Ch 17- Aldehydes and Ketones Homework: 17.10, 17.13, 17.17, 17.19, 17.28, 17.35, 17.40, 17.41, 17.49, 17.53, 17.69

In this chapter we study compounds that contain the carbonyl group. The carbonyl group is one of the most important functional groups in organic chemistry and Biochemistry because it is present in aldehydes, ketones, carboxylic acid and carboxylic acid derivatives. The Carbonyl group

Aldehydes & Ketones The functional group of an aldehyde is a carbonyl group bonded to a hydrogen In methanal, the simplest aldehyde, the carbonyl group is bonded to 2 H’s In all other aldehydes, it is bonded to one H and a carbon chain. The functional group of a ketone is a carbonyl group bonded to 2 carbon chains.

Nomenclature 1) Find the longest chain that contains the carbon of the carbonyl and number the chain to give that carbon the lowest number. –For aldehydes, drop the -e, and add -al –Aldehydes will always be at the end of a chain, so they will always be at carbon #1, so there is no need to put the 1.

Nomenclature (cont) For aldehydes that have carbon-carbon double bonds in them, the longest chain must contain the carbon of the carbonyl and BOTH carbons of the C-C double bond!! To name these, we drop the -ane ending and add -enal The -en- shows the double bond, the -al shows the aldehyde. –Remember to provide the locant for the double bond

Nomenclature of Ketones For ketones, we find the longest chain that contains the carbon of the carbonyl group, and number the chain to give that carbon the lowest number You drop the -e, from the parent name and add -one.

In naming aldehydes and ketones that also have an -OH in the molecule, find the longest chain that contains both the carbon of the carbonyl and the carbon bonded to the -OH group Number the chain to give the carbon of the carbonyl the lowest number The -OH will be named as a substituent! When the -OH group is named as a substituent, it is named as a hydroxy, and numbered and alphabetized will all other substituents present.

Examples

Physical Properties Oxygen is more electronegative than carbon therefore the carbon-oxygen double bond is polar with the Oxygen bearing a partial negative charge and the Carbon bearing a partial positive charge The only intermolecular forces are dipole- dipole forces and London Forces They can not Hydrogen bond to each other!!

Physical Properties As the groups bonded to the carbonyl increase in size, the solubility in water decreases Most aldehydes and ketones have strong odors and are used in perfumes and flavoring agents

Reactions 1)Aldehydes can be oxidized to the carboxylic acids -Ketones are resistant to oxidation Aldehydes can also be oxidized by O 2 !!

Reduction Reactions Just like C-C double bonds, C-O double bonds can be reduced by the addition of H 2 with a metal catalyst to produce an alcohol.

Sodium Borohydride Aldehydes and Ketones can also be reduced using Sodium Borohydride, NaBH 4 NaBH 4 contains hydrogen in the form of hydride ions, H - The advantage of using NaBH 4 is that it does not reduce C-C double bonds!

Examples

In Nature In Biological systems, nicotinamide adenine dinucleotide, a coenzyme abbreviated NADH, is used to provide the hydride ion to reduce aldehydes and ketones.

Reactions of Alcohols 3) Addition of Alcohols Addition of one molecule on an alcohol to an aldehyde or ketone form a hemiacetal Hemiacetal- a compound with a carbon bonded to 1 -OH group and 1 -OR group

Examples

Reactions of Hemiacetals Hemiacetals can react with another molecule of alcohol to form an acetal Acetal- a compound with a carbon bonded to 2 -OR groups

Info on Hemiacetals Hemiacetals are generally unstable and are only minor components of an equilibrium mixture The only exception is when the -OH group is part of the same molecule as the carbonyl group and a five or six member ring can form The compound exists almost entirely in the cyclic hemiacetal forms In this case, the -OH group adds to the C=O group of the same molecule

Examples

YOUR TURN!! Do the same thing with: NOTE: Six membered rings are more stable than five membered Rings. If both can form, the six membered ring will form over the Five membered ring

Keto-Enol Tautomerism A carbon atom adjacent to a carbonyl group is called an  -carbon, and a hydrogen atom bonded to it is called an  -hydrogen

Keto-Enol Tautomerism A carbonyl compound that has an  - hydrogen is in equilibrium with a constitutional isomer called an enol The name enol is derived from the IUPAC designation of it as having both an alkene (-en) and an alcohol (-ol)

Keto-Enol Tautomerism The Keto and Enol forms are examples of Tautomers. Tautomer- constitutional isomers in equilibrium with each other that differ in the location of a hydrogen atom relative to an oxygen atom This type of isomerism is called keto-enol tautomerism For any pair of keto-enol tautomers, the keto form generally predominates at equilibrium!