Until now, almost seven million organic compounds have been discovered. But the reactive groups present in these organic compounds are relatively smaller. These groups are called functional groups. Functional groups are the specific atoms, ions, or groups of atoms having identical reactive patterns. These functional groups are responsible for the reactivity, physical, and other chemical properties of the organic compounds.
To identify these functional groups, there are several basic chemical tests that will confirm the presence of any functional group in any organic compound. For each type of functional group, there is a specific test that will show the presence of that group through the change of color of the final product, gas formation, precipitate formation, etc.
There are almost seven million organic compounds discovered now. To memorize these seven million organic compounds is impossible. So scientists divided these compounds into groups to identify them easily. These groups are based on the functional groups they contain, so understanding the functional groups rather than the individual compound is easier. Students can identify other similar compounds if they know the groups according to the functional groups. For example, if any compound consists of the -OH group, it belongs to the alcohols. But students have to differentiate between R-O-H and C-O-O-H. Because if the OH group is attached to carbon, it is an alcohol, and if it is connected to oxygen, it is an acid. So the understanding of the groups to the connecting groups is necessary.
The first thing that may confuse the students is what basic chemical tests are and for what and why they are used. The second thing that may confuse the students is how to select a test for a specific compound; learning this point makes this topic more accessible. The third thing in which the students face difficulties is the results, which type of results are shown by the specific functional group. So it is very necessary that students may know how to select, perform and analyze any chemical tests for functional groups.
The functional groups and the basic chemical tests involve the color change, gas production, and precipitate formation of materials. Understanding it theoretically is challenging because students get confused about why and how the color change occurs. They have no idea about the colors of the reagents, so make them clear about the colors of the reagents as well. Tell them why and how the color changes, why the gas produces etc. If you teach in a classroom, use the color makers to express the final products. The use of flashcards makes it easy to memorize the functional groups and their structures.
Explain the common functional groups first; the following are organic compounds' most common functional groups.
Alkane: A hydrocarbon that has no functional groups. Alkanes are named with the suffix -ane, e.g., butane.
Alkene: A hydrocarbon that has at least one C-C double bond. The double bond is nucleophilic, which makes the alkenes substrates in electrophilic addition reactions. Alkenes are named with the suffix -ene, e.g., butene.
Alkyne: A hydrocarbon that has at least one C-C triple bond. The triple bond is nucleophilic, which makes the alkynes substrates for electrophilic addition reactions. If the alkyne is terminal, the proton is slightly acidic, and acetylide anions can be formed. Alkynes are named with the suffix -yne, e.g., butyne.
Arene: Also called aromatics are hydrocarbons that contain at least one phenyl group. Aromatic rings have a high electron density and are nucleophilic, which makes them substrates for electrophilic aromatic substitution. Aromatics can include other functional groups named with the prefix phenyl- or with the suffix -benzene, e.g., phenylamine and chlorobenzene.
Haloalkane: A compound that contains a halogen (main group VII in the periodic table) substituent. The halogens are more electronegative than carbon and are good leaving groups, making haloalkanes good substrates for SN1/SN2 and E1/E2 reactions. Haloalkanes are named with a derivation of the prefix halo-, e.g., bromobutane.
Aldehyde: A compound that contains a C-O double bond, where one of the substituents on the carbon atom is a hydrogen atom, and the other is a carbon atom. The C=O bond is polarized towards oxygen, making the carbon atom electrophilic, and thus aldehydes are substrates for nucleophilic addition reactions. Aldehydes are named with the suffix -al, e.g., butanal.
Ketone: A compound that contains a C-O double bond, where both of the substituents on the carbon atom are carbon atoms. The C=O bond is polarized towards oxygen, making the carbon atom electrophilic; thus, ketones are substrates for nucleophilic addition reactions. Ketones are named with the suffix -one, e.g., butanone.
Alcohol: A compound with a hydroxy (-OH) substituent to a saturated carbon. Alcohols can be deprotonated to create a good nucleophile or protonated to transform the -OH into a good leaving group (OH2), thereby making the alcohols suitable substrates for SN1/SN2 and E1/E2 reactions. Alcohols are named with the suffix -ol, e.g., butanol. Note that a hydroxy (-OH) group on an aromatic ring yields the functional group phenol, which has slightly different reaction properties.
Ether: A compound that contains a C-O-C bond. Ethers are generally not very reactive and are often used as solvents. Ethers are named with the suffix ether, e.g., diethyl ether.
Amine: A compound that contains an amino substituent. The amine can be primary (R-NH2), secondary (R-NHR'), or tertiary (R-NR'R''). Amines are alkaline and often used as bases or nucleophiles. Amines are named with the suffix amine, e.g., triethyl amine.
