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Notes on organic redox reactions: Advanced Level Theoretical-Physical Chemistry
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REDOX CHEMISTRY Part 3 Sections 8 to 10 Organic Oxidation & Reduction – Preparative Redox Reactions in Organic Chemistry Advanced Level Theoretical Organic Chemistry Revision Notes on REDOX reactions involving organic oxidation and reduction synthesis–preparations. Reactions involving the oxidation or reduction of the starting reactant molecules and the chemistry of the appropriate oxidising agents and reducing reagents is described and discussed. How do you work out the oxidation state/number of carbon in organic molecule compounds?
PART 12 ORGANIC REDOX REACTIONS
REDOX section–index: 2. Introducing oxidation state (with sub–index) * 3. Oxidation state rules–guidelines & inorganic examples * 4. Naming inorganic compounds * 5. Using oxidation states to describe redox changes in a given inorganic reaction equation (with sub–index) * 6. Constructing full inorganic redox equations from half–equations (with sub–index) * 7. Redox titrations * 8. Organic synthesis reductions (with summary table) * 9. Organic synthesis oxidations (with summary table) * 10. Other Organic Redox Reactions (with sub–index) * 11. Carbon's ox. state in selected organic compounds, functional group level
*
See also Equilibria Part 7
Redox Reactions for half cell equilibria, electrode potential, standard hydrogen electrode, Simple cells and notation,
Electrochemical Series, EØcell for reaction feasibility, 'batteries' and fuel cell systems
etc.
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Redox
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in Organic Chemistry A summary of some redox reactions used in organic synthesis is given below. Further details for selected reactions are given below the summary tables. Most of the reactions described are found in one or other of UK based GCE–AS–A2 or IB courses. The application of oxidation states to organic molecules can be tricky, but, (i) use of half–cell equations usually gets round the problem, and (ii) hopefully the oxidation state exemplars in the last section will help illuminate the situation if you are interested, but this knowledge is not required at this level? Important examination note: Unless hydrogen gas or oxygen gas is used directly in the redox synthesis reaction [O] and [H] should be used in simplified equations and examples will be quoted in each section and some syllabuses specifically state so. The concept of functional group level is dealt with in appendix 1. |
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8. Summary of some ORGANIC SYNTHESIS REDUCTION REACTIONS Guide notes: YES/NO denotes whether reaction possible. Lithium tetrahydridoaluminate(III), LiAlH4 (lithium aluminium hydride), is a more powerful reducing agent than sodium tetrahydridoborate(III), NaBH4 (sodium borohydride) and accounts for the NO/YES differences in columns (a) and (b). R = H, alkyl or aryl for aldehydes, carboxylic acids and nitriles, R = alkyl or aryl for ketones. Click on 8.1, 8.2, 8.3, 8.4 and 8.5 in the table for more details on (a), (b), (c) or (d) reaction reagents/conditions and the chemical outcome. Important examination note: Unless hydrogen gas or oxygen gas is used directly in the redox synthesis reaction [O] and [H] should be used in simplified equations and examples will be quoted in each section and some syllabuses specifically state so. |
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homologous series change on reduction | molecular structure change on reduction | reducing agent (a) NaBH4 in water (lab method) | reducing agent (b) LiAlH4 in e.g. ether (lab method) | reducing agent (c) reflux with Sn/conc. HCl(aq) (lab method) | reducing agent (d) Ni/H2 (industry), but many other hydrogenation catalysts based on Pt, Pd etc. |
8.1 alkene ====> alkane |
>C=C< ====> –CH–CH– |
NO | NO | NO | YES, Ni/H2 used |
8.2 aldehyde/ketone ====> primary/secondary alcohol |
RCHO
==> RCH2OH and R2C=O ==> R2CHOH |
YES | YES | NO but can be reduced with Na/C2H5OH or Zn/CH3COOH mixtures | NO with
Ni/H2 YES with other specialised catalysts |
8.3 carboxylic acid ====> primary aliphatic alcohol |
RCOOH ====> RCH2OH |
NO | YES | NO | NO with
Ni/H2 YES with other specialised catalysts |
8.4 nitrile ====> primary aliphatic amine |
RC ====> RCH2NH2 |
NO | YES | NO | YES with Ni/H2 |
8.5 nitro–aromatic ====> primary aromatic amine |
e.g. C6H5NO2 ====> C6H5NH2 |
NO | YES | YES | YES with Ni/H2 |
************************ | ********************* | ********** | *********** | ************* | ***************** |
Some further details of the organic reductions tabulated above
8.1 Reduction of alkenes to alkanes
8.2 Reduction of aldehydes to primary alcohols and ketones to secondary alcohols
8.3 Reduction of a carboxylic acid to a primary aliphatic alcohol
8.4 Reduction of nitriles to primary aliphatic amines
8.5 Reduction of nitro–aromatics to primary aromatic amines
2C6H5NO2(aq) + 14H+(aq) + 3Sn(s) ==> 2C6H5NH3+(aq) + 3Sn4+(aq) + 4H2O(l)
