CVEN 5424 Environmental Organic Chemistry Lecture 24 – Redox Reactions: Reduction

Announcements  Reading  Chapter 14, Sections  Problem sets  PS 8 due Thursday  PS 9 out Thursday Office hours (starting next week) Tuesday 11:30 am-1 pm Wednesday 9-10 am by appointment

Reduction Reactions  Predicting kinetics with rate LFERs  k R  redox potential (Marcus)  E H 1 ; usually first electron transfer (Eqn ) log k R E H 1 / 0.059

Reduction Reactions  Predicting kinetics with rate LFERs  k R  redox potential (Marcus)  E H 1 ; usually first electron transfer (Eqn ) log k R E H 1 / Reduction: faster at higher E H (lower {e - }); able to give e - to compound at lower {e - }

Reduction Reactions  Predicting kinetics with rate LFERs  k R  redox potential (Marcus)  E H 1 ; usually first electron transfer  k R   (Hammett) (Eqn ) log k R E H 1 / log k R  Reduction: faster at higher E H (lower {e - }); able to give e - to compound at lower {e - }

Reduction Reactions  Predicting kinetics with rate LFERs  k R  redox potential (Marcus)  E H 1 ; usually first electron transfer  k R   (Hammett) (Eqn ) log k R E H 1 / log k R  Reduction: faster at higher E H (lower {e - }); able to give e - to compound at lower {e - } Reduction: faster at higher  ; easier to give electrons if more e - - withdrawing

Reduction Reactions  reductive dehalogenation (C reduced, X released)  R—X  R—H  R—CHX—CHX—R  R—CH=CH—R

Reduction Reactions  Reduction of carbon: reductive dehalogenation  R—X  R—H CCl 4 + H e - = CHCl 3 + Cl - R—CHX—CHX—R  R—CH=CH—R

Reduction Reactions  Reduction of carbon: reductive dehalogenation  reducing conditions (e.g., anoxic)  promote breakdown of RX, removal of halogens  usually, products are less toxic  aliphatic (C 1 and C 2 ) – fast  olefinic (R=R-X) – relatively fast  aromatic (Ar-X) – slow (e.g., PCBs, chlorinated benzenes)  most reductants capable of reducing RX  high redox potentials mean easy to reduce

Reduction Reactions  Reductive dehalogenation  rate affected strongly by  ease of breaking the C—X bond  C—X bond strength  stability of carbon radical formed  electron-withdrawing substituents stabilize carbon radical

Reduction Reactions  Reductive dehalogenation  rate affected strongly by  ease of breaking the C—X bond  C—X bond strength  stability of carbon radical formed  electron-withdrawing substituents stabilize carbon radical  rate affected weakly by  tendency of C—X to accept electron (electronegativity)  stabilization of carbon radical by geminal halogen

Reduction Reactions  Rank the following three halogenated ethanes by order of reductive dehalogenation rate from slowest to fastest: compound X electro- negativity bond strength (kJ mol -1 ) 1,2-dibromoethane ,2-dichloroethane ,2-diiodoethane

compound t 1/2 (h) X electro- negativity bond strength (kJ mol -1 ) 1,2-dichloroethane>> ,2-dibromoethane ,2-diiodoethane << slowestfastest Reduction Reactions

 Predicting kinetics – one example Reductive dehalogenation k R  bond strength and electron withdrawal BS: C—X bond strength (kcal mol -1 )  *: Taft constant (Table 12.4, SGI, 1993) BE: C—C or C=C bond energy (kcal mol -1 ) Peijnenburg et al. (1991) Science of the Total Environment 109/110,

Reduction Reactions

compoundBS (kcal mol -1 ) ** BE (kcal mol -1 ) tetraiodoethene ,2-diodoethane ,2-dichloro-1,2-dibromoethane hexachloroethane ,3-dibromobutane ,2-dibromoethane  -hexachlorocyclohexane tetrachloromethane sym-tetrachloroethane tetrachloroethene  -hexachlorocyclohexane ,2-dichloroethane ,1-dichloroethene ,2-dichloroethene perfluorohexane perfluorodecaline Peijnenburg et al. (1991) Science of the Total Environment 109/110, Reduction Reactions

 Example: 1,1,2,2-tetrachloroethane a.k.a., “sym-tetrachloroethane”  C-X bond strength, BS = 81 kcal mol -1  stronger C-X bond (e.g., C-F)  slower rate  Taft constant,  * = 4.20  more electron-withdrawing  faster rate  C-C bond energy, BE = 63.2 kcal mol -1  stronger C-C bond  faster rate Reduction Reactions

