- Alkenes are unsaturated aliphatic hydrocarbons having one or more carbon-carbon double bonds i.e. C=C. Since alkenes form oily products with halogens, they are called olefins.
- They contain two hydrogen atoms less than the corresponding alkanes. The general formula for alkenes is CnH2n, Where n = number of carbon atoms.
- The aliphatic hydrocarbons containing two or three carbon-carbon double bonds are called alkadienes and alkatrienes respectively.
- Example of alkenes:
- CH2CH2 Ethene or Ethylene
- CH3 – CH=CH2 Propene or Propylene
Nomenclature of Alkenes:
Structural Isomerism in Alkenes:
- Chain isomerism is due to the difference in the structure of carbon chain in alkenes.
- It is due to the difference in the position of the double bond in the same carbon chain
Geometric Isomerism in Alkenes:
- The isomerism which is due to restricted rotation about carbon-carbon double bond is known as geometrical isomerism or cis-trans isomerism.
- In alkenes, it is seen that free rotation about the carbon-carbon single bond is possible. However, in alkenes, such a rotation is not possible because any rotation about the axis for carbon-carbon (sigma) bond would decrease the overlapping of orbitals and a large amount of energy necessary to do this is not available under ordinary conditions. This gives rise to restricted rotation about the carbon-carbon double bond. This restricted rotation about the carbon-carbon double bond causes the two different spatial arrangement in 2 – Butene.
- When two compounds having the same molecular formula, similar chemical structures and double bond possess different geometrical arrangements of the atoms or groups about the doubly bonded carbon atoms, the phenomenon is known as geometrical isomerism and such compounds are known as geometrical isomers.
- When the two identical atoms or groups are on the same side of the double bond the isomer is called cis isomer; while if they are on the opposite sides, the isomer is called trans – isomer.
- Cis-trans isomers have similar chemical properties but they differ in their physical properties.
Conditions for Geometrical Isomerism:
- There should be a double bond in the molecule.
- The two atoms or groups attached to each doubly bonded carbon atom should be different. Thus abC=Cab. abC=Ccd, abC=Cax can show cis-trans isomerism.
- Consider following examples
- In propene two hydrogen atoms (same atoms) are attached to one of the doubly bonded carbon atoms. Hence it can not show cis-trans isomerism.
- In 2-Methyl-but-2-ene, two methyl groups (same group) are attached to one of the doubly bonded carbon atoms. Hence it can not show cis-trans isomerism.
Examples of Geometrical Isomerism:
Comparision of Properties of Cis and Trans Isomers:
- cis-isomer is more polar than the trans isomer because in it the individual bond dipoles do not cancel each other. In trans-isomers, the individual bond dipoles almost cancel each other hence their dipole moment is almost zero.
- Due to greater polarity cis-isomers has a greater boiling point than its trans-isomer.
- Due to symmetry, the molecules of trans-isomers are closely packed and shows higher melting point than its cis isomer.
- trans-isomers are more stable than cis-isomers.
Cahn-Ingold-Prelog Priority Rules:
- For higher substituted stereoisomers, the concept of cis and trans isomers is not sufficient. Hence Cahn-Ingold and Prelog proposed priority rules using which higher substituted stereoisomers are expressed in a form called E-Z notation. If the two groups of higher priority are on the opposite sides of the double bond, then the double bond is assigned the configuration E (derived from German word entgegen means the opposite). If the two groups of higher priority are on the same side of the double bond, then the double bond is assigned the configuration Z (derived from German word zusammen means together). The rules of assigning priority are as follows.
- Locate the four atoms directly attached to the stereocenter (X). Assign priorities based on the atomic number to all four atoms. Priority 1 is assigned to the atom or group of highest atomic number, priority 4 to the lowest. Use the following sequence of elements arranged in decreasing order of atomic number for determining priority. I (Iodine) > Br (Bromine) > Cl (Chlorine) > F (Fluorine)> O (Oxygen)> N (Nitrogen) > C (Carbon) > T (Tritium) > D (Deuterium) > H (Hydrogen)
- If two or more atoms are identical (designated A and B ), Locate all the atoms directly attached to the identical atoms in questions (designated A-1, A-2, A-3 and B1, B-2, B-3). Assign priorities to all these atoms based on atomic number (1 is the highest priority, 3 the lowest). Compare the highest priority atoms, i.e. compare A-1 with B-1. If A-1 is a higher
priority atoms than B-1, then A is a higher priority than B. If A-1 and B-1 are the same atoms, then compare the second highest priority atoms directly bonded to A and B (A-2 with B-2); if A-2 is a higher priority atom than B-2, then A is higher
priority than B. If A-2 and B-2 are identical atoms, compare A-3 with B-3 and so on.
