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2.5: Functional groups containing mix of sp3- and sp2-, or sp-hybridized heteroatom

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    Learning Objectives
    • Identify, assign IUPAC name, and draw structure from the IUPAC name of carboxylic acids and their derivatives, including acid halides, acid anhydrides, esters, amides, and nitriles.
    • Predict the changes in the polarity and its effect on the reactivity of carboxylic acids and their derivatives, including acid halides, acid anhydrides, esters, amides, and nitriles.
    • Identify phosphoric acid, anhydrides of phosphoric acids, phosphate anions, and esters of phosphoric acids.

    What are carboxylic acids and carboxylic acid derivatives?

    Carboxylic acids have a carbonyl group (\(\ce{C=O}\)) and a hydroxyl group (\(\ce{-OH}\)) on the same carbon, i.e., \(\ce{-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}}\!\!-OH}\) group. The carboxylic acid group is represented as \(\ce{-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}}\!\!-OH}\), or as \(\ce{-COOH}\). Carboxyl acids have some characteristics of the \(\ce{C=O}\) group, some characteristics of the \(\ce{-OH}\) group, and some additional characteristics due to the interaction of the two groups. In carboxylic acid derivates, the \(\ce{-OH}\) group is replaced with another group, that includes acid halides (\(\ce{R-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}}\!\!-X}\)), acid anhydrides (\(\ce{R-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}}\!\!-O-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}-R'}}\)), easters (\(\ce{R-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}}\!\!-OR'}\)), and amides (\(\ce{R-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}}\!\!-NH2}\)). Nitrile group that has the carbonyl \(\ce{O}\) replaced with a \(\ce{N}\) and the \(\ce{-OH}\) group also replaced with the same \(\ce{N}\), i.e., \(\ce{R-C≡N}\) group is also classified as a carboxylic acid derivative. The nomenclature and physical characteristics of carboxylic acids and their derivatives are described in the following sections.

    Carboxylic acids (\(\ce{R-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}}\!\!-OH}\))

    Nomenclature of carboxylic acids

    The IUPAC nomenclature of the carboxylic acids follows the following rules.

    • The longest hydrocarbon chain containing the carboxylic acid group is chosen as the parent name, with the last letter 'e' of its suffix replaced with -oic acid. For example, \(\ce{HCOOH}\) is methanoic acid, \(\ce{CH3COOH}\) is ethanoic acid, and \(\ce{CH3CH2COOH}\) is propanoic acid.
      • If there are two carboxylic acid groups, the suffix changes to -dioic acid, e.g., \(\ce{HOOC-COOH}\) is ethanedioc acid, and \(\ce{HOOC-CH2-COOH}\) is propanedioic acid. (note that when the suffix begins with a consonant (the letter 'd' in this case), the last letter 'e' of the parent hydrocarbon name is not dropped)
    • Start numbering from the \(\ce{C}\) of the \(\ce{-COOH}\) group. The \(\ce{-COOH}\) group itself does not need a location number, as it is always at the end of the chain.
    clipboard_ee9aeae9092b84970246af9374b1adfaa.png2-methylpropanoic acid
    clipboard_ef8495ca9525ea69aef93d10d131f2459.pngprop-2-enoic acid
    • If the \(\ce{-COOH}\) group is bonded to a cyclic chain, the suffix -carboxylic acid is added to the name of the cyclic hydrocarbon. Numbering starts from the point of attachment of \(\ce{-COOH}\) to the ring.
    clipboard_e88a7e5fea92923a9f6135ef01d6ec56f.pngcyclohexanecarboxylic acid
    clipboard_e8da5f3cef57bbc6d66915cd0e9d28437.pngcyclohex-2-ene-1-carboxylic acid
    • If the \(\ce{-COOH}\) group is bonded to a benzene ring, the parent name "benzoic acid is used.
      • Numbering starts from the point of attachment of the \(\ce{-COOH}\) group to the ring.
    clipboard_e5288e4c812ab7956ef4a6e21260be23f.pngbenzoic acid
    clipboard_ed88bc91cf019c44b2383e43b5bdeaef5.png2-hydroxybenzoic acid
    Example \(\PageIndex{1}\)

    clipboard_eade49f79d2cb63899ba9ef0109531917.pngWhat is the IUPAC name of the compound shown on the right?

    Solution
    • The longest chain counting the \(\ce{-COOH}\) group is three \(\ce{C's}\) and a double bond, so the parent name is propene and replace the last letter 'e' with -oic acid, i.e., propenoic acid.
    • There is a methyl group attached that becomes a prefix, i.e., methylpropenoic acid.
    • A location number is needed for the methyl group and the double bond. Start numbering from the \(\ce{C}\) of the \(\ce{-COOH}\) group, double bond receives#2 and the methyl group receives #2.

    Answer: 2-methylprop-2-enoic acid

    The common name of 2-methylprop-2-enoic acid is methacrylic acid, and prop-2-enoic acid is acrylic acid, the monomers (the repeating units) in some polymers.

    Example \(\PageIndex{2}\)

    clipboard_e3e93f2cb3e8e1d94d0c6ce915e3d36cc.pngWhat is the IUPAC name of the compound shown on the right?

    Solution

    The \(\ce{-COOH}\) group is attached to a five \(\ce{C}\) cyclic chain, so the name of the cyclic chain becomes the parent name: cyclopentane.

