Chapter 13

Aldehydes and Ketones

1. Structure

The carbonyl group is a double bond between oxygen and carbon.

Carbonyl compounds include: Aldehydes: at least one hydrogen bonded to the carbonyl carbon

Ketones: no hydrogens bonded to the carbonyl carbon.

carboxylic acids and amides. [Tune in NEXT chapter…]

1. Properties

The carbonyl group is polar.

Intermolecular forces in carbonyl compounds are stronger than in alkanes (nonpolar) or ethers (only slightly polar). weaker than in alcohols, which are polar and can hydrogen bond.

1. Boiling points

Structure Name Molar mass Boiling point

CH3CH2CH2CH3 butane 58 -0.5 oCCH3-O-CH2CH3 methoxyethane 60 7.0 oCCH3CH2CH2-OH 1-propanol 60 97.2 oC

O ||

CH3CH2-C-Hpropanal 58 49 oC

O ||

CH3-C-CH3

propanone 58 56 oC

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1. Solubility

There is no hydrogen bonding between two carbonyl groups.

1. Solubility

Aldehydes and ketones can hydrogen bond with water.

Aldehydes and ketones with five or fewer carbon atoms are fairly soluble in water.

Large aldehydes and ketones dissolve only in nonpolar solvents.

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1. Solubility in water—general guidelines

Molecules with polar groups tend to be more soluble in water.

Molecules that can hydrogen bond with water are more soluble than those than cannot.

Increasing the number of polar groups on a molecule increases its water solubility.

Increasing the number of possibilities for hydrogen bonding increases solubility in water.

Increasing the molar mass of a molecule decreases its water solubility.

2. Nomenclature

Naming aldehydes

The parent compound is named for the longest continuous chain containing the carbonyl group.

The final –e of the parent alkane is replaced with –al.

The chain is numbered beginning at the carbonyl carbon.

Substituents are named and numbered as usual.

The aldehyde is always carbon 1, so no number is used for the carbonyl group.

2. Nomenclature

The simplest aldehydes

Draw structures for

7,8-dibromooctanal

trans-2-hexenal (component of olive oil)

methanal ethanal

pencast

2. Nomenclature

Name the following compounds.

pencast

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2. Nomenclature

Ketones

The rules are analogous to the rules for aldehydes.

The –e ending of the parent alkane is changed to –one.

The location of the carbonyl is indicated with a number. [This number is never 1- because that would be an aldehyde!]

The longest carbon chain is numbered to give the carbonyl the lowest possible number.

2. Nomenclature

The simplest ketones need no number for the carbonyl. (Why?)

Draw structures for the following compounds.

2-fluorocyclohexanone

3-chloro-2-pentanone

propanone butanone

pencast

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2. Nomenclature

Name the following compounds.

pencast

3. Important aldehydes and ketones

What is the relationship between embalming fluid and methanol poisoning?

What is the I.U.P.A.C. name of the following component of lemongrass?

pencast

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3. Important aldehydes and ketones

What is the I.U.P.A.C. name for cinnamaldehyde?

Chanel No. 5 became famous for containing synthetic aldehydes “in copious quantities”.*

*Perfume Shrine

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Follow the link for a fun description of properties of aldehydes and ketones.

3. Important aldehydes and ketones

Aldehydes in perfumery

heptanal: naturally occuring in clary sage and possessing a herbal green odour

octanal: orange-like

nonanal: smelling of roses

decanal: powerfully evocative of orange rind

citral (a more complicated 10-carbon aldehyde): odor of lemons

undecanal: naturally present in coriander leaf oil

unsaturated undecen-1-al

lauryl aldehyde (12 carbons): evocative of lilacs or violets

4. Reactions--oxidation

Preparation is usually by oxidation of an alcohol. Aldehydes are produced from primary alcohols.

Ketones are produced from secondary alcohols.

Tertiary alcohols don’t undergo oxidation (no –H to lose).

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4. Reactions--oxidation

Oxidation of aldehydes and ketones Aldehydes can be oxidized to carboxylic acids.

