Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Chemistry Class 11

Created by: Mohit Rajpoot

Class 11 : Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

The document Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev is a part of the Class 11 Course Chemistry Class 11.
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Alkenes:

1. Introduction

Alkenes are hydrocarbons with carbon-carbon double bonds, Alkenes are sometimes called olefins, a term derived from olefinic gas, meaning "oil forming gas". Alkenes are among the most important industrial compound and many alkenes are also found in plants and animals. Ethylene is the largest - volume industrial organic compound, used to make polyethylene and a variety of other industrial and consumer chemicals.

2. Structure and bonding in Alkenes:

(1) Alkenes are unsaturated hydrocarbons having at least one double bond.

(2) They are represented by general Formula (G.F.) CnH2n (one double bond)

(3) In Ethene, C = C bond length is 1.34 Å.

(4) Its bond energy is 146 kcal.mol-1

(5) The hybridization of (C = C) alkenic carbon is sp2

(6) The π e- cloud is present above and below the plane of s-bonded skeleton.

(7) They are also known as olefins since ethene, the first member of the homologous series forms oily liquid substance when treated with halogens.

(8) Compounds may exist as conjugated polyenes or as cumulated polyenes or as isolated polyenes.

Note : That angle a > b since repulsion due to p electrons (double bond - single bond repulsion > single bond single bond repulsion according to VSEPR theory.

Ex.1 Write IUPAC names of:

 

(a) Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev (b) Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

Ans. (a) 2, 3-Dimethylcyclohexene

(b) 1-(2-butenyl) cyclohex -1-ene

Ex.2 Give the structure for each of the following:

(a) 4-Methyl-1, 3-hexadiene

(b) 1-Isopropenylcyclopentene

Ans. (a) Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev (b) Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev


3. Physical Properties of Alkenes / Hydrocarbons:

Table : III

 

 

Physical properties

Homologous series

Isomers

1.

Physical state

C1 - C3 gases,

C4 - C20 liquids, >C20 : solids

 

2.

Dipole moment (n)

 

cis > trans

3.

Polarity

-

cis > trans (for Cab = Cab type of alkenes)

4.

Melting point

decreases as the homologous series increase.

trans > cis

(due to more packing capacity)

5.

Boiling point

increases as the homologous series increase. 

cis > trans

# branching decreases B.P.

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Polarity increases, boiling point increases

6.

Solubility

Practically insoluble in water but fairly soluble in non-polar solvents like benzene-petroleum ether, etc.

cis > trans

Polarity increases, solubility in polar solvents increases.

7.

Stability

 

trans > cis
(cis isomers has more Vander Waals repulsion)

 

4. Laboratory test of Alkenes:

Table - IV

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev


5. Methods of preparation of alkenes:

(I) BY PARTIAL REDUCTION OF ALKYNES:

(a) By Catalytic Hydrogenation of Alkynes in presence of poisoned catalyst(A Syn Addition of Hydrogen : Synthesis of cis-Alkenes: This is performed by:

(i) Lindlar's catalyst : Metallic palladium deposited on calcium carbonate with lead acetate and quinoline.

(ii) P-2 catalyst (Ni2B: nickel boride)

General Reaction: R - C º C - R Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

Mechanism of hydrogenation :

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Steps : The reactant alkyne molecules and hydrogen molecules get adsorbed at the surface of metal catalyst. It is chemical adsorption (chemisorption).

In this state, the reactants lie very close to each other and so the hydrogen atoms start forming bonds with carbon. Two hydrogen atoms are added to two triply bonded carbon atom from the same side of p bond and a cis or syn addition product is formed. The product alkene now escapes away from the surface of the catalyst. Quinoline occupies the metal surface inhibiting further reduction to alkanes. Quinoline therefore is called catalyst poison and palladium is called deactivated catalyst or poisoned catalyst.

 

e.g. Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

(b) Birch Reduction : (Anti Addition of Hydrogen : Synthesis of trans-Alkenes)

General Reaction: Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Mechanism : Reagents Na(or Li, K) + liq NH3 → Na+ + e (solvated electron)

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev
Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Note : This process of reduction is not eligible when terminal alkenes are taken. (R - C º CH) because terminal alkenes form sodium salt with Na metal.

CH3 - C = CH + Na / NH3 → CH3 - CH = C- Na+ + [H]+

Ex.3 Identify the reagent for following synthesis.

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Ans. H2 / Lindiar's catalyst.

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

Ex.4 Identify the products in the following reaction :

 

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Ans. Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

(II) BY DEHALOGENATION OF VICINAL DI-HALIDES:

There are two types of dihalides namely gem (or geminal) dihalides in which the two halogen atoms are attached to the same carbon atom and vicinal dihalides in which the two halogen atoms are attached to the adjacent carbon atoms.

Dehalogenation of vicinal dihalides can be effective either by NaI in acetone or zinc in presence of acetic acid or ethanol.

General Reaction:

(i) Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

(ii) CH3 - CHBr - CH2Br Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev CH3 - CH = CH2

Mech.