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9. Summary of some ORGANIC SYNTHESIS OXIDATION REACTIONS Guide notes Some direct catalytic oxidations at higher temperatures used in industry are included, but most are 'school laboratory' reactions. YES/NO – for a laboratory synthesis. R = alkyl or aryl for primary/secondary alcohols. Click on 9.1, 9.2, 9.3, 9.4 and 9.5 in the table for more details on (a), (b) etc. Important examination note: Unless hydrogen gas or oxygen gas is used directly in the redox synthesis reaction [O] and [H] should be used in simplified equations and examples will be quoted in each section and some syllabuses specifically state so. |
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homologous series change on oxidation | molecular structure change | (a) heat with mod conc. H2SO4 and K2Cr2O7(aq) (lab method) | (b) reflux with KMnO4/NaOH(aq) (lab method) | (c) oxygen + catalyst or thermal decomposition (industrial methods) |
9.1 primary alcohol ==> aldehyde ==> carboxylic acid | RCH2OH ==> RCHO ==> RCOOH | YES, can separate intermediate aldehyde, or allow complete oxidation to carboxylic acid | YES but only get RCOOH and of little synthetic use | e.g. CH3CH2OH ==> CH3CHO (Cu/500oC) |
9.2 secondary alcohol ==> ketone | R2CHOH ==> R2C=O | YES | YES but of little synthetic use | (CH3)2CHOH ==> CH3COCH3 (Cu/500oC) |
9.3 tertiary alcohol ==> ? | R3C–OH fairly stable (if oxidised C–C bonds broken ==> lower RCOOH, CO2, H2O products) | not readily oxidised – no synthetic use | not readily oxidised – no synthetic use | not readily oxidised – no synthetic use |
9.4 alkyl groups on benzene ring | e.g. C6H5CH3 ==> C6H5COOH | YES | YES | YES air/150oC/Co salt |
9.5 alkene ==> ? | details in appropriate box | NO | ethene ==> ethane–1,2–diol (at room temp.) | e.g. ethene ==> epoxyethane (Ag/250oC) |
Some further details of the organic oxidations tabulated above
9.1 Oxidation of primary alcohols to aldehydes and carboxylic acids (includes oxidation of aldehydes to carboxylic acids)
9.2 Oxidation of secondary alcohols to ketones
9.3 Oxidation of tertiary alcohols
9.4 Oxidation of alkyl–aromatic hydrocarbons to aromatic carboxylic acids
9.5 Oxidation of alkenes
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10. Other miscellaneous Organic Redox Reactions This is a 'collection' of reactions not dealt with in sections 8. and 9. They may/may not be useful reactions. Section 10. reaction sub–index: 10.1 Cannizzaro reaction * 10.2 aldehydes/ketones tests * 10.3 Combustion * 10.4 Fuel cells * 10.5 The use of 1,4–dihydroxybenzene (quinol, hydroquinol) in photography
11. Oxidation state and organic compounds Usually the oxidation state of hydrogen is +1, and oxygen –2 in organic compounds. This section should enable you to answer questions such as 'what is the oxidation state of carbon in methane?', 'what is the oxidation state of carbon in methanol?', ''what is the oxidation state of carbon in methanol?', 'what is the oxidation state of carbon in methanoic acid?', ''what is the oxidation state of carbon in ethane?', ''what are the oxidation states of carbon in ethanol?', ''what are the oxidation states of carbon in ethanal?', ''what are the oxidation states of carbon in ethanoic acid?', ''what is the oxidation state of carbon in ethene?', ''what are the oxidation states of carbon in propene?', (The quoted Pauling electronegativities are C = 2.5, H = 2.2 and O = 3.5 which gives the lead in assigning oxidation numbers in this section i.e. the highest oxidation state is assigned to the least electronegative atom and vice versa for hydrogen and oxygen BUT beware for carbon, logical deduction can give some surprising, but correct results!) On this basis you can achieve a useful oxidation number analysis of simple organic compounds in an oxidation sequence. e.g. the oxidation sequence below, with the oxidation state of carbon in () and in hydrogen in (). CH4 (–4) == ox'n ==> CH3OH (–2) == ox'n ==> HCHO (0) == ox'n ==> HCOOH (+2) ==> CO2 (+4) The above sequence can described in terms of the 'level of the functional group' which is equal the number of bonds the carbon atom forms with more electronegative atoms like oxygen. Therefore in the above sequence: alkane hydrocarbons are level zero (carbon and hydrogen have virtually the same electronegativity), alcohols are level 1 (C–OH, as are halogenoalkanes/haloalkanes e.g. C–Cl), carbonyl is level 2 (e.g. C=O in aldehydes/ketones), carboxylic acids are level 3 (C=O and C–OH) and finally the fully oxidised carbon in carbon dioxide is level 4. Note that a functional group level applies to a single carbon atom e.g. the carbon of the functional group, and not the full molecule. Similarly for the oxidation sequence from ethane to ethanoic acid ... CH3CH3 (–3,–3) = ox'n => CH3CH2OH (–3,–1) = ox'n => CH3CHO (–3,+1) = ox'n => CH3COOH (–3,+3) In terms of oxidation states, as far as I can tell, in most organic compounds, hydrogen always seems to be +1 and oxygen –2 Note the rise of carbon's oxidation state in increments of 2, see oxidation equations for acidified potassium dichromate(VI) reaction with alcohols and aldehydes in section 9.1(a) where the half–cell oxidation equations involve a 2 electron loss from the organic molecule. Other organic molecules and redox sequences can be similarly 'analysed' ethene H2C=CH2 (–2,–2) + H2 (0) == reduction/Ni ==> ethane CH3–CH3 (–3,–3), (+1) propene CH3–CH=CH2 (–3,–1,–2) + H2 == reduction/Ni ==> CH3–CH2–CH3 (–3,–2,–3) ethanol CH3–CH2–OH (–3,–1) == ox'n ==> ethanal CH3CHO (–3,+1) == ox'n ==> CH3COOH (–3,+3) Appendix 1. The Concept of Functional Group Level
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