Chapter 14: Aldehydes and Ketones - Structures, Properties, Naming, Reactions, Study notes of Chemistry

Download Chapter 14: Aldehydes and Ketones - Structures, Properties, Naming, Reactions and more Study notes Chemistry in PDF only on Docsity! Chapter 14 – Aldehydes and Ketones 14.1 Structures and Physical Properties of Aldehydes and Ketones Ketones and aldehydes are related in that they each possess a C=O (carbonyl) group. They differ in that the carbonyl carbon in ketones is bound to two carbon atoms (RCOR’), while that in aldehydes is bound to at least one hydrogen (H2CO and RCHO). Thus aldehydes always place the carbonyl group on a terminal (end) carbon, while the carbonyl group in ketones is always internal. Some common examples include (common name in parentheses): Simple aldehydes (e.g. formaldehyde) typically have an unpleasant, irritating odor. Aldehydes adjacent to a string of double bonds (e.g. 3-phenyl-2-propenal) frequently have pleasant odors. Other examples include the primary flavoring agents in oil of bitter almond (Ph- CHO) and vanilla (C6H3(OH)(OCH3)(CHO)). As your book says, simple ketones have distinctive odors (similar to acetone) that are typically not unpleasant in low doses. Like aldehydes, placing a collection of double bonds adjacent to a ketone carbonyl generally makes the substance more fragrant. The primary flavoring agent in oil of caraway is just a such a ketone. O propanone (acetone) nail polish remover H H O methanal (formaldehyde) preservative O H trans-3-phenyl-2-propenal (cinnamaldehyde) oil of cinnamon O 3-methylcyclopentadecanone (muscone) a component of one type of musk oil 2 Because the C=O group is polar, small aldehydes and ketones enjoy significant water solubility. They are also quite soluble in typical organic solvents. 14.2 Naming Aldehydes and Ketones Aldehydes The IUPAC names for aldehydes are obtained by using rules similar to those we’ve seen for other functional groups (e.g. –OH): 1) Locate the longest carbon chain in the molecule that includes the aldehyde group. Name it like an alkane, except use the ending –al in place of –e. 2) Number the carbonyl carbon “1” and name all other functional groups as you’ve seen previously. (Since aldehydes are always terminal, there is no need to number them.) Common names occur frequently for aldehydes. These fall into two broad classes. The first type of name is derived from the name used for a common carboxylic acid. The name of the carboxylic acid typically comes from a Latin origin. For example, formaldehyde (CH2O) is derived from formic acid (HCO2H). You may know of formic acid as the major component of an ant bite. The bite stings because the ant has injected formic acid into some of your cells and the acid causes those cells to die or be damaged. For a creature the same size as an ant, the effect is devastating. The beginning form- in formaldehyde comes from the Latin word for ant formica. O propanal O Cl 2-chlorobutanal HO O 3-hydroxypropanal O oil of carraway 5 more dangerous than the metals. As metals, they don’t form complex ions in the body. Thus they must first be oxidized and that generally doesn’t happen in your body. (Nonetheless, the metals are still bad for your health for other reasons and shouldn’t be ingested.) Large scale conversions of aldehydes to carboxylic acids frequently employ either potassium permanganate (KMnO4) or chromic acid (H2CrO4) as the oxidant. The metal by- products of this reaction are more readily recycled than either the Tollens’ or Benedict’s reagents. For example: 14.4 Reduction of Aldehydes and Ketones In reduction reactions of aldehydes and ketones we add hydrogen across the double bond. That is, a hydrogen atom will be added to each atom of the double bond, converting the aldehyde or ketone into an alcohol. We can add this hydrogen in one of two different ways. The first is to split apart a hydrogen molecule and add the two product hydrogen atoms or to use a hydride donor, followed by adding a proton (H+). For industrial scale reductions of small aldehydes and ketones the former reactions are frequently employed. Hydrogen is mixed with either an aldehyde or ketone in the presence of a metal catalyst, usually nickel, platinum, or palladium. Aldehydes reduce to 1º alcohols and ketones to 2º alcohols (and under extreme conditions (not shown) the hydroxy group can be removed altogether). Chemical reductions employing hydride reagents such as NaBH4 and LiAlH4 are also CH3(CH2)5CH O KMnO4 CH3(CH2)5COH O CH3CH2CH2CH H2 Ni CH3CH2CH2CH2OH O 6 common. Each acts as a source of the H- ion (although this ion never actually exists freely in solution). The reaction proceeds in two steps. In the first the electrons on the negatively charged hydride ion attack the positive end of the C=O dipole. The source of H+ ions may be either a dilute acid or even water. Reagents like NaBH4 and LiAlH4 could never survive most biological conditions and your body uses enzymes to accomplish the same reactions. (For example, LiAlH4 frequently ignites on exposure to water.) In the human body, the NADH unit serves as the hydride ion source (NAD+ = nicotinamide adenine dinucleotide) and water as the H+ source. But otherwise the mechanism of reaction is largely the same. 14.5 Reactions of Aldehydes and Ketones with Alcohols We will discuss a few other reactions of aldehydes and ketones now. The first is that with alcohols. This reaction is unusual in that the products of these reactions are normally unstable. We are interested in them because in one important biological case, the synthesis of carbohydrates, the products possess high stability. In this reaction an alcohol molecule adds across the carbonyl double bond with the alcoholic hydrogen atom attaching to the carbonyl oxygen. If you look back in your notes, you will see that this reaction resembles the addition of water to a C=C double bond to form an alcohol (Chapter 12 notes, p. 7). This molecule, one in which the same carbon is bound to both an OH R R :O: + :H- R H R O R H R OH H+δ+ δ- ....: : : - R H :O: + R O H O H R' R'O H .. .. δ+ δ- : : 7 and an OR group is called a hemiacetal. A nearly identical reaction takes place with ketones to yield a hemiketal. The difference is that the hemiacetal carbon is also bound to an H atom, while the hemiketal carbon is bound to an R group. R O H O H R' : : R O R O H R' : : hemiacetal hemiketal In sugars, the molecule has an aldehyde group at one end and an alcohol group on the other. The chain that connects them is 5 or 6 carbons long. If the molecule does an intramolecular (internal) reaction of this type, the resulting product is a 5- or 6-membered ring. Rings of this size are particularly stable and, in the case of sugars, can polymerize into carbohydrates in a way the straight chain molecules can’t. Even so, individual sugar molecules exist in an equilibrium between the ring-open and ring-closed forms. Hemiacetals and hemiketals can react with another equivalent of alcohol to yield acetals and ketals, respectively. The net effect is to replace the alcoholic hydrogen on the former with the R group of the alcohol. An acid catalyst and large excess of added alcohol are needed for this C C C C C H2C OH HO H H HO H OH HO H OH OH HO OH H H OHH OH CH2OH H OH HO H OH H OHH OH CH2OH H α-D-glucose + β-D-glucose