10E. Aldehydes + Ketones

Question 1

Aldehydes + Ketones

Answer 1

• compounds containing the carbonyl group C=O, one of the most important functional groups in organic chemistry
• aldehydes contain a carbonyl group bonded to a hydrogen atom, represented by RCH=O or RCHO
• ketones contain a carbonyl group bonded to two carbon atoms, represented by RCOR’

Question 2

Naming

Answer 2

• follows the pattern of including the functional group as part of the parent chain
• for saturated aldehydes the last -e of the parent alkane is replaced by -al. As the carbonyl group can only be at an end of the parent chain, + therefore its carbon is number 1, it’s location is not included in the name
• for unsaturated aldehydes, the carbon-carbon double bond location is included by the number of it’s first carbon. Also, the -ane from the parent alkane is replaced by -enal: -en to show the double bond and -al to show aldehyde
• for ketones, the parent chain is numbered from the end closest to the carbonyl group, and the last -e from the parent alkane is replaced by -one
• for aldehydes + ketones that also include -OH or -NH₂ groups, the parent chain is numbered to give precedence to the carbonyl group. The -OH substituent is indicated by hydroxy- + the -NH₂ substituent by amino-. They are numbered + alphabetized along with any other substituents
• common names for aldehydes are derived from the corresponding carboxylic acid. The word ‘acid’ is removed + the suffix -ic or -oic is replaced by -aldehyde
• common names for ketones are derived by naming each alkyl or aryl group bonded to the carbonyl group as a separate word, followed by ‘ketone’. The alkyl or aryl groups are generally listed in order of increasing moleular weight

Question 3

Physical Properties

Answer 3

• the carbonyl group is polar covalent due to the difference in electronegativity between carbon + oxygen, producing a partial negative charge on the oxygen + a partial positive charge on the carbon
• as no hydrogen bonding can occur between aldehyde or ketone molecules, they have lower boiling points than alcohols of similar molecular weight
• the carbonyl oxygen will form hydrogen bonds with water + therefore low molecular weight aldehydes + ketones are more soluble in water than non-polar compound. As the hydrocarbon portion of the molecule increases in size, however, they become less soluble
• aldehydes + ketones have strong odours. Ketones are generally pleasant with many used in perfumes + as flavouring agents. Aldehydes have varying odours - some very unpleasant + others used in perfumes

Question 4

Reactions

Answer 4

• note that the carbon of the carbonyl group has three electron density regions + is therefore a trigonal planar structure with bond angles of approxmately 120^o
• aldehydes + ketones can undergo oxidation +/or reduction

Question 5

Oxidation

Answer 5

• aldehydes are readily oxidised to carboxylic acids by a variety of oxidising agents including potassium dichromate + especially the oxygen in air
• ketones resist oxidation by most oxidising agents, including potassium dichromate + molecular oxygen
• the difference in oxidation capacity allows for a simple test to determine whether a substance is an aldehyde or ketone - by treating it with a mild oxidising agent
• Tollens’ reagent, Ag(NH3)2+ was used particularly for this purpose. Because of the high cost of silver, this reaction is now only used for making mirrors. If the reaction is carried out properly, the silver metal precipitates as a smooth, mirror like deposit on the inner surface of the reaction vessel

Question 6

Reduction

Answer 6

• aldehydes are reduced to primary alcohols + ketones are reduced to secondary alcohols, most commonly by the reagent sodium borohydride, NaBH4, which donates a hydride ion, H:-, to the carbon of the carbonyl group. This leaves a negative charge on the oxygen, which attracts a hydrogen from the aqueous acid
• an advantage of using NaBH4 is that it does not reduce carbon-carbon double bonds because they are non-polar + therefore do not attract the hydride ion
• in biological systems, the agent for reduction of aldehyds + ketones is the reduced form of the coenzyme nicotinamide adenine dinucleotide, NADH

Question 7

Hemiacetal

Answer 7

• a molecule containing a carbon bonded to one -OH group + one -OR group - the product of adding one molecule of alcohol to the carbonyl group of an aldehyde or ketone
• the H of the alcohol adds to the cabonyl oxygen + the -OR group of the alcohol adds to the carbonyl carbon
• generally unstable + are therefore only minor components of the equilibrium mixture, except when a hydroxyl group is part of the same hemiacetal + a five or six member ring can form. In this case the -OH group adds to the C=O group of the same molecule + forms a cyclic hemiacetal. Such molecules are impotant in the chemistry of carbohydrates

Question 8

Acetal

Answer 8

• a molecule containing a carbon bonded to two -OR groups - the result of the reaction between a hemiacetal + alcohol catalyzed by acid
• all steps in hemiacetal + acetal formation are reversible
• to form more acetal, an excess of alcohol is supplied or water is removed from the equilibrium mixture
• for hydrolisisof the acetal to the original aldehyde or ketone + alcohol, an excess of water is required

Question 9

Keto-enol tautomerism

Answer 9

• a carbon atom adjacent to a carbonyl group is called an alpha-carbon + a hydrogen atom bonded to it is called an alpha-hydrogen
• a carbonyl compound with an alpha-hydrogen is in equilibrium with a constitutional isomer known as an ‘enol’: a molecule containing an -OH group bonded to a carbon of a carbon-carbon double bond
• ‘enol’ is derived from alkene, -en, + alcohol, -ol
• tautomers are constitutional isomers that differ in the location of a hydrogen atom + a double bond
• a ketone + its enol form are tautomers. This type of tautomerism is known as keto-enol tautomerism
• for any pair of keto-enol tautomers, the keto form generally predominates at equilibrium