Ch 15. Group 15

2

N2 / O2 separation

BP MW main uses

N2 77K 28 inert gas/coolant

O2 90K 32 fuel/medial

3

Elemental Forms

N2 B(N≡N) = 946 KJ/mol (kinetically inert)

N2 fixation:

N2 + 3 H2 ↔ 2 NH3

Haber process, industrial source of all N compounds

400°C, 200 atm, Fe cat

4

N2 chemistry 6 Li + N2 → 2 Li3N

[Ru(NH3)5(H2O)]2+ (aq) → → Cat process to N cmpds?

N2 → NH4+

enzyme w/ Fe4S4 cage + MoFe7S8 cofactors (ferrodoxins)

bacteria

cat = nitrogenase

5

P allotropes

white P black P

Molecular, Td, rapidly oxidized to phosphate in air

red P many polymorphs, air stable hexagonal puckered sheets

prep from high P or Bi flux, air stable

6

History From “The 13th

Element: The Sordid Tale of Murder, Fire, and Phosphorus” by John Emsley

7

Elemental forms

As, Sb, Bi → incr. metallic character

Single vs. multiple bonds D(E-E) D(E=E) D(E≡E) N 163 409 946 N≡N P 201 -P-P- O 142 447 O=O S 264 431 -S-S- generally in the p-block, π-bonds are uncommon except with period 2 elements

8

Halides almost all group 15 halides are air sensitive:

PCl3 + ½O2 → O = PCl3 oxidation

PCl3 + 4H2O → H3PO4 + 3HCl + H2 oxidation + hydrolysis

all pentahalides hydrolyze rapidly and generate HX N forms endoergic halides NF3 to “NI3” show decreasing stability NF4

+ is isostructural to ammonium and is the only stable N(V) halide P to Bi MX3 MX5 MX6

− all are known for X = F, most for Cl, some for Br,I C3v D5h Oh

9

Halides PF5 to BiF5 show increasing Lewis acidity

ex : PF5 + F− → PF6− ΔH = - 340kJ/mol

SbF5 + F− → SbF6− - 500kJ/mol

SbF5 is an oligomeric, viscous, colorless liquid

(SbF5)4

Heavier congeners tend to higher CN

10

Group 15 Frost diagrams

11

Group 15 redox trends • NO3

− and Bi(V) are strong oxidants

• NO3− should be the strongest oxidant from general periodic trend

down a group (higher χ and higher IE result in less stable high oxidation state). But there is no regular trend.

• Bi(III) is unusually stable due to inert pair effect

• PO43− is unusually stable due to strong P=O bonding

• Low pH increases oxidation strength of nitrogen oxoanions and also often increases rate (via protonation of N-O bonds)

• most reactions are slow and many species are kinetically stable

ex: NO2− , N2O, NO, NO2 ↔ N2O4

12

N oxides

N2O4 is isoelectronic w C2O4

2− (oxalate). Since C has lower χ than N, oxalate has a stronger M-M bond and there is no appreciable equ w/ monomer

13

N oxides

14

N oxides 4 HNO3 (aq) → 4 NO2 (aq) + O2 (g) + 2 H2O (l)

More rapid for conc. HNO3 due to presence of undissociated acid

Easier to break N-OH vs N=O

N2O (g) + 2 H+ (aq) + 2 e- → N2 (g) + H2O (l)

E° = + 1.77 V at pH = 0, but it’s a poor oxidant due to slow reaction kinetics

NO+ (solv) + e- → NO (g)

E° ~ + 1.1 V, nitrosyl cation is a facile oxidant with rapid kinetics

15

Low oxidation state N

Ox state -3 -1 -2

Ammonia hydroxylamine hydrazine

pKb 4.8 8.2 7.9

also N3- (azide) which is isoelectronic with CO2 and N2O

NaN3

∆↓

Na (m) + 3/2N2 (g)

16

Pourbaix diagrams

17

P oxides

18

Phosphates

19

P oxides mostly strong reducing agents (except for P(V)), especially in base

Generally labile reactions

Ox state

+1 H2PO2− (hypophosphite) H3PO2 is monoprotic

+3 HPO32− (phosphite) H3PO3 is diprotic

+5 PO43− (phosphate) Td H3PO4 is triprotic

Anhydride acid

P4O6 ⇔ H3PO3

P4O10 ⇔ H3PO4

H2O

20

Sb2O4

21

PS compounds

Matches:

P4S3 + KClO3 + filler/glue/water = strike anywhere

KClO3 (head) and red P (stripe) = safety

P4S3

22

PN compounds

planar but not aromatic

P4(NR)6

Note that –P=N- is isoelectronic with –Si=O- (siloxanes)

nPCl3 + nNH4Cl → (Cl2PN)n + 4n HCl n = 3 or 4

dichlorophosphazene trimer or tetramer

oligomer

↓ 290 °C + Lewis acid initiator

(Cl2PN)n polydichlorophosphazene, elastomeric at RT

↓ 2n NaOR (can be OR− , NR2−)

[(RO)2PN]n

130 °C

These hydrolyze in air to form phosphate and HCl

An air stable poly-phosphazene

23

Arsine ligand 4 As + 6 CH3I → 3 (CH3)2AsI + AsI3 (CH3)2AsI + Na → Na+(CH3)2As- + NaI Na+(CH3)2As- → o-C6H4(As(CH3)2)2 soft LB, bidentate

o-C6H4Cl2 / THF

[PdCl6]2-

24

Organoarsine chemistry

As(CH3)3 + CH3Br → As(CH3)4+Br- oxidative addition As(III) -> As(V)

For As(Ph)3 , this does not work Ph3As=O + PhMgBr → Ph4As+Br- + MgO acid-base exchange (Td) LiPh AsPh5 + LiBr

As-As bonding

25

2As(CH3)2Br + Zn → (CH3)2As-As(CH3)2 + ZnBr2

As5(CH3)5