Download Group 13 and 14 Group 14: Carbon

Group 13 and 14
• Most dramatic variation in a
period from top to bottom
(non-metals to metals)
– Oxidation states +3 and +4
– Metals +1,+2
• Light members B,C, Si are
important cationic nonmetals
• Typically oxophiles and
fluorophiles
– borates, silicates, alumina,
carbonates
• Readily form alkyl and
hydrogen covalent bonds
– boranes, silanes, hydrocarbons
Group 14: Carbon
• Carbon has vast organic chemistry
– Forms bonds with H and electronegative
elements O, S, N, Halogens
• Lack of C-O extended structures
• Carbon cluster chemistry
– Fullerenes
– Nanotubes
• Carbide chemistry
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C-OH Condensation
• B, Si, Al and many metallic hydroxides
condense readily to form extended MO-M polymers and networks
– Siloxanes, alumina, borates
• One explanation:
– Condensation typically requires a
coordination+1 intermediate
• protonated or aquated
– i.e. Si(OH)4 forms Si(OH)4(H2O) to form SiO-Si bonds
Carbon Cluster Chemistry
• Relies on C-C covalent bond formation
• Simple examples: network structures
graphite and diamond
• Fullerenes: C2n ranging from n=30-48
– Exciting (overhyped)
new allotrope of C
– Relatively easily
synthesized
– C60 I point group
• E, 12 C5, 20 C3, 15 C2
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Reactivity: Halogens
• C60 reacts
according to
localized
double and
single
bonds, I.e. it
isn’t
aromatic
Organometallic Fullerenes
• Endohedral Fullerenes
– Caged metal ions Mx@Cn
• Synthesized by laser ablation of C/M surfaces
• M includes Sc, Y, La typically in C80, C82, C84
• Fullerene Ligands
– Most transition metals have been bonded
to fullerene in varying fashion,
– typically as a η2 ligand I.e. similar to double
bond not aromatic ring
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Pd(η2 –C60)(PPh3)2
Nanotubes: the Useful Fullerenes
• Discovered
shortly after
fullerenes
• Structuraly can
be thought of
fullerenes with
excess C6 rings
stretching out
the sphere in
one dimension
• Very high aspect
ratio materials
4
Applications
•
•
•
•
Nanowires
Nanocomposites
Nanoswitches
Nanocatalysts
• Macro Problem:
– too stable, won’t form bonds with matrices
– Requires reflux in conc. HNO3/H2SO4 to
“functionalize” nanotube surfaces
SWNTs and MWNTs
• “crude”
synthetic
methods
result
diverse
products
mostly
multi-wall
nanotubes
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Amorphous Carbons
• Non Crystalline Carbon
– carbon black
– activated carbon
– carbon fibers
– charcoal
– soot
• All are mixtures of graphite and
fullerenes, properties almost completely
determined by surface area, impurities
and oxidation
Carbides: C anion chemistry
• Compounds of C with electropositive
elements
• Three types
– Saline Carbides: Ionic Carbides
• Grps 1,2, and Al
– Metallic Carbides
• Carbides with the transition metals, most
common
– Metalloid Carbides
• Carbides with B and Si, exceedingly hard
materials
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Saline Carbides: Ionic
• Three types
– Intercalation: graphite
with Grp 1 metal vapor
– Dicarbides: contain the C22aceylide anion in the holes in
a metal ion lattice: CaC2
– Methides: very rare, C4anions from Na, Be, Al
carbides
• Na4C, Be2C, Al4C3
Intercalated Graphites
• Two types of intercalated graphite
– Buckled layers – pi system destroyed
• Colourless non-conductor
• Carbon monofluoride (CF)n :
lubricious material
– Retained pi systems
•
•
•
•
Coloured, electrical conductors
Properties vary with intercalant
Alkali metals with varying degrees of inclusion
Better conductors than graphite itself Æ addition of
electrons to the pi system
• KC8 Æ KC24 Æ KC36 Æ KC48Æ KC60
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Metallic Carbides
• Formed by combination of C with the
transition metals
• Formally are interstitial alloys of C
anions in the octahedral holes of metal
crystal structure
• However, some evidence for directional
bonding: i.e. the hard sphere model of
ionic lattices doesn’t always apply
Metalloid Carbides: Si, B
• Small electronegativity differences give
covalent C-Si or C-B bonding
• Silicon carbide: SiC carborundum
• Boron carbide: B4C
• extremely hard, inert materials, v. high
melting
• Industrial uses, particularly SiC
– Made by high temperature reduction of Si
or B oxides in the presence of graphite
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Grp 14: Silicon
• “If we weren’t made out of C, we would
be made of Si”
• Some very similar behaviour to C
• Significant differences
– Catenation vs Condensation
Bond
C-C
Si-Si
S-S
Energy
356
226
226
Bond
C-O
Si-O
S-O
Energy
336
368
330
Bond Stability vs Polarity
• Note: bond stability is not the last word
in reactivity,
– depends on reaction mechanism
– S-Cl (391 kJ mol-1) vs Si-C (275 kJ mol-1)
– Charge separation is important,
– Si-Cl is very polar, thus Si undergoes
nucleophilic attack by OH- to form Si-OH
– Si-C is stable to nucleophilic attack
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Silicates
• Useful, very common, stable
• Based on tetrahedral Si: SiO4 units,
attached at vertices to other SiO4 units
Si
HO
OH
OH
OH
OH
Si
O
Si
O
OH
OH
OH
SiO4 Structures
• SiO44-, Si2O72-, Si6O1812-
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Cages as Well
Silicate Glasses:
• amorphous structure of SiO42- units, a
random assembly of units
• Random assembly:
– Coordination of 3 or 4 for non O elements
(eg. B, Al, Si)
– Only one O atom shared between any two
non O atoms
• Inclusion or Al, B, alkali metals alters
the degree of crystallization in the glass,
alters physical and chemical
characteristics
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Siloxanes (silicones)
• Formed by the condensation of
(R)xSi(OH)4-x species
• Result is Si-O-Si bonds with other Si-R
functionality present
HO
Si
O
CH3
Si
CH3
CH3
CH3
CH3
O
CH3
Si
O
CH3
Si
O
CH3
CH3
n
Aluminosilicates
• Silicates which contain aluminum ions
are aluminosilicates
• Al may combine with silicate matrix in
two ways
– Occupying vacant Oh holes
– Occupying Td holes, displacing Si atom
• Al3+ and Si4+ are similar in size so replacement
is common
• Requires charge compensation, one additional
alkali cation
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Layered Aluminosilicates
• Common minerals
– talc, clay, mica
• Individual layers are
charge neutral,
weakly bound to each
other
• Minerals exhibit
cleavage planes and
water storage
Molecular Sieves & Zeolites
• Aluminosilicate crystalline materials with
cavities on a molecular scale
• Na12[(AlO2)12(SiO2)12]-xH2O A Type
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Group 14: Carbon and Silicon
• Carbon
– catenation
– cluster chemistry
• fullerenes and tubes, reactivity, uses
– carbides
• saline, metallic, metalloid
• Silicon
– reactivity, condensation vs catenation
– silicates, siloxanes, aluminosilicates
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