    Add the suffix -carboxylic acid to the parent name to indicate the carboxylic acid group attached to a cyclic chain.

    Answer: cyclopentanecarboxylic acid

    Example \(\PageIndex{3}\)

    clipboard_e5f39f0ed9bf6802d7621c41eb93f55bc.pngWhat is the IUPAC name of the compound shown on the right?

    Solution
    • A carboxylic acid bonded to a benzene ring takes "benzoic acid" as the parent name.
    • A \(\ce{-OH}\) group takes the prefix "hydroxy" in the presence of a \(\ce{-COOH}\) group, i.e., hydroxybenzoic acid.
    • Start numbering from the point of attachment of the \(\ce{-COOH}\) group: the \(\ce{-OH}\) group receives #4.

    Answer: 4-hydroxybenzoic acid.

    Common names of carboxylic acids

    Common names of carboxylic acids are derived from the names of natural sources of these acids. Table 1 lists some of the common names of carboxylic acids.

    Table 1: Common names and the sources of the common names of some of the carboxylic acids.
    Condensed formula IUPACE name Common name Source of the common name
    \(\ce{HCOOH}\) methanoic acid formic acid Latin: formica, ant
    \(\ce{CH3COOH}\) ethanoic acid acetic acid Latin: acetum, vinegar
    \(\ce{CH3CH2COOH}\) propanoic acid propionic acid Greek: propion, first fat
    (\ce{CH3(CH2)2COOH}\) butanoic acid butyric acid Latin: butyrum, butter
    (\ce{CH3(CH2)4COOH}\) hexanoic acid caproic acid Latin: caper, goat
    (\ce{CH3(CH2)14COOH}\) hexadecanoic acid palmitic acid Latin: palma, palm tree
    (\ce{CH3(CH2)16COOH}\) octadecanoic acid stearic acid Greek: stear, solid fat
    (\ce{CH3(CH2)18COOH}\) eicosanoic acid arachidic acid Greek: arachis, peanut

    The first syllable of the common names, e.g., form-, acet-, prop-, etc., are also used as the first syllable of the common names of related compounds. For example, \(\ce{HCOH}\) is formaldehyde, \(\ce{CH3COH}\) is acetaldehyde, etc.

    Physical properties of carboxylic acids

    The carboxylic acid group has a \(\ce{C=O}\) and a \(\ce{-OH}\) groups, i.e., an sp2- and an sp3 hybridized \(\ce{O}\) bonded to the same \(\ce{C}\). Both \(\ce{O's}\) have two lone pairs of electrons on them, i.e., \(\ce{-\!\!{\overset{\overset{\huge\enspace\!{:O:}}|\!\!\!\!|\enspace}{C}}\!\!-\overset{\Large{\cdot\cdot}}{\underset{\Large{\cdot\cdot}}{O}}-H}\). Lone pair of electrons are usually not shown except when needed, i.e., the carboxylic acid group is represented as \(\ce{-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}}\!\!-OH}\) or as \(\ce{-COOH}\).

    The carboxylic acid group has three polar bonds, i.e., \(\ce{\overset{\delta{+}}{C}{=}\overset{\delta{-}}{O}}\), \(\ce{\overset{\delta{+}}{C}{-}\overset{\delta{-}}{O}}\) and \(\ce{\overset{\delta{-}}{O}{-}\overset{\delta{+}}{H}}\), resulting in a polar group: \(\ce{-\!\!{\overset{\overset{\huge\enspace\!{\overset{\Large{\delta{-}}}{O}}}|\!\!|\enspace}{\overset{\delta{+}}{C}}}\!\!-\overset{\delta{-}}{O}-\overset{\delta{+}}{H}}\). This is because \(\ce{O}\) are more electronegative than \(\ce{C}\) (3.3-2.6 = 0.7) and \(\ce{H}\) (3.3-2.2 = 1.17), as shown in Figure \(\PageIndex{1}\). It makes \(\ce\overset{\delta{+}}{C}\) an electrophile, \(\ce\overset{\delta{-}}{O}\) a nucleophile or a base, and \(\ce\overset{\delta{+}}{H}\) an acid in reactivity. Due to the acid protons, carboxylic acids are also classified as organic acids.

    clipboard_e64bf0d8b0f8f86d05caf1e030502a672.png
    Figure \(\PageIndex{1}\): Electrostatic potential map of ethanoic acid \(\ce{CH3-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}}\!\!-OH}\), \(\ce{C's}\) are gray, \(\ce{O's}\) are red, and \(\ce{H's}\) are wight in the model, red region is \(\delta{-}\), blue is \(\delta{+}\) and green is neutrla in the map. (Copyright; Public domain)

    The polar \(\ce{\overset{\delta{+}}{C}{=}\overset{\delta{-}}{O}}\) and \(\ce{\overset{\delta{-}}{O}{-}\overset{\delta{+}}{H}}\) bonds allow dipole-dipole interactions and hydrogen bonding in addition to the London dispersion forces. Carboxylic acids have stronger intermolecular forces, higher melting points, higher boiling points, and higher solubilities in water compared to alcohols and aldehydes of comparable molar mass due to more intermolecular forces, as compared in Table 2.

