Oxidation - chem.wschem.ws/dl-1008/08a-oxidation.pdf · ‣ Tollens’ Test ‣ Benedict’s Test ‣ Reduction of Carbonyls ‣ Aldehydes to 1˚ alcohols ‣ Ketones to 2˚ alcohols

Oxidation

© Nick DeMello, PhD. 2007-2015

Stealing and loosing electrons.

Exploring carbonyls, an oxidized functional group.

Ch08

version 1.0

Oxidation & the Carbonyl Group

‣ Oxidation-Reduction ‣ What is oxidation?

‣ Oxidation Numbers

‣ Redox Reactions ‣ Single Displacement

‣ Combustion & Respiration

‣ The carbonyl group ‣ Structure

‣ sp2 trigonal planar

‣ Partial charge

‣ Substances with carbonyl groups ‣ Aldehydes

‣ Ketones

‣ Carboxylic Acids

‣ Organic Reactions, focus on carbon ‣ Adding O-C bonds is oxidation

‣ Adding H-C bonds is reduction

‣ Oxidation of Alcohols ‣ Combustion reactions

‣ Dehydration reactions ‣ structural & stereoisomers

‣ Oxidation of Thiols

‣ Oxidation to disulfides

‣ Oxidation of Carbonyls ‣ Tests for aldehydes

‣ Tollens’ Test

‣ Benedict’s Test

‣ Reduction of Carbonyls

‣ Aldehydes to 1˚ alcohols

‣ Ketones to 2˚ alcohols

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Oxidation & Reduction

‣ Oxidation and Reduction are complimentary chemical processes.

‣ The word reduction comes from the alchemical process of smelting ore. ‣ Brittle heavy metal ores were heated with coke

(carbon) and the result was pure metals.

‣ Iron, copper, tin, lead, mercury and other metals were prepared this way.

‣ The metal you got out, always weighed less than the ore that went in, so we called the process reduction.

‣ If that same metal was allowed to react with oxygen, it would produce metal ores. Iron swords rusted, silver tarnished, etc.

‣ The reverse process, reacting metal with oxygen, was called oxidation.

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Fe2O3 (iron ore) + C → Fe (iron) + CO2

Fe (iron) + O2 → Fe2O3 (rust)

Fe 3+ → Fe 0

Fe 0 → Fe 3+


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Fe2O3 (iron ore) + C → Fe (iron) + CO2

Fe (iron) + O2 → Fe2O3 (rust)

Reduction is adding electrons to an atom.

Oxidation is removing electrons from an atom.

Fe 3+ → Fe 0

Fe 0 → Fe 3+


‣ If an atom gains electrons, it’s said to be reduced.

Example: Fe 3+ ➞ Fe 0

Cl 0 ➞ Cl 1-

‣ If an atom looses electrons, it’s said to be oxidized.

Example Fe 0 ➞ Fe 3+

Cl 1- ➞ Cl 0

‣ Chemical reactions where electrons are transferred from one atom to another are called oxidation-reduction reactions.

Example: Fe + HCl ➞ FeCl3 + H2

‣ It can be tricky to figure out which atoms gained or lost electrons in a reaction.

‣ In the above reaction:

‣ Iron was oxidized.

‣ Chlorine neither gained nor lost electrons.

‣ Hydrogen was reduced.

‣ To help us explore oxidation-reduction reactions we assign oxidation numbers to each atom in the solution.

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‣ Partial charge


‣ Ketones









‣ Tollens’ Test





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Oxidation Numbers

‣ Who gains or looses electrons can seem complicated in reactions that involve covalent bonds.

‣ We use oxidation numbers to determine what’s happening to individual atoms in molecular compounds and ions.

‣ Oxidation numbers are assigned by imagining we put enough energy into that molecule to break all it’s atoms down into ions.

‣ What ions would be formed if we did that reveal the oxidation number of the atom in the molecule.

‣ The oxidation number is equal to the charge that the atoms would have had.

‣ We’ll use those oxidation numbers to show us what happens to atoms in more complicated reactions.

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NaHCO3 Na1+ H1+ O2-

O2-

O2-

C4+

Oxidation Numbers

‣ What ions would be formed if we did that reveal the oxidation number of the atom in the molecule.

