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Nov 1, 2018

General Properties of Alkenes

General Properties of Alkenes: 

The numbers containing two to four carbon atoms are gases; five to seventeen is liquid; eighteen on words solid at room temperature and they burns in air with a luminous flame. In general, the physical properties of the alkenes are similar to those of alkenes, since the alkenes are only weak van der Walls attractive forces. 
Owing to the presence of 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 olifinic bond is due to presence of two π- electrons, and when addition occurs, the trigonal arrangement in the alkene changed to the tetrahedral arrangement in the saturated compound produced. 
Alkenes addition reactions involve an electrophile in the first step. This is the rate – determining step, and the intermediate carbocation then reacts rapidly with a neuclophile.
Physical and Chemical Properties of Alkenes.
Addition Reaction of Alkenes

Stabilities of Alkenes:

One way of measuring the stability of an alkene is the determination of its heat of hydrogenation.

Physical and Chemical Properties of Alkenes.
Heat of Hydrogenation of Alkenes
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 same alkane on hydrogenation. This arises from the fact that the enthalpy of formation of alkenes is not purely additive properties; it is also depends on steric effects and these tend to vary from molecule to molecule. Since three n- butenes all gives the n-butane on reduction.
Thus the order of stabilities
trans but-2-ene cis but-2-enebut-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 group 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 other hand, hyperconjugation stabilises 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 greater stabilising effect then steric repulsion a desstabilising effect. 
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 
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 is:
R₂C=CR₂R₂C=CHRR₂C=CH₂ ~ RCH=CHRRCH=CH₂CH₂=CH₂

Reactions of alkenes: 

1. Addition Reaction:

An addition reaction, in 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.
(i). Catalytic Hydrogenation:
Alkenes are readily hydrogenated under pressure in the presence of catalyst. Finely divided platinum and palladium are effective at room temperature; nickel on a support requires a temperature between 200⁰C and 300⁰C; Raney nickel is effective at room temperature and atmospheric pressure.
CH₂ = CH₂  CH₃ - CH₃
Unsaturated hydrocarbon Saturated hydrocarbon
(ii) Electrophilic Addition of Halogens:
Alkenes form addition compounds with chlorine or bromine.
Example,
CH₂ = CH₂ + Br₂  BrCH₂ - CH₂Br
Ethylene Ethylene dibromide
Halogen addition can take place either by a heterolytic (polar) or a free-radical mechanism.
Heterolytic (Polar) Mechanism:
Halogen addition radially occurs in solution, in the absence of light or peroxides and is catalyzed by inorganic halides.
Evidence for the Polar Mechanism:
Eathyene reacts with bromine in aqueous sodium chloride solution, the products are eatylene dibromide and 1-bromo-2-chloroeathane, no eathylene dichloride is obtained.

Physical and Chemical Properties of Alkenes.
Evidence for the Polar Mechanism of Ethylene
These addition reaction are at least two stage process, but does not demonstrate the order of he addition, that is whether the halogen are electrophile or nucleophile reagents. However, coupled with the fact that ethylene does not react with these solutions in absence of bromine, this strong indication for electrophilic attack by halogen. A mechanism consist with these fact is,
Physical and Chemical Properties of Alkenes.
Polar Mechanism of Ethylene

Stereochemistry of halogen addition to alkenes:

Addition of halogen is usually predominantly trans, that is the addition is streoselective (one stereo-isomer predominates over the other).
As for example, the addition of bromine to maleic acid gives enantiomers of dibromosuccinic acid, which can result only from trans addition.
Physical and Chemical Properties of Alkenes.
Enantiomers of Dibromosuccinic Acid

Free-Radical Mechanism:

The addition of halogen to alkenes, occurs in the absence of light, is polar.
h⋎
Cl₂ 2Cl˙
     Cl₂
CH₂=CH₂ CH₂Cl - CH₂˙  CH₂Cl - CH₂Cl + Cl˙, etc.

(iii) Addition compounds with the Halogen Acids:

Ethylene add hydrogen bromide to from ethyl bromide.
CH₂=CH₂ + HBr CH₃⎯CH₂Br
The order of reactivity of the halogen acids is, 
hydrogen iodidehydrogen bromidehydrogen chloridehydrogen 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 is 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,
As for example propane might add on hydrogen iodide to from propyl iodide or isopropyl iodide.

CH₃ - CH = CH₂ + HI CH₃ - CH(H) - CH₂(I) (Propyl iodide)
or it might be,
CH₃ - CH = CH₂ + HI  CH₃ - CH(I) - CH₂(H) (Isopropyl iodide)
Markownikoff studied many reactions of this kind, and as a result of his work, formulated the following rule.

Markownikoff Rule:

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.

Explanation of Markownikoff’s Rule :

Markownikoff’s Rule is empirical, but may be explained theoretically on the basis that the addition occurs by a polar mechanism. As with the halogen, 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.
Physical and Chemical Properties of Alkenes.
Bridge Carbonium Ion Formation
Now consider the case of propene. 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.
Physical and Chemical Properties of Alkenes.
Mechanism for Addition of Hydrogen Iodide To Propene

Alternative Explanation of Markownikoff’s Rule :

An alternative explanation for Markownikoff's rule is in terms of of stabilities of carbonium ions. Represent as primary, Secondary and Tertiary carbonium ion.
Which of the following carbocation is more stable?
(i) CH₃CH₂ (ii) CH₃(CH₃)CH (iii) CH₃(CH₃)CCH₃.

Explanation for the order of the stability of these carbocation by hyperconjugation.
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 (iii) 〉(ii) 〉(i)