1. what is the carbonyl group?

Copyright © 2010 Pearson Education, Inc. Chapter Sixteen 1

Goals

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1. What is the carbonyl group? Be able to recognize the carbonyl group and describe its polarity and shape.

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2. How are ketones and aldehydes named? Be able to name the simple members of these families and write their structures, given the names.

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3. What are the general properties of aldehydes and ketones? Be able to describe such properties as polarity, hydrogen bonding, and water solubility.


Goals Contd.►

4. What are some of the significant occurrences and applications of aldehydes and ketones? Be able to specify where aldehydes and ketones are found, list their major applications, and discuss some important

members of each family.

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5. What are the results of the oxidation and reduction of aldehydes and ketones? Be able to describe and predict the products of the oxidation and reduction of aldehydes and ketones.

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6. What are hemiacetals and acetals, how are they formed, and how do they react? Be able to recognize hemiacetals

and acetals, describe the conditions under which they are formed, and predict the products of hemiacetal

and acetal

formation and acetal

hydrolysis.


16.1 The Carbonyl Group►

Carbonyl compound: Any compound that contains a carbonyl group, C=O.

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Carbonyl group: A functional group that has a C atom joined to an O atom by a double bond.

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The bond angles between the three substituents on the carbonyl carbon atom are 120°, or close to it.


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Aldehyde: A compound that has a carbonyl group bonded to at least one hydrogen.

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Ketone: A compound that has a carbonyl group bonded to two carbons in organic groups that can be the same or different.


16.2 Naming Aldehydes and Ketones►

The simplest aldehydes are known by their common names, which end in -aldehyde, for example, formaldehyde, acetaldehyde, and benzaldehyde.

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In the IUPAC system, the -e

ending of the alkane with the same number of C’s is replaced by -al.

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When substituents are present, the chain is numbered beginning with 1 for the carbonyl carbon. For example, 3-methylbutanal.


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Common names for ketones give the names of the two alkyl groups bonded to the carbonyl carbon followed by the word ketone.

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Ketones are named systematically by replacing the final -e of the alkane name with -one. The numbering of the chain begins at the end nearest the carbonyl group. The location of the carbonyl group is indicated by placing the number of the carbonyl carbon in front of the name. Using this nomenclature scheme, acetone would be named 2-propanone.

Chapter Seven

WORKED EXAMPLE 16.1 Naming a Ketone Given Its Structure

Analysis

Give both the systematic (IUPAC) name and the common name for the following compound:

The common name uses the names of the two alkyl groups.

Solution

The IUPAC name is 3-hexanone. The common name is ethyl propyl

ketone.

The compound is a ketone, as shown by the single carbonyl group bonded to two alkyl groups: an ethyl group on the left (CH3

CH2

—) and a propyl

group on the right (—CH2

CH2

CH3

). The IUPAC system identifies and numbers carbon chains to indicate where the carbonyl group is located, counting in the direction that gives the carbonyl carbon the lowest number possible.


16.3 Properties of Aldehydes and Ketones

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Aldehydes and ketones cannot hydrogen-bond with one another, so they are lower boiling than alcohols.

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Aldehydes and ketones are higher boiling than alkanes because of the polarity of the carbonyl group. Common aldehydes and ketones are liquids.


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Simple aldehydes and ketones are water-soluble due to hydrogen bonding with water molecules, and ketones are good solvents.

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Simple ketones are less toxic than simple aldehydes.

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Many aldehydes and ketones have distinctive odors.

►A perfumer sits at a mixing table testing new combinations of fragrances, many of which are aldehydes and ketones.


16.4 Some Common Aldehydes and Ketones

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Formaldehyde is a colorless gas. Low concentrations in air cause eye, throat, and bronchial irritation, and high concentrations trigger asthma attacks. Skin contact produces dermatitis. CH2

O is very toxic by ingestion, causing serious kidney damage, coma, and sometimes even death.

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CH2

O kills viruses, fungi, and bacteria by reaction with the groups in proteins, allowing for its use in disinfecting and sterilizing equipment and as a preservative for biological specimens.


► Acetaldehyde is a sweet-smelling, flammable liquid present in ripe fruits, and produced in the normal breakdown of carbohydrates.

► Acetaldehyde is a general narcotic, and large doses can cause respiratory failure. Chronic exposure produces symptoms like those of alcoholism.


