Functional Groups and Class of Organic Compounds

What are Functional Groups?

A functional group in organic chemistry is a substituent or moiety in a molecule that triggers the molecule's distinctive chemical processes. No matter how the rest of the molecule is made up, the same functional group will experience the same or a similar set of chemical events. This permits the design of chemical synthesis as well as the systematic prediction of chemical reactions and the behaviour of chemical molecules. Other functional groups close by can affect a functional group's reactivity. Retrosynthetic analysis can be used to design organic synthesis by using functional group interconversion.

It is referred to as a systematic nomenclature for naming organic compounds when the names of functional groups are combined with the names of the parent alkanes. The first carbon atom after the one that joins to the functional group is referred to as the alpha carbon in conventional nomenclature; the second is the beta carbon; the third is the gamma carbon, etc. According to IUPAC conventions, the position must be labelled numerically, for example, 4-aminobutanoic acid. Different qualifiers are used to identify isomers in conventional names; for instance, isopropanol (IUPAC name: propan-2-ol) is an isomer of n-propanol (propan-1-ol). The terms "functional group" and "moiety" are somewhat interchangeable. A moiety, on the other hand, is a complete "half" of a molecule and can be either a single functional group or a bigger unit made up of several functional groups. An "aryl moiety," for instance, could be any group that contains an aromatic ring, regardless of how many functional groups the aryl in question comprises.

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Different Functional Groups

The common functional groups are listed below. The symbols R and R' in the formulations often stand for a connected hydrogen atom or a hydrocarbon side chain of any length, though they can also occasionally stand for any combination of atoms.

Functional Groups Containing Hydrocarbons

1. The letter R stands for alkanes, alkenes, and alkynes (and occasionally benzene derivatives). Since they exclusively contain carbon and hydrogen atoms, these groups are also known as hydrocarbyl groups. They might differ, though, in the number of double or triple bonds that are between the two carbon atoms.
2. The kind of the carbon-carbon bond affects these groups' reactivity in different ways. Some groups have particular names because they contain an alkane that is lengthy and branched or has a ring structure. Names like bornyl and cyclohexyl are examples.
3. It's possible for the hydrocarbon functional groups to have an ionic charge. Carbocations are the name given to positively charged compounds, whereas carbanions are the name given to negatively charged hydrocarbons.

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Functional Groups containing Halogen

1. The functional groups that have a bond between a carbon atom and a halogen are known as haloalkanes, sometimes known as alkyl halides. "Halo-" is the prefix used to indicate a halogen. For instance, the prefix fluoro can be used to refer to the chemical CH3F as fluoromethane.
2. The word "halide" is the suffix used to identify a halogen. For instance, with the suffix fluoride, the identical substance, fluoromethane (CH3F), might alternatively be referred to as methyl fluoride.
3. Depending on the halogen, the carbon-halogen bond has varying degrees of stability and strength. Alkyl iodides, for instance, have a weak carbon-iodine connection while alkyl fluorides have a strong and permanent carbon-fluorine bond.
4. All the alkyl halides, with the exception of these alkyl fluorides, are easily subject to elimination reactions or nucleophilic substitution reactions.

Also read about- Salicylic Acid Structure and Fehling Test.

Functional Groups containing Oxygen

The hybridization of the carbon-oxygen bond determines all of the features of functional groups that contain this link.

This can be explained by the fact that the sp3 hybridised oxygen found in alcohols has an electron-donating impact, in contrast to the sp2 hybridised oxygen found in carbonyl groups with a carbon-oxygen double bond, which has an electron-withdrawing effect.

Below is a table with examples that lists the suffixes that are used in the naming of compounds that include functional groups that contain C-O bonds.

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Functional Groups containing Sulfur

Due to their capacity to make more bonds than oxygen, their lighter equivalent on the periodic table, sulfur-containing compounds display distinctive chemistry. For sulphides, disulfides, sulfoxides, and sulfones, substitutive nomenclature (noted as a prefix in the table) is preferable over functional class nomenclature (noted as a suffix in the table).

