Chapter 15: Aldehyde and Ketones

In this chapter, we discuss the hydrocarbon derivatives that contain the element oxygen directly with a double bond. In last chapter, we discussed the functional groups (alcohols phenols, and ethers) have the common feature of carbon-oxygen single bonds. Carbon oxygen double bonds are also possible in hydrocarbon derivatives. We will now consider the simplest types of compounds that contain this structural feature: aldehydes and ketones.

15.1 The Carbonyl Group

Both aldehydes and ketones contain a carbonyl functional group. A carbonyl group is a n atom double-bonded to an oxygen atom. The structural representation for a carbonyl group is

Carbon-oxygen and carbon-carbon double bonds differ in a major way. A carbon oxygen double bond is polar, and a carbon-carbon double bond is nonpolar. The electronegativity (Section 5.9) of oxygen (3.5) is much greater than that of carbon (2.5). Hence the carbon-oxygen double bond is polarized, the oxygen atom acquiring a fraction of negative charge and the carbon atom positive charge as shown in figure above.

15.2 Compounds Containing a Carbonyl Group

Aldehydes: Carbonyl is attached to at least one H atom in aldehydes

Ketones: Carbonyl is directly attached with two carbon atoms in ketones

Carboxylic acids: carbonyl carbon atom bonded to a hydroxyl group.

Esters: carbonyl carbon atom bonded to an oxygen atom

Amide: carbonyl carbon atom bonded to a nitrogen atom

Aldehydes  Carbon chain on one side of carbonyl and hydrogen on the other	Ketones  Carbon chain on both sides of the carbonyl

Other Carbonyl Derivatives

15.3 The Aldehyde and Ketone Funcitonal Groups

Common names of simple aldehydes and ketones

Aldehydes:

Formaldehyde: HCHO

    Acetaldehyde:                       CH₃CHO

    Propionaldehyde:             CH₃CH₂CHO

    Butyraldehyde:           CH₃CH₂CH₂CHO

Valeraldehyde:      CH₃CH₂CH₂CH₂CHO

Ketones:

Acetone:                                     CH₃COCH₃

Methyl ethyl ketone CH₃CH₂COCH₃

Butyl propyl ketone CH₃CH₂CH₂CH₂COCH₂CH₂CH₃

15.4 Nomenclature for Aldehydes

Rule 1: Select as the parent carbon chain the longest carbon chain that includes the carbon atom of the carbonyl group.

Rule 2: Name the parent chain by changing the “-e” ending of the corresponding alkane name to “-one.”

Rule 3: Number the carbon chain such that the carbonyl carbon atom receives the lowest possible number. The position of the carbonyl carbon atom is noted by placing a number immediately before the name of the parent chain.

Rule 4: Determine the identity and location of any substituents, and append this information to the front of the parent chain name.

Rule 5: Cyclic ketones are named by assigning the number 1 to the carbon atom of the carbonyl group. The ring is then numbered to give the lowest number(s) to the atom(s) bearing substituents.

Aldehydes: -al as suffix

Give the condensed structures of following aldehydes:

Aldehydes:
Ethanal (acetaldehyde) 
3-hydroxybutanal (aldol) 
3-bromo-5-methylhexanal 
1-bromo-5-methyl-3-heptanone 

15.5 Nomenclature for Ketones

Ketones: -one as suffix

Aromatic ketones

Give the condensed structures of following ketones:

Ketones:
propanal (propionaldehyde) 
2-butanone (methylethylketone) 
4,4-dimethyl-2-pentanone 
cyclopentanone 
4-hydroxy-2-cylohexenone 

15.6 Isomerism for Aldehydes and Ketones

Constitutional isomers exist for aldehydes and ketones

Isomers between aldehydes and ketones are called functional group isomers

Two types of isomers:

• Skeletal isomers: arrangements of atoms in space is different.

• Positional isomers: Position of the functional group is different.

The keto and enol forms of aldehydes and ketones
Aldehydes and ketones exsists in two different isomers also called tautomers. Regular carbonyl form of the aldehyde or the ketone is called keto form and the -OH containing form is called enol from.

 In biological systems keto form of aldehyde sugars (aldoses) are converted to ketone sugars (ketoses) via enediol (enol) froms as shown below. Therefore D-fructose which is a ketone or keto sugar (ketose) will give a positive test for Benedict's test because of the ability of ketoses to get converted to aldoses (aldehydes).

15.7 Selected Common Aldehydes and Ketones

Methanal or formaldehyde:

structure of fomaldehyde

Fomalin 40% water solution of formaldehyde has been widely used a preservative for biological specimens.  Formaldehyde is the simplest aldehyde. It is manufactured by oxidizing methanol with air over a metal catalyst in a temperature range of 400-650 degrees Celsius. The catalyst can be copper, silver, or molybdeum alloy.