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Acetone is one of the most widely used of all organic solvents. It dissolves most organic compounds and is also miscible with water. Acetone is the solvent in many varnishes, lacquers, and nail polish removers.

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When the metabolism of fats and carbohydrates is out of balance (for example, in starvation or diabetes mellitus), acetone is produced in the liver, a condition known as ketosis that in severe cases leaves the odor of acetone on a patient’s breath.


16.5 Oxidation of Aldehydes►

In aldehyde oxidation, the hydrogen bonded to the carbonyl carbon is replaced by an –OH group.

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Ketones, because they do not have this hydrogen, do not react with mild oxidizing agents.


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(a) In the Tollens’

test, colorless silver ion is reduced to metallic silver.–– TollensTollens reagent: Ag(NHreagent: Ag(NH33 ))22 NONO33 ((aqaq)) in in NaOHNaOH ((aqaq))

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(b) In the Benedict’s test, the blue copper(II) ion is reduced to copper(I) in brick-

red copper(I) oxide–– Commonly used as a test for reducing sugars (mostly Commonly used as a test for reducing sugars (mostly aldehydesaldehydes))–– Can be used to detect presence of glucose in urine.Can be used to detect presence of glucose in urine.

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In both tests, the aldehyde is oxidized to the carboxylic acid.


16.6 Reduction of Aldehydes and Ketones

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The reduction of a carbonyl group occurs with the addition of hydrogen across the double bond to produce an –OH group, a reaction that is the reverse of the oxidation of an alcohol.

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Aldehydes are reduced to primary alcohols, and ketones are reduced to secondary alcohols.

Chapter Seven

WORKED EXAMPLE 16.2 Writing the Products of a Carbonyl Reduction

Analysis

Solution

What product would you obtain by reduction of benzaldehyde?

First, draw the structure of the starting material, showing the double bond in the carbonyl group. Then rewrite the structure showing only a single bond between C and O, along with partial bonds to both C and O:

Finally, attach hydrogen atoms to the two partial bonds and rewrite the product.

The product obtained is benzyl alcohol


16.7 Addition of Alcohols: Hemiacetals and Acetals

The initial product of addition reactions of aldehydes and ketones with alcohols are known as hemiacetals. Compounds with both an -OH group and an -OR group bonded to the same carbon atom.


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Ethanol forms hemiacetals

with acetaldehyde and acetone.

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Hemiacetals

rapidly revert back to aldehydes or ketones by loss of alcohol and establish an equilibrium with the aldehyde or ketone.

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When equilibrium is reached, very little hemiacetal

is present.


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A major exception occurs when the alcohol and carbonyl functional groups that react are part of the same

molecule.

The resulting cyclic hemiacetal

is more stable than a noncyclic

hemiacetal.

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Most simple sugars exist mainly in the cyclic hemiacetal form, as shown below for glucose, rather than in the open-

chain form.


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If a small amount of acid catalyst is added to the reaction of an alcohol with an aldehyde or ketone, the hemiacetal

initially

formed is converted into an acetal

in a substitution reaction.

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An acetal is a compound that has two

-OR groups bonded to what was once the carbonyl carbon atom.


The aldehyde or ketone from which an acetal

is formed can be regenerated by reversing the reaction. Reversal requires an acid catalyst and a large quantity of water.

Hydrolysis: A reaction in which a bond or bonds are broken and the -H and -OH of water add to the atoms of the broken bond or bonds.

Chapter Seven

WORKED EXAMPLE 16.3 Predicting the Products of Hemiacetal and Acetal Formation

Analysis

Solution

Write the structure of the intermediate hemiacetal

and the acetal

final product formed in the following reaction:

First, rewrite the structure showing only a single bond between C and O, along with partial bonds to both C and O:

Next, add one molecule of the alcohol (CH3

OH in this case) by attaching —H to the oxygen partial bond and —OCH3

to the carbon partial bond. This yields the hemiacetal

intermediate:

Finally, replace the —OH group of the hemiacetal

with an —OCH3

from a second molecule of alcohol.

The reaction produces the acetal

and water:

Chapter Seven

WORKED EXAMPLE 16.4 Identification of Hemiacetals

Analysis

Solution

Which of the following compounds are hemiacetals?

