8885dc02_47-747/25/0310:05 AM Page48mac76mac76:385 Part I Structure and Catalysis TABLE 2-1 Melting Point, Boiling Point, and Heat of Vaporization of Some Common Solvents Melting point(C) Boiling point(C) Heat of vaporization(/g) Water 260 Methanol(CH3 OH) 1.100 Ethanol(CH3 CH2OH 117 78 Propanol( CH3 CH2 CH2OH) Butanol (CH3(CH2)2 OH) 117 Acetone(CH3COCH3) 95 Hexane(CH3(CH2)4CH3) Benzene(ceh6) Butane(CH3(CH2)2CH3 0.5 Chloroform( CHCl3) -63 61 247 The heat energy required to coert 1.0 g of a liquid at its boiling point, at atmospheric pressure, into its gaseous state at the sam temperature. It is a direct measure of the energy required to overcome attractive forces between molecules in the liquid phase. attractions between adjacent water molecules that give liquid water great internal cohesion. A look at the elec tron structure of the h,o molecule reveals the cause of these intermolecular attractions 8+ Each hydrogen atom of a water molecule shares an electron pair with the central oxygen atom. The geom- etry of the molecule is dictated by the shapes of the outer electron orbitals of the oxygen atom, which are similar to the sp3 bonding orbitals of carbon(see Fig 1-14). These orbitals describe a rough tetrahedron, with (a a hydrogen atom at each of two corners and unshared 104.5° electron pairs at the other two corners (Fig 2-la).The H-O-H bond angle is 104.5, slightly less than the 09. of a perfect tetrahedron because of crowding the nonbonding orbitals of the oxygen atom. The oxygen nucleus attracts electrons more strongly than does the hydrogen nucleus (a proton) that is, oxygen is more electronegative. The sharing of electrons between H and O is therefore unequal; the alent b electrons are more often in the vicinity of the oxygen 0965 atom than of the hydrogen. The result of this unequal electron sharing is two electric dipoles in the water mol- ecule, one along each of the h-o bonds; each hydro- gen bears a partial positive charge(8) and the oxygen atom bears a partial negative charge equal to the sum of the two partial positives(28). As a result, there is FIGURE 2-1 Structure of the water molecule. The dipolar nature of an electrostatic attraction between the oxygen atom of the H2O molecule is shown by (a)ball-and-stick and(b) space-filling models. The dashed lines in (a)represent the nonbonding orbitals. one water molecule and the hydrogen of another (fig There is a nearly tetrahedral arrangement of the outer-shell electron 2-1c), called a hydrogen bond. Throughout this book, pairs around the oxygen atom; the two hydrogen atoms have local- we represent hydrogen bonds with three parallel blue ized partial positive charges (8)and the oxygen atom has a partial lines, as in Figure 2-lc negative charge(28).(c) Two H2O molecules joined by a hydrogen Hydrogen bonds are relatively weak. Those in liq- bond(designated here, and throughout this book, by three blue lines) uid water have a bond dissociation energy (the en- between the oxygen atom of the upper molecule and a hydrogen atom ergy required to break a bond) of about 23 kJ/mol, com- of the lower one Hydrogen bonds are longer and weaker than cova- pared with 470 kJ/mol for the covalent O-H bond in lent O-H bondsattractions between adjacent water molecules that give liquid water great internal cohesion. A look at the elec￾tron structure of the H2O molecule reveals the cause of these intermolecular attractions. Each hydrogen atom of a water molecule shares an electron pair with the central oxygen atom. The geom￾etry of the molecule is dictated by the shapes of the outer electron orbitals of the oxygen atom, which are similar to the sp3 bonding orbitals of carbon (see Fig. 1–14). These orbitals describe a rough tetrahedron, with a hydrogen atom at each of two corners and unshared electron pairs at the other two corners (Fig. 2–1a). The HOOOH bond angle is 104.5, slightly less than the 109.5 of a perfect tetrahedron because of crowding by the nonbonding orbitals of the oxygen atom. The oxygen nucleus attracts electrons more strongly than does the hydrogen nucleus (a proton); that is, oxygen is more electronegative. The sharing of electrons between H and O is therefore unequal; the electrons are more often in the vicinity of the oxygen atom than of the hydrogen. The result of this unequal electron sharing is two electric dipoles in the water mol￾ecule, one along each of the HOO bonds; each hydro￾gen bears a partial positive charge (

) and the oxygen atom bears a partial negative charge equal to the sum of the two partial positives (2
). As a result, there is an electrostatic attraction between the oxygen atom of one water molecule and the hydrogen of another (Fig. 2–1c), called a hydrogen bond. Throughout this book, we represent hydrogen bonds with three parallel blue lines, as in Figure 2–1c. Hydrogen bonds are relatively weak. Those in liq￾uid water have a bond dissociation energy (the en￾ergy required to break a bond) of about 23 kJ/mol, com￾pared with 470 kJ/mol for the covalent OOH bond in 48 Part I Structure and Catalysis TABLE 2–1 Melting Point, Boiling Point, and Heat of Vaporization of Some Common Solvents Melting point (°C) Boiling point (°C) Heat of vaporization (J/g)* Water 0 100 2,260 Methanol (CH3OH) 98 65 1,100 Ethanol (CH3CH2OH) 117 78 854 Propanol (CH3CH2CH2OH) 127 97 687 Butanol (CH3(CH2)2CH2OH) 90 117 590 Acetone (CH3COCH3) 95 56 523 Hexane (CH3(CH2)4CH3) 98 69 423 Benzene (C6H6) 6 80 394 Butane (CH3(CH2)2CH3) 135 0.5 381 Chloroform (CHCl3) 63 61 247 *The heat energy required to convert 1.0 g of a liquid at its boiling point, at atmospheric pressure, into its gaseous state at the same temperature. It is a direct measure of the energy required to overcome attractive forces between molecules in the liquid phase. 104.5 Hydrogen bond 0.177 nm Covalent bond 0.0965 nm H 
  


(a) (b) (c) 2 H O FIGURE 2–1 Structure of the water molecule. The dipolar nature of the H2O molecule is shown by (a) ball-and-stick and (b) space-filling models. The dashed lines in (a) represent the nonbonding orbitals. There is a nearly tetrahedral arrangement of the outer-shell electron pairs around the oxygen atom; the two hydrogen atoms have local￾ized partial positive charges (
) and the oxygen atom has a partial negative charge (2). (c) Two H2O molecules joined by a hydrogen bond (designated here, and throughout this book, by three blue lines) between the oxygen atom of the upper molecule and a hydrogen atom of the lower one. Hydrogen bonds are longer and weaker than cova￾lent OOH bonds. 8885d_c02_47-74 7/25/03 10:05 AM Page 48 mac76 mac76:385_reb: