General Properties of Alkenes

Physical properties of alkenes

    Numbers containing two to four carbon atoms are gases; five to seventeen is liquid; eighteen on words solid at room temperature and they burn in air with a luminous flame.
    In general, the physical properties of alkenes are similar to those of alkenes, since the alkenes are only weak van der walls attractive forces.

Stabilities of alkenes

    One way of measuring the stability of an alkene is the determination of its heat of hydrogenation.
    CH₂=CH₂ (ΔH = -137 kJ mol⁻¹), MeCH=CH₂ (ΔH = -125.9 kJ mol⁻¹), MeCH₂CH=CH₂(ΔH = -126 kJ mol⁻¹), MeCH₂CH=CH₂(ΔH = -126.8 kJ mol⁻¹), cis- MeCH=CHMe(-119.7 kJ mol⁻¹), trans- MeCH=CHMe(-119.7 kJ mol⁻¹), Me₂C=CH₂(ΔH = -188.8 kJ mol⁻¹).
    Since the reaction is exothermic, the smaller ∆H is (numerically), the more stable is the alkene relative to its parent alkane.
    Thus, it is only possible to compare the stabilities of different alkenes which produce the same alkane on hydrogenation.
    This arises from the fact that the enthalpy of formation of alkenes is not purely additive properties; it also depends on steric effects and these tend to vary from molecule to molecule. Since three n- butenes all give the n-butane on reduction.
Thus the order of stabilities
trans but-2-ene 〉 cis but-2-ene 〉 but-1-ene
    This order may be explained in terms of steric effect and hyperconjugation.
    In but-1-ene, steric repulsion is virtually absent.
    In but-2-enes, the two methyl groups in cis isomer being closer together than in the trans isomer, experience greater steric repulsion and consequently the cis form is under greater strain than the trans. Thus steric repulsion destabilizes a molecule.
    On the other hand, hyperconjugation stabilizes a molecule, and is small in but-1- ene but much larger in but-2-enes.
    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 and give your reasons.(i) Me₂C=CH₂, (ii) cis-MeCH=CHMe, (iii) trans-MeCH=CHMe (iv) MeCH₂CH=CH₂
    Solution
    MeCH₂CH=CH₂(but-1-ene)ㄑcis-MeCH=CHMe(cis but-2-ene)ㄑtrans-MeCH=CHMe(trans but-2-ene) ㄑMe₂C=CH₂(isobutene)
    In general, the order of stability of alkenes are 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 is due to the presence of two π- electrons, and when addition reaction occurs, the trigonal arrangement in the alkene changed to the tetrahedral arrangement in the saturated compound produced.

Reactions of alkenes

Combustion reactions 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 = -ve

CH₂=CH₂ + 3O₂ → 2CO₂ + 2H₂O, ΔH = -ve

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 requires a temperature between 200⁰C and 300⁰C; Raney nickel is effective at room temperature and atmospheric pressure.
CH₃CH = CH₂(propene) → CH₂CH₃ - CH₃(propane)
    Addition of halogens to alkenes
    Alkenes from addition compounds with chlorine or bromine.
CH₂ = CH₂(ethylene) + Br₂ → BrCH₂ - CH₂Br(ethylene dibromide)
    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.

Properties of Alkenes
Reaction 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, 
    For example, 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. So 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.
Chemical properties of 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⁺

Physical and chemical properties of alkenes, stabilities of alkenes, reactions of alkenes and Markownikoff rule.

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