Two examples of these reactions are presented in the following diagram. It should be noted that, like acetal formation, these are acid-catalyzed reversible reactions in which water is lost. Consequently, enamines are easily converted back to their carbonyl precursors by acid-catalyzed hydrolysis. A mechanism for enamine formation may be seen by pressing the "Show Mechanism" button. Cyanohydrin formation is weakly exothermic, and is favored for aldehydes, and unhindered cyclic and methyl ketones.
Two examples of such reactions are shown below. The cyanohydrin from benzaldehyde is named mandelonitrile. The reversibility of cyanohydrin formation is put to use by the millipede Apheloria corrugata in a remarkable defense mechanism. This arthropod releases mandelonitrile from an inner storage gland into an outer chamber, where it is enzymatically broken down into benzaldehyde and hydrogen cyanide before being sprayed at an enemy. The distinction between reversible and irreversible carbonyl addition reactions may be clarified by considering the stability of alcohols having the structure shown below in the shaded box.
If substituent Y is not a hydrogen, an alkyl group or an aryl group, there is a good chance the compound will be unstable not isolable , and will decompose in the manner shown.
In all these cases addition of H— Y to carbonyl groups is clearly reversible. If substituent Y is a hydrogen, an alkyl group or an aryl group, the resulting alcohol is a stable compound and does not decompose with loss of hydrogen or hydrocarbons, even on heating.
It follows then, that if nucleophilic reagents corresponding to H: — , R: — or Ar: — add to aldehydes and ketones, the alcohol products of such additions will form irreversibly. Free anions of this kind would be extremely strong bases and nucleophiles, but their extraordinary reactivity would make them difficult to prepare and use.
Fortunately, metal derivatives of these alkyl, aryl and hydride moieties are available, and permit their addition to carbonyl compounds. Addition of a hydride anion to an aldehyde or ketone would produce an alkoxide anion, which on protonation should yield the corresponding alcohol. Two practical sources of hydride-like reactivity are the complex metal hydrides lithium aluminum hydride LiAlH 4 and sodium borohydride NaBH 4.
These are both white or near white solids, which are prepared from lithium or sodium hydrides by reaction with aluminum or boron halides and esters. Lithium aluminum hydride is by far the most reactive of the two compounds, reacting violently with water, alcohols and other acidic groups with the evolution of hydrogen gas. The following table summarizes some important characteristics of these useful reagents. Some examples of aldehyde and ketone reductions, using the reagents described above, are presented in the following diagram.
The first three reactions illustrate that all four hydrogens of the complex metal hydrides may function as hydride anion equivalents which bond to the carbonyl carbon atom.
In the LiAlH 4 reduction, the resulting alkoxide salts are insoluble and need to be hydrolyzed with care before the alcohol product can be isolated. In the borohydride reduction the hydroxylic solvent system achieves this hydrolysis automatically. The lithium, sodium, boron and aluminum end up as soluble inorganic salts.
The last reaction shows how an acetal derivative may be used to prevent reduction of a carbonyl function in this case a ketone. Remember, with the exception of epoxides, ethers are generally unreactive with strong bases or nucleophiles.
The acid catalyzed hydrolysis of the aluminum salts also effects the removal of the acetal. This equation is typical in not being balanced i. If the saturated alcohol is the desired product, catalytic hydrogenation prior to or following the hydride reduction may be necessary.
Before leaving this topic it should be noted that diborane, B 2 H 6 , a gas that was used in ether solution to prepare alkyl boranes from alkenes , also reduces many carbonyl groups. Consequently, selective reactions with substrates having both functional groups may not be possible. In contrast to the metal hydride reagents, diborane is a relatively electrophilic reagent, as witnessed by its ability to reduce alkenes. This difference also influences the rate of reduction observed for the two aldehydes shown below.
The first, 2,2-dimethylpropanal, is less electrophilic than the second, which is activated by the electron withdrawing chlorine substituents.
To see examples of these reactions, Click Here. The two most commonly used compounds of this kind are alkyl lithium reagents and Grignard reagents.
They are prepared from alkyl and aryl halides, as discussed earlier. These reagents are powerful nucleophiles and very strong bases pK a 's of saturated hydrocarbons range from 42 to 50 , so they bond readily to carbonyl carbon atoms, giving alkoxide salts of lithium or magnesium. Because of their ring strain, epoxides undergo many carbonyl-like reactions, as noted previously. Reactions of this kind are among the most important synthetic methods available to chemists, because they permit simple starting compounds to be joined to form more complex structures.
Examples are shown in the following diagram. A common pattern, shown in the shaded box at the top, is observed in all these reactions. The organometallic reagent is a source of a nucleophilic alkyl or aryl group colored blue , which bonds to the electrophilic carbon of the carbonyl group colored magenta. The product of this addition is a metal alkoxide salt, and the alcohol product is generated by weak acid hydrolysis of the salt.
The first two examples show that water soluble magnesium or lithium salts are also formed in the hydrolysis, but these are seldom listed among the products, as in the last four reactions. Two additional examples of the addition of organometallic reagents to carbonyl compounds are informative. The first demonstrates that active metal derivatives of terminal alkynes function in the same fashion as alkyl lithium and Grignard reagents.
