Chapter 17 Aldehydes and Ketones

I. Properties and Names of Aldehydes and KetonesA. Nomenclature

1) Carbonyl groups are the highest priority functional group we have seen

2) Aldehydes

a) Common Names modify the name of the corresponding carboxylic acid

b) IUPAC Names treat aldehydes as derivatives of alkanes

i. Suffix –al added to the alkane name: alkane becomes alkanal

ii. The carbonyl carbon is #1, but is not numbered in the name

iii. Complex aldehydes are named as “alkanecarbaldehydes”

C

O

H3CC

O

OH H3CC

O

H

Acetic Acid Acetaldehyde

HC

O

H

Formaldehyde

C

O

H

Benzaldehyde

CH3CH2CH

O

ClCH2CH2CH2CH

OH

O

H

O

propanal 4-chlorobutanal 4,6-dimethylheptanalcyclohexanecarbaldehyde

3) Ketones

a) Common Names come from the 2 R groups listed in alphabetical order, followed by “ketone”. Phenyl Ketones have common names ending in “-phenone”

b) IUPAC Names of ketones modify the alkane name with “-one”

i. The carbonyl carbon is assigned the lowest possible number

ii. Aromatic ketones are named as aryl substituted alkanones

iii. A ketone with an aldehyde is called an “oxo-” substituent

H3CC

O

CH3

dimethyl ketone acetone

CH3CH2

C

O

CH3

ethyl methyl ketone

H3CC

O

C

O

BenzophenoneAcetophenone

H3CC

O

CH3 O O

Cl OCH3 O

O

H

COOH CH3CCH2CH

O O

CH3CCH2CH=CHCH2CCH3

OH

CH3

O

C CH C

O

H

O

Br

propanone 2-pentanone 4-chloro-6-methyl-3-heptanone 2,2-dimethylcyclopentanone

1-phenylethanone

4-formylcyclohexanecarboxylic acid

3-oxobutanal

7-hydroxy-7-methyl-4-octen-2-onepropynal

5-bromo-3-ethynylcycloheptanone

4) Carbonyl groups named as a substitutent are called alkanoyl (or acyl) groups

5) Drawing Aldehydes and Ketones

a) Aldehyde = RCHO, Alcohol = RCOH

b) Ketone: RCH2COCH3

B. Carbonyl Structure and Physical Properties

1) C=O bond is similar to C=C bond

a) C(sp2)—O(sp2) -bond overlap, with a p-p overlap for the -bond

b) Planar group with 3 120o angles

formyl-

HC

O

ethanoyl-or acetyl-

H3CC

O

CH3CH2CH2CH

O

CH3CH2CH2CHO H

O

Butanal

CH3CH2CCH3

O

CH3CH2COCH3

O2-butanone

2) The C=O bond is fairly strong, 175-180 kcal/mol (ethene = 173 kcal/mol)

a) Oxygen is electronegative, so the bond is polar

b) The partially positive charged carbon is electrophilic

c) The partially negatively charged oxygen is nucleophilic and basic

d) Resonance structures:

3) Boiling points are higher than alkanes due to polarity

4) Small carbonyl compounds (< 7 C) are water soluble due to polarity (acetone)

C

O

C

O

C. Spectroscopy

1. 1H NMR

a) Aldehyde proton is extremely deshielded, = 9-10 ppm

i. Movement of -electrons reinforces magnetic field

ii. + C increases the deshielding beyond that effect

b) Hydrogens adjacent to the aldehyde are also slightly deshielded

c) Ketones also slightly deshield adjacent hydrogens

RCH2

C

O

H2.5 9.8

R2CHC

O

CH32.6 2.0

2) 13C NMR

a) Carbonyl carbons are observed at around 200 ppm due to + C

b) Adjacent carbons are somewhat effected as well

CH3CH

OCH3CH2CH

O

CH3CCH3

O

CH3CCH2CH2CH3

O

31.2 199.6 5.2 36.7 201.8 30.2 205.1 29.3 206.6 45.2 17.5 13.5

3) IR

a) C=O stretch is intense, 1690-1750 cm-1

b) Aldehyde carbonyl usually around 1735 cm-1

c) Ketone carbonyl usually around 1715 cm-1

d) Conjugation reduces the wavenumber by 30-40 cm-1

e) Small rings increase the wavenumber

O

CH3

1680 cm-1

O1745 cm-1

4) UV-Vis

a) Nonbonding oxygen lone pairs give n*

b) -bond gives * transitions

c) Acetone: n* = 280 nm ( = 15), * = 190 nm ( = 1100)

d) Conjugation shifts the absorbances to longer wavelenth (lower E)