    Table 1: Compares boiling points and solubility of a carboxylic acid, alcohol, and aldehyde of comparable molar mass.
    Condensed formula IUPAC name Molar mass (g/mol) Boiling point (oC) Solubility in water
    \(\ce{CH3(CH2)2COOH}\) Butanoic acid 88.1 163 Miscible
    \(\ce{CH3(CH2)3CH2OH}\) Pentan-1-ol 88.1 137 2.3 g/100 mL
    \(\ce{CH3(CH2)3CHO}\) Pentanal 86.1 103 Slightly soluble

    Two carboxylic acids can make two hydrogen bonds with each other, as illustrated in Figure \(\PageIndex{2}\), behaving as a dimer with two times higher molecular mass. It explains their higher boiling points than alcohols of the same molar mass. Carboxylic acids of up to five \(\ce{C's}\), i.e., methanoic acid, ethanoic acid, propanoic acid, butanoic acid, and pentanoic acid, are soluble in water. Hexanoic acid is slightly soluble, and higher acids are insoluble.

    clipboard_e7326fe690d056f61128f13c6fc34e9f8.png
    Figure \(\PageIndex{2}\): Illustration of hydrogen bonding and dimer formation using acetic acid as a model carboxylic acid compound. (left: molecular formula showing hydrogen bonds as dotted lines, right: electrostatic potential map showing attraction between oppositely charged regions of two molecules, i.e., hydrogen bonding in this case (Copyright; Public domain)

    Carboxylic acids have a sour taste because they are acids due to ionizable proton in their \(\ce{-O-H}\) groups. For example, the sour taste of citrus fruits is due to citric acid, and the sour taste of vinegar is due to ethanoic acid.

    Oxidation is i) loss of electrons, ii) gain of \(\ce{O}\), or loss of \(\ce{H}\); and the reduction is the opposite of these. Oxidation and reduction happen together and are collectively called Redox reactions. Most chemical reactions in biological systems are redox reactions, e.g., photosynthesis is a reduction of \(\ce{CO2}\) to convert solar energy into potential chemical energy, and digestion of food is the opposite, i.e., oxidation to release the energy for the activities of life.

    A major portion of organic compounds is hydrocarbon groups, gradually oxidized to alcohols, aldehydes or ketones, carboxylic acids, and finally, carbon dioxide and water, releasing energy. For example, methane (\(\ce{CH4}\)) oxidizes to methanol (\(\ce{CH3OH}\)), methanal (\(\ce{CH2O}\)), methanoic acid (\(\ce{HCOOH}\)), and finally to carbon dioxide (\(\ce{CO2}\)) that is exhaled, as shown below.

    clipboard_e8192686e959d79ea6d47138eaac44335.png

    The alcohols, aldehydes, ketones, and carboxylic acids also serve as intermediates for synthesizing compounds the body needs. Carboxylic acids commonly appear in the metabolic process. For example, glucose, a six \(\ce{C}\) compound, is first converted to two pyruvic acid molecules. Under low oxygen conditions (anaerobic), pyruvic acid is reduced to lactic acid.

    clipboard_e782d08ef8e541637900641e73de1c0db.png

    In the presence of oxygen (aerobic), pyruvic acid releases a \(\ce{CO2}\) and becomes a two \(\ce{C}\) group that joins a four \(\ce{C}\) compound oxalic acid to make a six \(\ce{C}\) compound citric acid. Citric acid releases a \(\ce{CO2}\) and becomes a five \(\ce{C}\) compound \(\alpha\)-ketoglutaric acid, which releases another \(\ce{CO2}\) and becomes four \(\ce{C}\) compound succinic acid, as shown below. This process goes on through several intermediate carboxylic acids, and either all the \(\ce{C's}\) of the starting compound convert to \(\ce{CO2}\), or the intermediate is utilized to synthesize compounds needed by the body.

    clipboard_ec1d102ef35e5e7179586134aee158d01.png, clipboard_ec069df44b97da1d005b5b1fbc7eb3c8e.png, and clipboard_e8bd6c5db82ff18f1608fbfe8edd6018f.png

    Note: The carboxylic acids are shown as neutral acids in the above example, but in the physiological conditions, they exist as anions, i.e., as a carboxylate group (\(\ce{-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}}\!\!-O^{-}}\)). These points will be discussed in a later chapter.

    Acid halides and acid anhydrides -the most reactive acid derivatives

    Nomenclature of acid halides

    Acid halides contain the (\(\ce{-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}}\!\!-X}\)) group, where \(\ce{X}\) can be \(\ce{F}\), \(\ce{Cl}\), \(\ce{Br}\), or \(\ce{I}\).

    • IUPAC name of the acid halide takes the name of the corresponding carboxylic acid with the suffix -oic acid replaced with -oil halide, as shown in the following examples.
    clipboard_e264c4acc4b797bd2f4a6326fa425d426.pngethanoyl chloride or acetyl chloride
    clipboard_e94f16b3e7e9ff2af03942ce8cb5c6c4a.pngpropanoyl bromide
    clipboard_e0a5a398956e4e0c7d487c7725d45bab9.pngprop-2-enoyl bromide

    Acid chlorides and acid bromides are the most common.