The second example again illustrates the use of acetal protective groups in reactions with powerful nucleophiles. Aldehydes and alcohols are organic compounds.
They have different functional groups, as well as different chemical and physical properties. An aldehyde has a carbonyl carbon atom a carbon atom attached to an oxygen atom through a double bond , but there are no carbonyl centres in alcohols. Both aldehydes and alcohols are very important in organic synthesis reactions, as precursors for other compounds such as ketones. Overview and Key Difference 2. What is Aldehyde 3. What is Alcohol 4. Aldehydes are organic compounds containing the —CHO group as the functional group.
Hence, the R group determines the reactivity of this organic molecule. Moreover, aromatic aldehydes are less reactive than aliphatic aldehydes. Acetone is the simplest and most important ketone. Because it is miscible with water as well as with most organic solvents, its chief use is as an industrial solvent for example, for paints and lacquers.
It is also the chief ingredient in some brands of nail polish remover. Acetone is formed in the human body as a by-product of lipid metabolism. Normally, acetone does not accumulate to an appreciable extent because it is oxidized to carbon dioxide and water. In certain disease states, such as uncontrolled diabetes mellitus, the acetone concentration rises to higher levels.
It is then excreted in the urine, where it is easily detected. In severe cases, its odor can be noted on the breath. Ketones are also the active components of other familiar substances, some of which are noted in the accompanying figure. Certain steroid hormones have the ketone functional group as a part of their structure. Two examples are progesterone, a hormone secreted by the ovaries that stimulates the growth of cells in the uterine wall and prepares it for attachment of a fertilized egg, and testosterone, the main male sex hormone.
These and other sex hormones affect our development and our lives in fundamental ways. The polar carbon-to-oxygen double bond causes aldehydes and ketones to have higher boiling points than those of ethers and alkanes of similar molar masses but lower than those of comparable alcohols that engage in intermolecular hydrogen bonding. How does the carbon-to-oxygen bond of aldehydes and ketones differ from the carbon-to-carbon bond of alkenes?
Learning Objectives Explain why the boiling points of aldehydes and ketones are higher than those of ethers and alkanes of similar molar masses but lower than those of comparable alcohols. The importance of molecular structure in the reactivity of organic compounds is illustrated by the reactions that produce aldehydes and ketones.
We can prepare a carbonyl group by oxidation of an alcohol—for organic molecules, oxidation of a carbon atom is said to occur when a carbon-hydrogen bond is replaced by a carbon-oxygen bond. The reverse reaction—replacing a carbon-oxygen bond by a carbon-hydrogen bond—is a reduction of that carbon atom. Recall that oxygen is generally assigned a —2 oxidation number unless it is elemental or attached to a fluorine. Since carbon does not have a specific rule, its oxidation number is determined algebraically by factoring the atoms it is attached to and the overall charge of the molecule or ion.
In general, a carbon atom attached to an oxygen atom will have a more positive oxidation number and a carbon atom attached to a hydrogen atom will have a more negative oxidation number. This should fit nicely with your understanding of the polarity of C—O and C—H bonds.
The other reagents and possible products of these reactions are beyond the scope of this chapter, so we will focus only on the changes to the carbon atoms:. Methane represents the completely reduced form of an organic molecule that contains one carbon atom.
Sequentially replacing each of the carbon-hydrogen bonds with a carbon-oxygen bond would lead to an alcohol, then an aldehyde, then a carboxylic acid discussed later , and, finally, carbon dioxide:. In this example, we can calculate the oxidation number review the chapter on oxidation-reduction reactions if necessary for the carbon atom in each case note how this would become difficult for larger molecules with additional carbon atoms and hydrogen atoms, which is why organic chemists use the definition dealing with replacing C—H bonds with C—O bonds described.
Indicate whether the marked carbon atoms in the three molecules here are oxidized or reduced relative to the marked carbon atom in ethanol:. There is no need to calculate oxidation states in this case; instead, just compare the types of atoms bonded to the marked carbon atoms:. Aldehydes are commonly prepared by the oxidation of alcohols whose —OH functional group is located on the carbon atom at the end of the chain of carbon atoms in the alcohol:. Alcohols that have their —OH groups in the middle of the chain are necessary to synthesize a ketone, which requires the carbonyl group to be bonded to two other carbon atoms:.
An alcohol with its —OH group bonded to a carbon atom that is bonded to no or one other carbon atom will form an aldehyde. An alcohol with its —OH group attached to two other carbon atoms will form a ketone. If three carbons are attached to the carbon bonded to the —OH, the molecule will not have a C—H bond to be replaced, so it will not be susceptible to oxidation.
Formaldehyde, an aldehyde with the formula HCHO, is a colorless gas with a pungent and irritating odor. Formaldehyde causes coagulation of proteins, so it kills bacteria and any other living organism and stops many of the biological processes that cause tissue to decay. Thus, formaldehyde is used for preserving tissue specimens and embalming bodies.
It is also used to sterilize soil or other materials. Formaldehyde is used in the manufacture of Bakelite, a hard plastic having high chemical and electrical resistance.
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