To identify a hemiacetal, look for a carbon atom with single bonds to 2 oxygen atoms, one in an —OR group and one in an —OH group. Note that the O of the —OR group can be part of a ring.

Compound (a) contains 2 O atoms, but they are bonded to different C atoms; it is not a hemiacetal. Compound (b) has 1 ring C atom bonded to 2 oxygen atoms, one in the substituent —OR group and one bonded to the rest of the ring, which is the R group; it is a cyclic hemiacetal. Compound (c) also contains a C atom bonded to one —OH group and one —OR group, so it too is a hemiacetal.

Chapter Seven

WORKED EXAMPLE 16.5 Identification of Acetals

Analysis

Solution

Which of the following compounds are acetals?

As in identifying hemiacetals, look for a carbon atom that has single bonds to 2 oxygen atoms, but in this case both of them will be —OR groups. Note that the O of the —OR group can be part of a ring.

In (a), the central carbon atom is bonded to —CH3

, —H and two —OCH2

CH3

groups, so the compound is an acetal. Compound (b) does have a carbon atom bonded to 2 oxygen atoms, but one of the bonds is a double bond rather than a single bond, so this is not an acetal. Compound (c) has an oxygen atom in a ring, making it also part

of an —OR group, where R is the ring. Since one of the carbons connected to the O in the ring is also connected to an —OCH2

CH3

group, compound (c) is an acetal.

Chapter Seven

WORKED EXAMPLE 16.6 Writing the Products Obtained from Acetal Hydrolysis

Analysis

Write the structure of the aldehyde

or ketone

that forms by hydrolysis of the following acetal:

The products are the aldehyde

or ketone

plus two molecules of the alcohol from which the acetal

could have been formed. First, identify the two C—O acetal

bonds, redrawing the structure if necessary:

Next, break the H—OH bond and one of the acetal

C—OR bonds (it does not matter which one); move the water OH to the acetal

carbon to form the hemiacetal

and the water H to the OR to form one molecule of HOR:

Chapter Seven

Remove the H and OR groups from the hemiacetal, and change the C—O single bond to a C=O double bond to give carbon the four bonds it must have. Combine the H and OR you removed from the second alcohol molecule.

In this example, the product is an aldehyde.

Solution

WORKED EXAMPLE 16.6 Writing the Products Obtained from Acetal HydrolysisContinued


Chapter Summary►

The carbonyl group is a C=O. The group is polar, with a partial (-) charge on O and a partial (+) charge on C. The O and the two substituents on the carbonyl-group C atom form a planar triangle.

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The simplest aldehydes and ketones are known by common names. Aldehydes are named systematically by replacing the final -e in an alkane name with -al.

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Ketones are named systematically by replacing the final -e in an alkane name with -one and numbering starting with 1 at the end nearer the group. The location of the carbonyl group is indicated by placing the number of its carbon before the name.


Chapter Summary Contd.►

Aldehyde and ketone molecules are polar, do not hydrogen-

bond with each other, but can hydrogen-

bond with water. Small ones are water-soluble. Aldehydes and ketones are higher boiling than alkanes but lower boiling than alcohols.

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Aldehydes and ketones are present in many plants, where they contribute distinctive, pleasant odors. Such natural aldehydes and ketones are widely used in perfumes and flavorings.

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Formaldehyde (an irritating and toxic substance) is used in polymers, is present in smog-laden air, and is produced biochemically from ingested methanol.



Acetone is a widely used solvent and is a by-product of food breakdown during diabetes and starvation. Many sugars are aldehydes or ketones.

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Mild oxidizing agents (Tollens’

and Benedict’s reagents) convert aldehydes to carboxylic acids but have no effect on ketones.

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With reducing agents, hydride ion adds to the C of the group in an aldehyde or ketone and hydrogen ion adds to the O to produce primary or secondary alcohols, respectively.



Aldehydes and ketones establish equilibria with alcohols to form

hemiacetals

or acetals.

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Hemiacetals, which have an -OH and an -OR on what was the carbonyl carbon, result from addition of one alcohol molecule to the C=O bond.

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The more stable acetals, which have two -OR groups on what was the carbonyl carbon, form by addition of a second alcohol molecule to a hemiacetal.

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The aldehyde or ketone can be regenerated from an acetal

by treatment with an acid catalyst and a large quantity of water, which is an

example of a hydrolysis reaction.

1. what is the carbonyl group?

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