Alkene
- Alkenes [ olefins (oil forming) ]
- Alkenes are unsaturated hydrocarbons containing at least one double bond.
- General formula for alkenes is CnH2n.
Structure of Double Bond
- Carbon-carbon double bond in alkenes consists of
- one strong sigma (σ) bond (bond enthalpy about 397 kJ mol–1) due to head-on overlapping of sp2 hybridised orbitals.
- One weak pi (π) bond (bond enthalpy about 284 kJ mol–1) obtained by lateral or sideways overlapping of the two 2p orbitals of the two carbon atoms.
- The double bond is shorter in bond length (134 pm) than the C–C single bond (154 pm).
- pi (π) bond is a weaker bond due to poor sideways overlapping between the two 2p orbitals.
- Alkenes are easily attacked by reagents or compounds which are in search of electrons. Such reagents are called electrophilic reagents.
- The presence of weaker π-bond makes alkenes unstable molecules.
- Strength of the double bond (bond enthalpy, 681 kJ mol–1) is greater than that of a carbon-carbon single bond in ethane (bond enthalpy, 348 kJ mol–1).
Isomerism :
• Alkenes show
1. Structural isomerism and
2. Geometrical isomerism.
Structural isomerism :
• As in alkanes, ethene (C2H4) and propene (C3H6) can have only one structure but alkenes higher than propene have different structures.
• Alkenes possessing C4H8 as molecular formula can be written in the following three ways:
• Structures I and III, and II and III are the examples of chain isomerism whereas structures I and II are position isomers.
Geometrical isomerism:
1. YX C = C XY
2. XYC = CXZ and
3. XYC = CZW
• The stereoisomers of this type are called geometrical isomers.
• The isomer of the type, in which two identical atoms or groups lie on the same side of the double bond is called cis isomer
• The other isomer of the type (b), in which identical atoms or groups lie on the opposite sides of the double bond is called trans isomer .
• Cis form of alkene is found to be more polar than the trans form.
• For example, dipole moment of cis-but-2-ene is 0.33 Debye, whereas, dipole moment of the trans form is almost zero or it can be said that trans-but-2-ene is non-polar.
• In the trans-but-2-ene, the two methyl groups are in opposite directions, Threfore, dipole moments of C-CH3 bonds cancel, thus making the trans form non-polar.
Preparation Of Alkenes :
1. From alkynes:
• Alkynes on partial reduction with calculated amount of dihydrogen in the presence of palladised charcoal partially deactivated with poisons like sulphur compounds or quinoline give alkenes.
• Partially deactivated palladised charcoal is known as Lindlar’s catalyst.
• Alkenes thus obtained are having cis geometry.
• However, alkynes on reduction with sodium in liquid ammonia form trans alkenes.
2. From alkyl halides:
- dehydrohalogenation
- β-elimination reaction
- iodine > bromine > chlorine
- for alkyl groups it is : tertiary > secondary > primary.
3. From vicinal dihalides / dehalogenation :
4. From alcohols by acidic dehydration:
Properties Physical properties :
• Alkenes as a class resemble alkanes in physical properties, except in types of isomerism and difference in polar nature.
• The first three members are gases, the next fourteen are liquids and the higher ones are solids.
• Ethene is a colourless gas with a faint sweet smell.
• All other alkenes are colourless and odourless, insoluble in water but fairly soluble in non-polar solvents like benzene, petroleum ether.
• They show a regular increase in boiling point with increase in size i.e., every – CH2 group added increases boiling point by 20–30 K.
• Like alkanes, straight chain alkenes have higher boiling point than isomeric branched chain compounds.
Primary > Secondary > tertiary
Chemical properties :
- show addition reactions. [ Due to pi (π) electrons ]
- alkenes also undergo free radical substitution reactions.
- Oxidation and ozonolysis reactions are also quite prominent in alkenes.
Addition of halogens :
• bromine or chlorine add up to alkene to form vicinal dihalides.
