Aldehydes and Ketones

Naming aldehydes Establish parent name based on longest chain with aldehyde included. Drop the suffix –e from the parent chain and add –al to indicate an aldehyde. Aldehydes are ALWAYS on the end of a molecule because of the terminal hydrogen, so you DO NOT need to tell the position of the aldehyde. It is assumed to be 1. Other branches are named and numbered just like before.

Practice 1 2 3

Priority of groups for IUPAC nomenclature We have pretty much learned how to name molecules in their order of priority…this means that each new type of nomenclature is the highest priority. You will need to refer to the sheet I am handing out to name molecules that are lower priority. For example, if you have a molecule with an alcohol and aldehyde, the aldehyde takes priority, so the branch term for the alcohol is hydroxy-

Common aldehydes 4.jpg 5.jpg

Naming Ketones Identify the longest continuous chain with the ketone present. Use the parent name but drop the suffix –e and add –one. If the chain is longer than 4 carbons, it is numbered so that the ketone has the smallest number possible. Other branches/groups are named as we have done before. Ketones are not branched… they have too high of a priority.

Bonding in Aldehydes and Ketones The carbon atom of the carbonyl (C=O), is sp 2 hybridized. This means there is a sigma (s overlap) and a pi (p overlap) bond present. The carbonyl group itself is polar due to the high electronegativity of oxygen with respect to carbon. This create a dipole moment, and it is this characteristic that dictates a lot of the reactivity and functionality.

Bonding and Physical Properties They CANNOT hydrogen bond to themselves! This is due to the fact that no hydrogen is directly attached to an oxygen atom. Therefore, they have lower boiling points than alcohols of similar molar masses. Solubility of aldehydes and ketones decreases markedly after lengthening the carbon chain to 5 or more. Low molar mass aldehydes have a rancid, permeable odor and are responsible for the bad taste and smell of rotten foods.

Fragrances It is high molar mass and aromatic aldehydes and ketones that are especially fragrant. These molecules are used widely in the flavoring and perfume industries.

Stereoisomers Stereoisomers are isomers that have the same element in relatively the same order, but they are still different in their orientation in space. Optical isomers are one form of stereoisomers because of the differences in the way they react with plane polarized light. Simple substances composed of two stereoisomers are called enantiomers. The enantiomer that rotates plane polarized light in a clockwise manner is a (+) enantiomer. The enantiomer that rotates plane polarized light in a counter clockwise manner is a (-).

Enantiomers Substances that consist of a mixture in equal proportions of these enantiomers are called racemic mixtures. Enantiomers have the same physical properties, but they are mirror images of each other. Biologically, the R and S may not both have the same activity. It is difficult to separate enantiomers, but luckily, the non- active ones are not harmful. They are merely inactive.

Chemical Reactions of Aldehydes and Ketones Oxidation: Aldehydes are easily oxidized to carboxylic acids. Reactants like potassium dichromate in sulfuric acid (K 2 Cr 2 O 7 + H 2 SO 4 ), and even milder oxidizing agents like Cu 2+ or Ag +. Most ketones do not undergo oxidation, so this is a way to differentiate them from one another. The Tollens test (silver mirror test) for aldehydes is based on the ability of silver ions to oxidize them. A silver mirror is deposited on the inside of the test tube when an aldehyde is added to silver nitrate and ammonia.

Other Aldehyde tests Fehling and Benedict tests- These solutions contain Cu 2+ ions in a basic solution. They oxidize the aldehyde to an acid and the blue ions (Cu 2+ ) are reduced to Cu 2 O, a brick red precipitate. The Benedict test is commonly used to detect carbohydrates (which often have an aldehyde group). Many aldehydes and ketones are used in industry because they are easily and cheaply made from petroleum, natural gas and oxygen in the air. Simple hydrocarbons are oxidized to form acetaldehyde, which can be further oxidized to acetic acid.

Reduction Reactions Both aldehydes and ketones are easily reduced to alcohols. This reaction is accomplished by elemental hydrogen in the presence of a catalyst or by reducing agents like, LiAlH 4 or NaBH 4 ). Aldehydes make primary alcohols and ketones make secondary alcohols. Reduction reactions

Biochemical processes

Addition Reactions Aldehydes and ketones can be added to alcohols to make hemiacetals and hemiketals. Hemiacetal- 1 alkoxy (ether) and 1 hydroxy (alcohol) group off the same carbon Aldehydes react with alcohols in a small amount of acid to make hemiacetals.

Acetals and Ketals To make acetals or ketals, 2 alkoxy (ether) groups are on the same carbon. If the carbon has another alkyl group and a hydrogen, it is an acetal. If it has 2 other alkyl groups, it is a ketal. These substances are unstable in acidic solutions, but stable in basic solutions.

Can you circle the hemiacetal in each?

Addition of Hydrogen Cyanide When you add hydrogen cyanide to aldehydes and ketones, you form cyanohydrins. The –CN and –OH are on the same carbon atom. This is a typical carbonyl addition reaction.

Aldol Condensation Reactions In a carbonyl compound, the carbons next to the carbonyl carbon are labeled with the Greek alphabet, starting with alpha, then beta… The Hydrogen atoms attached to the alpha carbon are more easily removed as protons than other hydrogens within the molecule. Molecules that contain this alpha carbon can add themselves to other molecules that also contain the alpha carbon. The result is a molecule with a carbonyl and alcohol group in the molecule. This must be done in diluted basic medium.

Aldol Condensation

Practice 1) Write the aldol condensation reaction of propanal. 2) Show the mechanism for the aldol condensation of 3-pentanone.

Condensation Polymers A polymer is a long chain of a molecule with many repeating units. Monomers are the individual units that repeat to form the polymer. When monomers join together to form polymers, a dehydration usually occurs to join them together. Important classes of polymers are: polyesters, polyamides, phenol-formaldehyde polymers, and polyurethanes.

Phenol-Formaldehyde Polymers