With NaI in acetone :

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

It involves an anti elimination of halogen atoms.

Remarks:

(1) Both are E2 elimination.

(2) Both are stereospecific anti elimination.


(III) DEHYDRO-HALOGENATION OF ALKYL HALIDES:

Dehydro halogenation is the elimination of a hydrogen and a halogen from an alkyl halide to form an alkene.

Dehydro halogenation can take place by Eand E2 mechanism.

(i) Hot alcoholic solution of KOH EtO- / EtOH.
(ii) NaNH2.
(iii) t-BuO-K+ in t-BuOH. 

(i) Dehydrohalogenation by the E2 mechanism: Second-order elimination is a reliable synthetic reaction, especially if the alkyl halide is a poor SN2 substrate. Edehydrohalogenation takes place in one step, in which a strong base abstracts a proton from one carbon atoms as the leaving group leaves the adjacent carbon.

General reaction :

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

e.g. Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Here β - H is eliminated by base hence called β-elimination following Saytzeff rule.

i.e, (Highly substituted alkene is major product). It also involves an anti elimination of HX.

 

e.g. 

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

e.g.

 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

e.g.

 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

e.g.

 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

(ii) Formation of the Hoffmann product:

Bulky bases can also accomplish dehydro halogenation that do not follow the saytzeff rule. Due to steric hindrance, a bulky base abstracts the proton that leads to the most highly substituted alkene. In these cases, it abstracts a less hindered proton, often the one that leads to formation of the least highly substituted product, called the Hoffmann product.

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Stereospecific E2 reactions:

The E2 is stereospecific because it normally goes through an anti and coplanar transition state. The products are alkene, and different diastereomers of starting materials commonly give different diastereomers of alkenes.

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Ex.5 What alkyl halide would yield each of the following pure alkene on reaction with alcoholic KOH ? 

(i) Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev 

(ii) CH3 - CH2 - CH2 - CH = CH2 

Ans. (i)

 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev 

(ii) CH3CH2CH2CH2 CH2Cl

(iii)

 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Ex.6 What are the various product due to loss of HBr from  Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Ans.  Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev


(IV) DEHYDRATION OF ALCOHOLS

Alcohols when heated in presence of following reagents undergo loss of water molecule and form alkenes. The elimination is β elimination.

(i) H2SO4 / 160ºC

(ii) H3PO4 / Δ

(iii) P2O5 / Δ

(iv) Al2O3 / 350ºC undergo loss of water molecule and form alkenes.

General Reaction: RCHCH2OH Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev R - CH = CH2  H2O

e.g. Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

(V) BY PYROLYSIS OF ESTERS:

Thermal cleavage of an ester involves formation of a six membered ring in the transition state leading to the elimination of an acid leaving behind an alkene.

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

As a direct consequence of cyclic transition state, both the leaving groups namely proton and carboxylate ion are eliminated from the cis position. This is an example of syn elimination.


(VI) BY HOFFMANN ELIMINATION METHOD:

Alkenes can be prepared by heating quaternary ammonium hydroxide under reduced pressure at a temperature between 100ºC and 200ºC.

Less substituted alkenes are formed as major product in this case, which are defined as Hoffmann alkenes.

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev


(VII) BY WITTIG REACTION:

The aldehydes and ketones are converted into alkenes by using a special class of compounds called phosphorus ylides, also called Wittig reagents.

The Triphenyl group of phosphorane has a strong tendency to pull oxygen atom of the aldehyde or ketone via a cyclic transition state forming an alkene.

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

(R, R', R" and R"' may be hydrogen or any alkyl group)

e.g.

 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

Ex.7 Complete the following reaction :

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Ans. Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

Ex.8 Identify the (X), (Y), and (Z) in the following reactions

(i) PhCH2Br CH3 -Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev - CH3 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev (X)

(ii) CH3I PhCOCH3 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev (Y)

(iii) PhCH2Br PhCH = CHCHO Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev (Z)

Ans. (X) = Ph - CH = C(CH3)2

(Y) = Ph - C(CH3) = CH2

(Z) = Ph - CH = CH - CH = CH - Ph

 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev


6. Chemical reactions of alkenes:

(I) CATALYTIC HYDROGENATION OF ALKENES : (HETEROGENEOUS HYDROGENATION)

Hydrogenation : The function of catalyst

Hydrogenation of a alkene is exothermic reaction (DHº = - 120 kJ mol-1)

R - CH = CH - R H2 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev R - CH2 - CH2 - R heat

As a consequence ,both hydrogen atoms usually add from the same side of the molecule. This mode of addition is called a syn addition.

Hydrogenation of an alkene is formally a reduction, with addition of H2 across the double bond to give an alkane.

The process usually requires a catalyst containing Pt, Pd or Ni.

e.g. CH3 - CH - = CH - CH3  H2 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev CH3 - CH2 - CH2 - CH3

e.g. Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

Ex.9 Complete the following reactions :

CH3CH = CH2  H2 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev ?