    Nomenclature of acid anhydrides

    Acid anhydride contains two acyl \(\ce{R-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}}\!\!-}\) groups bonded to a common \(\ce{O}\) atom, i.e., (\(\ce{-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}}\!\!-O-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}-}}\)) group. A carboxylic acid anhydride is derived by condensing two carboxylic acids by losing a \(\ce{H2O}\) molecule. The acid anhydride is symmetric anhydride if both the acids are the same and mixed or asymmetric anhydride if two different acids are condensed to form the anhydride,

    • Symmetric acid anhydrides are named using the name of the corresponding acid with the last word 'acid' replaced with 'anhydride', as shown in the following examples.
    clipboard_efd4be90008dd1ee7657dea940d8cc3ef.pngethanoic anhydride or acetic anhydride
    clipboard_e1b6fccb71d3437900e718b2b3f955d7b.pngbenzoic anhydride
    • Mixed or asymmetric anhydrides are named by listing the names of the two acids in alphabetic order without the last word 'acid', followed by the word 'anhydride', as shown in the following examples.
    clipboard_e2c9aeb43a8d8c4c41d9b4c41870ba8fb.pngethanoic propanoic anhydride
    clipboard_e20a4fbbcb99791a9696b8467f2fbb60c.pngaccetic benzoic anhydride

    Physical properties of acid halides and acid anhydrides

    The acid halides group has two polar bonds, i.e., \(\ce{\overset{\delta{+}}{C}{=}\overset{\delta{-}}{O}}\), and \(\ce{\overset{\delta{+}}{C}{-}\overset{\delta{-}}{X}}\) resulting in a polar group: \(\ce{-\!\!{\overset{\overset{\huge\enspace\!{\overset{\Large{\delta{-}}}{O}}}|\!\!|\enspace}{\overset{\delta{+}}{C}}}\!\!-\overset{\delta{-}}{X}}\). The acid anhydrides have four polar bonds i.e., two \(\ce{\overset{\delta{+}}{C}{=}\overset{\delta{-}}{O}}\), and two \(\ce{\overset{\delta{+}}{C}{-}\overset{\delta{-}}{O}}\) resulting in a polar group: \(\ce{-\!\!{\overset{\overset{\huge\enspace\!{\overset{\Large{\delta{-}}}{O}}}|\!\!|\enspace}{\overset{\delta{+}}{C}}}\!\!-\overset{\delta{-}}{O}-\!\!{\overset{\overset{\huge\enspace\!{\overset{\Large{\delta{-}}}{O}}}|\!\!|\enspace}{\overset{\delta{+}}{C}}}-}\). This polarity of the group can be observed in the electrostatic potential maps of acid halide, acid anhydride, and an acid shown in Figure \(\PageIndex{3}\)

    clipboard_ead17bd07b58c107c044dea262f9b6677.png
    clipboard_ee71317dc23dd6577993ab75c1385916e.png
    clipboard_eb68c0df2e80d0455ba3265e325455e0f.png
    Figure \(\PageIndex{3}\): Electrostatic potential maps of acetyl chloride (\(\ce{CH3COCl}\)), acetic anhydride (\(\ce{CH3COOCOCH3}\)), and acetic acid (\(\ce{CH3COOH}\)). \(\ce{C's}\) are gray, \(\ce{H's}\) are white, \(\ce{O's}\) are red, and \(\ce{Cl}\) is green in the model. Blue area is \(\delta{+}\), read is \(\delta{-}\), and green is neutral in the map. (Copyright: Public domain)

    It is apparent from the comparison of the electrostatic potential maps shown in Figure \(\PageIndex{3}\) that the carbonyl \(\ce{C}\) is more bluish, i.e., higher \(\delta{+}\) and stronger nucleophile, in the case of acid halide and acid anhydride than in carboxylic acid. The question is \(\ce{Cl}\) in the acid halide is less electronegative than \(\ce{O}\) in carboxylic acid, then why is the carbonyl \(\ce{C}\) more \(\delta{+}\) in the acid halide than in the acid? The answer is in the fact that the heteroatom not only draws the bonding electron away from the carbonyl \(\ce{C}\), it also donates its lone pair through resonance that diminishes the \(\delta{+}\) character on the carbonyl \(\ce{C}\):

    • The 2p-orbital of \(\ce{O}\) and 2p-orbital of \(\ce{C}\) of carboxylic acid are of similar size and overlap well for the reasonce to happen and diminish its \(\delta{+}\) character.
    • The 3p-orbital of \(\ce{Cl}\) or 4p orbital of \(\ce{Br}\) overlaps poorly with 2p-orbital of carbonyl \(\ce{C}\) due to the size difference and does not diminish its \(\delta{+}\) character.
    • The (\ce{O}\) is shared between to \(\ce{C=O}\) groups in the case of acid anhydride. So it diminish \(\delta{+}\) character of \(\ce{C=O}\) less than in carboxylic acids.

    Due to the highly nucleophilic carbonyl \(\ce{C}\), the acid halides and acid anhydrides are very reactive and primarily used as reactive intermediates in chemical synthesis. Again due to their high reactivity, they can not survive in biological systems. Biochemical systems use carboxylic acid derivatives containing \(\ce{S}\) or phosphate groups as reactive intermediates, which will be described later.

    Easters

    Esters have an acyl group ((\(\ce{R-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}}\!\!-}\)) bonded with an alkoxy group (easters (\(\ce{-OR'}\)), i.e., an easter (\(\ce{R-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}}\!\!-OR'}\)) group.