II. Preparation of Aldehydes and KetonesA. Oxidation of Alcohols

1) Cr(VI) reagents (like CrO3) oxidize alcohols to carbonyls

2) Secondary ROH gives ketonesCH3C

H

CH3

OHCrO3

H3CC

O

CH3

3) Primary ROH gives aldehydes

a) Must be done under anhydrous conditions to prevent overoxidation

b) PCC = pyridinium chlorochromate, pyridine, and CH2Cl2 conditions

c) Manganese dioxide (MnO2) oxidizes only allylic alcohols; it won’t react with ordinary alcohols.

B. Ozonolysis of Alkenes

C. Hydration of Alkynes

1. Markovnikov hydration yields ketones

CH3CH2OHCrO3 CH3CH3COOH CH3CH2OH

PCC, py

CH2Cl2CH3CHO

HOCH2CH2CH=CHCH2OHMnO2

CHCl3HOCH2CH2CH=CHCH

O

CH3 1. O3

2. ZnCH3CCH2CH2CH2CH2CH

O O

CR CHH2O, H+, Hg2+

C C

R

HO H

H

tautomerizationCR

O

CH3

2. Anti-Markovnikov hydration yields aldehydes

D. Aryl Ketones Via Friedel Crafts Alkanoylation (Acylation)

III. Additions to CarbonylsA. Three regions of Carbonyl Reactivity

1) The :O: is nucleophilic and will attack electrophiles

2) The C is electrophilic and will be attacked by nucleophiles

3) The -C has acidic protons (NEXT CHAPTER)

CR CHR2BH

C C

R

H BR2

H

H2O2

HO- C C

R

H OH

H

tautomerizationRCH2CH

O

CH3O 1. CH3CCl, AlCl3

O

2. HCl, H2O

CH3O

C

O

CH3

C

O

CH2

C

O

CH2

B. Hydrogenation

1. Catalytic Hydrogenation reduces carbonyls

2. Reaction is slower than for alkenes: higher H2 pressure and temp. needed

3. C=C bonds can be selectively hydrogenated in the presence of C=O

C. Ionic Additions

1. Polar +X—Y- molecules will add to C=O

2. Hydrides add 2 H atoms to the carbonyl, but won’t reduce alkenes

CH3CCH2CH3

OH2

Ra NiCH3CHCH2CH3

OH

O

H2 (1 atm)

Pt, 25 oC O

CH3CCH2CH3

O NaBH4

CH3CHCH2CH3

OH

O

1. LiAlH4, Et2O

2. H+, H2OOHH

C

O X+Y-

C

OX

Y

3. Grignard Reagents add R, H to the carbonyl

4. Milder Reagents

a) Hydrides and Grignard Reagents are strong bases. They irreversibly add to the carbonyl

b) Less basic reagents can also add to carbonyls, but the reactions are reversible: H2O, ROH, RSH, RHN2, etc…

c) The conditions used in the reaction of these milder bases determine how the reaction proceeds

5. Nucleophilic Addition-Protonation (Basic Conditions)

a) Mechanism

CH3CCH2CH3

OH

R

CH3CCH2CH3

ORMgBr

THF

C O+ -

Nu-C O

Nu

H OHC

Nu

OH

Alkoxide ion

+ OH-

b) Nucleophile approaches, causing C to rehybridize

c) -bond electrons move to Oxygen, producing an alkoxide anion

d) Protonation from solvent yields the product

e) The new Nu—C bond has both electrons from Nu- (like in SN2)

f) An electron pair is the “leaving group”

g) Strongly basic nucleophiles typically follow this mechanism

6. Electrophilic Protonation-Addition (Acidic Conditions)

a) Mechanism

b) C=O is a weak base (strong acid) so small, but reactive, amount present

c) Nu attacks carbon electrophile to give product. This removes intermediate and shifts the equilibrium to the right.

d) Weakly basic nucleophiles typically follow this mechanism. Strongly basic nucleophiles would just get protonated and couldn’t react.

-+

C OH+

C OH C OHNu C

Nu

OH

pKa of C=OH is -8