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Ketones | | | Structure | Ketones have as their functional group the carbonyl group, which has a carbon atom double bonded to an oxygen atom. Also, in order to be a ketone, the carbon atom has to be bonded to two other carbon atoms. Another way of putting this is that the carbonyl group cannot be at the end of a carbon chain - it must be somewhere in the middle. | | Properties - The molecules of ketones are polar.
- The large ones are slightly polar because the polar carbonyl group becomes progressively less dominant as the number of carbon atoms increases.
- They have dipole-dipole intermolecular bonds.
- They do not have hydrogen bonding because they do not have a hydrogen atom bonded to the oxygen atom.
- The small ones, up to about five carbon atoms, are soluble in water . As the carbon chains get longer, the molecules become less soluble in water.
| Reactions that Yield Aldehydes & Ketones | - Ketones can be formed by the oxidation of secondary alcohols.
- A specific example of this reaction is shown below .
| H H H H
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H-C--C--C--C-H
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H OH H H 2-butanol | K2Cr2O7
¾¾¾¾¾¾®
H2SO4, heat | H H H
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H-C--C--C--C-H
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H O H H 2-butanone | + 2[H] | | | | | | Ketones have as their functional group a double bond between a carbon and an oxygen. The reactions that are characteristic of ketones are the reactions that involve this carbon-to-oxygen double bond. Reactions Addition The most common reaction or class of reactions for ketones is addition, although these reactions are generally classified differently. The addition reactions that ketones undergo are not as extensive as addition reactions that alkenes undergo. But these are the characteristic type of reactions for ketones. | Consider the reaction in which H2 is added across the C=O double bond. Note that, in a way, the addition of 2 H's across the carbon-oxygen double bond to give an alcohol as a product is just the opposite of the reaction in which a ketone is formed from an alcohol by taking two hydrogens away. | | | O
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R-C-R | + H2 | ® | O-H
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R-C-R
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H | | ReductionIt is not at all necessary for both of the hydrogen atoms to come from an H2 molecule. One of the most common ones is the addition of 2 H's across the carbon-oxygen double bond to give an alcohol as a product. This is just the opposite of the reaction in which a ketone is formed from an alcohol by taking two hydrogens away, as shown below.
H H H H
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H-C--C--C--C-H
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H OH H H 2-butanol | ¾¾¾® | + 2 H· | H H H
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H-C--C--C--C-H
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H O H H 2-butanone | | - The reaction in which the alcohol is changed into a ketone involves removing two hydrogen atoms. This is an oxidation reaction.
- Ketones can be reduced to form alcohols by putting the two hydrogen atoms back on. The hydrogen atoms bring electrons with them. Thus, the compound is gaining electrons, therefore it is a reduction reaction .
- When both hydrogen atoms come from H2, you ONE also call it an addition reaction.
- An equation for such a reaction is shown here
H H H
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H-C--C--C--C-H
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H O H H 2-butanone | + 2 H· | ¾¾¾® | H H H H
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H-C--C--C--C-H
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H OH H H 2-butanol | | . We can emphasize that relationship by using reversible arrows, as shown below. Note that reversing the direction of the reaction requires changing the conditions of the reaction. To reduce a ketone, you need a reducing agent. To oxidize an alcohol, you would need an oxidizing agent. H H H
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H-C--C--C--C-H
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H O H H 2-butanone | + 2 H· | reduction
¾¾¾¾¾¾®
¬¾¾¾¾¾¾
oxidation | H H H H
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H-C--C--C--C-H
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H OH H H 2-butanol | | Aldehydes Aldehydes are compounds that are very similar to ketones. The essential difference is that the carbonyl group is at the end of a carbon chain. | | Structure | Since the carbonyl group is at the end of the carbon chain, the carbon atom in that group is not bonded to two other carbon atoms, as in the ketones. Instead, it is bonded to one carbon atom and one hydrogen atom. (There is one important exception.) This is often written as -CHO to save time and space. Note that the H is before the O. That is to distinguish aldehydes from alcohols (COH). Properties - Aldehydes, like ketones, are polar molecules.
- The bond angles and hybridization are the same.
- They also have dipole-dipole intermolecular bonding. Because of this, they are held together with greater attraction than that with alkanes of similar size and molecular weight.
- Thus they would have slightly higher melting points and boiling points than comparable alkanes, which have van der Waals bonding.
- Aldehydes do not have hydrogen bonding as alcohols do. Therefore, aldehydes have lower melting points and boiling points than comparable alcohols.
- Aldehydes are also soluble in water .
- The solubility in water generally decreases as the length or size of the nonpolar portion (the alkyl part) gets larger. At about five carbon atoms aldehydes are only slightly soluble, and less so with more carbon atoms.
Formation - Aldehydes can be formed by a few different kinds of reactions. The one shown here is the oxidation of a primary alcohol.
H H H H H
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H-C-C-C-C-C-OH
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H H H H H 1-pentanol | | | | oxidation
¾¾¾¾¾¾® | | H H H H O
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H-C-C-C-C-C + 2[H]
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H H H H H pentanal | - Aldehydes can also be formed by the reduction of carboxylic acids.
Reactions of Aldehydes & Ketones - The Grignard Reaction: The reaction of an alkyl, aryl or vinyl halide with magnesium metal in ether solvent, produces an organometallic complex of uncertain structure, but which behaves as if it has the structure R-Mg-X and is commonly referred to as a Grignard Reagent.