    Nomenclature of esters

    • IUPAC name of an ester starts with the name of the alkyl group that is part of the alkoxy (\(\ce{-OR'}\)) group, followed by the name of the acid corresponding to the acyl ((\(\ce{R-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}}\!\!-}\)) group with the suffix -oic acid replaced with the suffix -oate, as shown in the following examples.
    clipboard_e8d08c72634fb8fb13dc617ca49c18e6d.pngmethyl propanoate
    clipboard_e668cc5a755d750f9d39a97b4fb913b4b.pngethyl benzoate
    clipboard_e282be61281a2d9c2d5984117229f4087.pngpropyl acetate
    Example \(\PageIndex{1}\)

    clipboard_ec0c946c2720f881fc8c5fd3609ec170c.pngWhat is the IUPAC name of the compound shown on the right?

    Solution
    • Alkyl group in the alkoxy (\(\ce{-OR'}\)) group is four (\(\ce{C's}\)) i.e., butyl.
    • The acyl group is three (\(\ce{C's}\)), i.e., propanoic acid; replace acid with -oate, i.e., propanoate.

    Answer: butyl propanoate

    Example \(\PageIndex{2}\)

    clipboard_e62b46cc17aebaf17acac678cba0f68c2.pngWhat is the IUPAC of the compound shown on the right?

    Solution
    • It is an easter where the alkyl group in the alkoxy (\(\ce{-OR'}\)) group is one (\(\ce{C's}\)), i.e., methyl.
    • The acyl group contains benzene, i.e., benzoic acid, and replace the -ic acid with -oate, i.e., benzoate.

    Answer: methyl benzoate

    Physical properties of esters

    Esters group has a polar bonds, i.e., \(\ce{\overset{\delta{+}}{C}{=}\overset{\delta{-}}{O}}\), and two polar \(\ce{\overset{\delta{+}}{C}{-}\overset{\delta{-}}{O}}\) groups, resulting in a polar group: \(\ce{-\!\!{\overset{\overset{\huge\enspace\!{\overset{\Large{\delta{-}}}{O}}}|\!\!|\enspace}{\overset{\delta{+}}{C}}}\!\!-\overset{\delta{-}}{O-R'}}\), as shown in Figure \(\PageIndex{4}\). The reactivity of an ester's carbonyl \(\ce{C}\) is almost the same as that of a carboxylic acid.

    clipboard_e35586b2a8e5d18c58f5d2585accaf738.png
    Figure \(\PageIndex{4}\): Electrostatic potential maps of methyl acetate (\(\ce{CH3COOCH3}\)),. \(\ce{C's}\) are gray, \(\ce{H's}\) are white, and \(\ce{O's}\) are red in the model. Blue area is \(\delta{+}\), read is \(\delta{-}\), and green is neutral in the map. (Copyright: Public domain)

    Easters are pretty common in nature. Small esters are volatile and soluble in water, making them easier to smell and taste. The fragrances of many perfumes and flavors of several fruits are due to esters, as shown in Table 3.

    Table 3: Some esters and the flavor/taste of the fruit they are associated with are shown in the next row.
    clipboard_e645c6092d484513e20fabfb7e9facc6f.pngpentyl acetate clipboard_e7150340360c39db2ce1562fbef77ebec.pngpentyl methanoate clipboard_e879b45e0380cd142b4aa661dd6e4b811.pngethyl heptanoate clipboard_eee58ce6ffc679b4ee066f704b9a15c7e.pngpropyl acetate

    clipboard_ebd33a984a4645b1e6074452fbd6e4e0b.png

    Banana

    clipboard_ed4614f22ff03735bf0009cbd402b66aa.png

    Plum

    clipboard_e0f50c8099be352047415f516bf939216.png

    Grape

    clipboard_ed62948fbe7c8854794a3d54f024513bb.png

    Pear

    clipboard_e27e1e430463dc4ccea81d1df1074ed75.pngoctyl acetate clipboard_e1f7e2da46a7542faa5951ea1dbeab03f.pngethyl butanoate clipboard_ea33965e828d6d7fbbe0a0dad29400028.pngpentyl butanoate clipboard_e772e7b010c9fbbea3bd7133dff371941.pngpropyl pentanoate

    clipboard_e3ca55e42e7447fef7154fc94fe880f04.png

    Orange

    clipboard_e7768b2c9280eab821ec7d7a49eef92b0.png

    Pineapple

    clipboard_e2194c4fbb5a5d27fcbb4128c199dff78.png

    Apricot

    clipboard_e79e36dfcba7815ef52dd3f7673e1f936.png

    Apple

    Esters in fats

    Fats are esters of carboxylic acids that contain a long chain hydrocarbon, an alkane or alkene with cis double bonds, called fatty acids, and propane-1,2,3-triol also called glycerol, as shown in one example in Figure \(\PageIndex{5}\).

    clipboard_e82ccba5dafd545ea6f332503b9096817.png
    Figure \(\PageIndex{5}\): Representative triglyceride found in linseed oil, a triester (triglyceride) derived from linoleic acid (green), \(\alpha\)-linoleic acid (red), and oleic acid (blue). (Copyright;Neutroic, Public domain, via Wikimedia Commons)
    Aspirin -an ester in medical use

    Aspirin is an ester of salicylic acid found in a willow tree's bark. Salicylic acid reduces pain and fever, but it irritates the stomach lining. Aspirin, an ester of salicylic acid, overcomes this problem and is commonly used to reduce pain and fever and as an anti-inflammatory agent. Methyl salicylate is another ester of salicylic acid found in wintergreen oil and used as skin ointments to soothe sore muscles.

    clipboard_e3e1735e3d10859882ec7220836e910c4.png clipboard_e5482ed55b1907f39874b71f222bae47e.png clipboard_e757a65be954291134207b611675414a4.png
    Salicylic acid Aspirin Methyl salicylate

    Amides

    Amidess have an acyl group ((\(\ce{R-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}}\!\!-}\)) bonded with a nitrogen group (\(\ce{-NRR'}\)), i.e., an amide (\(\ce{-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}}\!\!-NRR'}\)) group, where \(\ce{R}\) and \(\ce{R'}\) may be \(\ce{H}\) or a hydrocarbon group.

