following rule.
Markovnikov rule for alkenes
The negative part of the addendum adds on to the carbon atom that is joined to the less number of hydrogen atoms.
For halogen acids, the halogen atom is the negative part. 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 electrons are 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 as primary, secondary and tertiary carbonium ion.
Chemical properties of alkenes
Many compounds including alkenes contain one double bond that exists in the two forms which differ in most of their physical and chemical properties.
Vant Hoff suggested, if we assume there is no free rotation about a double bond, two structural arrangements are possible for this molecule.
In ethylene, we have used SP2 trigonal hybridization to describe the double bond. In this case, two SP2 electrons of ethylene to form one banana bond and the other two-electron to form a second banana bond.