- Multiple bonds are considered as an equivalent number of single bonded atoms.
- Example – 1: 1-Bromo-1-chloro-2-iodopropene:
Priority: I > Br > Cl > C
- In the first configuration I and Br the high priority groups are on opposite sides of the double bond, hence it is E configuration and the compound is named as (E)-1-Bromo-1-chloro-2-iodopropene.
- In the second configuration I and Br the high priority groups are on the same side of the double bond, hence it is Z configuration and the compound is named as (Z)-1-Bromo-1-chloro-2-iodopropene.
- Example – 2: 2-Iodo-2-pentene
Priority: I > CH3CH2 > CH3 > C
- In the first configuration I and CH3CH2 the high priority groups are on opposite sides of the double bond, hence it is E configuration and the compound is named as (E)-2-Iodo-2-pentene.
- In the second configuration I and CH3CH2 the high priority groups are on the same side of the double bond, hence it is Z configuration and the compound is named as (Z)-2-Iodo-2-pentene.
Structure of Ethylene (Ethene):
Hybridization of Carbon in Ethylene Molecule:
- Ethene is built from hydrogen atoms (1s1) and carbon atoms (1s22s22px12py1). Carbon atom undergoes sp2 hybridization. It has (1s22s22px12py1) configuration in its ground state. It promotes one of the electrons from 2s2 pair into the empty 2pz orbital. This state has (1s22s12px12py12pz1) configuration. One s and 2 p ( px and py) mix and form three sp2 hybridized orbitals. pz orbital do not take part in the hybridization. The three sp2 hybrid orbitals arrange themselves as far apart as possible – which is at 120° to each other in a plane. The remaining pz orbital is at right angles to them.
- Two sp2 hybridized orbitals of each carbon atoms undergo axial overlap with 1s orbital of two hydrogen atoms to form sigma (σ) bonds. Thus there are 4 C-H overlaps (sigma bonds). The remaining sp2 hybrid orbital of each carbon overlap axially to form a C-C bond (sigma bond). Unhybridized pz orbitals of the two carbons overlap laterally to form C-C (pi bond). Thus between two carbons, there is a double bond ( 1 sigma bond and another pi bond).
Dot and Dash Structure of Ethylene Molecule:
- In Ethene, each carbon atom utilizes two electrons in making two covalent bonds with two hydrogen atoms, and one electron in making one covalent bond with another carbon atom. The fourth electron of each carbon forms the double bond between the two carbon atoms. This is the conventional formula for Ethene.
Bond Lengths and Bond Angles in Ethylene Molecule:
Ball and Stick Model of Ethylene Molecule:
π Cloud of Ethylene Molecule:
Preparation of Alkenes:
By Dehydration of Alcohols:
- Alcohols on heating in the presence of dehydrating agents Iike concentrated sulphuric acid or phosphorous pentoxide or anhydrous alumina etc. undergoes a loss of water molecule to form an alkene. As water molecule is removed from alcohol molecule, the reaction is called dehydration reaction.
- The ease of dehydration in alcohol is in the following order: Tertiary alcohol > Secondary alcohol > Primary alcohol
- General Reaction:
- Example – 1: Preparation of ethene from ethyl alcohol (Ethanol):
- Example – 2: Preparation of propene from n-propyl alcohol (Propan-1-ol):
- Example – 3: Preparation of propene from iso-propyl alcohol (Propan-2-ol):
- Example – 4: Preparation of isobutylene (2-Methylprop-1- ene) from tert – butyl alcohol (Propan-2-ol):
- Reaction Mechanism:
- Acid is proton donor. A proton (H+) from acid acts on the most electronegative atom in alcohol. i.e. oxygen and gets attached to it. This process is called protonation of alcohol.
- A water molecule is eliminated by protonated alcohol to give a positively charged species called carbocation.
- The carbocation formed is unstable and loses a proton to form an alkene.
By Dehydro-Halogenation of Alkyl Halides:
- Alkyl halide on refluxing with alcoholic KOH, undergoes a loss of hydrogen halide, to form an alkene.
- The ease of dehydro-halogenation in alcohol is in the following order.
- For same alkyl group and different halagen R-I > R-Br > R-Cl
- For different alkyl group but same halogen Tertiary alkyl halide > Secondary alkyl halide > Primary alkyl halide
- General Reaction:
- Example – 1: Preparation of Ethene From Ethyl Bromide (Bromoethane):
- Example – 2: Preparation of propene From n-Propyl chloride (1-Chloropropane):
- Example – 3: Preparation of propene From iso-Propyl Iodide (2-Iodopropane):
- Example – 4: Preparation of 2-Methylpropene From tert-Butyl bromide:
- Saytzeff’s Rule:
- In dehydrohalogenation or in dehydration, an alkene formed by elimination has a greater number of alkyl groups attached to the doubly bonded carbon atom.
