Hydroboration Oxidation of Alkenes

Alkenes react with diborane (B2H6) and form trialkyl boranes.
Trialkylboranes then react with H2O2 and give alcohols.

Due to this reaction Propene is converted to alcohol

H3C-CH=CH2 is converted to H3C-CH2-CH2OH

The final result of this raction is the addition of a water molecule to the double bond of the molecule in a direction opposite to that of Markownikov rule.

This reaction was discovered by Herbert C. Brown, who was awared Nobel prize in 1980 for the synthetic use of the reaction.

IIT JEE Chemistry - Study Guide - Study Plan - 15. Hydrocarbons

15.1 Classification of Hydrocarbons
15.2 Alkanes
15.3 Nomenclature of Alkanes
15.4 Conformations in Hydrocarbons
15.5 Preparation and Properties of Alkanes

Chemistry of Alkenes
15.6 Nomenclature of Alkenes
15.7 Isomerism in Alkenes
15.8 Stability of of Alkenes
15.9

Chemistry of Alkynes


15.10 Isomerism in Alkynes
15.11 Preparation and Properties of Alkynes

Chemistry of Alkadienes

15.12 Dienes
15.13 Stability of Conjugated Dienes
15.14 Delocalization of Electrons
15.15 Electrophilic addition to Conjugated Dienes

Chemistry of Aromatic Hydrocarbons

15.16 Arenes or Aromatic Hydrocarbons
15.17 Nomenclature
15.18 Stability and Structure of Benzene
15.19 Isomerism in Arenes
15.20 Aromaticity (Huckel Rule)
15.21 Sources of Aromatic Hydrocarbons
15.22 Preparation of Benzene and Its Homologues
15.23 Properties of Benzene and Its Homologues
15.24 Mechanism of Electrophilic Substitution Reactions of Benzene
15.25 Directive Influence of Substituents and Their Effect on reactivity
15.26 Polynuclear Hydrocarbons

Chemistry of Petroleum and Petrochemicals

15.27 Petroleum and Composition of Crude Oil
15.28 Fractional Distillation of Crude Oil
15.29 Quality of Gasoline – Octane Number
15.30 LPG and CNG
15.31 Cracking and Reforming
15.32 Petrochemicals

Conceptual Questions with Answers: 15
Additional Numerical Problems for Practice:
Revision Exercises
Very Short Answer questions 45
Short Answer Questions 62
Long Answer Questions 10

Competition File
Numerical Problems
Objective Questions: 65
Fill in the blanks: 15
True or False: 18

Study Plan


Day 1

15.1 Classification of Hydrocarbons
15.2 Alkanes
15.3 Nomenclature of Alkanes
15.4 Conformations in Hydrocarbons

Day 2
15.5 Preparation and Properties of Alkanes (Preparation)

Day 3

15.5 Contd. Properties of alkanes

Day 4
Revision
Practice problems 15.3 to 15.12

Day 5

Chemistry of Alkenes
15.6 Nomenclature of Alkenes
15.7 Isomerism in Alkenes
PP. 15.13, 15.14

Day 6
15.8 Stability of of Alkenes
15.9 Preparation and Properties

Day 7
Revision
PP. 15.15 to 15.20
Conceptual Questions 5,8,9,11,


Day 8
Chemistry of Alkynes

15.10 Isomerism in Alkynes
15.11 Preparation and Properties of Alkynes

day 9
Revision
PP. 15.21 to 15.26

Day 10

Chemistry of Alkadienes

15.12 Dienes
15.13 Stability of Conjugated Dienes
15.14 Delocalization of Electrons
15.15 Electrophilic addition to Conjugated Dienes

Day 11

Chemistry of Aromatic Hydrocarbons

15.16 Arenes or Aromatic Hydrocarbons
15.17 Nomenclature
15.18 Stability and Structure of Benzene
15.19 Isomerism in Arenes

Day 12
15.20 Aromaticity (Huckel Rule)
15.21 Sources of Aromatic Hydrocarbons
15.22 Preparation of Benzene and Its Homologues

Day 13
15.23 Properties of Benzene and Its Homologues
15.24 Mechanism of Electrophilic Substitution Reactions of Benzene
15.25 Directive Influence of Substituents and Their Effect on reactivity
15.26 Polynuclear Hydrocarbons

