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

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

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)

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

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 Ag 2+ 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 (CuSO 4 ) can be used to test for this type of aldehyde; the blue Cu 2+ forms Cu 2 O, a red solid

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 H 2, LiAlH 4 (lithium aluminum hydride) and NaBH 4 (sodium borohydride) However, NaBH 4 is usually the reagent of choice - hydrogenation will also reduce alkenes and alkynes if present - LiAlH 4 is more reactive than NaBH 4, but reacts violently with water and explodes when heated above 120º C

Addition of Water to Aldehydes and Ketones H 2 O can add across the carbonyl of an aldehyde or a ketone, similar to the addition of H 2 O 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

Mechanism of Acid-Catalyzed Hydration of Formaldehyde First, the carbonyl O is protonated by the acid catalyst Next, H 2 O attacks the carbonyl carbon to form a protonated hydrate Finally, H 2 O removes the proton to form the hydrate

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 H 2 O favors acetal - addition of H 2 O favors aldehyde or ketone Acetals are often used as protecting groups in organic synthesis

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)

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)

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)

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