Ethanal or acetaldehyde:

Acetaldehyde also called Ethanal (CH₃CHO), an aldehyde used as a starting material in the synthesis of acetic acid, n-butyl alcohol, ethyl acetate, and other chemical compounds. It is manufactured by the oxidation of ethyl alcohol and by the hydration of acetylene. Pure acetaldehyde is a colourless, flammable liquid.

2-Propanone or acetone:

Acetone also called 2-propanone, or dimethyl ketone  (CH₃COCH₃), organic solvent of industrial and chemical significance, the simplest and most important of the aliphatic (fat-derived) ketones. Pure acetone is a colourless, somewhat  aromatic, flammable, mobile liquid that boils at 56 C. It is also used as nail polish.

2-Butanone or methyl ethyl ketone:
Important idustrial sovent.

Oil of almonds or benzaldehyde:

Found in almond nuts.

Oil of Cinnamon or cinnamaldehyde:

Found in cinnamon.

Berry Flavoring a-Demascone:

Found in berry.

Oil of vanilla beans or vanillin:

Found in vanilla beans.

Mushroom flavoring or 2-octanone:

used as mushroom flavoring.

Oil of lemongrass or citral:

Found in lemon grass.

15.8 Physical Properties of Aldehydes and Ketones

15.9 Preparation of Aldehydes and Ketones

Preapration of aldehydes:
Partial oxidation of primary  alcohols with H₂CrO₄:

(primary alcohol)          ----                 (aldehyde)

Preapration of ketones:
Oxidation of secondary alcohols with KMnO₄, H₂CrO₄

(Secondary alcohol)          ----                 (ketone)

Tertiary alcohols doesn't under go oxidation:

                (Tertiary alcohols)          ----                 (no reaction)

15.10 Oxidation and Reduction of Aldehydes and Ketones

Aldehyde are readily oxidized to corresponding  carboxylic acids  by number of different oxidizing agents.  The symbol [O] is used to indicate the oxidation process.
Benedict's Test for aldehydes:

  Aldehyde loses an e- (aldehyde is oxidized to an acid)  Cu²⁺ gains an e- (Cu²⁺ is reduced to Cu⁺)

Oxidizing Agents and Reducing Agents in the Benedict's Test

Oxidation = loss of an e- 
Reduction = gain of an e-

The aldehyde is a reducing agent. 
The aldehyde is oxidized, but it's oxidation facilitates the reduction of the Cu²⁺.

The Cu²⁺ is  an oxidizing agent. 
The Cu²⁺ is reduced, but it's reduction facilitates the oxidation of the aldehyde.

The travel agent does not travel, but the travel agent facilitates the travel of someone else.

Glucose is an aldehyde sugar which show positive test for the Benedict's test. This test have been used in old days to detect excess bolld sugsr in diabetic patients.

Tollen's Test:

A basic Ag(NH₃)₂⁺ in aqueous ammonia reduces  to metallic silver (mirror) with aldehyde oxidized to carboxylic acid.

The commercial manufacture of silver mirrors uses a similar process.

Reduction of carbonyl compounds to alcohols:
Reduction aldehydes and ketones to primary sceondary alcohols Reduction Reactions: Aldehyde and ketones are readily reduced to corresponding alcohols by number of different reducing agents. The symbol [H] is used to indicate the reduction process.  One of the methods is hydrogenation where H₂ in the presence of a catalyst such as Pt rduces the alcohol. The  methods of  reduction include:

Pt(H₂) platinum catalyzed hydrogenation
LiAlH₄ lithium aluminum hydride
NaBH₄ (sodium borohydride)

Reduction of aldehydes:

(aldehyde)     ----   (primary alcohol)

Reduction of ketones:

(Secondary alcohol)          ----                 (ketone)

15.11 Reaction of Aldehydes and Ketones with Alcohols

Addition reactions to carbonyl double bond in aldehydes and ketones:

Hemiacetal and hemiketal formation

Hemiacetals

Hemiketals

ketone  + 1st alcohol  ---  hemiketal

Acetals, and Ketals.

In biological systems sugars which are complex aldehydes and ketones are found in one of these forms: hemiacetals,  and hemiketals or acetals and ketals.

Cyanohydrin formation

Hydrate formation

Aldol condensation reactions
Aldol condensation is a reaction in which aldehydes  or ketones react to form larger molecules. In biological systems this reaction is catalysed by an enzyme named aldolase.

aldehyde                aldehyde                                                         aldol

15.12 Formaldehyde-Based Polymers

Formaldehyde condense with phenols  under acidic or basic  condition; polymerization gives a network of phenol rings held  together by methylene groups at the ortho and para positions.

This polymer called bakelite is a  stiff, three dimensional network with very little solubility in organic solvents and a high resistance to electricity and heat. It is  used in a wide variety of household objects and electrical fixtures.

15.13 Sulfur-Containing Carbonyl Groups