Day 14

Chemistry of Petroleum and Petrochemicals

15.27 Petroleum and Composition of Crude Oil
15.28 Fractional Distillation of Crude Oil
15.29 Quality of Gasoline – Octane Number
15.30 LPG and CNG
15.31 Cracking and Reforming
15.32 Petrochemicals

Day 15

Examples 15.13 to 15.28

Revision period


Day 16

Conceptual Questions with Answers: 15

Day 17


Revision Exercises: Very Short Answer questions 1 to 30

Day 18
Revision Exercises: Very Short Answer questions 31 to 45
Revision Exercises: Short Answer questions 1 to 15

Day 19

Revision Exercises: Short Answer questions 16 to 45

Day 20
Revision Exercises: Short Answer questions 46 to 62
Competition File-Objective Questions: 1 to 15

Day 21
Competition File-Objective Questions: 16 to 45

Day 22
Competition File-Objective Questions: 46 to 65
Competition File-Fill in the blanks: 15

Day 23

Competition File-True or False: 18

Day 24 - 30
Revision and test paper problem solving

Alkenes- Study Guide - IIT JEE

Preparation, properties and reactions of alkenes: Physical properties of alkenes (boiling points, density and dipole moments); Acid catalysed hydration of alkenes (excluding the stereochemistry of addition and elimination); Reactions of alkenes with KMnO4 and ozone; Reduction of alkenes; Preparation of alkenes by elimination reactions; Electrophilic addition reactions of alkenes with X2, HX, HOX and H2O (X=halogen); Metal acetylides.

Alkenes - Revision Facilitator

Alkenes Introduction

Nomenclature

Isomerism

Methods of Preparation

Partial reduction of alkynes

Dehydrohalogenation of alkyl halides.

Dehalogenation of vicinal dihalides (dihalogen compounds)

Dehydration of alcohols

Kolbe's electrolytic method (alkenes from salts dicarboxylic acids)

Preparation of alkenes by elimination reactions

Alkenes - Physical properties

1. State

2. Melting points

3. Boiling points

4. Dipole moments

5. Solubility


Alekenes Chemical Properties

Electrophilic addition reactions

Halogen acids
Water
Hypohalous acid
Sulphuric acid

oxidation,

Combustion
Hydroboration oxidation
Potassium permanganate - cold and hot
Catalytic oxidation
Reaction with Ozone

Reduction

Polymerisation
of ethene
of vinyl chloride
of styrene
of tetrafluoroethylene



Replacement reactions.

Addition of sulphuric acid

Acidity of alkenes

Acid catalysed hydration of alkenes

Reactions of alkenes with KMnO4

Reactions of Alkenes with Ozone

Reduction of Alkenes

IIT JEE Revision Ch. 22 Alkenes - Core Chapter Points

Syllabus

Preparation, properties and reactions of alkenes:

Preparation of alkenes by elimination reactions;
Physical properties: boiling points, density and dipole moments
Acidity;
Acid catalysed hydration of alkenes(excluding the stereochemistry of addition and elimination);
Reactions of alkenes with KMnO4 and
Reactions of alkenes with ozone;
Reduction of alkenes;
Electrophilic addition reactions of alkenes with X2, HX, HOX and H2O (X=halogen);
---------------

Introduction

Alkenes are unsaturated hydrocarbons having carbon-carbon double bond(C=C) in their molecules.

Their general formula is C-nH-2n.
The simplest alkene is ethene, C-2H-4

Methods of Preparation

1. Dehydrohalogenation of alkyl halides.

2. Dehydration of alcohols

3. Dehalogenation of vicinal dihalides

Physical properties

State: Ethene, propene and butene are gases at room temperature. From pentene onwards till alkenes having 18 carbon atoms, they are liquids. Still higher members of the family are solids.



Addition of water

water adds to alkenes in the presence of mineral acids. Hence it is termed catalytic hydration of alkenes. Addition occurs in accordance with Markownikov's rule. We get alcohols from this addition.

Oxidation with potassium permanganate (specially mentioned in syllabus)
Alkenes react with cold dilute potassium permanganate solution(alkaline) to form 1,2-diols called glycols. The glycols contain two -OH groups on adjacent carbon atoms.

