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Topic 9

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Department
Chemistry
Course
Chemistry 2223B
Professor
Felix Lee
Semester
Fall

Description
Chemistry 2213a  Fall 2012  Western University Topic 9. Aldehydes & Ketones A. Structure and Nomenclature  The carbonyl group is present in aldehydes and ketones and is the most important group in bio-organic chemistry. O O O R H R R carbonyl group aldehyde ketone  Both the C and O are sp , and their p orbitals overlap to form a  O bond. Angles are 120°. Most importantly, the C=O is a polar doCb+ bond. It can react with both electrophiles and nucleophiles.  In IUPAC nomenclature, for a O compound named as an O Br H aldehyde, the C=O is always position #1 (takes precedence Br over other groups) and the H suffix al is used. propanal 3,3-dibromo-2-methylbutanal Aldehydes & Ketones  2  When the aldehyde group is a substituent on a ring, the suffix carbaldehyde is added to the ring name, and the point of attachment is given #1. CHO CHO cyclopropanecarbaldehyde cis-2-methylcyclopentanecarbaldehyde  In ketones, the C=O is given the lowest possible number in the carbon chain and the suffix one is added. O O 3-methyl-2-butanone 4,4-dimethyl-2-cyclohexenone  In compounds with two or more functional groups, it mayObe O necessary to name the carbonyl group as an ordinary substituent using oxo. H 3-oxobutanal + Aldehydes & Ketones  3  Carbonyl compounds don’t have a  hydrogen, so intermolecular hydrogen bonding is not possible. However, the polar carbonyl group results in higher BP than ethers, but lower than that of alcohols. O 35° 80° 118° O OH B. Spectroscopy  In IR spectroscopy, the carbonyl group has a strong, distinctive absorption between 1705 – 1780 cm . (As we’ll see later, the C=O stretch appears in all functional groups containing the C=O bond, including acids and derivatives). Aldehydes & Ketones  4 1  In H NMR, the H aldehydes is shifted far downfield to about 9.5 – 10 ppm.  Although the carbonyl H (d) splits (and is also split by) the two H atoms c, the splitting is very small. Therefore, d looks like a singlet. Similarly, the signal for c looks like it is split only by the H atoms b.  Note that ketones do not have an H on the C=O, so the peak near 9.5 – 10 ppm does not appear. Aldehydes & Ketones  5 C. Nucleophilic Addition Reactions  The reactivity of carbonyl compounds can be explained by the structure of the carbonyl group. The bond is polar not only because of a difference in electronegativity between the two atoms, but also because of resonance. O O R R' R R'  Nucleophiles will always add to the carbon, and electrophiles (typically a proton) will add to the oxygen. Therefore, only one regioisomer is formed. OH R C R' Nu  The mechanism of the reaction depends on whether the solution is acidic or basic. Regardless of the mechanism, these are termed nucleophilic addition reactions because the RDS is the breaking of the  bond by the nucleophile. Aldehydes & Ketones  6  In basic solution, where anions exist, nucleophilic attack occurs first. The electrophile is weakly electrophilic, while the nucleophile is strongly nucleophilic. A neutrally charged species, such as water, is the proton donor. H OH O slow O fast OH R C R' R C R' R R' Nu Nu Nu  In acidic solution, where cations exist, protonation occurs to generate a strong electrophile prior to the addition of a weak nucleophile. Notice that a protonated C=O is just like a resonance-stabilized carbocation. H OH H O 2 2 O OH OH OH fast slow fast R' R R' R R R' R R' Nu H H Nu Nu H OH H2O 2 Aldehydes & Ketones  7  Some important considerations: 2 3 o As in alkene reactions, an sp carbon changes to sp , and a stereocentre may be formed, in which case both stereoisomers are formed in equal amounts. o Carbonyl reactions are not as exothermic as alkene reactions, so the reaction is easily reversed (this is a very useful biochemical property). o Aldehydes (less hindered) react faster than ketones. 1. Addition of carbon nucleophiles: Grignard Reaction  In the late 19 century, Victor Grignard discovered that alkyl halides (Cl, Br, or I) reacted with Mg to form a highly polar, almost-ionic C–Mg bond. + - - +   ether   R X Mg R MgX functionally equivalent to R MgX  These organomagnesium compounds, called Grignard reagents, behave as negatively charged carbons. Aldehydes & Ketones  8  Grignard reagents are therefore nucleophilic. O - + O MgX   ether R MgX R'' R' R'' R' R  The ionic product is easily converted to an alcohol using some sort of proton donor, such as acid in water. O MgX H / H O OH 2 R'' R' R'' R' HOMgX R R  However, because carbanions are basic, they are destroyed by the presence of compounds that can act as acids. Acid is therefore only added in the second part of the sequence. OH O ether H / H2O MgBr Aldehydes & Ketones  9  The Grignard reaction is one of the most useful ways to create new carbon- carbon bonds. The starting materials (carbonyl compounds and alkyl halides) are readily available, and the product is formed in good yield.  There is also exceptional versatility: Two R groups come from the carbonyl compound, and one from the alkyl halide. It doesn’t matter which R groups are from the carbonyl compound or from the alkyl halide. OH  For example, two ways to make: CH 3 O ether H / H2O CH3MgBr product O ether H / H2O CH 3 MgBr product Aldehydes & Ketones  10  Grignard reagents even react with carbon dioxide, to yield carboxylic acids, and with epoxides, to give alcohols.  Reaction with epoxides occurs because of the polar C–O bond in addition to the reactivity of the strained 3-atom ring. O ether H / H2O R–MgX O C O RCOH - O  OH ether H / H2O R–MgX + C C C  R Aldehydes & Ketones  11 2. Addition of hydrogen nucleophiles: reduction  The addition of a hydride reduces an aldehyde or a ketone to an alcohol. H OH OH O slow O fast OH R C R' R C R' R R' H H H  In the laboratory, there are two common hydride sources: o Weaker: NaBH4in alcohol solvent, followe2 by H O o Stronger: LiAlH in ether solvent, followed by H O 4 2  Aldehydes and ketones can also be reduced by catalytic hydrogenation, similar to the hydrogenation of alkenes. However, metal hydrides usually do NOT reduce alkenes because C=C bonds are typically non-polar. Aldehydes & Ketones  12 O OH H 2 Ni + 2 H2 O 1. LiAlH OH 4 2. H O 2 3. Addition of oxygen (alcohol) nucleophiles  The two previous reactions involved the addition of a good nucleophile (carbanion or hydride) to a neutrally charged aldehyde or ketone (a weak electrophile).  Alcohols, which are weak nucleophiles, do not react very fast with aldehydes or ketones unless either an acid or a base catalyst is present. O H or OH OH ROH R 1 C R 2 R 1 R 2 OR Aldehydes & Ketones  13  The product of the reaction contains both OR and OH bonded to the same C, and is ca
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