‣ Rules: ‣ Elements in their natural state (Fe, Cl2, S8) have an oxidation number zero.

‣ If it’s ionic or has an ionic component, break the molecule into those ions first.

‣ Then look at hydrogen:

‣ If hydrogen is attached to a metal, it get’s -1

‣ Otherwise it get’s +1

‣ Then go through the remaining atoms in order of decreasing eletrongativity, let each atom take from the molecule whatever electrons it needs to attain the charge you expect from it’s location on the periodic table.

‣ The last element get’s whatever is left.

‣ Molecules have a charge of zero, so that last element will not get what it wants, it’s like musical chairs — the last guy get’s left out.

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NaHCO3 Na1+ H1+ O2-

O2-

O2-

C4+

-2+1+1 +4

Chlorine’s oxidation number?

Cl2

NaCl (Na+ Cl1-)

ClO1-

HClO3

ClOF5

Cl2O9

O

-1

+1

+5

+7

+9








‣ Partial charge


‣ Ketones









‣ Tollens’ Test





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Identifying Red-Ox Reaction

‣ When an atoms oxidation number goes up in a reaction, it’s been oxidized (lost electrons).

‣ When an atoms oxidation number goes down in a reaction, it’s been reduced (gained electrons).

‣ Sometimes atoms are neither oxidized nor reduced.

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NO3 → NO2 + O2

O2-O2-

O2-

-2

N6+O2-

O2-

-2

N4+ O0

heat

+6 +4

Nitrogen’s oxidation number went down, it was reduced in this

reaction.

Note: no ions are formed in this reaction. This is just an

accounting trick we do to find out where the electrons moved to

within the molecule.

Identifying Red-Ox Reaction

‣ When an atoms oxidation number goes up in a reaction, it’s been oxidized (lost electrons).

‣ When an atoms oxidation number goes down in a reaction, it’s been reduced (gained electrons).

‣ For underlined atom in each reaction below, determine if it’s been oxidized, reduced, or neither.

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MnO41-(aq) + I1−(aq)→Mn2+(aq) + I2 (s)

2 AgCl(s) + H2 (g) → 2 H1+(aq) + 2 Ag(s) + 2 Cl1-(aq)

Iron rusting to Iron (III) oxide.

Precipitating gold metal from gold ions in sea water.

Na3PO4 (aq) + H2SO4 (aq) → H3PO4 (aq) + Na2SO4 (aq)

Oxidized

Oxidized

Reduced

Neither

Reduced

Oxidation-Reduction Reactions

‣ Atoms that gain electrons (negative charges) are reduced.

‣ Atoms that loose electrons are oxidized.

‣ Electrons always end up somewhere. If something in the reaction is getting oxidized, something else is getting reduced.

‣ Red-ox processes are not an equilibrium processes — someone wins; someone looses; end of story. No trade-backs.

‣ You can drive equilibrium with red-ox processes, just like you drive it with other precipitation, gas formation or water formation.

‣ Metals can be oxidized by acids and salts (rust is an example).

‣ Metal oxidation often occurs by a single displacement mechanism.

A + BC ➞ AC + B

Zn(s) + HBr(aq) ➞ ZnBr2(aq) + H2(g)

Zn(s) + H1+(aq) + Br1-(aq) ➞ Zn2+(aq) + Br1-(aq) + H2(g)

oxidation number0 +1 -1 +2 -1 0

Zn is oxidized (0 goes to +2)Hydrogen is Reduced (+1 goes to 0)

Bromine is neither.

Oxidation-Reduction Reactions

‣ Atoms that gain electrons (negative charges) are reduced.

‣ Atoms that loose electrons are oxidized.

‣ Electrons always end up somewhere. If something in the reaction is getting oxidized, something else is getting reduced.

‣ Red-ox processes are not an equilibrium processes — someone wins; someone looses; end of story. No trade-backs.

‣ You can drive equilibrium with red-ox processes, just like you drive it with other precipitation, gas formation or water formation.

‣ Metals can be oxidized by acids and salts (rust is an example).

‣ Metal oxidation often occurs by a single displacement mechanism.

‣ How do you know if the reaction will occur?