Reaction with ozone

Ozone, O3, is an allotrope of oxygen that adds rapidly to carbon-carbon double bonds. Since the overall change in ozonolysis is more complex than a simple addition reaction, its mechanism has been extensively studied. Reactive intermediates called ozonides have been isolated from the interaction of ozone with alkenes, and these unstable compounds may be converted to stable products by either a reductive workup (Zn dust in water or alcohol) or an oxidative workup (hydrogen peroxide).


Reduction
Addition of hydrogen to a carbon-carbon double bond is called hydrogenation. The overall effect of such an addition is the reductive removal of the double bond functional group.

Polymerisation
-- polymerisation of ethene
-- polymerisation of vinyl chloride
-- polymerisation of styrene

Addition of hydrogen to a carbon-carbon double bond is called hydrogenation. The overall effect of such an addition is the reductive removal of the double bond functional group.

IITJEE Revision Alkenes Introduction

Hydrocarbons having at least one double bond

Their general formula is C_nH_2n.

The carbon carbon double bond is made of a σ bond and a π bond.

The bond dissociation enthalpy of double bond is 610 kJ mol^-1.

The carbon-carbon bond length in ethene is 134 pm.

Revision Alkenes Nomenclature

Ethene
Propene
Butene
Pentene

The rules used for naming alkenes under IUPAC system are similar to alkanes with some special ones.

i)The suffix to be used is -ene.

ii)The longest continuous chain should include both the carbon atoms of the double bond.

iii) The chain is to be numbered from the end that gives the lower number to the first carbon atom of the double bond.

iv) If there are two or more double bonds, the suffix used is -adiene or -atriene.


Some more examples

Buta-1,3-diene

Revision Alkenes Isomerism

Isomerism in alkenes

1. Structural isomerism: Alkenes show chain isomerism and position isomerism.

a. Chain isomerism: C-4H-8 exists as two chain isomers, n-Butene and Isobutene.
b. Position isomerism: The isomers differ in the position of the double bonds. C-4H-8 can have the double bond as the terminal bond or it can be in the middle. The one with the terminal bond is n-Butene and the one with the double bond in the middle is but-2-ene. Thus butane with the structural formula C-4H-8 has three structural isomers.

2. Geometrical isomerism: Alkenes exhibit geometrical isomerism.

Molecules of the type C₂A₂B₂ are available as geometrical isomers. The two atoms attached to the same carbon atom are different. In this symbols, A and B are different.

The molecule in which similar atons or groups lie on the same side of the double bond is called cis-isomer. Both As are attached to the carbon on one side of the double bond. Simiarly two Bs.


If they are attached on the opposite sides, it is a trans-isomer

Examples
CH₃HC=CHCH₃ exhibits geometrical isomerism.

As CH₃ and H are different groups

Maleic acid and Fumaric acid are geometrical isomers.
HOOCHC=CHCOOH

Cis isomer is maleic acid and trans isomer is fumaric acid.

If they are attached on the opposite sides, it is a trans-isomer

Revision Alkenes - Methods of Preparation

From
alkynes
alkyl halides
dihalogen derivatives (dihalides)
alcohols
potassium salts of dicarboxylic acids

1. Partial reduction of alkynes
Alkynes can be reduced to alkenes using palladium-charcoal catalyst in catalytic hydrogenation (addition of hydrogen). The reduction (addition of hydrogen) can also be done using sodium in liquid ammonia.

2. Dehydrohalogenation of alkyl halides.
From alkyl halides, an atom of hydrogen and an atom of halogen are removed by treating it with alcoholic KOH.

As a hydrogen atom and a halogen atom are removed from the molecule, the reaction is called dehydrohalogenation.

3. Dehalogenation of vicinal dihalides (dihalogen compounds)
Vicinal dihalides are converted to alkenes by heating with zinc dust in ethyl alcohol.


4. Dehydration of alcohols
Alcohols are heated with sulphuric acid or phosphoric acid at about 443 K. A H2O molecule gets removed from the alcohol giving alkene.

The reaction occurs in multiple steps.

5. Kolbe's electrolytic method (alkenes from salts dicarboxylic acids)
The electrolysis of potassium salts of dicarboxylic acids gives alkenes.

