Aldehydes and Ketones

Part II

Carbonyl vs. Cyano Group

C

O

C N

carbonyl cyano

These groups appear to be totally different.

Carbonyl vs. Cyano Group

C

N

They are similar: Carbon bonded to a heteroatom. O makes two bonds, N makesthree bonds. So, we simply have an extra bond in the cyano group.

C

O

Discussion Questions for Online Students

Answer the following questions for both the carbonyl and cyano groups.

Where would H+ bond?Where would a nucleophile attack?How would H2O add?How many H2O molecules can add?

Online Students Answer Questions Offline, Send

Solutions to Instructor as an Attachment to an e-mail

Not accepted after Feb 8, 2002

Hydrolysis of a Nitrile

• An alcohol contains a hydroxyl (-OH) group.

• A nitrile contains a cyano (-CN) group.

• Nitrile is the family name; a nitrile contains a cyano group. You recognize that a nitrile contains nitrogen.

Hydrolysis of a Nitrile

• Hydrolysis means clevage (olysis) by water (hydro).

• How does water cleave a nitrile?• Water adds to bonds. A cyano group

has two bonds, so two water molecules add to one cyano group, making a gem-diol.

• What do gem-diols do?

How does Water Add to a Nitrile?

• Water can be considered H+ and OH-.

• The OH- adds to the C of the cyano.

• The H+ adds to the N of the cyano.

• A bond remains, so the process is repeated: OH- to C and H+ to N.

• The result: a gem-diol (and amino group).

Addition of Water to a NitrileAddition of Water to a Nitrile

R C NH++

Step 1(OH addsto C and H adds to N)

Step 2(OH addsto C and Hadds to N)

geminal diolnitrile

H2OR C NH2

O

amide

acid-catalyzedhydrolysis

H OHH OH

H+

Step 3(gem diollosses water)geminal diol

H2O+R C OH

OH

OH HNH2-

geminal triol

H2O

carboxylic acid

The gem diol goes to an amidein limited water and toan acid in excess water.

The amide canbe converted toan acid by hydrolysis.

R C NH

OH

R C NH2

OH

OH

R C OH

O

R C NH2

OH

OH

Compare

• The addition of water to a carbonyl group of an aldehyde or ketone

• With

• The addition of water to the cyano group of a nitrile

C

O

+ H OH

C

OH

OH

4 bonds

Bonding occurs between + and - poles.

C

N 4 bonds

+ H OH

C

NH

OH

4 bonds

4 bonds

Go Back to the Previous Slide and Ask Yourself the Following

Question

• What are the similarities and differences in the addition of water to a carbonyl and cyano group?

Key to Both Reactions

• Identifying the positive and negative poles of the reactants

• Getting the first step correct by bonding the positive end (H+) of the reagent water to the negative end (O) of the substrate and the negative end (OH-) of the reagent to the positive end (C) of the substrate, while keeping four bonds to carbon.

Comparison

• Similarities• Mode of addition is

the same.• H adds to heteroatom• OH adds to carbon• A bond is cleaved

• Differences• Carbonyl has one

bond, cyano group has two

• Water (H, OH) add twice

For Cyano: A Second HFor Cyano: A Second H22O AddsO Adds

4 bonds

C

NH

OHH OH

+ C

NH2

OH

OH

gem-diol

4 bonds

When you ask, "how does the substrate react?"The answer is: just like any addition to a polardouble bond.

A Geminal Diol Loses Water

• Two groups bonded to the same carbon atom are geminal (twins).

• A geminal or gem-diol is two ols (OH groups) bonded to the same C atom.

• Gem-diols are generally less stable than the corresponding carbonyl group, so gem-diols lose water to form or reform a carbonyl group.

C

NH2

OH

OH

- H2OC

NH2

O

= C NH2

O

gem-diol acid amide

General Principle: Gem-diols lose H2O.

Oxidation-Reduction Reactions

General Principles:

• Oxidation is the gain of oxygen or loss of hydrogen.

• Reduction is the gain of hydrogen or loss of oxygen.

Oxidation is an Increase in Oxidation Number

The oxidation number of any covalently bonded atom is found by assigning valence electrons to that atom and subtracting that number of valence electrons from the atom’s group number in the periodic table.