    Nomenclature of amides

    • IUPAC name of an amide is the name of the acid corresponding to the acyl ((\(\ce{R-\!\!{\overset{\overset{\huge\enspace\!{O}}|\!\!|\enspace}{C}}\!\!-}\)) group with the suffix -oic acid replaced with the word amide.
      • If a hydrocarbon group is bonded to nitrogen, its name appears as a prefix preceded by N-.
      • If two identical hydrocarbon groups are bonded to nitrogen, the group name is preceded by N, N-di.
      • If two different hydrocarbon groups are bonded to nitrogen, the group names, each preceded by N-, are listed alphabetically as prefixes. Some examples are shown below.
    clipboard_e885b3526a553c8565f4a9c7aae4279b6.pngethanamide or acetamide
    clipboard_e4e99d19a4fd7d6612253088615097d68.pngN-methylpropanamide
    clipboard_e543e8d77a30275f8a01c72cffbb3648c.pngN,N-dimethylmethanamide or N,N-dimethylformaide
    clipboard_e1927d3ad1336ff8526aff9c34d2e336a.pngN-ethyl-N-methylpropanamide
    • An amide group attached to a benzene ring takes the base name 'benzamide,' as shown in the following examples.
    clipboard_e1897a422e745614e4f6700c751cc6eae.pngbenzamide
    clipboard_e50f70b2153fe730a8fee59bb6bf7c48b.png3-methylbenzamide
    Example \(\PageIndex{1}\)

    clipboard_ef4598dab55434cff1da70bb2da14a698.pngWhat is the IUPAC name of the compound shown on the right?

    Solution
    • There is an amide group on a four \(\ce{C}\) chain, i.e., butanoic acid, which changes to butanamide.
    • There is a methyl group on nitrogen that becomes the prefix N-methyl.

    Answer: N-methylbutanamide

    Example \(\PageIndex{2}\)

    clipboard_e19f537b99f83b3cb42338548806dfee2.pngWhat is the IUPAC name of the compound shown on the right?

    Solution
    • There are two amide groups, so it takes the suffix -diamide.
    • The corresponding acid is a six \(\ce{C}\) diacid named hexanedioic acid. Replace -oic acid with amide.

    Answer: hexanediamid

    Physical properties of amides

    Amide group has a polar bonds, i.e., \(\ce{\overset{\delta{+}}{C}{=}\overset{\delta{-}}{O}}\), \(\ce{\overset{\delta{+}}{C}{-}\overset{\delta{-}}{N}}\), and \(\ce{\overset{\delta{-}}{N}{-}\overset{\delta{+}}{H}}\) groups, resulting in a polar group: \(\ce{-\!\!{\overset{\overset{\huge\enspace\!{\overset{\Large{\delta{-}}}{O}}}|\!\!|\enspace}{\overset{\delta{+}}{C}}}\!\!-\overset{\delta{-}}{N}-R'R"}\), as shown in Figure \(\PageIndex{5}\).

    clipboard_eb68c0df2e80d0455ba3265e325455e0f.png
    clipboard_e789e10c0118cfcc8d9b106d9ece7958d.png
    clipboard_ebb03ba543b032e3aac627a5e22e0f47c.png
    Figure \(\PageIndex{5}\): Electrostatic potential maps of acetic acid (\(\ce{CH3COOH}\)), acetamide (\(\ce{CH3CONH2}\)), and acetonitrile (\(\ce{CH3C≡N}\)). \(\ce{C's}\) are gray, \(\ce{H's}\) are white, \(\ce{O's}\) are red, and \(\ce{Cl}\) is green in the model. Blue area is \(\delta{+}\), read is \(\delta{-}\), and green is neutral in the map. (Copyright: Public domain)

    Comparison of electrostatic potential maps in Figure \(\PageIndex{5}\) shows that the carbonyl \(\ce{C}\) of amide is less \(\delta{+}\), i.e., less electrophilic than that of the corresponding carboxylic acid. This is because of two reasons i) \(\ce{N}\) is less electronegative and draws electrons away less than \(\ce{O}\) and ii) being less electronegative, \(\ce{N}\) sends its lone pair of electrons more to the carbonyl \(\ce{C}\) neutralizing its \(\delta{+}\) by resonance than \(\ce{O}\). Both of these factors make the carbonyl \(\ce{C}\) less \(\delta{+}\) and less electrophilic, which makes amides one of the lest reactive and the most stable carboxylic acid derivatives that are found commonly in nature. The resonance effect is illustrated in Figure \(\PageIndex{6}\) below. Due to the resonance, the \(\ce{C-N}\) bond has a significant double bond character.

