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Bond Making and Bond Breaking

Addition and elimination reactions are exactly opposite. A  bond is formed in elimination reactions, whereas a  bond is broken in addition reactions. Bond Making and Bond Breaking.

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Bond Making and Bond Breaking

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  1. Addition and elimination reactions are exactly opposite. A  bond is formed in elimination reactions, whereas a  bond is broken in addition reactions.

  2. Bond Making and Bond Breaking • A reaction mechanism is a detailed description of how bonds are broken and formed as starting material is converted into product. • A reaction can occur either in one step or a series of steps.

  3. Regardless of how many steps there are in a reaction, there are only two ways to break (cleave) a bond: the electrons in the bond can be divided equally or unequally between the two atoms of the bond.

  4. Homolysis and heterolysis require energy. • Homolysis generates uncharged reactive intermediates with unpaired electrons. • Heterolysis generates charged intermediates.

  5. To illustrate the movement of a single electron, use a half-headed curved arrow, sometimes called a fishhook. • A full headed curved arrow shows the movement of an electron pair.

  6. Homolysis generates two uncharged species with unpaired electrons. • A reactive intermediate with a single unpaired electron is called a radical. • Radicals are highly unstable because they contain an atom that does not have an octet of electrons. • Heterolysis generates a carbocation or a carbanion. • Both carbocations and carbanions are unstable intermediates. A carbocation contains a carbon surrounded by only six electrons, and a carbanion has a negative charge on carbon, which is not a very electronegative atom.

  7. Three reactive intermediatesresulting from homolysisandheterolysis of a C – Z bond

  8. Radicals and carbocations are electrophiles because they contain an electron deficient carbon. • Carbanions are nucleophiles because they contain a carbon with a lone pair.

  9. Heterolytically cleave each of the carbon-hetratom bonds and label the organic intermediate as a carbocation or carbanion a) carbocation b) carbanion

  10. Bond formation occurs in two different ways. • Two radicals can each donate one electron to form a two-electron bond. • Alternatively, two ions with unlike charges can come together, with the negatively charged ion donating both electrons to form the resulting two-electron bond. • Bond formation always releases energy.

  11. Relative stabilities of carbocations

  12. Relative stability of radicals

  13. Bond Dissociation Energy • The energy absorbed or released in any reaction, symbolized by H0, is called the enthalpy change or heat of reaction. • Bond dissociation energy is the H0 for a specific kind of reaction—the homolysis of a covalent bond to form two radicals.

  14. Because bond breaking requires energy, bond dissociation energies are always positive numbers, and homolysis is always endothermic. • Conversely, bond formation always releases energy, and thus is always exothermic. For example, the H—H bond requires +104 kcal/mol to cleave and releases –104 kcal/mol when formed.

  15. Comparing bond dissociation energies is equivalent to comparing bond strength. • The stronger the bond, the higher its bond dissociation energy. • Bond dissociation energies decrease down a column of the periodic table. • Generally, shorter bonds are stronger bonds.

  16. Which has the higher bond dissociation energy? • H-Cl or H-Br b) c)

  17. Bond dissociation energies are used to calculate the enthalpy change (H0) in a reaction in which several bonds are broken and formed.

  18. Bond dissociation energies have some important limitations. • Bond dissociation energies present overall energy changes only. They reveal nothing about the reaction mechanism or how fast a reaction proceeds. • Bond dissociation energies are determined for reactions in the gas phase, whereas most organic reactions occur in a liquid solvent where solvation energy contributes to the overall enthalpy of a reaction. • Bond dissociation energies are imperfect indicators of energy changes in a reaction. However, using bond dissociation energies to calculate H° gives a useful approximation of the energy changes that occur when bonds are broken and formed in a reaction.

  19. Calculate H for each of the following reactions, knowing H of O2 and O-H = 119 kcal/mol, H of C-H = 104 kcal/ml and H of one C=O = 128 kcal/mol. a) Bonds Formed Bonds Broken C-H = 4 x 104 kcal/mol = 416 kcal/mol C-O = 2 x -128 kcal/mol = -256 kcal/mol O-O = 2 x 119 kcal/mol = 238 kcal/mol O-H = 4 x -119 kcal/mol = -476 kcal/mol H = 416 + 238 = +654 kcal/mol H = -256 + -476 = -732 kcal/mol H = 654 + -732 kcal/mol = -78 kcal/mol

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