Carboxylic acid: A compound with a -COOH substituent. Carboxylic acids and their derivatives (below) can be transformed into each other through nucleophilic substitution reactions. Carboxylic acids are named with the suffix -oic acid, e.g., butanoic acid.
Acid anhydride: A compound with a -COOCO- component. Acid anhydrides are derivatives of carboxylic acids, structurally resembling two carboxylic acids that have been merged with the elimination of a water molecule. Acid anhydrides are named with the suffix -oic anhydride, e.g. butanoic anhydride.
Ester: A compound with a -COOR substituent. Esters are derivatives of carboxylic acids, structurally resembling a carboxylic acid that has been merged with alcohol by eliminating a water molecule. Esters are named with -yl -oate, e.g., butyl butanoate.
Amide: A compound with a -CONHR substituent. Amides are derivatives of carboxylic acids, structurally resembling a carboxylic acid that has been merged with an amine by eliminating a water molecule. Amides are named with the suffix amide, e.g., butanamide.
Acyl halide: A compound with a -COX substituent, where the X is a halogen. Acid halides are derivatives of carboxylic acids, where the hydroxy substituent has been replaced with a halogen atom. Acyl halides are named with the suffix -oyl halide, e.g., butanoyl chloride.
Students can easily memorize the functional groups by using mnemonics. Following are some common mnemonics for functional groups:
Vowels: The vowels "A", "E", and "Y" before the last two words “ne” of any alkyl group represent alkane, alkene, and alkyne, respectively.
Alcohol: If there is any C-O-H group, the compound is an alcohol.
Ether: If there is a C on either sides of any O, the compound is an ether.
Amine: The “N’’ in amine represents nitrogen; amine means the compound has a nitrogen functional group.
Aldehyde: remember the Y of an aldehyde; it represents the C=O as C<O.
Ketone: remember the diagonal strokes of K; it represents the C₂-C=O group
Carboxylic Acid: In carboxylic acid, the "Box" represents the boxed wine (C-O-H) or alcohol, and "Y" indicates a C=O double bond as C<O.
Amide: ‘’Ami’’ represents the Amine with a "D". D for the double bond to oxygen.
Students can easily memorize the functional groups by making flash cards. The process of making flashcards is very easy. Ask the students to write the name of functional groups on a page along with the prefix and suffix and draw the structure of the functional group on the other side of the same page.
Through the flash card, you can also take a test of students. Give them different cards of names and structures of functional groups and ask them to join the characters with the respective structure of the functional group.
Explain some basic chemical tests and perform them in front of students.
The ceric ammonium nitrate test is a way to examine a solution for the presence of either alcohols or phenols. In solution, the orange-yellow ceric ammonium nitrate makes a complex with the alcohol or phenol, which results in a color change. Alcohols cause a red color change, whereas phenols induce a more dark-red to dark-brown color change, depending on the phenol involved. From labster theory, you can find the complete procedure of the Ceric ammonium nitrate test.
Sodium hydrogen carbonate, also known as sodium bicarbonate, reacts with acidic solutions to form carbon dioxide, released as gas from the solution, resulting in so-called brisk effervescence. Sodium hydrogen carbonate can be used to test for carboxylic acids. If effervescence occurs, then an acid group is present. Phenol is a weak acid that dissolves in sodium hydroxide solution but does not dissolve in sodium hydrogen carbonate solution. Stronger acids, such as carboxylic acids, dissolve in both solutions. This can be useful for separating mixtures of acids. From labster theory, you can find the complete procedure of the Sodium bicarbonate test.
The bromine test is used for an unsaturated carbon-carbon bond, such as an alkene or alkyne. The test uses a chemical reaction called addition, where a reactant, bromine, is added to an organic compound to break a double or triple bond. Bromine has an orange-brownish color when in solution, so the color of the solution is lost when an alkene or alkyne is present for bromine to react with. Bromine will also react with aromatic compounds, such as phenol, but it can't react with alkanes as they contain only single bonds; therefore, there is no color change when mixed. Benzene can react with bromine in the presence of a catalyst, but not without a catalyst, since it is not reactive enough. Phenol is more reactive than benzene so it can react with bromine without a catalyst. This is because the alcohol group donates electron density into the delocalized benzene ring. From labster theory, you can find out the complete procedure of the Bromine test.
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In Labster’s interactive Functional Groups and Basic Chemical Tests Virtual Lab, students will learn the basics of functional groups within organic chemistry and perform simple chemical tests to validate their presence.
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