 Example: 1,1,2,2-tetrachloroethane a.k.a., “sym-tetrachloroethane”  BS = 81 kcal mol -1   * = 4.20  BE = 63.2 kcal mol -1 Reduction Reactions

 reductive dehalogenation (C)  R—X  R—H  R—CHX—CHX—R  R—CH=CH—R  quinone reduction (C reduction)  O=Ar=O  HO—Ar—OH

Reduction Reactions  Reduction of carbon: quinone reduction  O=Ar=O  HO—Ar—OH

Reduction Reactions  reductive dehalogenation (C)  R—X  R—H  R—CHX—CHX—R  R—CH=CH—R  quinone reduction (C)  O=Ar=O  HO—Ar—OH  nitroaromatic reduction (N reduction)  Ar—NO 2  Ar—NH 2  aromatic azo reduction (N reduction)  Ar-N=N-Ar  2 Ar—NH 2

Reduction Reactions  Reduction of nitrogen: nitroaromatic reduction  Ar—NO 2  Ar—NH 2  aromatic azo reduction  Ar-N=N-Ar  2 Ar—NH 2

Reduction Reactions  Reduction of nitrogen: nitroaromatic reduction

Reduction Reactions  reductive dehalogenation (C)  R—X  R—H  R—CHX—CHX—R  R—CH=CH—R  quinone reduction (C)  O=Ar=O  HO—Ar—OH  nitroaromatic reduction (N)  Ar—NO 2  Ar—NH 2  aromatic azo reduction (N)  Ar-N=N-Ar  2 Ar—NH 2  sulfoxide reduction (S reduction)  R—S(=O)—R  R—S—R

Reduction Reactions  Reduction of sulfur  sulfone reduction  R—S(=O) 2 —R  R—S(=O)—R  sulfoxide reduction  R—S(=O)—R  R—S—R sulindac (a NSAID) sulfide sulfoxide sulfone

Reduction Reactions  Need electron donors for reduction  aqueous species  H 2 S, HS -  Fe(II)  Mn(II)  NH 3  minerals, etc.  sulfide minerals (e.g., pyrite)  Fe(II) minerals (e.g., biotite)  Fe(0) in remediation

reduction of organic contaminants oxidation of reductants

Reduction Reactions  Zero-valent iron  permeable reactive barrier remediation  other metals also (Sn, Zn, etc.)  iron metal is oxidized  “rusting”  corrosion  Fe 0 = Fe e -  E H 0 (W) ~ -0.6 V  Promotes reductive dechlorination

Reduction Reactions  Permeable reactive barrier  maintain redox potential (Fe 0 supply)  avoid clogging  Commercialization  ARS Technologies ARS Technologies

CCl 4 + H e - = CHCl 3 + Cl - Fe(0) = Fe e - CCl 4 + H + + Fe(0) = CHCl 3 + Cl - + Fe 2+ CHCl 3 + H + + Fe(0) = CH 2 Cl 2 + Cl - + Fe 2+ CH 2 Cl 2 + H + + Fe(0) = CH 3 Cl + Cl - + Fe 2+ CH 3 Cl + H + + Fe(0) = CH 4 + Cl - + Fe 2+

Reduction Reactions  Demonstrations for ZVI  halogenated alkanes, alkenes  DDT, DDD, DDE  pentachlorophenol  triazines (e.g., atrazine, RDX)  metals  Cr(VI), As(V)  U(IV)

Fe

 Fe(0) reduction of chlorinated compounds:  methanes  ethanes  propanes  ethenes (Johnson et al., 1996, ES&T 30, ) Reduction Reactions

 Accounting for Fe(0) surface area  better correlation indicates surface process (Johnson et al., 1996, ES&T 30, ) Reduction Reactions

 Rate of reduction by Fe(0)  depends on compound concentration  depends on iron surface area, a s (m 2 g -1 )  k obs (h -1 )= k SA (L m -2 h -1 )  a Fe(0) (m 2 L -1 ) (Johnson et al., 1996, ES&T 30, ) Reduction Reactions

 Fe(0) mechanism(s)?  A) direct electron transfer from Fe(0) at surface  B) reduction by Fe 2+ generated by Fe(0), resulting in metal corrosion  C) catalyzed hydrogenolysis by H 2 formed by reduction of H 2 O during corrosion iron surface, mineral surface, or organic matter Reduction Reactions

 LFER?  relating k SA to two-electron redox potential (E) of compounds ? (Johnson et al., 1996, ES&T 30, ) Reduction Reactions

 More representative redox potentials (Scherer et al., 1998, ES&T 32, ) Reduction Reactions

 LFERs  E LUMO, R 2 = 0.83  E 1, R 2 = 0.81 (Scherer et al., 1998, ES&T 32, ) Reduction Reactions

Next Lecture  Oxidation Reactions