Boron: Uses, Borides, Boranes, Boron Trihalides, Boric Acid (11/20)

is boron a metal?

no

Describe the structure of B12

icosahedron

What is an icosahedron?

20 faces, 30 edges, 12 vertices

is boron easy to purify?

no

why is boron difficult to purify?

high MP, corrosive liquid

easier method for purifying boron?

can reduce B2O3 by Mg, resulting in 98% pure powder

harder method for purifying boron?

boron halide + reducing agent + catalyst → pure boron + compound

B2O3 / B(OH)3 are ___ (acidic/basic/amphoteric)

acidic

Borates/silicates have ___ structures

similar

B/Si halides are ___ hydrolyzed

readily

B/Si ___ are volatile/flammable/hydrolyzed

hydrides

name two uses of boron

pyrex, cleaning agents

describe thermal shock

when something is heated to a high temperature and then immediately cooled, the immediate attempt to shrink can cause breakage (ex. glass)

why is pyrex better than glass?

has more thermal shock resistance

what structure is pyrex similar to?

SiO2

describe boride structure

hard, high MP, chemically resistant

what boron compound is used in armor & bicycle frames?

boron carbide

how do you make boron carbide?

reduction of B2O3 with C

describe the structure of titanium boride

hexagonal sheets of boron ions, each boride has -1 oxidation

how do you make titanium boride?

reduction of TiO2 with C in presence of B4C

is magnesium boride good at conducting?

yes - superconducts usually

describe boranes’ bonding

electron deficient

draw the diborane structure

Describe diborane

highly reactive, toxic, colorless gas

borane + moisture =

boric acid, dihydrogen

diborane used in ___ of organic compounds

reduction

what are the five class structures of borane?

closo, nido, arachno, hypho, conjuncto

describe the polyhedron of closo borane

complete, regular

describe the polyhedron of nido borane

(n+1) vertex polyhedron

describe the polyhedron of arachno borane

(n+2) vertex polyhedron

describe the polyhedron of hypho borane

(n+3) vertex polyhedron

Wade’s Rules: closo

F = 2n + 2

Wade’s Rules: nido

F = 2n + 4

Wade’s Rules: arachno

F = 2n + 6

what type of borane structures might see a B-H-B bridge bond?

nido, arachno

what type of borane structures might see a B-H terminal atom?

nido, arachno

what type of borane structures might see a BH2 group?

arachno

B4H10: give the borane nomenclature

tetraborane(10)

B3H7: give the borane nomenclature

triborane(7)

give the equation for Wade’s Rules

F = 3b + 4c + h + x - 2n

What kind of acids are boron trihalides?

Lewis acids

Why are boron trihalides Lewis acids?

vacant p orbital?

If an acid has more overlapping of p orbitals, it is ___

weaker

Give the orbital mixing explanation for why BF3 is a weaker acid than BBr3.

more overlapping = more orbital mixing is more favorable with a good energy max (more likely to get this with 2p F then a 4p Br orbital)

Give the bond distance explanation for why BF3 is a weaker acid than BBr3.

smaller halide = less bond distance = more overlapping = weaker acid

What type of bond character do we see in BCl3?

single

What type of bond character do we see in BF3?

single, double

What kind of acid is boric acid?

Lewis acid