• The group number gives that atom’s normal number of valence electrons.

• The oxidation number is the difference between the normal number of valence electrons and the number of assigned electrons.

Procedure for Finding an Oxidation Number of a Covalent Atom

• Show all valence electrons with a Lewis Structure.

• Assign bonding valence electrons to the more electronegative atom of the pair sharing those electrons.

• Assign nonbonding valence electrons to the atom that has (owns) them .

• Subtract the sum of assigned bonding and nonbonding electrons from the atom’s group number in the periodic table.

Oxidation Numbers for Water

H O H

Lewis Structure

Assign bonding electrons: O is more electronegativethan H, so in any bond between O and H, O is assignedor given all of the bonding electrons. For water, O sharesfour bonding electrons with H. O gets all = 4. H = 0.Assign nonbonding electrons. O has 4, H = O.Sum of assigned electrons for O = 4 + 4 = 8O's group number = 6.Oxidation Number of oxygen = 6 - 8 = -2Oxidation Number of each H = 1 - 0 = +1

Oxidation Numbers for MethaneLewis Structure

Assign bonding electrons: C is more electronegativethan H, so in any bond between C and H, C is assignedor given all of the bonding electrons. For methane, C shareseight bonding electrons with H. C gets all = 8. H = 0.Assign nonbonding electrons. C has 0, H = O.Sum of assigned electrons for C = 8 + 0 = 8C's group number = 4.Oxidation Number of carbon = 4 - 8 = -4Oxidation Number of each H = 1 - 0 = +1

C HHH

H

Oxidation Numbers for Carbon Dioxide

Lewis Structure

Assign bonding electrons: O is more electronegativethan C, so in any bond between C and O, O is assignedor given all of the bonding electrons. For CO2, C shareseight bonding electrons with O. O gets all = 8. C = 0.Assign nonbonding electrons. C has 0, each O = 4.Sum of assigned electrons for C = 0, O = 8C's group number = 4. O's group number = 6.Oxidation Number of carbon = 4 - 0 = +4Oxidation Number of each O = 6 - 8 = -2

C OO

Alternative Method

• H will be +1 in hydrocarbons and derivatives, and O will be –2.

• Since the sum of oxidation numbers is the overall charge of the chemical species, the oxidation state of a single carbon can be found mathematically.

• However, for two or more carbon atoms this method gives an average oxidation number.

Oxidation Numbers of Underlined Atoms

CH3CH2CH3 H3C C CH3=

assigned =

H

H6, Ox.No. = -2

CH3CCH3

O

= H3C C CH3

O

assigned = 2, Ox. No. = +2

Oxidation Numbers of Underlined Atoms

CH3OH

CH2O

sum = 0 = 4 - 2 + xx = -2

sum = 0 = 2 - 2 + xx = 0

x = oxidation number of carbon.

Find the Oxidation Number of the Underlined Atom in Each of the following Structures.

CH4 CH3OH CH2O HCO2H CO2

Solutions

CH4 CH3OH CH2O HCO2H CO2

-4 -2 0 +2 +4

Preparation of Aldehydes and Ketones

• Oxidation of Io alcohols to aldehydes

• Oxidation of IIo alcohols to ketones

Preparation of Aldehydes from Io Alcohols

R CH2 C H

OH

HIo alcohol

-1

mildoxidation R CH2 C

O

H

aldehyde

+1

R CH2 C

O

OH

acid

+3strongoxidation

Preparation of Aldehydes from Io Alcohols

• Requires a mild oxidizing agent.

• A strong oxidizing agent makes an acid.

• The oxidation number of C changes from –1 to + 1.

• The alcohol loses two H’s.

• Loss of H’s is oxidation.

Mild Oxidizing Agents

• PCC = pyridinum chlorochromate (pyridine poisons chromium +6 somewhat, reducing its oxidizing power)

• Swern (a man’s name) = DMSO (dimethylsulfoxide), oxalyl chloride, and triethylamine.

Complete the following Equations

OHPCC

OHSwern

Preparation of Aldehydes from Io Alcohols

R CH2 C H

OH

HIo alcohol

-1

R CH2 C

O

H

aldehyde

+1PCC

or Swern

-2Hmild oxidation

Oxidation is an increase in oxidation number! (-1 to +1 is an increase)

Oxidation Methodology

• Carbon atoms that contain oxygen and hydrogen are candidates for further oxidation.

