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

400C, 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 bondsD(E-E) D(E=E) D(E≡E)

N 163 409 946 NNP 201 -P-P-O 142 447 O=OS 264 431 -S-S-

generally in the p-block, -bonds are uncommon except with period 2 elements

8

Halidesalmost 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 BiMX3 MX5 MX6

all are known for X = F, most for Cl, some for Br,IC3v 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 C2O42

(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 oxidesmostly 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