Functional Groups containing Nitrogen

This group of nitrogen-containing compounds includes those that can have C-O bonds, like amides.

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Commonly Encountered Functional Groups 

Following are some of the most commonly encountered functional groups. Keep in mind that "R" stands in for a general-purpose carbon substituent.

1. Alkanes, Alkenes, Alkynes, and Aromatic Rings

The hydrocarbon functional groups have a tendency to be exceedingly weak acids and are highly non-polar. In contrast to molecules containing more polar functional groups, the only intermolecular interactions that occur in hydrocarbons are London dispersion forces, hence their boiling temperatures are often rather low.

Alkanes are hydrocarbons with a single bond alone. Alkyl groups, which relate to alkanes without a C-H link like methyl, ethyl, or propyl, are what are known as alkane substituents.

1. Methane, ethane, propane, butane, and octane are typical alkanes.
2. Alkanes have a very low polarity and a high covalent C-H bond. Water does not mix them.
3. Alkyl carbons have tetrahedral shape around the carbon and are sp3 hybridised.
4. Alkyl groups, denoted by the symbol R-, are frequently thought of as the "spectator" functional groups of organic chemistry. Alkanes don't go through a lot of distinct reactions, with the exception of free-radical substitution and, of course, burning. They frequently serve as the core of organic compounds.

Alkenes are hydrocarbons that have one or more double bonds between carbon atoms.

1. Ethene, propene, and butene are typical illustrations.
2. Alkenyl groups are what are known as alkene substituents; -CH=CH2 is frequently referred to as vinyl.
3. Alkenyl carbons have a trigonal planar shape and are sp2 hybridised.

Alkynes have a triple bond made of carbon and carbon.

1. The most basic alkyne is ethyne (acetylene); acetylenes are another name for alkynes.
2. Alkynyl groups are the name for alkyne substituents.
3. Alkynes with terminals have a C-H. C-C bonds are present at each end of internal alkynes.
4. Alkynyl carbons undergo linear geometry sp hybridization.

Benzoene rings-  Six-membered rings with three double bonds make up benzoene rings. Because of their exceptional stability and a characteristic known as aromaticity (which has nothing to do with scent), benzene rings are frequently found in nature. possibly be depicted as a hexagon with a circle as well.

1. Model aeroplane glue stinks because of benzene and methylbenzene (toluene).
2. Phenyl groups are the C6H5- group substitutes for benzophenone.
3. Benzene has sp2 hybridised carbons with trigonal planar shape.

Also read about- First 20 elements of Periodic Table

2. Alcohols, Ethers, Amines, Thiols, Alkyl Halides

Because of the higher difference in electronegativities between the bonding atoms, these functional groups are noticeably more polar.

These functional groups have stronger intermolecular pressures because of the interactions between the dipoles. The ability of alcohols and amines to form hydrogen bonds also raises the boiling temperatures of these compounds.

Alcohols with the prefix R-OH have carbon linked to the hydroxyl group.

1. Typical examples include isopropanol, methanol, and methanol.
2. In hydrogen bonding, the O-H bond takes part and is highly polarised.
3. Additionally, hydroxyl groups improve water solubility.
4. Alcohols can function as Lewis bases in addition to being weak acids.
5. C=O-bonded hydroxyl groups are regarded as carboxylic acids, a distinct functional group.

Ethers R-O-R are composed of two carbon bonds surrounding oxygen atoms.

1. Among the ethers that are frequently employed as lab solvents are dioxane, tetrahydrofuran, and diethyl ether.
2. Ethers' boiling temperatures are lower than those of alcohols with comparable molecular weight because they cannot act as hydrogen-bond donors, but higher than those of hydrocarbons because of stronger dipole-dipole forces.

Alkyl halides- Functional groups for alkyl halides include R-F, R-Cl, R-Br, and R-I, where R is an alkyl group.

