CHEM 0310 Lecture Notes - Lecture 3: Hyperconjugation, Pyrolysis, Zeolite

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6 Feb 2017
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Chapter 3: Reactions of Alkanes
Radicals
-Combustion of alkanes releases most of the energy that powers modern industrialized society
-Radical reactions begin with bond dissociation
-Alkanes lack functional groups so they do not undergo the kinds of electrophile-nucleophile reactions
typical of functionalized molecules
-Alkanes are typically unreactive undergo homolytic bond cleavage
-The energy needed to break bonds is called bond-dissociation energy (DHo)
-Homolytic cleavage: bond breaks in such a way that the two bonding electrons divide equally between
the two participating atoms these fragments thus have unpaired electrons (radicals and free atoms)
very, very reactive when they become free radicals
-Heterolytic cleavage breaking a bond in which the entire bonding electron pair is donated to one of
the atoms - these fragments form ions
-Homolytic cleavage occurs in nonpolar solvents while heterolytic cleavage occurs in polar solvents
-Dissociation energies, DHo, refers only to hemolytic cleavages
-The larger the DHo, the stronger the corresponding bond
-Bonds are strongest when made by overlapping orbitals that are closely matched in energy and size
(CH3-F>CH3-I)
Stability of Radicals
-Bond strength decreases as carbons have more attachments (primary>secondary)
-Radical stability increases along the series from primary secondary; consequently the energy to
create them decreases
-If both radicals are the same carbon amounts, look for the one with a more electronegative atom which
makes it less stable
-Due to the unpaired electrons, free radicals do not have an electron octet - usually instable and highly
reactive but become more stable with more connections (tertiary>secondary)
Structure of Alkyl Radicals
-Hyperconjugation: an arrangement that allows the bonding pair of electrons in the sigma orbital to
delocalize into the partly empty p lobe (example: CH3CH2)
-Both hyperconjugation and resonance are forms of
electron delocalization but distinguished by the type of
orbital
-Resonance normally refers to pi-type overlap while
hyperconjugation incorporates overlap with the orbitals of
sigma bonds
-Radicals are stabilized by hyperconjugation
-Addition of an alkyl group increases the hyperconjugation
interaction
-The more hyperconjugation there is, the greater the stabilization of the system
Pyrolysis
-Alkanes are produced naturally by the slow decomposition of animal and vegetable matter in the
presence of water and absence of oxygen
-When alkanes are heated to a high temperature, both C-H bond and C-C bonds rupture (pyrolysis)
-The resulting radicals can combine to form new higher or lower molecular weight alkanes
-Single-barbed (fishhooks) show the formation of a new covalent combination and sharing an electron
this process usually requires a catalyst
Catalyst
-Zeolite catalyst speeds up the pyrolysis to enable the process to occur at low temperatures
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