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Showing posts with label ORGANIC CHEMISTRY. Show all posts
Showing posts with label ORGANIC CHEMISTRY. Show all posts

Nov 1, 2018

Alkenes or Olifins

Alkenes or Olifins: 

Unsaturated Compounds:

The compounds contains at least one pair of adjacent carbon atoms linked by a multiple bond, then that compound is said to be unsaturated.
Unsaturated Hydrocarbon
1.Ethylene(C2H4):
This Compound contain a double bond. There are only four univalent hydrogen atoms present in ethylene, therefore ethylene said to be unsaturated Compound.
H2C = CH2
2.Acetylene(C2H2):
This Compound contain a triple bond and there are only two univalent hydrogen atoms.
HC CH

Alkenes or Olefins

The alkenes are the unsaturated hydrocarbons that contain one double bond. They have the general formula CnH2n, as they contain two hydrogen atoms less than the alkanes, alkenes are called unsaturated hydrocarbons. The double bond is called 'olifinic'bond or 'ethylenic bond'. 
The name olefin arose from the fact that ethylene was called 'olefiant gas'( oil-forming gas), since it forms oily liquids when treated with chlorine or bromine. The original name given to this homologous series was olefine, but it was latter decided to reserve the suffix -ine for basic substances only.

Nomenclature of Alkenes or Olifins:

1.Common naming system of Alkenes or Olifins:
In the common naming system of an olefin is named according to the following rules,
1. The total number of carbon atom in the olefin is counted and the name of the corresponding alkane is determined.
H2C=CH2 
Corresponding alkane is ethane.
2. By changing the name of the corresponding alkane, the suffix -ane of the latter into -ylene.
Alkenes H2C=CH2
Corresponding alkane ethane
Name of the Alkene ethylene
3.The position of the double bond is indicated by Number 1, 2, 3, 4...., or Greek Letters α, β, , , ...., the end carbon atom nearest to the double bond is denoted by 1, next 2,and so on or α, next β, and so on. These letters are then known as locants.
  2 1
CH3 CH = CH2
3   2 1
CH3 CH2 CH = CH2
3   2 1
CH2 CH = CH CH2
  β ɑ
CH3 CH = CH2
ɣ   β ɑ
CH3 CH2 CH = CH2
ɣ   β ɑ
CH2 CH = CH CH2
3. The locants of the double bond carbon atom is then placed before the name of the olifin as obtain from rule 1 and 2. A hyphen is written in between the locants and the name. The locants are used only to name olefins containing more then three carbon atom. Olifins of low molecular weights only have common names.
  2 1
CH3 CH = CH2
Parent alkane is propane
Thus the name of the Compound is
1 - propylene
or α - propylene
1. CH3 - CH2 - CH = CH2
1 - butylene
or 
α - butylene
2. CH3 - CH = CH - CH3 
2 - butylene 
or β - butylene

Write down the common names of the following compounds:
(i) CH3CH2CH2CH=CH2 (ii) CH3CH=CHCH2CH3.

(i) CH3CH2CH2CH=CH2
3   2 1
CH3 CH2 CH2 CH = CH2
Parent alkane is pentane.
Thus the name of the Compound is, 
1 - pentylene 
or α - pentylene
(ii) CH3CH=CHCH2CH3
Parent alkane is pentane
Thus the name of the Compound is 
2 - pentylene 
or β - pentylene
Formula Structural Formula Common Name
C4H8 (CH3)2C=CH2 isobutylene
C5H10 CH3CH2CH2C=CH2 ɑ-pentylene
or,
ɑ-amylene
C5H10 CH3CH=CHCH2CH3 β-pentylene
or,
β-amylene
C5H10 (CH3)2C=CHCH3 isopentylene
or,
isoamylene
2.Substituted or Derived Nomenclature:
Another method of nomenclature is to consider ethylene as the parent substance and higher member is derivatives of ethylene. If the compound is mono-substituted then no difficulty arises in naming. But the compound is a di- substituted derivatives of ethylene isomerism is possible. Since the alkyl groups are of attached same or different carbon atoms. 
When the groups are attache to the same carbon atom of the olifins named as the the asymmetrical compound.
When the groups are attache to the different carbon atom of the olifins named as the the symmetrical compound.
1. CH3CH = CH2 
Methylethylene 
2. CH3ㄧCH2CH = CH2
Ethylethylene
3. CH3 ㄧ(H3C)C = CH2
as - dimetylethylene
4. CH3CH = CHㄧCH3
sym - dimethylethylene
3. I.U.P.A.C. System of Nomenclature:
According to the I.U.P.A.C. system of nomenclature, the class suffix of the olifins is - ene, and so the series becomes the alkene series.
1.The longest carbon chain containing the double bond is chosen as the parent alkene.
CH3C(CH3)2CH2CH(CH3)CH=C(CH3)CH2CH3
CH3 CH CH = C CH3

CH2
CH2
CH3 C CH3
CH3

CH3
The parent part is here base chain, it consists of 8 carbons. The base name, therefore, is to be derived from octane. 
2. The position of the double bond and side chains are indicated by numbers, the lowest number possible being given to the double bond, and this is placed before the suffix.
5 4 3
CH3 CH CH = C CH3
6 CH2 2 CH2
CH3 C CH31 CH3
8 CH3
To give the lowest number possible double bonded carbon is numbered 3. 
The name of which is obtained by changing the suffix - ane of the corresponding alkane into - ene.
There are four branches:
One methyl branch on carbon atom number 3, three methyl branches on 5th and 7th carbon atoms. These are to be indicated as prefixes to the base name. Their names with their locants are, 3,  5, 7, 7 - trimethyl
Hence the full name is,
3, 5, 7, 7 - tetramethyl - 3 - octene

What are the names of the following compounds in the IUPAC system ? 
(i) CH3 - CH2 - CH = CH2 
(ii) C(CH3)2 = CH2 
(iii) CH3 - CH = C(CH3) - CH2 - CH3
(iv) CH2 = C(C2H5) - CH(CH3)2

(i) CH3 - CH2 - CH = CH2 
but-1-ene 
(ii) C(CH3)2 = CH2 
2-methylprop-1-ene 
(iii) CH3 - CH = C(CH3) - CH2 - CH3
3-methylpent-2-ene
(iv) CH2 = C(C2H5) - CH(CH3)2
2-ethyl-3-methylbut-1-ene

Write out the (ignoring stereo-chemistry) of the isomeric pentenes, and name them by the IUPAC system. Give the structures of the products formed from each on Ozonolysis.

The Molecular formula of the pentene is C5H12.
Now take each one in turn and introduce one double bond, starting at the least substituted end and shifting the double bond inwards.
(i) CH3CH2CH2CH=CH2
pent-1-ene 
(ii) CH3CH2CH=CHCH3
pent-2-ene
(iii) CH3CH(CH3)CH=CH2
3-methylbut-1-ene
(iv) CH3C(CH3)=CHCH3
2-methylbut-2-ene
(iv) CH3CH2C(CH3)=CH2
2-methylbut-1-ene
The product obtained from the ozonide depends on the nature of the reagents used. Here we small use of Zn and acid to give aldehyde and/or ketones.
CH3CH2CH2CH=CH2
CH3CH2CH2CHO + HCHO
CH3CH2CH=CHCH3
CH3CH2CHO + CH3CHO
CH3CH(CH3)CH=CH2
CH3(CH3)CHCHO + HCHO
CH3C(CH3)=CHCH3
CH3(CH3)C=O + CH3CHO
CH3CH2C(CH3)=CH2
HCHO + CH3COCH2CH3



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-enea

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)