    clipboard_efbabfc993ff4e9aa124aad66305356d8.png
    Figure \(\PageIndex{6}\): Illustration of the resonance shown by the dotted lines showing partial double bonds formed by the overlap of three consecutive p-orbitals (yellow lobes) on \(\ce{O}\), carbonyl \(\ce{C}\), and \(\ce{N}\). Recall that the \(\ce{N}\) when it is next to an sp2 hybridized atom converts from sp3 to sp2 hybridization for the resonance to happen. The blue lobs represent the sp2 orbitals (Copyright; Public domain)

    Since the lone pair of electrons of amide are occupied in the resonance, they are less available to protons of acids. Therefore, amides are much less basic than amines. Amides have \(\ce{\overset{\delta{-}}{N}{-}\overset{\delta{+}}{H}}\) bonds that allow them to make hydrogen bonds with water molecules. Therefore, amides containing up to five \(\ce{C's}\) are soluble in water due the hydrogen bonding. Those with larger alkyl groups, i.e., with more than five \(\ce{C's}\) are slightly soluble or insoluble due to the hydrophobic character of the alkyl groups dominating over the hydrophilic nature of the amide group.

    Amides in nature and medicines

    Proteins are made of small repeat units, called amino acids, joined through amide groups, as illustrated in Figure \(\PageIndex{7}\), which makes amide one of the most common groups present in nature.

    clipboard_e795fea2683555fd35889a3d86b75319b.png
    Figure \(\PageIndex{7}\): Skeletal formula of teriparatide—the biologically active region of the 84-amino acid human parathyroid hormone. (Copyright; Vaccinationist, Public domain via Wikimedia commons)

    During the digestion of proteins, \(\ce{N's}\) end up in urea excreted by kidneys. If kidneys malfunction, urea may build to a toxic level, resulting in uremia. Urea is also used as a fertilizer.

    Barbiturates derived from barbituric acid have sedative and hypnotic effects and are used in medicines for these effects. Barbiturate drugs include phenobarbital and pentobarbital. Phenacetin and acetaminophen are amides used in Tylenol as alternatives to reduce fever and pain but with little anti-inflammatory effect. The structures of these amides are shown below.

    clipboard_e36ae5a320e1d0aaebf1d6f36ce425427.png

    urea

    clipboard_ed49181927fb89cb5626c6d0c61c63f0a.png

    barbituric acid

    clipboard_e2d6179caadf037f0e27d78c48ae35925.pngphenobarbital clipboard_e897322f69b2074b1ac9ed9a049fcd1a5.pngpentobarbital clipboard_e3ad4c34a0c25201f0810fb91a9e3206b.pngphenacetin clipboard_ee19270e2097d00eef09d46a610a4d598.pngacetaminophen

    cyclic amides are called lactams. A four-member lactam is a common feature in the structure of Penicillin and related synthetic antibiotics, as shown below.

    clipboard_eceea2c3ec3ff4bf6879bc62b35b66bf6.png clipboard_e29cecd172df3b1c3870188cf5c0488a3.png clipboard_ed18ea6a62d4528f200ed25c755ea80b7.png clipboard_e59868a2b9623ebc6475ee0a1e8004476.png
    A four-membered
    lactam
    Penicillin G Amoxicillin Cephalexin

    Nitriles

    Nitriles have cyano group that is a polar bond \(\ce{-\overset{\delta{+}}{C}{≡}\overset{\delta{-}}{N}\!:}\) with a lone pair in one of the sp orbital of \(\ce{N}\), as illustrated in Figure \(\PageIndex{5}\).

    IUPAC name of nitriles is composed of the name of the hydrocarbon skeleton, including the \(\ce{C}\) in the nitrile group with the suffix -nitrile.

    Another way of naming them is to take the name of the corresponding carboxylic acid but replace the suffix -oic acid with -onitrile, as shown in the following example.

    clipboard_e4ac399c3afd01b5f493473c7b30c0105.pngacetonitrile or ethannitrile
    clipboard_ec41ff52a8c18bdacefdfb2ecf2318d96.pngpropanenitrile
    clipboard_eb75e60aeedd44a6e9886e22e081e14f7.pngbenzonitrile

    Nitriles are not very common in nature but are important as intermediates in synthesizing organic compounds.

    Phosphorous groups

    Phosphorous (\(\ce{P}\)) is in the same group with \(\ce{N}\) in periodic table. Like \(\ce{N}\) in ammonia (\(\ce{{H}-\overset{\bullet\bullet}{\underset{\underset{\huge{H}} |}{N}}\!-H}\)), the \(\ce{P}\) can have three bonds and a lone pair, as in phosphine (\(\ce{{H}-\overset{\bullet\bullet}{\underset{\underset{\huge{H}} |}{P}}\!-H}\)) with eight valence electrons, i.e., octet complete. Unlike \(\ce{N}\), the \(\ce{P}\) is in fourth row in the periodic table and, like other elements of the fourth and higher row, can have more than eight valence electrons in its compounds, e.g., ten valence electrons in phosphoric acid (\(\ce{HO-\!\!\!\!\!{\overset{\overset{\huge\enspace{O}}|\!\!|\enspace}{\underset{\underset{\huge\enspace\,{OH}} |}{P}}}\!\!\!\!-OH}\)). Phosphorous groups are important in biological systems, e.g., they are part of DNA molecules, phospholipids in cell membranes, and energetic molecules like adenosine triphosphate that are used as energy currency in biochemical reactions.

