Chapter 18

 Oxygen compounds are known of all elements  Except: He,Ne and possibly Ar  Oxygen Chemisrty involves  Obtaining Ne configuration by one of the following means

 1.) Electron gain to form the Oxide O 2-  2.) Formation of 2 single bonds usually in bent systems such as water or ethers.  3.) Formation of a double bond (org. Compounds)  4.) Formation of a single bond, as well as electron gain such as OH -  5.) Formation of three covalent bonds such as in H 3 O +  6.) Formation in rare cases of 4 covalent bonds

 Oxygen has the possibility of a broad range of bond types from ionic to covalent, which is seen in the binary oxides.  Highly ionic ones  alakli and alkaline earh metal oxides  Completely covalent ones  CO 2  Intermediates  B,Al, Si

 Formation of Oxide ion from Oxygen requires 1000 kJ/mol.  The stability of ionic metal oxides comes from the high lattice energies with the small and highly charged oxide ion.

 Wen the lattice energy is not sufficient to offset the eneries for ionisation, oxides with more covalent character are formed.  Oxides with some covalent character.  BeO  SiO 2  B 2 O 3

 Covalent oxides are compounds such as:  CO 2  SO 2  SO 3  NO 2  Dominated by Covalent bonding  An important aspect of bonding of molecular oxides is the use of p-orbitals in π bonding wit other atoms.

 p π - p π bonding is found in ketones (organic compound)  p π - d π bonding is found in  phosphine oxides (R 3 P=O)  or  linear M=O=M systems  Terminal Oxygen atoms with 3 lone pairs  (OH) -  These Oxygen atoms are sp 3 hybridized

 Covalent compounds with oxides with 2 lone pairs of electrons are typically angular molecules.  Water (104.5˚)  Alcohols  Ethers  Oxygen atoms are sp 3 hybridized.

 Atoms with d orbitals  p π –d π is often present in the bond to oxygen.  Si-O-Si in quartz 142˚  H 3 Si-O-SiH 3 is greater than 150˚  Linear molecules are found in systems with transition metals.

 The Oxonium Ion :OH 3 +  sp 3 hybridized  Oxonium Ion formation is analogous to the formation of ammonium ions.  Oxygen is less basic than nitrogen, oxonium ions are less stable.  OH 4 2+ is unlikely  repulsion towards proton  Oxonium ions undergo rapid inversion

 Basic Oxides  Dissolve in dilute acids:

 Acidic Oxides  Covalent non metal oxides are acidic  Unsoluble oxides will dissolve in bases

 Amphoteric Oxides  Other Oxides  Redox than acid-base

 Oxygen has  3 isotopes  2allotropes (O 2 ) and (O 3 )  O 2 is paramagnetic has a high dissociation energy  Ozone  Is diamagnetic  Electrical discharge produces Ozone

 Ozone is an oxidizing agent.  Oxidizing Potential

 Colorless Liquid  Resembles water  Igher hydrogen bound  40% denser than water  High Dielectric constant  Strong oxidizing agent  Heavy metal traces act as catalysts in dissociation reactions

 In dilute aqueous solutions:  More acidic than water  Production of peroxide:

 S,Se,Te,Po have lower electronegativities than oxygen  Compounds less ionic  Relative stabilities of bonds is different  H-bonding is lower  S---H--S bonds exist but are very weak  Like all others S has d π - p π bonds  SO 4 2- has multiple d π - p π bonds.

 The valence for S,Se,Te,Po is not confined to 2, and d orbitals can be used to form more than 4 bonds to other elements.  SF 6  S has a strong tendency to catenation  It forms polysulfide ions, polythionate ions

 Changes in properties from S to Po  Size of atoms increase  Electronegativity decreases  1. The decreasing stability of the hydrides.  2. The increasing tendency to form complex ions such as SeBr  3. The appearance of metallic properties for Te and Po atoms. Thus the oxides M0 2 are ionic and basic, reacting with HCI to give the chlorides.

 Sulfur occurs widely in nature as the element, as H 2 S and S02, in metal sulfide ores, and as sulfates [e.g., gypsum and anhydrite (CaS04), magnesium sulfate, and so on].

 Selenium and tellurium are less abundant but frequently occur as selenide and telluride minerals in sulfide ores, particularly those of Ag and Au.  Polonium occurs in U and Th minerals as a product of radioactive decay series. The most accessible isotope, 21OpO (a, days), can be made in gram quantities by irradiation of Bi in nuclear reactors.

 On melting, Ss first gives a yellow, transparent, mobile liquid that becomes dark and increasingly viscous above about 160°C. The maximum viscosity occurs about 200 °C, but on further heating the mobility increases until the boiling point ( C), where the liquid is dark red.

 Sulfur vapor contains S 8 and at higher temperatures S 2 molecules. The latter, like 0 2 are paramagnetic with two unpaired electrons, and account for the blue color of the hot vapor.  The elements S, Se, and Te burn in air on heating to form the dioxides; they also react on heating with halogens, most metals, and nonmetals. They are attacked by hot oxidizing acids like H 2 S0 4 or HNO 3.

 The S,Se and Te 2+ ions are square

All reactions of S 8 must involve initial ring opening to give sulfur chains or chain compounds. Many involve nucleophilic reactants, for example,

 The hydrides are extremely poisonous gases with revolting odors. The toxicity of H 2 S far exceeds that of HCN. The thermal stability and bond strengths decrease down the series, whereas the acidity in water increases.  Hydrogen sulfide dissolves in water to give a solution about 0.1 M at 1 atm.

 It is a colorless fuming liquid (bp 80 degC) that is readily hydrolyzed as in Reaction  Since the products are volatile it is used to prepare IronChloride

 The dioxides are obtained by burning the elements in air. Sulfur dioxide is produced when many sulfides are heated in air. Selenium and tellurium dioxides are also obtained by treating the elements with hot nitric acid to form H 2 Se0 3 and 2 Te0 2 ·HN0 3 heating these drives off water or nitric acid.

 Sulfur dioxide has lone pairs and can act as a Lewis base. However, it also acts as a Lewis acid giving complexes, for example, with amines, as in (CH3hNS0 2 and with electron-rich transition metal complexes.

 Sulfur dioxide is quite soluble in water; such solutions, which possess acidic properties, have long been referred to as solutions of sulfurous acid, H 2 S0 3

 Sulfur trioxide is obtained by reaction of S0 2 with 0 2 reaction needs a catalyst such as platinum sponge, V or N0.  Sulfur trioxide reacts vigorously with water to form sulfuric acid.  Industrially, S0 3 is absorbed in concentrated H 2 S0 4 to give oleum, which is then diluted.