By Cracking Alkanes:
- The higher alkane when heated strongly in the absence of air decomposes to give a lower alkene and lower alkane or hydrogen. Temperature is maintained at 500 °C and catalyst used is silica-alumina.
- Example -1: Ethane when heated strongly in the absence of air, decomposes to give ethene.
- Example-2: When Propane when heated in the absence of air, using silica- aluminium as catalyst decomposes to give a mixture of propene and methane.
By Dehalogenation of Vicinal Dihalides:
- Dihalogen derivatives of alkanes in which the two halogen atoms are attached to two neighbouring carbon atoms are called vicinal dihalides.
- When vicinal dihalides are heated with zinc dust in the presence of ethanol, alkenes are obtained.
- General Reaction:
- Example – 1: Preparation of Ethene:
- Example – 2: Preparation of Propene:
By Kolbe’s Electrolysis:
- Electrolysis of aqueous solutions of sodium or potassium salts of saturated dicarboxylic acids gives an alkene. When an aqueous solution of sodium or potassium salt of a dibasic acid is electrolyzed, an alkene is produced. For example, electrolysis of sodium succinate gives ethene.
By Controlled Hydrogenation of Alkynes:
- Use of Catalyst and Heat:
- Use of Lindlar’s Catalyst: Lindlar’s catalyst is palladium (Pd) supported over calcium carbonate or charcoal and partially deactivated with poisons such as sulphur or quinoline. When Lindlar’s catalyst is used major product is cis alkene
- Use of sodium and liquid ammonia: When sodium and liquid ammonia is used the major product is trans alkene
Physical Properties Alkenes:
- Physical State:
- First three members of alkenes viz. Ethene, Propene and Butene are colourless gases. Alkenes containing 5 to 17 carbons are liquids and higher members are solid at room temperature.
- Odour (Smell):
- Alkenes are odourless except Ethene which has a pleasant smell. Higher alkenes are odourless and colourless.
- Alkenes are insoluble in water but are readily soluble in non-polar organic solvents like benzene, hexane, CCl4, etc.
- Alkenes burn in air with luminous flame, producing carbon dioxide and water.
- Melting and Boiling Points:
- Alkenes have low melting and boiling points. The melting point and boiling point rises with increases in the molecular weight. For every -CH2 added the boiling point increases by 20- 30 K. Cis isomers have a higher boiling point than corresponding trans isomer. Generally melting point of cis isomer is lower than corresponding trans isomer.
- There is a gradual increase in densities with the increase in the molecular mass. Alkenes are lighter than water and their maximum density is 0.8.
- The alkene gases and vapours form an explosive mixture with air.
Stability of Alkenes:
- The stability of alkenes is measured in terms of its heat of hydrogenation. The heat of hydrogenation of an alkene is defined as the amount of heat evolved when one mole of alkene is completely hydrogenated to form alkane. An alkene having the greater value of the heat of hydrogenation is less stable.
- The more the number of alkyl groups attached to the doubly bonded carbon atoms, greater is the stability of the alkene. The greater stability is due to the fact that greater the number of alkyl groups attached to the doubly bonded carbon atoms, greater is the delocation of π electrons through hyperconjugation which leads to the stability of alkenes.
- R2C=CR2 (4 alkyl groups) > R22C=CHR (3 alkyl groups)> R2C=CH2 (2 alkyl groups) > RCH=CHR (2 alkyl groups) > RCH=CH2 (1 alkyl group) > CH2=CH2 (No alkyl group).
- Trans isomers (E configuration) are more stable than cis-isomers (Z configuration)
Reactivity of Alkenes:
- Alkenes have carbon-carbon double bond. one of the bonds is sigma (σ) bond while another bond is pi (π) bond. The extent of sigma bond is more (due to axial overlapping) than that of a pi bond (due to sideways overlap). The bond energy of sigma bond is 83 Kcal/mol while that of the pi bond is 63 Kcal/mol. The electron density of pi bond is diffused and exposed more to an attacking electrophile.
- The pi bond is under strain and tries to convert into two sigma bonds. Hence they attract electrophile easily. Hence alkenes undergo electrophilic addition reactions.
Reactions of Alkenes:
- Unlike alkanes, alkenes are very reactive and they undergo addition reactions to form a saturated compound.
Mechanism of Electrophilic Addition:
- Let us consider an attacking molecule XY, such that the part Y of the molecule is more electronegative than X. Being more electronegative than X, Y pulls the shared pair of electrons towards it. Hence the molecule can be represented as Xδ+Yδ-.