A + BC ➞ AC + B

Zn(s) + HBr(aq) ➞ ZnBr2(aq) + H2(g)

Mn(s) + Pb(NO3)2(aq) ➞ Mn(NO3)2(aq) + Pb(s)

Cu(s) + Pb(NO3)2(aq) ➞ N/R

‣ How do we know if the reaction happens? Look at the complete ionic equation.

‣ Remove the spectator ions to see the net ionic equation.

‣ There are two half reactions which make up the net ionic equation.

‣ The two half reactions show that we’re looking at a competition for electrons. It’s basically a tug of war.

‣ You can turn around one equation to compare them side to side. We need to decide who’s gonna win the fight over those two electrons.

‣ We could look up numbers for whose is better at holding electrons, or we could just reference a list of “who beat’s who” — the activity series.

Mn(s) + Pb2+ + NO31-➞ Mn2+ + NO31- + Pb(s)

Oxidation-Reduction Half Reactions

Mn(s) + Pb(NO3)2(aq) ➞ Mn(NO3)2(aq) + Pb(s)

Mn(s) + Pb2+ ➞ Mn2+ + Pb(s)

Mn(s) ➞ Mn2+ + 2e-

Pb2+ + 2e- ➞ Pb(s)

Pb2+ + 2e- ➞ Pb(s)

Mn2+ + 2e- ➞ Mn(s)

X XMolecular Equation

Net Ionic Equation

Complete Ionic Equation

Half Reaction Equations

The Activity Series

‣Which metal (oxidation zero) is more “active”?

‣We look at the half reactions.

‣ An atom of an element in the activity series will displace an atom of an element below it from one of its compounds.

The Activity Series

‣Which metal (oxidation zero) is more “active”?

‣ An atom of an element in the activity series will displace an atom of an element below it from one of its compounds.

Metals KCaNaMg AlZn FeNiSn Pb H

Cu AgAu

Grouped Metals

K Ca Na Mg

Ga Al Zn Fe Co Ni

Sn Pb H

Cu Ag Au

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1 2

3

4

5

6

Metal(ca0on)Ac0vitySeries


‣ How do we know which metal gives up it’s electrons? Check “activity.” The more active ion is the one more likely to turn into a cation (give up it’s electrons).

‣ Which is more active (more likely to loose it’s electrons)?

Sodium or Iron?

Al or Co?

H2 or Mg?

Hydrogen or Gold?

Sodium or Zinc?

Pb or Cu?

Nickel or Calcium?


‣ How do we know which metal gives up it’s electrons? Check “activity.” The more active ion is the one more likely to turn into a cation (give up it’s electrons).

‣ Which reactions will occur?

A + BC ➞ AC + B

Na(s) + FeBr3(aq) ➞ ?

Fe(s) + Zn(ClO3)2(aq) ➞ ?

Sn(s) + HNO3(aq) ➞ ?

Na more active than Fe? Yes.

Na(s) + FeBr3(aq) ➞ Fe(s) + NaBr (aq)

Fe more active than Zn? No.

Fe(s) + Zn(ClO3)2(aq) ➞ N/R

Sn more active than H? Yes.

Sn(s) + HNO3(aq) ➞ H2(g) ↑ + Sn(NO3)4(aq)








‣ Partial charge


‣ Ketones









‣ Tollens’ Test





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Ch08

Combustion Reactions

‣ Burning something is causing it to combust.

‣ Combustion reactions are reacting any substance with oxygen to form the most stable binary compounds of it’s elements and oxygen.

‣ The most common products are CO2 and H2O. Other common products are NO2 and P2O5.

‣ Combustion reactions are red-ox reactions, in which oxygen is reduced.

‣ The driving force in combustion reactions is oxygens fierce demand for electrons. Harnessing that property of oxygen is what gave us the internal combustion engine and is at the heart of most of fuels humans use.

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X + O2 ➞ H2O + CO2 + NO2 + P2O5 + …








‣ Partial charge


‣ Ketones









‣ Tollens’ Test





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Ch08

The Carbonyl Group

‣ A carbonyl group is an oxygen double bonded to a carbon skeleton.

‣ The the carbonyl group is present in and responsible for the chemistry of many classes of organic compound including: ‣ Ketones

‣ Aldehydes

‣ Carboxylic Acids … and more we’ll talk about in Chapter 13 and 14.

