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Organic Chemistry I Organic Chemistry I Mohammad Jafarzadeh Faculty of Chemistry, Razi University Organic Chemistry, Structure and Function (7th edition) By P. Vollhardt and N. Schore, Elsevier, 2014 1 98 CHAPTER 3 Reactions of Alkanes 3-1 STRENGTH OF ALKANE BONDS: RADICALS 98 CHAPTER 3 Reactions of Alkanes In Section 1-2 we explained how bonds are formed and that energy is released on bond formation. For example, bringing two hydrogen atoms into bonding distance produces 104 kcal mol21 (435 kJ mol21) of heat (refer to Figures 1-1 and 1-12). 3-1 STRENGTH OF ALKANE BONDS: RADICALS Bond making 21 21 In Section 1-2 we explained how bonds are formed and that energy is released on bond H? 1 H? uuuuy HOH DH 8 5 2104 kcal mol (2435 kJ mol ) formation. For example, bringing two hydrogen atoms into bonding distance produces Released heat: exothermic 104 kcal mol21 (435 kJ mol21) of heat (refer to Figures 1-1 and 1-12). Bond making Consequently, breaking such a bond requires heat—in fact, the same amount of heat that uuuuy 21 21 H? 1 H? HOH DH 8 5 2104 kcal mol (2435 kJ mol ) was released when the bond was made. This energy is called bond-dissociation energy, DH8, Released heat: exothermic 3. Reaction of Alkanes and is a quantitative measure of the bond strength. Consequently, breakingHomolytic suchCleavage a bond requires: Bonding heat—inElectrons fact, theSeparate same amount of heat that was released when the bond was made. This energy is called bond-dissociation energy, DH8, Bond breaking and is a quantitativeThe bondmeasureBreaks of thein bondsuch strength.a way that the two bonding electrons divide equally between the HOH uuuuy H? 1 H? DH 8 5 DH 8 5 104 kcal mol21 (435 kJ mol21) two participating atoms or fragments. This process is called homolytic cleavage or bond Consumed heat: endothermic Bondhomolysis breaking . HOH uuuuy H? 1 H? DH 8 5 DH 8 5 104 kcal mol21 (435 kJ mol21) The separation of the two bondingConsumedelectrons heat: endothermicis denoted by two single-barbed or “fishhook” arrows that point from the bond to each of the atoms. Radicals are formed by homolytic cleavage Radicals are formed by homolytic cleavage In our example, the bond breaks in such a way that the two bonding electrons divide equally . In our example,The the bondfragments breaks inthat suchform a wayhave that thean twounpaired bondingelectron electrons(H divide, Cl equally, CH3 , and CH3CH2 ). When between the two participating atoms or fragments. This process is called homolytic cleavage between the twothese participatingspecies atomsare composedor fragments.of Thismore processthan isone calledatom, homolyticthey are cleavagecalled radicals. or bond homolysis. The separation of the two bonding electrons is denoted by two single- or bond homolysis. The separation of the two bonding electrons is denoted by two single- Because of the unpaired electron, radicals and free atoms are very reactive and usually barbed or “" shhook” arrows that point from the bond to each of the atoms. barbed or “" shhook”cannot arrowsbe isolated that point. from the bond to each of the atoms. A single-barbed arrow Homolytic Cleavage: Bonding Electrons Separate A single-barbed arrow Homolytic Cleavage: Bonding Electrons Separate shows the movement of a shows the movement of a single electron. ABO A ϩ B j j single electron. Radicals 2 ABO AjϩjB Radicals The fragments that form have an unpaired electron, for example, H?, Cl?, CH ?, and ðCl"šj 3 CH3CH2?. When these species are composed of more than one atom, they are called radicals. Chlorine atom Because of the unpaired electron, radicals and free atoms are very reactive and usually ðClšj The fragments that form have an unpaired electron, for example, H?, Cl?, CH3?, and H cannot be isolated. However, radicals and free atoms are present in low concentration as " CH3CH2?. When these species are composed of more than one atom, they are called radicals. unobserved intermediates in many reactions, such as the production of polymers (Chapter 12)Chlorine atom ÄC HH and the oxidation of fats that leads to the spoilage of perishable foods (Chapter 22). Because of the unpaired electron, radicals and free atoms are very reactive and usually Methyl radical In Section 2-2 we introduced an alternative way of breaking a bond, in which the entire H cannot be isolated. However, radicals and free atoms are present in low concentration as bonding electron pair is donated to one of the atoms. This process is heterolytic cleavage H unobserved intermediates in many reactions, such as the production of polymers (Chapter 12) and results in the formation of ions. ÄC HH and the oxidation of fats that leads to the spoilage of perishable foods (Chapter 22). H3C Cj Methyl radical In Section 2-2 we