Chemical properties of alkenes

Physical properties of alkenes or olefins

The name olefin coming from the ethylene was called oil-forming gas or olefiant gas. Ethylene 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’s attractive forces.

Hydrogenation of alkenes

The heat released for hydrogenation of alkenes to form alkane known as the heat of hydrogenation.

CH_{3}\, CH=CH_{2}\xrightarrow{Pt\, or\, Pd}CH_{3}\, CH_{2}-CH_{3}+\Delta H

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 olefinsΔH
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 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 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.

Problem
Arrange the following alkenes in order of increasing stability.

  1. Me2C=CH2
  2. cis-MeCH=CHMe
  3. trans-MeCH=CHMe
  4. MeCH2CH=CH2

Solution
MeCH2CH=CH2 < cis-MeCH=CHMe < trans-MeCH=CHMe < Me2C=CH2

In general, the order of stability of alkenes

R2C=CR2>R2C=CHR>R2C=CH2~RCH=CHR>RCH=CH2>CH2=CH2

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 the olefinic 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 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 addition reaction, organic chemistry, is in its simplest terms an organic reaction where two or more molecules combine to form the larger one and the product is called additive compound.

Catalytic hydrogenation of alkenes

Alkenes are readily hydrogenated under pressure in the presence of a catalyst.

Finely divided platinum and palladium are effective at room temperature, nickel on support require a temperature between 200⁰C and 300⁰C, Raney nickel is effective at room temperature and atmospheric pressure.

CH_{3}CH=CH_{2}+H_{2}\xrightarrow[or\, Ni/\Delta ]{Pt\, or\, Pd}CH_{3}CH_{2}CH_{3}

Addition of halogens to alkenes

Alkenes from addition compounds with chlorine or bromine.

CH_{2}=CH_{2}+Br_{2}\xrightarrow{CCl_{4}}BrCH_{2}-CH_{2}Br

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 and is catalyzed by inorganic halides as for example aluminum chloride or by polar surfaces. These facts lead to the conclusion that reaction occurs by a polar mechanism.

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.

Chemical properties of alkenes
Properties of alkenes

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 is,

hydrogen iodide 〉hydrogen bromide 〉hydrogen chloride 〉hydrogen fluoride 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)

or it might be,

CH₃ – CH = CH₂ + HI → CH₃ – CH(I) – CH₂(H)

Markownikoff studied many reactions of this kind, and as a result of his work, formulated the following rule.

Markownikoff rule for alkenes

The negative part of the addendum acids on to the carbon atom that is joined to the less number of hydrogen atoms.

In the case of the halogen acids, the halogen atom is the negative part. According to Markownikoff rule, isopropyl halide is obtained.

Markownikoff’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, the proton adding fast, followed by halide ion. Also, the addition is predominantly trans, and this may explain in terms of the formation of a bridge carbonium ion.

Markownikoff rule for alkenes
Markownikoff rule

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 Markownikoff’s rule is in terms of the stabilities of carbonium ions. Represent as primary, secondary and tertiary carbonium ion.

Problem
Which of the following carbocation is more stable? (i) CH₃CH₂⁺ (ii) CH₃(CH₃)CH⁺ (iii) CH₃(CH₃)C⁺CH₃.

Solution
The number of hydrogen atoms available for hyperconjugation is 3 for (i), 6 for (ii) and 9 for (ii). consequently, (iii) would be expected to be the most stable.
Thus the stability order is

CH₃(CH₃)C⁺CH₃ 〉CH₃(CH₃)CH⁺ 〉CH₃CH₂⁺