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Chapter 16 Aldehydes and Ketones Aldehydes and Ketones

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  • Chapter 16 Aldehydes and Ketones

  • StructureThe functional group of an aldehyde is a carbonyl group bonded to a hydrogen atomThe functional group of a ketone is a carbonyl group bonded to two carbons

  • NomenclatureIUPAC names for aldehydesto name an aldehyde, change the suffix -e of the parent alkane to -althe aldehyde carbon must be carbon-1for unsaturated aldehydes, indicate the presence of a carbon-carbon double bond and an aldehyde by changing the ending of the parent alkane from -ane to -enal; show the location of the carbon-carbon double bond by the number of its first carbon

  • Nomenclaturethe IUPAC system retains common names for some aldehydes, including these three

  • NomenclatureIUPAC names for ketonesparent = longest chain that contains the carbonylindicate with parent alkane as -one carbonyl carbon gets the smaller numberthe IUPAC retains the common name acetone for 2-propanone

  • NomenclatureTo name an aldehyde or ketone that also contains an -OH or -NH2 groupgive the carbonyl carbon the lower numberindicate an -OH substituent by hydroxy-, and an -NH2 substituent by amino-hydroxy and amino substituents are numbered and alphabetized along with other substituents

  • NomenclatureCommon namesderived from the common name of the corresponding carboxylic acid; drop the word "acid" and change the suffix -ic or -oic to -aldehyde name each alkyl or aryl group bonded to the carbonyl carbon as a separate word, followed by the word "ketone;

  • Physical PropertiesA C=O bond is polar, with oxygen bearing a partial negative charge and carbon bearing a partial positive charge

  • Physical Propertiesin liquid aldehydes and ketones, the intermolecular attractions are polarno hydrogen bonding is possible between aldehyde or ketone moleculesaldehydes and ketones have lower boiling points than alcohols and carboxylic acids, compounds in which there is hydrogen bonding between molecules

  • Physical Propertiesformaldehyde, acetaldehyde, and acetone are infinitely soluble in wateraldehydes and ketones become less soluble in water as the hydrocarbon portion of the molecule increases in size,

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

    liquid aldehydes are sensitive to oxidation by O2

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

  • ReductionThe carbonyl group of an aldehyde or ketone can be reduced to an -CHOH group by hydrogen in the presence of a metal catalyst

  • ReductionThe most common laboratory reagent for the reduction of an aldehyde or ketone is sodium borohydride, NaBH4 hydrogen in the form of hydride ion, H:-in a reduction by sodium borohydride, hydride ion adds to the partially positive carbonyl carbon which leaves a negative charge on the carbonyl oxygenreaction of this intermediate with aqueous acid gives the alcohol

  • Reductiondoes not affect a carbon-carbon double bond

  • ReductionIn biological systems, the agent for the reduction of aldehydes and ketones is NADH (Section 26.3)this reducing agent also delivers a hydride ionreduction of pyruvate, the end product of glycolysis, by NADH gives lactate

  • Addition of AlcoholsAddition of a molecule of alcohol to the carbonyl group of an aldehyde or ketone forms a hemiacetal (a half-acetal)the functional group of a hemiacetal is a carbon bonded to one -OH group and one -OR groupin forming a hemiacetal, H of the alcohol adds to the carbonyl oxygen and OR adds to the carbonyl carbon

  • Addition of Alcoholshemiacetals are generally unstable and are only minor components of an equilibrium mixtureIf a five- or six-membered ring can form, the compound exists almost entirely in a cyclic hemiacetal form

  • Addition of Alcohols A hemiacetal can react further with an alcohol to form an acetal plus waterthis reaction is acid catalyzedthe functional group of an acetal is a carbon bonded to two -OR groups

  • Addition of Alcoholsall steps are reversibleLe Chatelier's principleto drive it to the right, we either use a large excess of alcohol or remove water from the equilibrium mixtureto drive it to the left, we use a large excess of water

  • Keto-Enol TautomerismA carbon atom adjacent to a carbonyl group is called an a-carbon, and a hydrogen atom bonded to it is called an a-hydrogen

  • Keto-Enol TautomerismA carbonyl compound that has a hydrogen on an a-carbon is in equilibrium with a constitutional isomer called an enol

    in a keto-enol equilibrium, the keto form generally predominates

  • Keto-Enol Tautomerismexample: draw structural formulas for the two enol forms for each ketone

  • Keto-Enol Tautomerismexample: draw structural formulas for the two enol forms for each ketonesolution:

  • Aldehydes and KetonesEnd Chapter 17

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