Ch.12 Alkanes
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Sigma bonds form when
2 orbitals, 1 from each bonding atom overlap head on. Each overlapping orbital has 1 unpaired electron
Why don’t alkanes react with most common reagents
. High bond enthalpy of C-C C-H bonds (require a lot of energy to break). . C-C, C-H bonds are non polar
What reactions can alkanes have. And what conditions
Combustion, react with Cl, Br in presence of UV light (sunlight). I is too unreactive
Moles of O2 needed for complete combustion of alkane
3n+1 _____ 2
Initiation of the halogenation of alkanes
Covalent bond in Cl broken though homolytic fission. Cl2 –> 2Cl•
Propagation of the halogenation of alkane
Cl radical removes H from alkane forming HCl and an alkyl radical C2H6 + Cl• —> C2H5• + HCl Alkyl radical can then react with another Cl2 molecule. Removes Cl forming a haloalkane and a Cl radical C2H5• + Cl2 —> C2H5Cl + Cl• Then the process starts again as It is a chain reaction
Termination of the halogenation of alkane
2 radicals collide forming a molecule where all the electrons are paired. 2Cl• —> Cl2 2C2H5• —> C4H10 Cl• + C2H5• —> C2H5Cl
Limitations of radical substitution
. Can happen repeatedly on same molecule . Can happen at different positions in a carbon chain . Leads to a mixture of products which need to be separated