© Prentice Hall 2001Chapter 81 Alkanes: Fuels from Petroleum Branched-chain hydrocarbons tend to perform better in internal combustion engines

© Prentice Hall 2001Chapter 82 Reactivity of Alkanes Alkanes have only strong, nonpolar  bonds No reaction with nucleophiles or electrophiles Not much reactivity - paraffins (little affinity)

© Prentice Hall 2001Chapter 83 Chlorination and Bromination of Alkanes

© Prentice Hall 2001Chapter 84 Chlorination and Bromination of Alkanes Initiation: Homolytic cleavage radicals Note that when an arrowhead with a single barb is used, it denotes movement of a single electron

© Prentice Hall 2001Chapter 85 Chlorination and Bromination of Alkanes

© Prentice Hall 2001Chapter 86 Product Distribution

© Prentice Hall 2001Chapter 87 Relative Stabilities of Alkyl Radicals

© Prentice Hall 2001Chapter 88 Reactivity–Selectivity Principle The very reactive chlorine atom will have lower selectivity and attack pretty much any hydrogen available on an alkane The less reactive bromine atom will be more selective and tends to react preferentially with the easy targets, i.e. tertiary hydrogens

© Prentice Hall 2001Chapter 89 Radical Substitution of Benzylic and Allylic Hydrogens

© Prentice Hall 2001Chapter 810 Radical Substitution of Benzylic and Allylic Hydrogens Benzylic and allylic radicals are even more stable than tertiary alkyl radicals It should be easy for a halogen radical to abstract a benzylic or allylic hydrogen

© Prentice Hall 2001Chapter 811 Radical Substitution of Benzylic and Allylic Hydrogens Problem is that for the allyl radical there is a greater likelihood that the halogen will add electrophilically to the adjacent double bond

© Prentice Hall 2001Chapter 812 Radical Substitution of Benzylic and Allylic Hydrogens Electrophilic addition can be minimized by maintaining the halogen at a very low concentration Under these conditions, halogens can substitute for allylic and benzylic hydrogens

© Prentice Hall 2001Chapter 813 Radical Substitution of Benzylic and Allylic Hydrogens N-Bromosuccinimide (NBS) is a good reagent for supplying low concentrations of bromine radical

© Prentice Hall 2001Chapter 814 Radical Substitution of Benzylic and Allylic Hydrogens Bromine radical comes from the homolytic cleavage of the N–Br Bond Low concentration of Br 2 is generated by the reaction of NBS with HBr Neither HBr nor Br 2 accumulate, so electrophilic addition is slow

© Prentice Hall 2001Chapter 815 Radical Substitution of Benzylic and Allylic Hydrogens When a radical abstracts an allylic or benzylic hydrogen, a radical that is stabilized by resonance is obtained

© Prentice Hall 2001Chapter 816 Radical Substitution of Benzylic and Allylic Hydrogens If the resonance hybrid is not symmetrical, more than one product is obtained 3-bromo-1-butene 1-bromo-2-butene

© Prentice Hall 2001Chapter 817 Stereochemistry of Radical Substitution

© Prentice Hall 2001Chapter 818 Stereochemistry of Radical Substitution

© Prentice Hall 2001Chapter 819 Stereochemistry of Radical Substitution If a chirality center already exists, it may affect the distribution of products A pair of diastereomers will be formed, but in unequal proportions

© Prentice Hall 2001Chapter 820 Stereochemistry of Radical Substitution

© Prentice Hall 2001Chapter 821 Reactions of Cyclic Compounds Cyclic alkanes react with halogens in much the same way as acyclic compounds

© Prentice Hall 2001Chapter 822 Reactions of Cyclic Compounds Cyclopropane undergoes electrophilic addition much like an alkene

© Prentice Hall 2001Chapter 823 Reactions of Cyclic Compounds The bond angles in cyclopropane are 60 o, which is considerably smaller than the ideal of 109 o The sp 3 hybrid orbital cannot overlap head-to-head - the bonds are weaker than normal Consequently three-membered rings undergo ring opening with electrophilic reagents

© Prentice Hall 2001Chapter 824 Radical Reactions in Biological Systems Alkanes (toxic) are converted to alcohols (nontoxic) in the liver via a radical mechanism An iron-containing enzyme, cytochrome P 450, catalyzes the reaction

© Prentice Hall 2001Chapter 825 Radical Reactions in Biological Systems A radical reaction also is involved in the reduction of a ribonucleotide to a deoxyribonucleotide

© Prentice Hall 2001Chapter 826 Radical Reactions in Biological Systems Protection from radical reaction is possible if a compound is present that reacts with the radical and forms a less reactive radical

© Prentice Hall 2001Chapter 827 Radical Reactions in Biological Systems

© Prentice Hall 2001Chapter 828 Radicals and Stratospheric Ozone Ozone (O 3 ) is a major constituent of smog In the stratosphere, a layer of ozone shields the Earth from harmful solar radiation The ozone layer is thinnest at the equator and thickest in polar regions

© Prentice Hall 2001Chapter 829 Radicals and Stratospheric Ozone Ozone is formed in the stratosphere by interaction of short-wavelength ultraviolet light with oxygen

© Prentice Hall 2001Chapter 830 Radicals and Stratospheric Ozone The stratospheric ozone layer acts as a filter for biologically harmful ultraviolet radiation Scientists have noted a precipitous drop in the ozone concentrations over Antarctica since 1985 Circumstantial evidence links the depletion in ozone to synthetic chlorofluorocarbons (CFCs) - used as refrigerants

© Prentice Hall 2001Chapter 831 Radicals and Stratospheric Ozone Chlorofluorocarbons (CFCs) are exceptionally stable, but under the intense ultraviolet radiation present in the stratosphere, they undergo a radical dissociation

© Prentice Hall 2001Chapter 832 Radicals and Stratospheric Ozone The chlorine radicals are ozone- removing reagents It has been estimated that each chlorine radical destroys 100,000 ozone molecules in a radical chain reaction Overall

© Prentice Hall 2001Chapter 833 Radicals and Stratospheric Ozone Production and use of CFCs has been slowed, but because these materials have a half-life of years they will be around in the stratosphere for a long time The ozone hole over Antarctica was observed in October 1999 to be a little smaller than in October 1998