introduced an alternative way of breaking a bond, in which the entire Heterolytic Cleavage: Bonding Electrons Move as Pair H bonding electron pair is donated to one of the atoms. This process is heterolytic cleavage Ethyl radical H ABO Aϩ ϩ ðBϪ and results in the formation of ions. Ions H3C Cj Heterolytic Cleavage: Bonding Electrons Move as Pair A normal, double-barbed Homolytic cleavage may be observed in nonpolar solvents or even in the gas phase. In H curved arrow shows the contrast, heterolytic cleavage normally occurs in polar solvents, which are capable of sta-Ethyl radical ϩ Ϫ movement of a pair of bilizing ions. Heterolytic cleavage is also restricted to situations where the electronegativies ABO A ϩ ðB electrons. of atoms A and B and the groups attached to them stabilize positive and negative charges, Ions respectively. Dissociation energies, DH8, refer only to homolytic cleavages. They have characteristic values for the various bonds that can be formed between the elements. Table 3-1 lists disso- ciation energies of some common bonds. The larger the value for DH8, the strongerA thenormal, cor- double-barbed Homolytic cleavage may be observed in nonpolar solvents or even in the gas phase. In responding bond. Note the relatively strong bonds to hydrogen, as in H–F and H–OH.curved However, arrow shows the contrast, heterolytic cleavage normally occurs in polar solvents, which are capable of sta- even though these bonds have high DH8 values, they readily undergo heterolytic cleavage in bilizing ions. Heterolytic cleavage is also restricted to situations where the electronegativies water to H1 and F2 or HO2; do not confuse homolytic with heterolytic processes. movement of a pair of electrons. of atoms A and B and the groups attached to them stabilize positive and negative charges, respectively. Dissociation energies, DH8, refer only to homolytic cleavages. They have characteristic values for the various bonds that can be formed between the elements. Table 3-1 lists disso- ciation energies of some common bonds. The larger the value for DH8, the stronger the cor- responding bond. Note the relatively strong bonds to hydrogen, as in H–F and H–OH. However, even though these bonds have high DH8 values, they readily undergo heterolytic cleavage in water to H1 and F2 or HO2; do not confuse homolytic with heterolytic processes. 98 CHAPTER 3 Reactions of Alkanes 3-1 STRENGTH OF ALKANE BONDS: RADICALS In Section 1-2 we explained how bonds are formed and that energy is released on bond formation. For example, bringing two hydrogen atoms into bonding distance produces 21 21 98 CHAPTER 3 Reactions of Alkanes 104 kcal mol (435 kJ mol ) of heat (refer to Figures 1-1 and 1-12). Bond making H? 1 H? uuuuy HOH DH 8 5 2104 kcal mol21 (2435 kJ mol21) 3-1 STRENGTH OF ALKANE BONDS: RADICALS Released heat: exothermic Consequently, breaking such a bond requires heat—in fact, the same amount of heat that In Section 1-2 we explained how bonds are formed and that energy is released on bond was released when the bond was made. This energy is called bond-dissociation energy, DH8, formation. For example, bringing two hydrogen atoms into bonding distance produces and is a quantitative measure of the bond strength. 104 kcal mol21 (435 kJ mol21) of heat (refer to Figures 1-1 and 1-12). Bond breaking Bond making 21 21 H? 1 H? uuuuy HOH DH 8 5 2104 kcal mol21 (2435 kJ mol21) HOH uuuuy H? 1 H? DH 8 5 DH 8 5 104 kcal mol (435 kJ mol ) Released heat: exothermic Consumed heat: endothermic Consequently, breaking such a bond requires heat—in fact, the same amount of heat that Radicals are formed by homolytic cleavage was released when the bond was made. This energy is called bond-dissociation energy, DH8, In our example, the bond breaks in such a way that the two bonding electrons divide equally and is a quantitative measure of the bond strength. between the two participating atoms or fragments. This process is called homolytic cleavage or bond homolysis. The separation of the two bonding electrons is denoted by two single- Bond breaking HOH uuuuy H? 1 H? DH 8 5 DH 8 5 104 kcal mol21 (435 kJ mol21) barbed or “" shhook” arrows that point from the bond to each of the atoms. Consumed heat: endothermic A single-barbed arrow Homolytic Cleavage: Bonding Electrons Separate Radicals are formed by homolytic cleavage shows the movement of a In our example, the bond breaks in such a way that the two bonding electrons dividesingle equally electron. ABO AjϩjB between the two participating atoms or fragments. This process is called homolytic cleavage Radicals or bond homolysis. The separation of the two bonding electrons is denoted by two single- barbed or “" shhook” arrows that point from the bond to each of the atoms.
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