Chapter 3 - Reactions of Alkanes

Chapter 3 - Reactions of Alkanes - Chapter 3: Reactions of...

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Chapter 3: Reactions of Alkanes Bond-Dissocation Energies, Radical Halogenation, and Relative Reactivity Radical reactions begin with breaking of bond (bond dissociation ); examples are halogenation and combustion; do not involve acid-base chemistry Halogenation is a radical reaction in which a hydrogen in an alkane is replaced by a hydrogen; halogens are very reactive 3.1 Strength of Alkane Bonds: Radicals Bond-dissociation energy or bond strength , DH°, is the energy released when a bond is made Radicals are formed by hemolytic cleavage o Hemolytic cleavage or bond homolysis is equal distribution of two bonding electrons to two participating atoms when bond is broken; forming radicals which are species with more than one atom and an unpaired electron; they are very reactive Usually found in nonpolar solvents or gas phase o Heterolytic cleavage is an unequal distribution of bonding electrons to produce ions ; usually occurs in polar solvents where electronegativities stabilize ions o Dissociation energies refer only to hemolytic cleavages o Bonds are strongest when made by overlapping orbitals that are closely matched in energy and size The stability of radicals determines the C-H bond strengths o Radical stability increases along series from primary to tertiary so energy required to create them decreases which accounts of the decrease in bond energies 3.2 Structure of Alkyl Radicals: Hyperconjugation Hyperconjugation is when singly occupied p orbitals (radicals) overlap with a C-H bond of a CH 3 group so that the bonding pair of electrons in the bond delocalizes into the partly empty p lobe; this has a stabilizing effect Similar to resonance which is also a form of electron delocalization except resonance applies to π- type overlap of p orbitals whereas hyperconjugation refers to overlap with orbitals of σ bonds Each additional alkyl group increases hyperconjugation interactions further Stabilization of radicals by resonance is greater Secondary and tertiary radicals are more stable because of steric crowding between substituent groups 3.3 Conversion of Petroleum: Pyrolysis High temperatures cause bond homolysis o Pyrolysis is when C-H and C-C bonds rupture; occurs at high temperatures o Radicals can combine in any way to form new higher or lower molecular weight alkanes o Hydrogen abstraction is the process in which radicals remove hydrogen atoms from carbon atom adjacent to another radical center to form alkenes
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Chapter 3 - Reactions of Alkanes - Chapter 3: Reactions of...

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