Ch 17- Aldehydes and Ketones

Homework: 17.10, 17.13, 17.17, 17.19, 17.28, 17.35, 17.40, 17.41,

17.49, 17.53, 17.69

• In this chapter we study compounds that contain the carbonyl group.

• The carbonyl group is one of the most important functional groups in organic chemistry and Biochemistry because it is present in aldehydes, ketones, carboxylic acid and carboxylic acid derivatives.

C

O

The Carbonyl group

Aldehydes & Ketones

• The functional group of an aldehyde is a carbonyl group bonded to a hydrogen

• In methanal, the simplest aldehyde, the carbonyl group is bonded to 2 H’s

• In all other aldehydes, it is bonded to one H and a carbon chain.

• The functional group of a ketone is a carbonyl group bonded to 2 carbon chains.

Nomenclature

• 1) Find the longest chain that contains the carbon of the carbonyl and number the chain to give that carbon the lowest number.– For aldehydes, drop the -e, and add -al– Aldehydes will always be at the end of a chain,

so they will always be at carbon #1, so there is no need to put the 1.

O

H

O

H

O

H

Nomenclature (cont)• For aldehydes that have carbon-carbon

double bonds in them, the longest chain must contain the carbon of the carbonyl and BOTH carbons of the C-C double bond!!

• To name these, we drop the -ane ending and add -enal

• The -en- shows the double bond, the -al shows the aldehyde.– Remember to provide the locant for the double

bondO

H

O

H

O

H

Nomenclature of Ketones

• For ketones, we find the longest chain that contains the carbon of the carbonyl group, and number the chain to give that carbon the lowest number

• You drop the -e, from the parent name and add -one.

O

O

O

• In naming aldehydes and ketones that also have an -OH in the molecule, find the longest chain that contains both the carbon of the carbonyl and the carbon bonded to the -OH group

• Number the chain to give the carbon of the carbonyl the lowest number

• The -OH will be named as a substituent! • When the -OH group is named as a

substituent, it is named as a hydroxy, and numbered and alphabetized will all other substituents present.

Examples

H

OOHOOH OH

H

O

O

Br

OH

HOO

Physical Properties

• Oxygen is more electronegative than carbon therefore the carbon-oxygen double bond is polar with the Oxygen bearing a partial negative charge and the Carbon bearing a partial positive charge

• The only intermolecular forces are dipole-dipole forces and London Forces

• They can not Hydrogen bond to each other!!

Physical Properties

• As the groups bonded to the carbonyl increase in size, the solubility in water decreases

• Most aldehydes and ketones have strong odors and are used in perfumes and flavoring agents

Reactions

1) Aldehydes can be oxidized to the carboxylic acids

-Ketones are resistant to oxidation

Aldehydes can also be oxidized by O2!!

H

O

H2SO4OH

OK2Cr2O7

H

O

O2 OH

O

Reduction Reactions

• Just like C-C double bonds, C-O double bonds can be reduced by the addition of H2 with a metal catalyst to produce an alcohol.

H

O

Pd, Pt, or Ni

OPd, Pt, or Ni

H2OH

OHH2

Sodium Borohydride

• Aldehydes and Ketones can also be reduced using Sodium Borohydride, NaBH4

• NaBH4 contains hydrogen in the form of hydride ions, H-

• The advantage of using NaBH4 is that it does not reduce C-C double bonds!

Examples

O

H2, Pd

OH

O OH

H3O

NaBH4

In Nature

• In Biological systems, nicotinamide adenine dinucleotide, a coenzyme abbreviated NADH, is used to provide the hydride ion to reduce aldehydes and ketones.

H3C COO

ONADH

H3C COO

O

H

H3O

H3C COO

OH

HPyruvate Lactate

Reactions of Alcohols

3) Addition of Alcohols

Addition of one molecule on an alcohol to an aldehyde or ketone form a hemiacetal

Hemiacetal- a compound with a carbon bonded to 1 -OH group and 1 -OR group

Examples

H

O

HOCH2CH3

O

CH3OH

OH

O CH3

H

OH

O CH2CH3+

+

Reactions of Hemiacetals• Hemiacetals can react with another

molecule of alcohol to form an acetal

• Acetal- a compound with a carbon bonded to 2 -OR groups

H

OH

O CH2CH3HOCH2CH3

CH3OH

OH

O CH3

O

O CH3

CH3

H

O

O CH2CH3

CH2CH3

+

+

Info on Hemiacetals• Hemiacetals are generally unstable and are

only minor components of an equilibrium mixture

• The only exception is when the -OH group is part of the same molecule as the carbonyl group and a five or six member ring can form

• The compound exists almost entirely in the cyclic hemiacetal forms

• In this case, the -OH group adds to the C=O group of the same molecule

Examples

H

O

HO

1234

5

4-Hydroxypentanal

Redraw to show-OH and carbonyl

close to each other

H

OOH

1

23

45

H

OOH

1

23

45

O OH

H1

23

4

5

A Cyclic Hemiacetal

YOUR TURN!!

• Do the same thing with:

H

O

OH

OH

OH

OH

OH

NOTE: Six membered rings are more stable than five memberedRings. If both can form, the six membered ring will form over theFive membered ring

Keto-Enol Tautomerism

• A carbon atom adjacent to a carbonyl group is called an -carbon, and a hydrogen atom bonded to it is called an -hydrogen

H3C

C

CH2

CH3

O

Alpha Carbons

Alpha Hydrogens

Beta Carbon (

Keto-Enol Tautomerism

• A carbonyl compound that has an -hydrogen is in equilibrium with a constitutional isomer called an enol

• The name enol is derived from the IUPAC designation of it as having both an alkene (-en) and an alcohol (-ol)

H3C CH3

O

H3C CH2

OH

Keto-Enol Tautomerism• The Keto and Enol forms are examples of

Tautomers.• Tautomer- constitutional isomers in

equilibrium with each other that differ in the location of a hydrogen atom relative to an oxygen atom

• This type of isomerism is called keto-enol tautomerism

• For any pair of keto-enol tautomers, the keto form generally predominates at equilibrium!