• Remove an H bonded to the C that contains O and replace the H with OH.

• When you get a gem-diol, dehydrate (lose water and make a carbonyl group.

Oxidizable Carbon Atoms

C C

OH

H

H C

O

HC C

OH

H

C

Io alcohol IIo alcohol aldehyde

Carbon atoms in the red square are oxidizable.

Mild Oxidation of Io Alcohol

C C

OH

H

H

Io alcohol

mild (O)

(O) = oxidation (net loss of 2 Hs)

C C

OH

H

OH

gem-diol

- H2OC C

O

H

aldehyde

Strong Oxidizing Agents

• KMnO4 and K2Cr2O7 are strong oxidizing agents.

• They contain Mn+7 and Cr+6, respectively.

• The transition metal ions are reduced, generally to Mn+4 and Cr+3.

• KMnO4 is purple and K2Cr2O7 is orange.

• Mn+4 is brown and Cr+3 is green.

Strong Oxidation of Io Alcohol

C C

OH

H

H

Io alcohol

strong (O)

(O) = oxidation (net loss of 2 Hs, gain 1 O)

C C

OH

OH

OH

gem-diol

- H2OC C

O

OH

acid

Strong Oxidation of Aldehyde

C

O

H

aldehyde

strong (O)

C

O

OH

acid

(O) = Oxidation (net gain of 1 O)

Mild or Strong Oxidation of IIo Alcohol

C C

O

CC C

OH

H

C

IIo alcohol gem-diol

(O)C C

OH

OH

C

ketone

(O) = Oxidation (net loss of 2 Hs)

Summary of Oxidation Reactions

• Io Alcohols can be oxidized to aldehydes with mild oxidizing agents and to acids with strong oxidizing agents.

• Aldehydes can be oxidized to acids with strong oxidizing agents.

• IIo Alcohols can be oxidized to ketones with mild or strong oxidizing agents.

• Methodology is the same for all.

Ozone (O3) as an Oxidizing Agent

• Ozone, an allotrope of oxygen, is a powerful oxidizing agent.

• It cleaves both the sigma and pi bonds of a double bond.

• Each half of the clevage gains an oxygen with a double bond (i.e., a carbonyl group).

• Aldehydes formed by this reaction will be oxidized to acids unless a reducing agent is added before workup of the reaction.

• Reducing agents are zinc (dust) or dimethylsulfide (Me2S).

Ozonolysis

• Ozonolysis literally means clevage with ozone. Step 1 of an ozonolysis is a reaction of a double bond with O3.

• If Zn/HCl or (CH3)2S is used in Step 2, then any aldehydes produced will not be oxidized to acids.

• If Zn/HCl or (CH3)2S is not used in Step 2, then any aldehydes produced will be oxidized to acids.

Oxidative vs. Reductive Workup

• When Zn/HCl or Me2S is not added, the reaction is an oxidative workup (aldehydes will be oxidized, thus oxidative).

• When Zn/HCl or Me2S is added, the reaction is a reductive workup (aldehydes will not be oxidized, because these reducing agents neutralize any excess ozone, thus reductive).

Ozonolysis of (Z)-2-Butene with Oxidative Workup

C C

CH3

HH

CH3 (1) O3

(2) H2OC

H

CH3

C

CH3

H

CH

CH3

O C

CH3

H

OCHO

CH3

O C

CH3

OH

O

aldehydesacids

Summary Ozonolysis

• Ozone cleaves double bonds C=C to give two carbonyl compounds.

• If one or both products are aldehydes, they may be isolated with a reductive workup, or they may be further oxidized to acids with an oxidative workup.

• If one or both products are ketones, they are not further oxidized.

• Thus, ozonolysis is a way to convert alkenes into aldehydes and ketones.

Ozonolysis of (Z)-2-Butene with Reductive Workup

C C

CH3

HH

CH3 (1) O3

(2) Zn/HClor (CH3)2S

CH

CH3

C

CH3

H

CH

CH3

O C

CH3

H

Oaldehydes