Alkenes

• Alkenes


  Hydrocarbons containing C=C double bonds
• Alkenes


  • They have greater chemical reactivity compared to alkanes due to the presence of the C=C double bond
  • They have the general formula CnH2n where n is an integer greater than or equal to 2
• Hybridisation of carbon atoms in C=C


  Each carbon atom is sp2 hybridised and bonded in a trigonal planar manner with a bond angle of 120°
• Strength of π bond in C=C


  It is stronger than a C-C single bond but weaker than twice the strength of a C-C single bond
• Alkenes can exhibit constitutional isomerism and cis-trans isomerism
• Cis-trans isomerism


  Exists due to restricted rotation about the C=C double bond and each carbon atom being joined to two different groups
• Cis-trans isomerism is possible only in cycloalkenes with ring size larger than seven carbon atoms
• Relative stability of cis-trans isomers


  Trans isomers are usually more stable than the corresponding cis isomers due to lower steric strain
• Enthalpy change of combustion

  A more exothermic enthalpy change indicates the isomer is more stable due to its higher energy content
• Boiling point and melting point of alkenes


  • Increase as the number of carbon atoms increases due to stronger instantaneous dipole-induced dipole forces of attraction between molecules
  • Decrease with greater degree of branching due to weaker instantaneous dipole-induced dipole forces
• Alkenes are non-polar molecules
• Alkenes experience instantaneous dipole-induced dipole forces of attraction between molecules
• Boiling point and melting point of alkenes


  • For straight chain alkenes, the boiling point/melting point increases as the number of C atoms increases
  • The greater the degree of branching, the lower the boiling point
• Solubility of alkenes


  Alkenes are insoluble in water, but soluble in non-polar solvents
• Increasing number of electrons (as seen from the Mr of alkenes)

  Increases the boiling point
• Increasing degree of branching in the isomer


  Decreases the boiling point
• Alkenes are non-polar molecules which experience instantaneous dipole-induced dipole forces of attraction between molecules
• Branched alkenes are more spherical in shape, hence have lesser surface area of contact between molecules
• Cis-but-2-ene has a net dipole moment while trans-but-2-ene has zero dipole moment
• The intermolecular forces in cis-but-2-ene (permanent dipole–permanent dipole and instantaneous dipole–induced dipole interactions) are stronger than those in trans-but-2-ene (only instantaneous dipole–induced dipole interactions)
• The more symmetrical trans-but-2-ene molecules pack into the crystal lattice better, allowing closer approach and larger attractive forces, thus resulting in higher melting points
• Preparation of alkenes


  1. Cracking of alkanes

  2. Elimination of H2O from alcohols

  3. Elimination of HX from halogenoalkanes
• Alcohols with H and OH on adjacent carbons atoms can be dehydrated to yield alkenes
• Alkyl halides with H and X (e.g. Cl or Br or I) on adjacent carbon atoms are dehydrohalogenated (eliminate HX) to yield alkenes
• Saytzeff's Rule


  If an elimination results in the formation of more than one type of alkenes, the more highly substituted alkenes (i.e. the one with more alkyl groups attached to the double bonded carbon atoms) will be the most stable alkene formed and the major product
• Reactions of alkenes


  • Combustion
  • Reduction (catalytic hydrogenation / catalytic addition of hydrogen)
  • Electrophilic addition reactions
• Combustion of alkenes


  Alkenes undergo complete combustion in air to form carbon dioxide and water
• Reduction (catalytic hydrogenation / catalytic addition of hydrogen)

  • Alkenes are hydrogenated to give alkanes
  • Heterogeneous catalysis using catalysts like Pt and Pd at room temperature
  • LiAlH4 cannot be used as it will be repelled by the electron-rich π electron cloud in C=C bond
• Electrophilic addition reactions


  • Most organic reactions are polar reactions which take place between an electron-poor site (electrophile) and an electron-rich site (nucleophile)
  • Electrophiles are electron-poor species that form a covalent bond by accepting an electron pair
  • Nucleophiles are electron-rich species that form a covalent bond by donating an electron pair
• The strength of an electrophile depends on the size and stability of the positive charge, while the strength of a nucleophile depends on the availability of lone pair of electrons
• Cations


  More powerful electrophiles than neutral molecules
• Carbon atoms


  Electrophilic when attached to electronegative atoms. The more electronegative the atom(s) bonded to carbon, the more electrophilic the carbon atom
• Anions


  More powerful nucleophiles than neutral molecules
• Electronegativity of atom bearing negative charge or lone pair


  The tighter the electrons are held to the nucleus, the weaker the nucleophile
• Down the group


  Electrons are held less tightly to the nucleus as the atom size increases, and hence they are more available for forming bonds, making the nucleophile stronger
• Alkenes, though generally non-polar, are highly reactive as compared to the alkanes because of the π electron cloud between the doubly bonded carbon atoms
• These loosely held π electrons will attract electrophiles or even induce dipoles, creating electrophilic sites in some nearby neutral molecules
• Alkenes are unsaturated, thus they undergo addition reactions. The reactions of alkenes mainly involve electrophilic addition reactions
• During the reaction, the weaker π bond (due to less effective overlapping of p orbitals) is broken instead of the σ bond. In place, two strong σ bonds are formed in the saturated product
• The shape of the C atom in C=C now changes from trigonal planar to tetrahedral (the hybridisation of the C atom changes from sp2 to sp3)