The connection between the structures of alkenes and alkanes was established in Section O1.6, which noted that we can transform an alkene into an alkane by adding an H2 mole-cule across the CPC double bond.
The driving force behind the reaction is the difference between the strengths of the bonds that must be broken and the bonds that form in the reaction. In the course of the hydro-genation reaction, a relatively strong HOH bond (435 kJ/mol) and a moderately strong carbon–carbon bond (~270 kJ/mol) are broken, but two strong COH bonds (439 kJ/mol) are formed. The reduction of an alkene to an alkane is therefore an exothermic reaction.
What about the addition of an H2molecule across a CPO double bond?
Once again, a significant amount of energy has to be invested in the reaction to break the HOH bond (435 kJ/mol) and the carbon–oxygen bond (~375 kJ/mol). The overall re-action is still exothermic, however, because of the strength of the COH bond (439 kJ/mol) and the OOH bond (498 kJ/mol) that are formed.
The addition of hydrogen across a CPO double bond raises several important points.
First, and perhaps foremost, it shows the connection between the chemistry of primary al-cohols and aldehydes. But it also helps us understand the origin of the term aldehyde. If a
H
H2 C C H
H
H OH
H
H C C H
H
H O
Pt
B
H
H2 C C H
H
H H H
H
H H
H C
GC G D
D P Ni
CH3CH2CH2BrCH O3 88n CH3CH2CH2OCH3 Br 2 CH3CH2OH H CH3CH2OCH2CH3
CH3CH2OH(aq)H2O(l) H3O(aq)CH3CH2O(aq)
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FUNCTIONAL GROUP CHEMISTRY 17
reduction reaction in which H2 is added across a double bond is an example of a hydro-genation reaction, then an oxidation reaction in which an H2molecule is removed to form a double bond might be called dehydrogenation. Thus, using the symbol [O] to represent an oxidizing agent, we see that the product of the oxidation of a primary alcohol is liter-ally an “al-dehyd” or aldehyde. It is an alcohol that has been dehydrogenated.
The reaction also illustrates the importance of differentiating between primary, sec-ondary, and tertiary alcohols. Consider the oxidation of isopropyl alcohol, or 2-propanol, for example.
The product of this particular reaction was originally called aketone, although the name was eventually softened to azetone and finally acetone. Thus, it is not surprising that any substance that exhibited chemistry that resembled “aketone” became known as a ketone.
Aldehydes can be formed by oxidizing a primary alcohol; oxidation of a secondary al-cohol gives a ketone. What happens when we try to oxidize a tertiary alal-cohol? The an-swer is simple: Nothing happens.
There aren’t any hydrogen atoms that can be removed from the carbon atom carrying the OOH group in a 3° alcohol. And any oxidizing agent strong enough to insert an oxygen atom into a COC bond would oxidize the alcohol all the way to CO2and H2O.
A variety of oxidizing agents can be used to transform a secondary alcohol to a ketone.
A common reagent for the reaction is some form of chromium(VI)—chromium in the 6 oxidation state—in acidic solution. The reagent can be prepared by adding a salt of the chromate (CrO42) or dichromate (Cr2O72) ion to sulfuric acid. Or it can be made by adding chromium trioxide (CrO3) to sulfuric acid. Regardless of how it is prepared, the ox-idizing agent in the reactions is chromic acid, H2CrO4.
The choice of oxidizing agents to convert a primary alcohol to an aldehyde is much more limited. Most reagents that can oxidize the alcohol to an aldehyde carry the reaction one step further—they oxidize the aldehyde to the corresponding carboxylic acid.
CH3CH2OH CH3CH OB
H2CrO4
CH3COH OB CH
CH3 CH2CH3 OHA
C
CH3 CH2CH3 OB
H2CrO4
C CH3 CH3
CH3 OHA
A
[O]
CH
CH3 CH3 CH3CCH3 OB OHA [O]
CH2
CH3 OH CH3C O
H
[O] B
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18 FUNCTIONAL GROUP CHEMISTRY
A weaker oxidizing agent, which is just strong enough to prepare the aldehyde from the primary alcohol, can be obtained by dissolving the complex that forms between CrO3and pyridine, C6H5N, in a solvent such as dichloromethane that doesn’t contain any water.
The Nomenclature of Aldehydes and Ketones
The common names of aldehydes are derived from the names of the corresponding car-boxylic acids.
The systematic names for aldehydes are obtained by adding -al to the name of the parent alkane.
The presence of substituents is indicated by numbering the parent alkane chain from the end of the molecule that carries the OCHO functional group. For example,
The common name for a ketone is constructed by adding ketone to the names of the two alkyl groups on the carbon of the CPO double bond, listed in alphabetical order.
The systematic name is obtained by adding -one to the name of the parent alkane and us-ing numbers to indicate the location of the CPO group.
Common Aldehydes and Ketones
Formaldehyde has a sharp, somewhat unpleasant odor. The aromatic aldehydes in Figure O2.7, on the other hand, have a very pleasant “flavor.” Benzaldehyde has the characteristic
CH3CCH2CH3
OB
2-Butanone
CH3CCH2CH3
OB
Ethyl methyl ketone
BrCH2CH2CH OB
3-Bromopropanal
HCH CH3CH
OB
OB
Ethanal Methanal
HCOH OB
HCH OB
CH3COH CH3CH OB
OB
Formic acid Formaldehyde
Acetaldehyde Acetic acid
CH3CH2OH CrO3/pyridine CH3CH OB
CH2Cl2
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FUNCTIONAL GROUP CHEMISTRY 19
odor of almonds, vanillin is responsible for the flavor of vanilla, and cinnamaldehyde makes an important contribution to the flavor of cinnamon.
FIGURE O2.7 Aromatic alde-hydes with characteristic odors.
CH