  The "R" group in this complex (alkyl, aryl or vinyl), acts as if it was a stabilized carbanion and Grignard reagents react with water and other compounds containing acidic hydrogens to give hydrocarbons (just as would be expected for a well-behaved, highly basic carbanion). In the absence of acidic hydrogens, the Grignard reagent can function as a powerful nucleophile, and is most often used in addition reactions involving carbonyl compounds, as shown above. The product of these addition reactions is typically a secondary or tertiary alcohol (primary alcohols can be formed by reaction with formaldehyde), as shown in the examples below; in these the carbonyl and halide portions of the molecules have been colored blue and red, respectively, to assist in understanding the component parts of the final products.  - Hydration of Aldehydes & Ketones: The hydration of carbonyl compounds is an equilibrium process and the extent of that equilibrium generally parallels the reactivity of the parent aldehyde or ketone towards nucleophilic substitution; aldehydes are more reactive than ketones and are more highly hydrated at equilibrium.
  - Formation of Cyanohydrins: The reaction of carbonyl compounds with HCN is an equilibrium process and, again, the extent of that equilibrium generally parallels the reactivity of the parent aldehyde or ketone towards nucleophilic substitution.
 - Reaction with Amines: The reaction of carbonyl compounds with amines involves the formation of an intermediate carbinolamine which undergoes dehydration to form an immonium cation which can loose a proton to form the neutral imine.
  - Some examples of common imine-forming reactions are given below:
 - Imines formed from secondary amines can loose a proton from the a-carbon to form an enamine. Because of resonance, enamines maintain a partial carbanion character on the a-carbon and can be utilized as nucleophiles, as will be discussed in the section on "alpha alkylations".
  - Ketal and Acetal Formation: Ketones and aldehydes react with excess alcohol in the presence of acid to give ketals and acetals, respectively. The mechanism of acetal formation involves equilibrium protonation, attack by alcohol, and then loss of a proton to give the neutral hemiacetal (or hemiketal). The hemiacetal undergoes protonation and loss of water to give an oxocarbonium ion, which undergoes attack by another mole of alcohol and loss of a proton to give the final product; note that acetal (or ketal) formation is an equilibrium process.
 - Some examples of acetal and ketal formation are given below:
 - The Wittig Reaction: Ketones and aldehydes react with phosphorus ylides to form alkenes. Phosphorus ylides are prepared by an SN2 reaction between an alkyl halide and triphenylphosphine, followed by deprotonation by a strong base such as n-butyllithium. The mechanism of the Wittig reaction involves nucleophilic addition to give an intermediate betaine, which decomposes to give the alkene and triphenylphosphine oxide. The Wittig reaction works well to prepare mono- di- and tri-substituted alkenes; tetra-substituted alkenes cannot be prepared by this method.
  Oxidation & Reduction of Aldehydes and Ketones - Preparation of Alcohols by Reduction of Aldehydes and Ketones: Reduction of simple aldehydes and ketones with BH4- yields the corresponding alcohol directly. The reaction works well for simple compounds, but reaction of BH 4- with a-b-unsaturated aldehydes and ketones can result in significant reduction of the double bond.
 - A much more powerful reductant is LiAlH4, which will reduce aldehydes, ketones, esters, carboxylic acids and nitriles. Some sample reactions are shown below:
 As seen in the first example, the reduction of carboxylate esters results in the addition of two moles of hydride to the carbonyl carbon, with loss of the alcohol portion of the ester, forming the corresponding primary alcohol.  Although the reduction of esters with LiAlH4 proceeds to produce the alcohol, reduction of carboxylate esters by diisobutylaluminum hydride (DIBAH) stops at the aldehyde.  - Wolff-Kishner Reduction: The imine formed from an aldehyde or ketone on reaction with hydrazine (NH2NH2) is unstable in base, and undergoes loss of N 2 to give the corresponding hydrocarbon.
 - Clemmensen Reduction: Carbonyl compounds can also be reduced by the Clemmensen reduction using zinc-mercury amalgam in the presence of acid; the mechanism most likely involves free radicals.
 - The Formation of Thioketal and Thioacetals: Ketones and aldehydes react with excess thiol in the presence of acid to give thioketals and thioacetals, respectively. These compounds are smoothly reduced by Raney-Nickel to give the corresponding hydrocarbons.
 - Oxidation of Aldehydes by Silver Oxide: Reaction of simple aldehydes with aqueous Ag2O in the presence of NH3 yields the corresponding carboxylic acid and metallic silver. The silver is generally deposited in a thin metallic layer which forms a reflective "mirror" on the inside surface of the reaction vessel. The formation of this mirror forms the basis of a qualitative test for aldehydes, called the Tollens Test.
 - Oxidation of Aldehydes to form Carboxylic Acids: Reaction of simple aldehydes with acidic MnO4-, or CrO3/H2 SO4 yields the corresponding carboxylic acid. Aldehydes oxidize very easily and it is often difficult to prevent oxidation, even by atmospheric oxygen.
 - Oxidation of Ketones: Ketones are more resistant to oxidation, but can be cleaved with acidic MnO4- to yield carboxylic acids.
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"It does not do to dwell on dreams and forget to live, remember that." ~Albus Dumbledore, Sorcerer's Stone.
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Aldehydes and Ketones - reactions
![Models of acetone and water. [63mod08.JPG]](http://dl.clackamas.cc.or.us/ch106-03/images/63mod08.JPG)
