Chapter 16: Aldehydes and Ketones

Carbonyl group bonded to a hydrogen atom

functional group of an aldehyde

Methanal (formaldehyde)

• simplest aldehyde

• its carbonyl group is bonded to 2 hydrogen atoms

Other aldehydes

bonded to 1 hydrogen and 1 carbon group

Functional group of ketone

carbonyl group bonded to 2 carbon groups

Naming an aldehyde

change the suffix -e of the parent alkane to -al

Unsaturated aldehydes

• indicate the presence of a carbon-carbon double bond by changing the ending of the parent alkane from -ane to -enal

• Numbering the carbon chain begins with the aldehyde carbonyl carbon

• Show the location of the carbon-carbon double bond by the number of its first carbon

Retained common names for some aldehydes by IUPAC system

• trans-3-Phenyl-2-propenal (cinnamaldehyde; in oil of cinnamon)

• Benzaldehyde (in almonds)

• Vanillin (from vanilla beans

IUPAC names for ketones

1. The parent alkane is the longest chain that contains the carbonyl group

2. Indicate the presence of the carbonyl group by changing the -ane of the parent alkane -one

3. Number the parent chain from the direction that gives the carbonyl carbon the smaller number

4. The IUPAC retains the common name acetone for 2-propanone

To name an aldehyde or ketone that also contains an –OH (hydroxyl) or –NH2 (amino) group

• Number the parent chain to give the carbonyl carbon the lower number

• Indicate an –OH substituent by hydroxy-, and an –NH2 substituent by amino-

• Hydroxyl and amino substituents are numbered and alphabetized along with other substituents

C=O bond

• polar

• oxygen bearing a partial negative charge and carbon bearing a partial positive charge

  • aldehydes and ketones are polar molecules

Aldehydes and ketones

• polar molecules

• no hydrogen bonding is possible between them because they have no polar O-H or N-H bonds to participate in hydrogen bonding

• have lower boiling points than alcohols and carboxylic acids

• become less soluble in water as the hydrocarbon portion of the molecule increases in size

In liquid aldehydes and ketones

there are weak intermolecular attractions between the partial positive charge on the carbonyl carbon of one molecule and the partial negative charge on the carbonyl oxygen of another molecule

Alcohols and carboxylic acids

compounds in which there is hydrogen bonding between molecules

Formaldehyde, acetaldehyde, and acetone

infinitely insoluble in water

Aldehydes

oxidized to carboxylic acids by a variety of oxidizing agents, including potassium dichromate

Liquid aldehydes

so sensitive to oxidation by O2 in the air that they must be protected from contact with air during storage

Ketones

resist oxidation by most oxidizing agents, including potassium dichromate and molecular oxygen

Tollens’ reagent

• specific for the oxidation of aldehydes

• If done properly, silver deposits on the walls of the container as a silver mirror

Carbonyl group of an aldehyde or ketone

reduced to a – C(H)OH group by hydrogen in the presence of a transition metal catalyst

Primary alcohol

product of reduction of an aldehyde

Secondary alcohol

product of reduction of a ketone

sodium borohydride (NaBH4)

• most common laboratory reagent for the reduction of an aldehyde or ketone

• This reagent contains hydrogen in the form of hydride ion, H:–

• in its reduction, hydride ion adds to the partially positive oxygen

• Reaction of this intermediate with aqueous acid gives the alcohol

Reduction by NaBH4

does not affect a carbon-carbon double bond or an aromatic ring

reduced form of nicotinamide adenine dinucleotide (NADH)

In biological systems, the agent for the reduction of aldehydes and ketones is the _____

hemiacetal

• formed from addition of molecule of alcohol to the carbonyl group or ketone

• its functional group is a carbon bonded to one -OH group and one -OR group

• In forming it, –H of the alcohol adds to the carbonyl oxygen and –OR adds to the carbonyl carbon

• generally unstable and are only major components of an equilibrium mixture except in one very important type of molecule

Cyclic hemiacetal form

When a hydroxyl group is part of the same molecule that contains the carbonyl group and a five- or six-membered ring can form, the compound exists almost entirely in a _________

Acetal and water

• product of hemiacetal when it reacts further with an alcohol

  • this reaction is acid-catalyzed

• its functional is a carbon bonded to 2 -OR groups

Reversible

all steps in hemiacetal and acetal formation are ________

Le Chatelier’s principle

Formation of hemiactel and acetal can be driven in either direction by using this

Drive to the right

use a large excess of alcohol or remove water from the equilibrium mixture

Drive to the left

use a large excess of water

Adding a proton to the carbonyl oxygen

makes the carbonyl carbon stronger electrophile and more susceptible to attack by a nucleophile

Oxonium ion

the intermediate formed when the first -OR group required is added for acetal formation

Mechanism of Acetal formation

1. Add a proton

2. Reaction of an electrophile and a nucleophile to form a new covalent bond

3. Proton transfer to another oxygen

4. Break a bond to form stable molecules or ions

5. Reaction of an electrophile and a nucleophile to form a new covalent bond

6. Take a proton away

a-carbon

carbon atom adjacent to a carbonyl group

a-hydrogen

hydrogen atom bonded to a-carbon

Enol

• An aldehyde or ketone that has a hydrogen on an α-carbon is in equilibrium with this constitutional isomer

• its name is derive from the IUPAC designation of it as both an alkene (-en-) and an alcohol (-ol)

Keto-enol equilibrium

the keto form generally predominates