Revision Preparation of alkenes by elimination reactions

One of the principal methods for alkene synthesis in the laboratory is the elimination of alkyl halides, alcohols and similar compounds.

When an alkyl halide is used, the reaction is called a dehydrohalogenation.

Alkenes can be synthesized from alcohols via dehydration, in which case water is lost. For example, the dehydration of ethanol produces ethene:
C2H5OH --> C2H4 + H2O

IIT JEE Revision - Alkenes - Physical properties

1. State: Ethene, propene and butene are gases at room temperature.
From pentene onwards till alkenes having 18 carbon atoms, they are liquids.
Still higher members of the family are solids.

2. Melting points: Alkenes have higher melting points than the corresponindg alkanes. Intermolecular forces of attraction in double bond are stronger.

The melting points increase with molecular mass of alkenes.

Among isomers, trans-alkenes have higher melting points than their corresponding cis alkenes.

3. Boiling points: The boiling points increase with increase in carbon atoms. The branched chain alkenes have lower boiling points than the corresponding straight chain alkenes.

Among geometric isomers, cis-alkenes have higher boiling points than the corresponding trans-isomers.

4. Dipole moments: Alkenes are weakly polar. Their dipole moments are higher than those of alkanes.

In case of geometricl isomers, symmetrical trans alkenes are nonpolar and have zero dipole moment due to symmetry. But unsymmetrical trans alkenes are polar. Cis isomers are polar and have dipole moments.

Unsymmetrical terminal alkenes such as propene and but-1-ene have some dipole moment.

5. Solubility: Alkenes are lighter than water. These are insoluble in water. They readily dissolve in organgic solvents like alcohol, benzenes, ether, carbon tetrachloride etc.

Alekenes Chemical Properties

Alkenes undergo electrophilic addition reactions.
They also participate in oxidation, polymerization and some replacement reactions.

Addition

Halogen acids
Water
Hypohalous acid
Sulphuric acid

Oxidation reactions

Combustion
Alkens burn in oxygen or air to give carbon dioxide , water and large amount of heat.

Hydroboration oxidation
Potassium permanganate - cold and hot
Catalytic oxidation
Reaction with Ozone

Reduction

Polymerisation
of ethene
of vinyl chloride
of styrene
of tetrafluoroethylene


Addition of sulphuric acid

Cold and concentrated sulphuric acid adds to alkenes forming alkyl hydrogen sulphate.

This adds to water to give alcohol. Actually the final reaction is same as acid catalysied hydration of alkenes.

IIT JEE Revision - Acidity of alkenes

Alkynes are weakly acidic.

Compared to alkynes, alkenes are still weak in acidic behaviour. Practically they do not shown any acidic behaviour.

The acidic property is because of s character.

Alkynes are sp hybridized. (50% s character)
Alkenes are sp² hybridized (33% s character)
Because of lesser s character, these carbon atoms are much less electronegative than the carbon atoms taking part in the triple bond.

Therefore, the release of H⁺ ion from an alkene molecule is difficult and they do not show acidic character.

In term of K_a

Alkynes have 10^-26 and

alkenes have 10^-36.

Revision Acid catalysed hydration of alkenes

water adds to alkenes in the presence of mineral acids. Hence it is termed catalytic hydration of alkenes. Addition occurs in accordance with Markownikov's rule. We get alcohols from this addition.

Ethene gives ethanol
Propene gives Propan-2-ol as the major product.

IIT JEE Revision - Reactions of alkenes with KMnO4

Oxidation of Alkenes with potassium permanganate

Alkenes react with cold dilute potassium permanganate solution(alkaline) to form 1,2-diols called glycols. The glycols contain two -OH groups on adjacent carbon atoms.

Ethene gives ethanediol (glycol).
Propene gives propane-1,2 diol (propylene glycol)

The alkaline potassium permanganate (known as Baeyer's reagent) has bright pink colour. Glycols have no colour and the pink colour will disappear after reaction. So Baeyer's test is used to find the presence of double bond.

Reaction with hot potassium permanganate: In this case, the alkene gets split up at the double bond forming acids or ketones.

