Introduction to Third-Row Functional Groups
INTRODUCTION TO THIRD-ROW FUNCTIONAL GROUPS
SOME SIGNIFICANT DIFFERENCES BETWEEN SECOND AND THIRD-ROW ELEMENTS:
(1) Electronegativity:
Group IV (14):Group V (15):Group VI (16):
C(2.5)N(3.1)O(3.5)
Si(1.7)P(2.1)S(2.4)
Bond polarity:
Toleration of positive charge:
R3O+:Oxonium cation - very reactive, powerful alkylation agent.
R3S+:Sulphonium cation - relatively stable species.
(2) Greater tendency towards catenation (chain formation):
R-O-O-ROrganic peroxide - extremely reactive oxidising agent.
R-S-S-R:Organic disulphide - relatively stable compound group.
(3) Decrease in stability of p-p multiple bonds:
[Ar-NN]+Arene diazonium functional group, stable, synthetically important reagent.
[Ar-PP]+Unknown in the free state.
R2C=OCarbonyl compound - very stable.
R2C=SThiocarbonyl compound - very reactive and unstable.
The striking absence of stable p-p multiple bonding for the third (and subsequent) row elements is - at least in part - due to the larger atomic radii of the heavier elements. This results in longer E–E -bonds and therefore weaker -overlap between adjacent p-orbitals.
(4) 'Hypervalent' Compounds - Compounds in which Third-Row Elements have 'expanded' valence shells containing >8 electrons:
(C6H5)3PP(III)Triphenylphosphine8e
(C6H5)3POP(V)Triphenylphosphine oxide10e
(C6H5)5PP(V)Pentaphenylphosphorane10e
(CH3)2SS(II)Dimethylsulphide8e
(CH3)2SOS(IV)Dimethylsulphoxide10e
(CH3)2SO2S(VI)Dimethylsulphone12e
Are 3d orbitals involved in P and S hypervalent compounds?
SUMMARY OF SOME IMPORTANT SULPHUR FUNCTIONAL GROUPS BY OXIDATION STATE.
(1) Sulphur 0:
RSH Thiol
R(CS)SH Dithiocarboxylic acid (SH sulphur)
(2) Sulphur +1:
RS-SRDisulphide
(3) Sulphur +2:
R2SSulphide
RHC=SThioaldehyde
R2C=SThioketone
R(CS)SHDithiocarboxylic acid (thiocarbonyl sulphur)
RSOHSulphenic acid
RSXSulphenyl halide
(4) Sulphur +4:
R3S+ Sulphonium cation
Sulphoxide
Sulphinic acid
(5) Sulphur +6:
Sulphone
Sulphonic acid
Sulphonyl chloride
(1)Reaction of an organic halide with hydrosulphide anion:
(2)Preparation from Thiourea:
No sulphide by-products.
(3)Preparation from Organolithium or Grignard reagents - t-BuSH and ArSH only.
PROPERTIES AND REACTIVITY OF THIOLS
The most important single group of sulphur compounds - the synthetic gateway to most other organosulphur species.
Volatile liquids or solids with offensive odours. (The stench of EtSH can be detected by the nose at a concentration in air of 1 part in 5.0 x 1013!)
(1) Acidity:
Thiols are much stronger acids than alcohols:
ROHpKa ca.16-18; RSHpKa ca.9-12
Thiolate anion, RS-, is a weaker base than alkoxide anion RO-.
(2) Nucleophilicity:
Thiolate anion, RS-, is a much stronger nucleophile than alkoxide anion RO-.
(a) Polarisability:
Larger, less tightly-held S valence shell more easily polarised, i.e. distorted towards E+ than more compact O valence shell.
(b) Solvation:
INTERCONVERSIONS OF SULPHUR FUNCTIONAL GROUPS
(1) Reactions based on the nucleophilicity of organic sulphur:
(2)Reactions involving oxidation or reduction of sulphur functional groups.
(3) Reactions based on nucleophilic displacement at divalent sulphur.
APPLICATIONS OF Sulphur-containing Functional Groups in Organic Synthesis
(1) Reductive cleavage of C-S bonds - desulphurisation of thiols and sulphides:
(2) Nucleophilicity of thiolate anions - demethylation of methyl(aryl) ethers, sterically crowded methyl esters and methylammonium cations:
(3) Sulphur-stabilised cations:
(4) Sulphur-stabilised -Carbanions