Sol. CH3CH2CH3

(II) ELECTROPHILIC ADDITION REACTIONS :

Mechanism:

Step 1 : Attack of the electrophile on p bond forms a carbocation.

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Step 2 : Attack by a nucleophile gives the product of addition

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

(i) Acid-Catalyzed Hydration of Alkenes

Alkenes add water in the presence of an acid catalyst to yield alcohols. The addition takes place with Markovnikov's rule. The reaction is reversible, and the mechanism for the acid-catalyzed hydration of an alkene is simply the reverse of that for the dehydration of an alcohol.

The carbocation intermediate may rearrange if a more stable carbocation is possible by hydride or alkanide migration. Thus, a mixture of isomeric alcohol products may result.
 

General Reaction: Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Mech. 

Step 1. : Protonation of the double bond forms a carbocation-

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Step 2 : Nucleophilic attack by water-

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Step 3 : Deprotonation to the alcohol- 

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

e.g.

 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

e.g.

 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Ex.10 Identify the product in following reaction:

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev
 

Ans. Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

(ii) (a) Oxymercuration - Demercuration:

Alkenes react with mercuric acetate in a mixture of water and tetrahydrofuran (THF) to produce (hydroxyalkyl) mercury compounds. These can be reduced to alcohols with sodium borohydride and water :

Oxymercuration:

General Reaction: Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

In the oxymercuration step, water and mercuric acetate add to the double bond; in the demercuration step, sodium borohydride reduces the acetoxymercury group and replaces in with hydrogen. Then net addition of H -and -OH takes place with Markovnikov's rule and generally takes place without the complication of rearrangements.

e.g.

 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

(b) Alkoxymercuration - demercuration:

General reaction: Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

e.g. Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

Ex.11 Supply the structures for (X) and (Y) in the following two - step reaction: :

C3H7CH = CH2 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Sol. (X) = C3H7CH(OH)CH2-HgOAC (Y) = C3H7CH(OH)CH3

(An organomercurial alcohol) 

Ex.12 Identify final product in the following :

(a) Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

(b) Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

Ans. (a) Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev (b) Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

(iii) Hydroboration-oxidation (SYN ADDITION): 

General Reaction:
  Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

An alkene reacts with BH3 : THF of diborane to produce an alkylborane. Oxidation and hydrolysis of the alkylborane with hydrogen peroxide and base yields an alcohol.

e.g.

 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Oxidation:

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

In the first step, boron and hydrogen undergo syn addition to the alkene in the second step, treatment with hydrogen peroxide and base replaces the boron with -OH with retention of configuration. The net addition of -H and -OH occurs with anti Markovnikov regioselectivity and syn stereoselectivity. Hydroboration -oxidation therefore, serves as a useful regiochemical complement to oxymercuration demercuration.
 

e.g.

 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

e.g. 

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

(i) Hydration with dil. H2SO4 proceeds via carbocation rearrangement.

(ii) Hydration with Hg(OAc)2, H2O, following by NaBH4 proceeds via Markonikov's rule.

(ii) Hydration with (BH3)2 followed by H2O2 / OH- proceeds via Anti Markonikov's rule.


(iv) Addition of hydrogen halides:

General Reaction:

 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Note:(1) Anti-Markovnikov addition is valid only for HBr in presence of peroxide and light only.
(2) HF, HCl and HI give only polar addition and give Markovnikov product only.

 

e.g. Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

e.g. Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

Ex.13 Predict the major products of the following reactions and propose mechanism to support your predictions.

(A) Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

(B) Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev CH3 - CH2 - O - O - CH2 - CH3 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev


 

(C) Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Sol. (A) Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev 
(B) Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev 
(C) Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

 

Ex.14 Identify the products in the following reactions :

(a) F3C - CH = CH2  HCl Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev 
(b) O2N - CH = CH2  HCl Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev
(c) CH3O - CH = CH2  HCl Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev 
(d) PhCH = CHCH3  HCl Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev
(e) Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Q.6 Give the products of the following reactions : -

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Q.7 Give the reactant (alkene) of the following products.

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

(v) Addition of halogen

Halogen add to alkenes to form vicinal dihalides.

General Reaction: Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

(X2 = Cl2, Br2)

The nucleophile attacks the electrophilic nucleus of one halogen atom, and the other halogen serves as the leaving group, departing as halide ion. Many reactions fit this general pattern.

Note : (i) F2 is not added because F+  is never generated. Morever reaction is explosive giving CO2 & H2O.

(ii) I2 is not added because reaction is reversible with equilibrium in backward direction.

(iii) Reaction with bromine is basis for test of alkenes.

(iv) Halogen addition is stereospecific anti addition.

(v) Halogens can also be added in presence of sun light and give free radical addition.

(Reactivity of halogen addition in sunlight is F2 (explosive) > Cl2 > Br2 > I2)

Mech.

Step-1 Formation of a halonium ion

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

Step-2 Opening of the halonium ion

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

X. -attacks from the back side of halonium ion.

e.g.

 Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

e.g. 

Alkenes (Properties and Nomenclature) Class 11 Notes | EduRev

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