=C(R-R') carbon attached to two alkyl groups gets oxidized to ketone
=C(R-H) carbon attached to one alkyl group and one hydrogen gets oxidized to carboxylic acid
=C(H-H) carbon attached to two hydrogen atoms gets oxidized to CO2

IIT JEE Revisin - Reactions of Alkenes with Ozone

Ozone, O3, is an allotrope of oxygen that adds rapidly to carbon-carbon double bonds.

Reactive intermediates called ozonides form from the interaction of ozone with alkenes.

These unstable compounds may be converted to stable products by either a reductive workup (Zn dust in water or alcohol) or an oxidative workup (hydrogen peroxide).

Reductive workup gives an aldehyde product when hydrogen is present on a double bond carbon atom, whereas oxidative workup gives a carboxylic acid or carbon dioxide in such cases.

Ozonide formation: a process that is believed to involve initial syn-addition of ozone, followed by rearrangement of the extremely unstable molozonide addition product.

In the reaction with ozone, carbon-carbon double bonds breaks and carbon-oxygen double bonds form in the two separate compounds.

The double bond of the original molecule is obtained by joining the carbon atoms of the two carbonyl compounds which are the final products of this reaction.

Hence ozonolysis helps in locating the double bond in the alkene.

IIT JEE Revision - Reduction of Alkenes

Alkenes react readily with hydrogen in the presence of finely divided nickel, platinum or palladium to give alkanes.

This reaction is used in the manufacture of vanaspati ghee from vegetable oils.

Ethene - Ethane
Propene - Propane

IIT JEE Revision - Electrophilic Addition Reactions of Alkenes with X2, HX, HOX and H2O (X=halogen)

Reaction mechanism

Reaction takes place in two steps.

Step 1. Attacking molecule gets partially polarised and as it becomes closer to the pi bond of the double bond, the electron cloud of the pi bond repels the electron cloud of the attacking molecule further. As a result, the nearer end of the attacking molecule acquires partial positive charge. At the same time, electromeric effect comes into operation in the double bond and the pi electron pair shifts to one of the carbon atoms making it negatively charged. Thus the partially positively charge atom of the attacking molecule attacks the negatively charged carbon of the alkenes and a new bond is formed. This leaves the other carbon atom with positive charge and also the other atom of the attacking molecule with negative charge.

Step 2: The negatively charged atom of the attacking molecule reacts with positively charged carbon of the alkene to complete the formation of addition product.



Electrophilic Addition Reactions of Alkenes with X2, HX, HOX and H2O (X=halogen)


Halogens (particularly chlorine and bromine) react with alkenes in the presence of an inert solvent (e.g. CCl4) to form dihalogen derivatives:

The reaction with flourine is explosive whereas iodine reacts very slowly.

Alkenes react with halogen acids (HCL, HBr, or HI) to form alkyl halides.

In these reactions one part of the molecule attaches itself to one carbon atom of the double bond whereas the other part to the second carbon atom of the double bond.

However,if the alkene is unsymmetrical, then two products are possible depending upon the carbon atom to which the halogen atom is attached.

Markonikov rule: during the addition across unsymmetrical multiple bond, the negative part of the attacking reagent joins with the carbon atom which carries smaller number of hydrogen atoms while the positive part goes to the carbon atom with more hydrogen atoms.

Due to fact that the reaction proceeds according to Markonikov's explanation, addition of HBr to Propene gives 2-Bromopropene as the major product up to 90%.

Exception to Markonikov rule - Kharasch effect - Peroxide effect: During the addition of HBr to an unsymmetrical alkene in the presence of organic peroxids (e.g., benzoyl peroxide), Br atom will join to the carbon carrying more hydrogen atoms while H atom will go to the other carbon atom.

Only HBr shows peroxide effect. HF, HCl and HI do not exhibit peroxide effect.

Alkenes react with hypohalous acids (HOX) or halogen Cl2 or Br2 in the presence of H2O to give halohydrins. In this reactin, markonikov;s rule is followed and halogen is the positive aprtg and OH is the negative part.

Water adds to alkenes in the presence of mineral acids (catalytic hydration of alkenes). Addition occurs in accordance with Markownikov's rule and we get alcohols from this addition.

Ethene gives ethanol
Propene gives Propan-2-ol as the major product.