Ketones can’t be oxidized further.

loss of a bond to H, gain of a bond to O

N.R. (no –H to lose)

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4. Reactions--oxidation

Tollen’s test is used to distinguish between aldehydes and ketones based on their ability to be oxidized.

When one substance is oxidized, another must be reduced.

If an aldehyde is present, a “silver mirror” forms on the inside of the glass container.

If a ketone is present, there is no reaction because it won’t undergo oxidation.

+ Ag(NH3)2+ + Ago

Tollen’s reagent elemental silver

4. Reactions--oxidation

The same process used in Tollen’s test used to be used to produce silvered mirrors.

This is an article published in 1911 describing the process, if you are interested!

Go here to see a 2-liter flask turned into a mirror.

For EXTRA CREDIT, explain what precautions need to be taken when the Tollen’s test is carried out. You’ll have to watch the video. Post your answer in the Module 4 Journal Answers journal with the tag Tollen.

4. Reactions--oxidation

Draw the structure of the following compounds and their oxidation products.

2-methyl-2-propanol

2-nonanol

1-decanol pencast

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4. Reactions--oxidation

What reaction will pentanal undergo, if any, with Tollen’s reagent?

+ Ag(NH3)2+ + Ago

pencast

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4. Reactions--oxidation

Benedict’s test is used to distinguish between reducing and non-reducing sugars.

A reducing sugar can be oxidized. The substance reduced is Cu+2

.

+ Cu+2

+ Cu2O

4. Reactions—blast from the past

Can you remember Chapter 12? Preparation of alcohols by hydrogenation of aldehydes and ketones

(slide 19)

4. Reactions--reduction

Hydrogenation is a reduction reaction. More bonds to hydrogen, fewer bonds to oxygen

Reduction of an aldehyde or ketone requires a Ni, Pt, or Pd catalyst. [sound familiar?]

Reduction of an aldehyde produces a primary alcohol. Reduction of a ketone produces a secondary alcohol.

4. Reactions--reduction

What is the product of reduction of

1-chloropropanone?

2-methylpropanal? pencast

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4. Reactions--addition

Take the time machine back to Chapter 11 now. Alkenes undergo addition reactions:

Hydrogenation (addition of H2) Halogenation (addition of X2) Hydration (addition of H2O) Hydrohalogenation (addition of HX)

General addition reaction

4. Reactions--addition

Addition of an alcohol to an aldehyde:

The product is called a hemiacetal (-OH and –OR attached to the same carbon).

Hemiacetals are very reactive. They react with an additional alcohol molecule, losing –OH and

adding another –OR.

H+

4. Reactions--addition

The final product is an acetal (2 –OR groups attached to one carbon).

hemiacetal acetal

4. Reactions--addition

Ketones undergo analogous addition reactions with alcohols.

The initial product is a reactive hemiketal (two –R groups, one –OH, and one –OR).

An additional –OR group is added to the hemiketal to produce a ketal.

hemiketal ketal

4. Reactions--addition

4. Reactions

Hemiacetal, acetal, hemiketal, or ketal?

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4. Reactions

Monosaccharide addition reactions

1

23

4

5

6 ]alcohol

aldehyde

D-glucose

]

4. Reactions

Monosaccharide addition reactions

The cyclic form is more stable than the linear form and no further oxidation takes place in this case.

1

23

4

5

6Hemiacetal: one –H one –OH one –OR one -R

4. Reactions: keto-enol tautomers

Tautomers differ from each other in the placement of one hydrogen and one double bond.

The two forms exist in an equilibrium mixture, mostly in the keto form, which is more stable.

Keto form (ketone)

Enol form (alcohol + alkene)

4. Reactions: keto-enol tautomers

Draw the keto and enol forms of

propanone.

propanal. pencast

4. Reactions: aldol condensation

In an aldol condensation, aldehydes or ketones react to make a larger molecule by forming a new carbon-carbon bond between two molecules.

+

OH- or enzyme

4. Reactions: aldol condensation

Write an equation for the aldol condensation of two molecules of propanal.

pencast