- XY molecule attack on alkene and adds to the π electrons cloud forming a bond through Xδ+(electrophilic part). During this, a π complex is formed. The π complex undergoes rearrangement to form unstable carbocation which on the addition of Y– gives the addition product.
Hydrogenation of Alkenes or Addition of Dihydrogen:
- When vapours of alkene are mixed with dihydrogen and are passed over catalyst like Nickel, Platinum or Palladium corresponding alkane is formed.
- General Reaction:
- Example – 1: Hydrogenation of Ethene:
- Example – 2: Hydrogenation of Propene:
- Pt and Pd catalyst are effective at room temperature. Alkenes cannot be hydrogenated by nascent Hydrogen.
- Hydrogenation is a very important process because it is used in the manufacturing of vanaspati from vegetable oils.
Halogenation of Alkenes:
- Alkenes combine with halogen in presence of inert solvent CCl4 at room temperature readily to form alkylene dihalide.
- Iodine does not give this reaction.
- General reaction:
- Examples – 1: Action of Bromine on Ethane:
- In such reactions, bromine (reddish brown colour) gets decolourised, and it is used as a test for alkenes (test for unsaturation).
- Example – 2: Action of Bromine on Propene:
Action of Haloacids on Symmetric Alkenes:
- The addition of halogen acids like HCl, HBr or HI to a compound containing multiple bond is known as hydrohalogenation. Alkenes react with halogen acid to form corresponding alkyl halide.
- The order of reactivity is HI > HBr > HCl. HCl reacts as per Markownikoff’s rule only.
- General reaction:
R–CH=CH2 + HX → R–CH2–CH2X
Alkene Alkyl halide
- Examples With Symmetric Alkenes:
- Example – 1: Action of HBr on Ethene:
CH2=CH2 + HBr → CH3–CH2Br
Ethene Ethyl bromide
- Example – 2: Action of HBr on But-2-ene:
CH3–CH=CH–CH3 + HBr → CH3–CH2–CHBr–CH3
Action of Haloacids on Unsymmetric Alkenes:
- Markownikoff’s rule: When an unsymmetrical alkene is treated with an unsymmetrical reagent like HBr, then the negative part of, the reagent goes to that carbon atom which has less number of hydrogen atoms attached to it.
- Example-1: Action of HBr on Propene:
- Example-2: Action of HBr on But-1-ene:
- Anti-Markownikoff’s rule (Peroxide effect or Kharasch effect): When an unsymmetrical alkene is treated with an unsymmetrical reagent like HBr in presence of a peroxide, then the negative part of, the reagent goes to that carbon atom which has more number of hydrogen atoms attached to it.
- Example-1: Action of HBr on Propene in Presence of Peroxide:
- Example-2: Action of HBr on But-1-ene in Presence of Peroxide:
Action of Water:
- Alkenes add a molecule of water in the presence of mineral acids such as H2SO4. Addition takes place as per Makonikoff’s rule.
- General Reaction:
- Example – 1: Action of water on ethene:
- Example – 2: Action of water on propene (Asd per Markownikoff’s rule):
- Mechanism of Addition of Water to Alkene:
Addition of Hypohalous Acids:
- Alkenes add hypohalous acid (HO-X) across the double bond to give halohydrins. The addition takes as per Markonikoff’s rule and OH acts as negative part of the reagent.
- General Reaction:
- Example – 1:
- Example – 2:
Combustion of Alkenes:
- Alkenes are combustible and burn with luminous flame in air or oxygen to form carbon dioxide and water with evolution of large amount of heat.
CH2=CH2 + 3O2 → 3 CO2 + 2H2O ; ΔH° = -1411 kJ mol-1.
Oxidation with potassium permanganate:
- With Cold and dilute KMnO4 solution:
- This reaction is Baeyer’s test.
- General Reaction:
- Example – 1:
- Example – 2:
Ozonolysis of Alkene:
- Alkene on treatment with ozone in presence of organic solvents like CCl4 forms alkylene ozonide, which on hydrolysis in presence of reducing agent like zinc give aldehydes or ketones or a mixture of both. This reaction is called an ozonolysis.
- Examples – 1: Action of Ozone on Ethene:
- Examples – 2: Action of Ozone on Propene:
Uses of Alkenes:
- Ethene gas is used in early ripening and storage of fruits and vegetables.
- Oxy-ethylene gas is used for cutting and welding of metal.
- Ethene is used for the preparation of ethyl alcohol and acetaldehyde.
- Ethene is used in the preparation of Polythene and resins polymers.
- Ethene gas is used as an anaesthetic.
- Ethene is used for the preparation of mustard gas.