• However, iodine does not show addition reaction under normal conditions.
• The reddish orange colour of bromine solution in carbon tetrachloride is discharged when bromine adds up to an unsaturation site. This reaction is used as a test for unsaturation.
• Addition of halogens to alkenes is an example of electrophilic addition reaction involving cyclic halonium ion formation.
Addition of hydrogen halides:
• Hydrogen halides (HCl, HBr,HI) add up to alkenes to form alkyl halides.
• The order of reactivity of the hydrogen halides is HI > HBr > HCl.
• Like addition of halogens to alkenes, addition of hydrogen halides is also an example of electrophilic addition reaction.
Addition reaction of HBr to symmetrical alkenes :
Addition reaction of HBr to unsymmetrical alkenes (Markovnikov Rule) :
• Negative part of the addendum (adding molecule) gets attached to that carbon atom which possesses lesser number of hydrogen atoms.
• Thus according to this rule, product I i.e., 2-bromopropane is expected.
Mechanism :
• Hydrogen bromide provides an electrophile, H+, which attacks the double bond to form carbocation
• The carbocation (b) is attacked by Br– ion to form the product as follows :
Anti Markovnikov addition or peroxide effect or Kharash effect :
• In the presence of peroxide, addition of HBr to unsymmetrical alkenes like propene takes place contrary to the Markovnikov rule.
• This happens only with HBr but not with HCl and Hl.
• Mechanism : Peroxide effect proceeds via free radical chain mechanism
• Peroxide effect is not observed in addition of HCl and HI.
• H–Cl bond being stronger (430.5 kJ mol–1) than H–Br bond (363.7 kJ mol–1), is not cleaved by the free radical.
• The H–I bond is weaker (296.8 kJ mol–1) and iodine free radicals combine to form iodine molecules instead of adding to the double bond.
Addition of sulphuric acid :
• Cold concentrated sulphuric acid adds toalkenes in accordance with Markovnikov rule to form alkyl hydrogen sulphate by the electrophilic addition reaction.
Addition of water : Markovnikov rule.
Oxidation:
• Alkenes on reaction with cold, dilute, aqueous solution of potassium permanganate (Baeyer’s reagent) produce vicinal glycols.
• Decolorisation of KMnO4 solution is used as a test for unsaturation.
•
Ozonolysis :
• Ozonolysis of alkenes involves the addition of ozone molecule to alkene to form ozonide, and then cleavage of the ozonide by Zn-H2O to smaller molecules.
• This reaction is highly useful in detecting the position of the double bond in alkenes or other unsaturated compounds.
Polymerisation:
• Polythene is obtained by the combination of large number of ethene molecules at high temperature, high pressure and in the presence of a catalyst.
• The large molecules thus obtained are called polymers. This reaction is known as polymerisation.
Alkynes :
• Acetylene is used for arc welding purposes in the form of oxyacetylene flame obtained by mixing acetylene with oxygen gas.
• Alkynes are starting materials for a large number of organic compounds.
• There are two possible structures for butyne – (i) but-1-yne and (ii) but-2-yne.
• Two compounds differ in their structures due to the position of the triple bond, they are known as position isomers.
• Isomers of C5H8
• C6H10.
Structure of Triple Bond :
• sp hybridised orbitals.
• H-C-C bond angle is of 180°
• ethyne molecule consists of one C–C σ bond, two C–H σ bonds and two C–C π bonds.
• The strength of C≡C bond > C=C bond > C–C bond
• The C≡C bond length is shorter (120 pm) than those of C=C (133 pm) and C–C (154 pm).
• ethyne is a linear molecule.
Preparation
1. From calcium carbide: On industrial scale, ethyne is prepared by treating calcium carbide with water.
From vicinal dihalides : dehydrohalogenation.
Properties Of Alkyne :
Physical properties
• First three members are gases, the next eight are liquids and the higher ones are solids.
• All alkynes are colourless.
• Ethyene has c