‣ The carbon in a carbonyl group is sp2.

‣ The bonds to it form a trigonal planar shape.

‣ The double bond allows electron density to shift more easily between the oxygen and carbon.

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‣ Partial charge


‣ Ketones









‣ Tollens’ Test





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Ch08

Ketones & Aldehydes

‣ The ketone family includes any substances that have a carbonyl group attached to two carbons.

‣ The aldehyde family includes any substances that a hydrogen attached to the carbonyl.

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Carboxylic Acid

‣ Carboxylic acids are substances that have a hydroxyl group attached to a carbonyl group.

‣ We will discuss carboxylic acids in more detail next. (including how to name them).

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Some uses of Aldehydes & Ketones

‣ Acetone (CH3)2CO is the simplest ketone. Acetone and other ketones made good organic solvents, able to dissolve both polar and non-polar substances.

‣ Formaldehyde (H2CO) is a gas at room temperature. The simplest aldehyde possible, it’s used in many manufacturing processes. As an aqueous solution (40%) it’s used to preserve biological specimens.

28Acetone

Formaldehyde

Naming Aldehydes

‣ You do not need to know the common names of aldehydes.

‣ To name aldehydes using IUPAC use the family suffix -al.

‣ You do not need to indicate the address of the aldehyde, because it is always in one one end of the chain.

‣ The carbonyl in an aldehyde is always carbon #1.

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Naming Aldehydes

‣ You do not need to know the common names of aldehydes.

‣ To name aldehydes using IUPAC you use the family suffix -al.

‣ You do not need to indicate the address of the aldehyde, because it is always terminal.

‣ In an aldehyde carbon #1 is always a carbonyl.

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3-Pentynal

3-Isopropyl-5-hexenal

3-Bromo-4-ethylheptanedial

Naming Ketones

‣ Common names for ketones are made by listing the two chains attached to the carbonyl in alphabetical order (like you do for the common name of ethers).

‣ To name ketones using IUPAC use the family suffix -one. ‣ Giving the ketone(s) the smallest address numbers is the first priority.

‣ Ketones can have a cyclic backbone.

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Common Names for Ketones



‣ Ketones can have a cyclic backbone.

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Methyl pentyl ketone

Dipropyl Ketone

tert-Butyl Ethyl Ketone

IUPAC Naming Ketones



‣ Ketones can have a cycle backbone.

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tert-butyl propyl ketoneor

2,2-Dimethyl-3-hexanone

4-Methyl-1,3-cyclopentanedione

6,6-Dimethyl-2,4,7-octanetrione








‣ Partial charge


‣ Ketones









‣ Tollens’ Test





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Ch08

Oxidation-Reduction Organic Reactions

‣ Oxidation is the process of removing electrons.

‣ Reduction is the process of adding electrons.

‣ For organic molecules, we will focus on the oxidation state of carbon.

‣ It can be complicated to calculate what happened to the carbon atoms in an organic reaction, but there is a short cut.

‣ Adding O-C bonds or loosing H atoms oxidizes the organic molecule.

‣ Adding H-C bonds or loosing O atoms reduces the organic molecule.

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‣ Oxidation is the process of removing electrons.

‣ Reduction is the process of adding electrons.

‣ For organic molecules, we focus on the oxidation state of carbon.

‣ It can be complicated to calculate what happened to the carbon atoms in an organic reaction, but there is a short cut.

‣ Adding O-C bonds or loosing H atoms oxidizes the organic molecule.

‣ Adding H-C bonds or loosing O atoms reduces the organic molecule.

‣ Oxidizing reagents can be used to convert a primary alcohol into an aldehyde.

‣ Reducing agents can convert an aldehyde into an alcohol into an alkane…

36


‣ Are these reactions oxidations or reductions?

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[OX][RED]

N/R

[RED] [OX]

Breaking Carbon bonds is more than oxidation/reduction,tertiary alcohols are very stable.

[OX]

‣ What would happen if the alcohol was tertiary?




‣ For organics, we focus on carbon ‣ Adding O-C bonds is oxidation




‣ The carbonyl group

‣ Structure


‣ Partial charge


‣ Ketones







‣ Tollens’ Test





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Ch08

Combustion Reactions

‣ Alcohols burn easily.

