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Chapter 13

CHMB31 Chapter 13

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Alen Hadzovic

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Chapter 13: The Group 13 Elements The Essentials • Group 13 Elements: boron, aluminum, gallium, indium and thallium. • Boron is the only nonmetal in the group. • Aluminum is the most abundant Group 13 element. • There is an increase in metallic character from B to Tl; B is a nonmetal, Al is a metalloid and Ga, In and Tl are all metals. • Boron’s diagonal relationship with silicon. o Boron and silicon form acidic oxides, B O 2n3 SiO ; al2minum forms an amphoteric oxide. o Boron and silicon form many polymeric oxide structures and glasses. o Boron and silicon form flammable, gaseous hydrides; aluminum hydride is a solid. • Aluminum is an electropositive metal but it is very inert because of the presence of a passivating surface oxide film, not oxidized by air. • Gallium is brittle at low temperatures but liquefies at 30°C and has the widest liquid range of 30-2420°C. • All of the elements form hydrides oxides and halides in the +3 oxidation state. o Many of the compounds made from lighter Group 13 elements have incomplete octets and act as Lewis acids (electron pair acceptor). o Boranes: binary hydrogen compounds of B. o Boron trihalides are monomeric in the gas, liquid and solid state unlike the rest of the halides in Group 13. o 4 B(OH) (3) + heat  2 B O (2)3+ 6 H O (l2 o B 2 3l) + 2 NH +3heat  2 BN (s) + 3 H O (g)2  BN structure consists of planar sheets of atoms like graphite.  BN and graphite have similar physical properties, both used as lubricants but BN is a white, nonconducting solid. • The +1 oxidation state becomes more stable down the group and is the most stable oxidation state for compounds of thallium (due to inert pair effect). o Because the F ion is so small, the trifluorides are mechanically hard ionic solids that have much higher melting points and sublimation enthalpies than other halides; they are not very soluble and do not act as a Lewis acid. o The heavier trihalides of Al, Ga and In are soluble in a wide variety of polar solvents and are excellent Lewis acids. • The most important oxosalts of Group 13 are called alums, MAl(SO ) • 124 2O, 2 where M is a univalent cation like Na , K , Rb , Cs , Tl or NH . 4+ o Gallium and indium can also form analogous series of salts but B and Tl do not because B is too small and a Tl atom is too large. o Alum is used widely to describe other compounds with the general I III formula MM’ (SO ) •4 2 H O, 2here M’ is often a d metal, such as Fe. • Boron forms an extensive range of polymeric, cage-like compounds which include the borohydrides, metallaboranes and the carboranes. o Closo Structure: borohydrides with the formula [B H ]n. n2- o Nido Structure: when boron clusters have the formula [B H n n+4] formula. o Arachno Structure: when clusters of boron have the formula [B H n n+6. o Metallaboranes: borons that form many metal-containing clusters. o Carboranes: a large family of clusters that contain both B and C atoms. The Detail • Aluminum is highly abundant; thallium and indium are the least abundant of the Group 13 elements. • 2 BBr 3g) + 3 H (g2  2 B (s) + 6 HBr (g) • The most useful compound of boron is borax (Na B O (OH) 2 4 5); th4 most2 commercially important element is aluminum because it its light and resistant to corrosion. • Diborane (B H) can be synthesized by metathesis between a boron halide and 2 a hydride source; many of the higher boranes can be prepared by the partial pyrolysis of diborane. o 3 LiBH 4et) + 4 BF (e3)  3 LiBF (et4 + 2 B H (g)2 6 o All the boranes are colorless and diamagnetic. o They range from gases (B H an2 6 H ) th4o8gh volatile liquids (B H ) to 5 9 sublimable solid (B H10.14 o B2H 6g) + 3 O (2)  2 B(OH) (s) 3 o B2H 6g) + 6 H O2(l)  2 B(OH) (aq3 + 6 H (g) 2 o B2H 6s a Lewis acid and the mechanism of this hydrolysis reaction involves coordination of H O 2cting as a Lewis base. • All the boron hydrides are flammable, sometimes explosively, and many of them are susceptible to hydrolysis. • Soft and bulky Lewis bases cleave diborane symmetrically; more compact and hard Lewis bases cleave the hydrogen bridge unsymmetrically. o Symmetric Cleavage: B H 2 6 is broken symmetrically into two BH 3 fragments; each of which forms a complex with a Lewis base. o Unsymmetrical Cleavage: generally observed when diborane and a few other boron hydrides react with strong, sterically uncrowded bases at low temperatures (NH ). 3 • Although it reacts with hard Lewis bases, diborane is best regarded as a soft Lewis acid. • Hydroboration, the reaction of diborane with alkenes in ether solvent, produces organoboranes that are useful intermediates in synthetic organic chemistry. • The tetrahydridoborate ion is useful intermediate for the preparation of metal hydride complexes and borane adducts. o B2H 6 2 LiH  2 LiBH 4 o
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