1 Lecture 4: Aldehydes, Ketones, and Chiral Molecules 14.1 Aldehydes and Ketones Copyright © 2007...

1

Lecture 4: Aldehydes, Ketones, and Chiral Molecules

14.1

Aldehydes and Ketones

Copyright © 2007 by Pearson Education, Inc.Publishing as Benjamin Cummings

2

Carbonyl Group in Aldehydes and KetonesA carbonyl group (C=O) In an aldehyde is attached

to at least one H atom.

In a ketone is attached to two carbon groups.


3

Naming Aldehydes

An aldehyde Has an IUPAC name in which the -e in the alkane

name is changed to -al. Has a common name for the first four aldehydes that

use the prefixes form(1C), acet(2C), propion(3C), and butyr(4C), followed by aldehyde.

O O O

║ ║ ║

H−C−H CH3−C−H CH3−CH2−C−H

methanal ethanal propanal

(formaldehyde) (acetaldehyde) (propionaldehyde)

4

Naming Aldehydes


5

Aldehydes in Flavorings

Several naturally occurring aldehydes are used as flavorings for foods and fragrances.

H

O

C

H

O

CCH=CH

Benzaldehyde(almonds)

Cinnamaldehyde (cinnamon)


6

Naming Ketones

Ketones are named In the IUPAC system by replacing the -e in the alkane

name with one. The carbonyl carbon is indicated by a number.

With a common name by indicating the alkyl groups attached to the carbonyl group in alphabetical order

followed by ketone.

O O

║ ║

CH3−C−CH3 CH3−C−CH2−CH3

propanone 2-butanone

(dimethyl ketone) (ethyl methyl ketone)

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Ketones in Common Use

Nail polish remover, Solvent

Propanone, Dimethylketone, Acetone

Butter flavoring


8

Classify each as 1) aldehyde or 2) ketone. O O

|| ||A. CH3—CH2—C—CH3 B. CH3—C—H

CH3 O | ||

C. CH3—C—CH2—C—H D. | CH3

Learning Check

O

9

Learning Check

Classify each as an aldehyde (1), ketone (2), alcohol (3),or ether (4). O ║ A. CH3─CH2─C─CH3 B. CH3─O─CH3

CH3 O OH

│ ║ │C. CH3─C─CH2─C─H D. CH3─CH─CH3

│ CH3

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Learning Check

Name each of the following: O ║1. CH3─CH2─CH2─CH2─C─H

O

║ 2. Cl─CH2─CH2─C─H O ║ 3. CH3─CH2─C─CH3

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Learning Check

Draw the structural formulas for each:

A. 4-methylpentanal

B. 2,3-dichloropropanal

C. 3-methyl-2-butanone

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14.2

Physical Properties



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Polar Carbonyl Group

The polar carbonyl group Provides dipole-dipole interactions.

+ - + -

C=O C=O

Does not have H on the oxygen atom. Cannot form hydrogen bonds.

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Boiling Points

Aldehydes and ketones have Polar carbonyl groups (C=O).

+ -

C=O Attractions between polar groups.

+ - + -

C=O C=O Higher boiling points than alkanes and ethers of

similar mass. Lower boiling points than alcohols of similar mass.

15

Comparison of Boiling Points

58

Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

16

Learning Check

Select the compound in each pair that would have the

Higher boiling point.

A. CH3—CH2—CH3 or CH3—CH2—OH

B.

C. CH3—CH2—OH or CH3—O—CH3

or

O

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Solubility in Water

The electronegative O atom of the carbonyl group in aldehydes and ketones forms hydrogen bonds with water.


18

Learning Check

Indicate if each is soluble or insoluble in water.A. CH3—CH2—CH3

B. CH3—CH2—OH O

||C. CH3—CH2—CH2—C—H

O ||

D. CH3—C—CH3

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Lecture 4: Aldehydes, Ketones and Chiral Molecules

14.3

Oxidation and Reduction


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Oxidation

Aldehydes are easily oxidized to carboxylic acids. O O

|| [O] || CH3—C—H CH3—C—OH Acetaldehyde Acetic acid

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Tollens’ Test

Tollens’ reagent, which contains Ag+, oxidizes aldehydes, but not ketones.

Ag+ is reduced to metallic Ag, which appears as a “mirror” in the test tube.


22

Benedict’s Test

In the Benedict’s Test,

Cu2+ reacts with aldehydes that have an adjacent OH group.

Aldehydes are oxidized to carboxylic acids.

Cu2+ is reduced to give Cu2O(s).


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Learning Check

Write the structure and name of the oxidized product

when each is mixed with Tollens’ reagent.

1. butanal

2. acetaldehyde

3. ethyl methyl ketone

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Lecture 4: Aldehydes, Ketones and Chiral Molecules

14.4

Addition Reactions

|+- +-| —C=O + X—Y —C—O—X

|Y

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Addition Reactions

When a polar molecule adds to the carbonyl group ofan aldehyde or ketone The negative part of the added molecule bonds to the

positive carbonyl carbon. The positive part of the added molecule bonds to the

negative carbonyl oxygen.

|+- +-| —C=O + X—Y — C—O—X

|Y

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Acetal Formation

Alcohols add to the carbonyl group of aldehydes and ketones.

The addition of two alcohols forms acetals.


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Hemiacetal Formation

The addition of one alcohol to an aldehyde or ketone forms an intermediate called a hemiacetal.

Usually, hemiacetals are unstable and difficult to isolate.


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Cyclic Hemiacetals

A stable cyclic hemiacetal forms when the C=O group and the —OH are both part of a five- or

six-atom carbon compound.


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Learning Check

Identify each as a 1) hemiacetal or 2) acetal.

O—CH3|

A. CH3—CH2—C—H | OH

O—CH2CH3|

B. CH3—CH2—C—CH2—CH3 | O—CH2CH3

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Learning Check

Draw the structure of the acetal formed by adding CH3OH to butanal.

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15.5

Chiral Molecules



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Chiral Compounds

Chiral compounds Have the same number of atoms

arranged differently in space. Have one or more chiral carbon

atoms bonded to four different groups.

Are mirror images like your hands. Try to superimpose your thumbs, palms, back of hands, and little fingers. Copyright © 2007 by Pearson Education, Inc.

Publishing as Benjamin Cummings

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Mirror Images

The mirror images of Chiral compounds cannot be superimposed. Two compounds of bromochloroiodomethane can align

the H and I atoms, which places the Cl and Br atoms on opposite sides.

Copyright © 2007 by Pearson Education, Inc Publishing as Benjamin Cummings

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Achiral Structures are Superimposable

When the mirror image of an achiral structure is rotated, the structure can be aligned with the initial structure. Thus this mirror image is superimposable.


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Learning Check

Identify each as a chiral or achiral compound.

A

CH2CH3

H CH3

Cl

C

B

H

H CH3

Cl

C

C

Br

H CH3

Cl

C

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Fischer Projections

A Fischer projection

Is a 2-dimensional representation of a 3-dimensional molecule.

Places the most oxidized group at the top.

Uses vertical lines in place of dashes for bonds that go back.

Uses horizontal lines in place of wedges for bonds that come forward.

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Drawing Fischer Projections


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D and L Notations

By convention, the letter L is assigned to the structure with the —OH on the left.

The letter D is assigned to the structure with the —OH on the right.


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Learning Check

Indicate whether each pair is a mirror image that

cannot be superimposed.

B.

A.

and

and

Br

CH3 H

CH2OH

C

H

CH3 H

Cl

C

Br

H CH3

CH2OH

C

H

H CH3

Cl

C

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