    Phosphoric acid and phosphoric anhydrides

    Phosphoric acid or orthphosphosporic acid has three \(\ce{-OH}\) groups and one \(\ce{=O}\) bonded to a \(\ce{P}\) atom, i.e., \(\ce{HO-\!\!\!\!\!{\overset{\overset{\huge\enspace{O}}|\!\!|\enspace}{\underset{\underset{\huge\enspace\,{OH}} |}{P}}}\!\!\!\!-OH}\). Like carboxylic acid anhydride, which is two carboxylic acids joined through a common \(\ce{O}\), two phosphoric acids joined through a common \(\ce{O}\) is a diphosphoric acid or a pyrophosphoric acid, and three phosphoric acids condensed in this way is a triphosphoric acid. Their corresponding anions formed after ionizing the acidic protons from the \(\ce{-OH}\) groups are called phosphates, and alkoxy (\(\ce{-OR}\) replacing one or more \(\ce{-OH}\) groups are phosphate esters, as shown below.

    \(\ce{HO-\!\!\!\!\!{\overset{\overset{\huge\enspace{O}}|\!\!|\enspace}{\underset{\underset{\huge\enspace\,{OH}} |}{P}}}\!\!\!\!-OH}\)

    Phosphoric acid or orthphosphosporic acid

    \(\ce{HO-\!\!\!\!\!{\overset{\overset{\huge\enspace{O}}|\!\!|\enspace}{\underset{\underset{\huge\enspace\,{OH}} |}{P}}}\!\!\!\!-O-\!\!\!\!\!{\overset{\overset{\huge\enspace{O}}|\!\!|\enspace}{\underset{\underset{\huge\enspace\,{OH}} |}{P}}}\!\!\!\!-OH}\)

    diphosphoric acid or a pyrophosphoric acid

    \(\ce{HO-\!\!\!\!\!{\overset{\overset{\huge\enspace{O}}|\!\!|\enspace}{\underset{\underset{\huge\enspace\,{OH}} |}{P}}}\!\!\!\!-O-\!\!\!\!\!{\overset{\overset{\huge\enspace{O}}|\!\!|\enspace}{\underset{\underset{\huge\enspace\,{OH}} |}{P}}}\!\!\!\!-O-\!\!\!\!\!{\overset{\overset{\huge\enspace{O}}|\!\!|\enspace}{\underset{\underset{\huge\enspace\,{OH}} |}{P}}}\!\!\!\!-OH}\)

    triphosphoric acid

    \(\ce{^{-}O-\!\!\!\!\!{\overset{\overset{\huge\enspace{O}}|\!\!|\enspace}{\underset{\underset{\huge\enspace\,{O^{-}}} |}{P}}}\!\!\!\!-O^{-}}\)

    Phosphate ion or orthphosphate ion

    \(\ce{^{-}O-\!\!\!\!\!{\overset{\overset{\huge\enspace{O}}|\!\!|\enspace}{\underset{\underset{\huge\enspace\,{O^{-}}} |}{P}}}\!\!\!\!-O-\!\!\!\!\!{\overset{\overset{\huge\enspace{O}}|\!\!|\enspace}{\underset{\underset{\huge\enspace\,{O^{-}}} |}{P}}}\!\!\!\!-O^{-}}\)

    diphosphate ion or a pyrophosphate ion

    \(\ce{^{-}O-\!\!\!\!\!{\overset{\overset{\huge\enspace{O}}|\!\!|\enspace}{\underset{\underset{\huge\enspace\,{O^{-}}} |}{P}}}\!\!\!\!-O-\!\!\!\!\!{\overset{\overset{\huge\enspace{O}}|\!\!|\enspace}{\underset{\underset{\huge\enspace\,{O^{-}}} |}{P}}}\!\!\!\!-O-\!\!\!\!\!{\overset{\overset{\huge\enspace{O}}|\!\!|\enspace}{\underset{\underset{\huge\enspace\,{O^{-}}} |}{P}}}\!\!\!\!-O^{-}}\)

    triphosphate ion

    Phosphoric esters

    Like carboxylic acid changes to an ester when its \(\ce{-OH}\) group is replaced with an alkoxy (\(\ce{-OR}\)) group, phosphoric acid changes to mono phosphoric ester when one of its \(\ce{-OH}\) groups are replaced with alkoxy (\(\ce{-OR}\)), to diester when two \(\ce{-OH}\) groups are replaced with alkoxy (\(\ce{-OR}\)), and to triester when three \(\ce{-OH}\) groups are replaced with alkoxy (\(\ce{-OR}\)).

    • The phosphoric esters are named by listing the names of the alkyl parts of the alkoxy groups in alphabetic order, followed by the world phosphate.
    • In more complex phosphodiesters, it is common practice to name the organic compound followed by the word 'phosphate' or prefixed phospho-, as shown in the following examples.

    clipboard_e4e562cc5e6c1dbca70958c91fdb45344.png

    dimethyl phosphate

    clipboard_ec3153e12643afeeaa0a4e28924a688aa.png

    ethyl methyl phosphate

    clipboard_e1c799d416312907c75f3433528aa787c.png

    dihydroxyacetone phosphate

    clipboard_e57f2f682920d4a0879ff1725447b99e6.png adenosine triphosphate

    This page titled 2.5: Functional groups containing mix of sp3- and sp2-, or sp-hybridized heteroatom is shared under a Public Domain license and was authored, remixed, and/or curated by Muhammad Arif Malik.