Nomenclature of Aldehydes & Ketones

Aldehydes & Ketones Nomenclature of Aldehydes & Ketones: The highest priority groups gets the #1 carbon on it (for the case of aldehydes and ketones) Including the suffix -al means it is an aldehyde group You also want to give substituents the lowest number possible (determining if you go clockwise or counterclockwise) You also order the functional groups by alphabetical order, not numerical order The suffix for ketones is -one (takes precedence over double bonds) Physical Properties of Aldehydes & Ketones: A way to make ketones and aldehydes is to oxidize alcohols Acetone is the simplest ketone (two methyl groups on it) Alcohols exhibit hydrogen bonding, but ketones and aldehydes only have dipole-dipole interactions which aren’t as strong as hydrogen bonding (so alcohols have higher boiling points) Ketones and aldehydes do have higher boiling points than simple alkanes which only have london dispersion forces Reactivity of Aldehydes & Ketones: Steric number is the number of s bonds and number of lone pairs of electrons Ketones are a little more stable than aldehydes because alkyl groups donate electron density in order to stabilize the partial positive on the carbonyl carbon Since aldehydes have less stable carbonyl carbons, then they are a little more prone to nucleophilic attack (also the H atom on aldehydes is smaller than an R group on ketones, making it more prone to nucleophilic attack due to lower hindrance) Formation of Hydrates: Adding H2O across a ketone/aldehyde forms a hydrate (or a gem diol) In order to increase reactivity of an aldehyde or ketones, you can add a halogen, which will pull electron density, and therefore making the carbonyl carbon much less stable Formation of Hemiacetals & Hemiketals: In order to generate hemiacetals (HOCOR), you add an alcohol instead of water For formation of rings (intramolecular reaction) it actually favors the ring formation as opposed to intermolecular formation of hemiacetals However, since the carbon is sp2 hybridized, the nucleophile can attack from either side and can therefore form different anomers Acid & Base Catalyzed Formation of Hydrates & Hemiacetals: By forming a carbocation, this makes the compound more prone/reactive to nucleophilic attack Acetals are ROCOR OH- is a much better nucleophile than H2O (However, OR- is more nucleophilic than the OH- by itself) In base a hemiacetal will not react to form an acetal Formation of Acetals: In acid, formation of acetal can take place (remember you protonate the hemiacetal in order to produce OH2 which is a great leaving group) The product is the acetal and H2O (7 step reaction) Ethylene glycol (C2H6O2) forms an intramolecular reaction (shown on the right) Acetals as Protecting Groups & Thioacetals: You can use LiAlH4 and H+ in order to go from a carboxylic acid to an alcohol (however, it reduces all carbonyls) and therefore you need a protecting group for any ketone you don’t want to be reduced (which you can do by forming a cyclic acetal using ethylene glycol) Thioacetal is RSCSR and you can use a dithioacetal in order to protect the group as well (it also takes away the other ketone away if you don’t want it) and is shown below Formation of Imines & Enamines: Imine is a double bonded nitrogen to a carbon with an H or R groups attached to the Nitrogen atom Enamine is a NR2 group single bonded to an alkene group (is the result of secondary amines reacting with a carbonyl) since the H was needed to be stripped from some other part of the molecule Formation of Oximes & Hydrazones: If the Y group of the imine is an OH group, then it is an oxime and if the Y group is NH2 or NHR’, then it is a hydrazone (oxime is more stable than imine because of resonance stabilization) Addition of Carbon Nucleophiles to Aldehydes & Ketones: By having a R group attached to Li or Mg, it makes the carbon nucleophilic and therefore able to attack the electrophilic carbonyl carbon (known as a Grignard reagent) This is important because you can now form carbon-carbon bonds!! Formation of Alcohols Using Hydride Reducing Agents: NaBH4 allows for the reduction of a ketone/aldehyde (adds an H atom to the carbonyl) LiAlH4 is another hydride reducing agent NaBH4 is less reactive than LiAlH4 but is otherwise similar. It is only powerful enough to reduce aldehydes, ketones and acid chlorides to alcohols but esters, amides, acids and nitriles are largely untouched (because boron is more electronegative than aluminum and therefore wants to hold onto the electrons more) Oxidation of Aldehydes Using Tollens’ Reagent: Tollens’ reagent is Ag+, NH3 and OH- and oxidizes aldehydes to a carboxylic acid (the silver cation is being reduced to solid Ag which is known as a silver mirror) → only aldehyde can react with Tollens’ reagent Chromium (Cr) compounds will aldehydes and alcohols while Tollens’ reagent will only oxidize aldehydes Cyclic Hemiacetals & Hemiketals: When you go from Haworth projection to a chair conformation, the groups pointing upwards become equatorial and the groups point downward become axial respectively Mutarotation describes the process of ring opening and switching of the anomer from alpha to beta (back and forth) The beta anomer is preferred because it is in the equatorial and cis to the CH2OH group