Structures of Aldehydes and Ketones

1. Structures of Aldehydes and Ketones • Both aldehydes and ketones contain a carbonyl group • Aldehydes have at least one H attached, while ketones have two C’s attached to the carbonyl • A carbonyl consists of a C double-bonded to an O • Like in an alkene, the double bond consists of one sigma and one pi bond • The carbonyl is a very polar group - O is more electronegative than C, so C-O bonds are polar - Also, the carbonyl has two resonance forms - This polarity makes carbonyls chemically reactive

2. Naming Ketones • Parent name ends in -one • Find longest chain containing the carbonyl group • Number C’s starting at end nearest carbonyl group • Locate and number substituents and give full name - use a number to indicate position of carbonyl group - cyclic ketones have cyclo- before the parent name; numbering begins at the carbonyl group, going in direction that gives substituents lowest possible numbers - use a prefix (di-, tri-) to indicate multiple carbonyl groups in a compound

3. Naming Aldehydes • Parent name ends in -al • Find longest chain containing the carbonyl group • Number C’s starting at end nearest carbonyl group • Locate and number substituents and give full name - aldehydes take precedence over ketones and alcohols in naming - ketones are called oxo as a secondary group - alcohols are called hydroxy as a secondary group - the smallest aldehydes are usually named with common names - we will not name cyclic aldehydes (except benzaldehyde)

4. Physical Properties of Aldehydes and Ketones • Because the carbonyl group is polar, aldehydes and ketones have higher boiling points than hydrocarbons • However, they have no H attached to the O, so do not have hydrogen bonding, and have lower boiling points than alcohols • Like ethers, aldehydes and ketones can hydrogen bond with water, so those with less than 5 carbons are generally soluble in water • Aldehydes and ketones can be flammable and/or toxic, though generally not highly so • They usually have strong odors, and are often used as flavorings or scents

9. Oxidation of Aldehydes • Recall that aldehydes and ketones are formed by the oxidation of primary and secondary alcohols, respectively • Also recall that aldehydes are readily oxidized to carboxylic acids, but ketones are not • Tollens’ reagent (silver nitrate plus ammonia) can be used to distinguish between ketones and aldehydes - with aldehydes the Ag2+ is reduced to elemental silver, which forms a mirror-like coat on the reaction container • Sugars (like glucose) often contain a hydroxy group adjacent to an aldehyde - Benedict’s reagent (CuSO4) can be used to test for this type of aldehyde; the blue Cu2+ forms Cu2O, a red solid

12. Reduction of Aldehydes and Ketones • Reduction can be defined as a loss in bonds to O or a gain in bonds to H • Aldehydes and ketones can be reduced to form alcohols - Aldehydes form primary alcohols - ketones form secondary alcohols • Many different reducing agents can be used, including H2, LiAlH4 (lithium aluminum hydride) and NaBH4 (sodium borohydride) • However, NaBH4 is usually the reagent of choice - hydrogenation will also reduce alkenes and alkynes if present - LiAlH4 is more reactive than NaBH4, but reacts violently with water and explodes when heated above 120º C

13. Addition of Water to Aldehydes and Ketones • H2O can add across the carbonyl of an aldehyde or a ketone, similar to the addition of H2O to an alkene • A partial positive H from water bonds to the partial negative carbonyl O, and the partial negative O from water bonds to the partial positive carbonyl C • The product of this reversible reaction is a hydrate (a 1,1-diol) • In general, the equilibrium favors the carbonyl compound, but for some small aldehydes the hydrate is favored • The reaction can be catalyzed by either acid or base

14. Mechanism of Acid-Catalyzed Hydration of Formaldehyde • First, the carbonyl O is protonated by the acid catalyst • Next, H2O attacks the carbonyl carbon to form a protonated hydrate • Finally, H2O removes the proton to form the hydrate

15. Addition of Alcohols to Aldehydes and Ketones • Alcohols can add to aldehydes and ketones using an acid catalyst • Addition of 2 alcohols produces an acetal (a diether) • The reaction intermediate, after addition of one alcohol, is a hemiacetal (not usually isolated) • This is a reversible reaction - removal of H2O favors acetal - addition of H2O favors aldehyde or ketone • Acetals are often used as protecting groups in organic synthesis

16. Formation of Cyclic Hemiacetals • When an aldehyde or a ketone is in the same molecule as an alcohol, a cyclic hemiacetal can form • These are more stable than the non-cyclic ones and can be isolated • Sugars, like glucose and fructose, exist primarily in the cyclic hemiacetal form • When an alcohol adds to a cyclic hemiacetal, a cyclic acetal is formed (this is how sugars bond together in polysaccharides)

18. Stereoisomers • Recall that constitutional isomers have the same molecular formula, but the atoms are bonded in a different order • Stereoisomers have the same molecular formula, and the same bonding order, but the atoms are arranged differently in 3-D space • There are two types of stereoisomers: - enantiomers are non-superimposable mirror images - diasteriomers are stereoisomers that are not mirror images (cis-trans isomers a type of diastereomers)

19. Chirality • An object, or a molecule, is chiral if it has a mirror image that is not superimposable • The most familiar chiral objects are your hands - the right hand is the mirror image of the left hand - no matter how you turn them, they can’t be superimposed • Many organic compounds are also chiral - most biomolecules (amino acids, sugars, etc.) are chiral and usually only one of the stereoisomers is used • In order for a carbon in an organic compound to be chiral, it must have 4 different groups attached (otherwise the mirror image will be superimposable)

24. Fischer Projections • Fischer projections are a simple way to represent chiral molecules (especially sugars) • The bonds to a chiral carbon are shown as crossed perpendicular lines, with the chiral C at the center - Horizontal bonds are coming towards you (like wedges) - Vertical bonds are going away from you (like dashes) • The D and L classification of sugars is based on the simplest sugar, glyceraldehyde • Compounds with more than one chiral carbon, such as larger sugars, can also be represented as Fischer projections - Each place where a horizontal line crosses the vertical line represents a carbon