‣ Combustion is an oxidation reaction.

39

[OX]

O2

HeatCO2 + H2O

[OX]

O2

HeatCO2 + H2O

[OX]

O2

HeatCO2 + H2O

Unsaturated hydrocarbonsburn hotter!

Dehydration Reactions

‣ Alcohols heated with an acid catalyst can dehydrate (loose water).

‣ This reaction has moderate selectivity in the kinds of alcohols that react.

‣ Tertiary alcohols react more readily, than secondary, react more readily than primary.

‣ Selectivity can be achieved with temperature control.

40

1° alcohols: 170° - 180 °C 2° alcohols: 100° - 140 °C 3° alcohols: 25° - 80 °C



‣ This reaction has moderate selectivity in the kinds of alcohols that react.

‣ Tertiary alcohols react more readily, than secondary, react more readily than primary.

‣ Selectivity can be achieved with temperature control.

41

+ H2OH+

Heat

+ H2OH+

Heat

+ H2OH+

Heat

1° alcohols: 170° - 180 °C 2° alcohols: 100° - 140 °C 3° alcohols: 25° - 80 °C

primary (1˚)

tertiary (3˚)

secondary (2˚)



‣ This reaction has poor selectivity in the products it forms.

‣ Depending on the structure of the starting material, it may produce: ‣ Stereoisomers

‣ (these are different products)

‣ Structural isomers ‣ (these are different products)

42

+ H2OH+

Heat

H+

Heat

cis-2-Pentene

+

trans-2-Pentene

+ H2O+

3-Methylcyclohexene2-Methylcyclohexene








‣ Partial charge


‣ Ketones









‣ Tollens’ Test





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Ch08

Oxidation of Disulfides

‣ Sulfides form disulfides when exposed to oxidation.

‣ Reminder: ‣ Adding O-C bonds or loosing H atoms oxidizes the

organic molecule.

‣ These disulfide bonds between and within hairs are what cause it to curl when oxidized (permed).

‣ PCC or KMnO4 can be used.

44

[OX]

[OX]








‣ Partial charge


‣ Ketones









‣ Tollens’ Test





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Ch08

Testing for Aldehydes

‣ When something get’s oxidized, something else get’s reduced.

‣ One way to test for the presence of aldehydes, is to try and reduce them with copper or silver ions.

‣ Tollen’s solution (AgNO3 & NH3) oxidizes aldehydes to carboxylic acids but has no effect on ketones.

‣ Tollen’s solution will produce silver metal when exposed to an aldehyde, but nothing will happen in the presence of a ketone. ‣ Tollen’s test demonstrates if an aldehyde is present.

N/RAgNO3

NH3

AgNO3

NH3

+ Ag (s)

Tollen’s Test








‣ Partial charge


‣ Ketones









‣ Tollens’ Test





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Ch08

Reducing Carbonyls

‣ Carbonyls can be reduced to alcohols with H2 and catalyst (Ni, Pd, or Pt).

48

Reducing Carbonyls

‣ Carbonyls can be reduced to alcohols with H2 and catalyst (Ni, Pd, or Pt).

49

H2 / NiH2 / Ni

H2 / Ni H2 / Ni

(ketone) (aldehyde)(2˚ alcohol) (1˚ alcohol)

Summary of Reactions

50

(reference handout on the website summarizes all reactions Ch11-12)


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52









‣ Partial charge


‣ Ketones









‣ Tollens’ Test





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Ch08

Predict the Product

‣ Each reaction below is a reduction or oxidation, predict the product.

54

H2 / Ni

[RED]

CrO42-

[OX]

CrO42-

[OX]

N/R

Predict the Product

‣ Each reaction below is a reduction or oxidation, predict the product.

55

H2 / Ni

[RED]

[OX]

Ag+ or Cu2+

[OX]

N/R

Ag+ or Cu2+








‣ Partial charge


‣ Ketones









‣ Tollens’ Test





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Ch08

Questions?

Documents

Oxidation - chem.wschem.ws/dl-1008/08a-oxidation.pdf · ‣ Tollens’ Test ‣ Benedict’s Test ‣ Reduction of Carbonyls ‣ Aldehydes to 1˚ alcohols ‣ Ketones to 2˚ alcohols