1. Alkyl halides include substances like chloroform, bromobutane, and methyl bromide.
2. Higher boiling temperatures than those seen in alkanes are the result of dipole-dipole interactions.
3. They are alkenyl halides if R is an alkene.
4. Functional groups that are crucial for substitution and elimination processes.

Amines- The functional group -NH2, -NHR, or NR2 is present in amino acids, and R is typically a hydrocarbon.

1. Just three of the numerous well-known compounds that include amines are morphine, codeine, and cocaine.
2. The term "amino group" refers to amino substituents.
3. Amines containing N-H bonds can form hydrogen bonds, which raises their boiling temperatures and increases their solubility in water.
4. The nitrogen's single pair may function as a base.

Thiols- Alcohols' sulfur-containing cousins are thiols (mercaptans) R-SH.

1. The S-H bond is far less polarised because the sulphur atom is not nearly as electronegative as oxygen.
2. Stronger than alcohols, thiols can function as weak acids.
3. Ethanethiol is added to natural gas to give it its distinctive fragrance because thiols are infamous for having strong odours.

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3. Aldehydes, Ketones, Carboxylic Acids, Esters

The carbonyl group is another name for the C=O group. The carbon contains a partial positive charge and the C=O bond is strongly polarised towards oxygen.

Aldehydes, ketones, esters, and carboxylic acids all contain carbonyls.

Aldehydes RCHO have carbon and C-H bonded by the compound C=O.

1. Examples include benzaldehyde, acetaldehyde, and formaldehyde.
2. Despite having polar covalent bonds, they do not act as hydrogen bond donors.

Ketone- C=O is linked to two carbons in the ketone RC(O)R.

1. Acetone (2-propanone) is used to remove nail polish.

Carboxylic Acid, a carbonyl is attached to the -OH in RCOOH. They differ from alcohols in terms of functional groups.

1. The two simplest carboxylic acids are formic acid and acetic acid (vinegar). Pentanoic and butanoic acids, two more short-chain acids, are infamous for their locker-room odours (or worse).
2. As a result of the hydroxyl group's involvement in hydrogen bonding, carboxylic acids have higher boiling temperatures.
3. Contrary to their name, carboxylic acids often exhibit only partial dissociation in water (as compared to strong acids such as HCl and H2SO4).

Esters- In contrast to carboxylic acids, esters RCOOR have an O-C bond in place of the O-H bond.

1. Esters are distinguished by their pleasant scents.
2. Possess polar bonds but are not involved in hydrogen bonds.

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4. Amides, Acid Halides, Anhydrides, Nitriles

Actually, there are quite a few significant functional groups that contain carbonyls. All of these functional groups are thought to be derivations of carboxylic acids since they can be produced by substituting other groups for OH.

Amides have an amino group and a carbonyl carbon linked.

1. Peptides are groups of amino acids joined together by the production of an amide.
2. Hydrogen bonding can occur between amides with N-H bonds.

Acid Halides- F, Cl, Br, or I are used in place of -OH in acid halides.

Anhydrides- An oxygen is surrounded by two carbonyls in anhydrides. different than esters.

1. The word "anhydrides" comes from the fact that two equivalents of a carboxylic acid can be converted into them while also losing water.

Nitriles- At first glance, nitriles don't appear to be derivatives of carboxylic acids, but they can be created by dehydrating amides.

1. Acetonitrile is a typical solvent.
2. Nitrile rubber, a co-polymer of butadiene and acrylonitrile, is used to create "nitrile gloves."
3. Sometimes the cyanide term is used to describe the -CN substituent.

5. Miscellaneous: Epoxides, Thioethers, Nitro, Imine, Azide

There is no overarching theme here; only a few more notable functional groups that are worth knowing.

Epoxides are technically ethers, but since they take part in several reactions that ethers typically avoid, they merit their own category.

Thioethers- The sulfur equivalents of ethers are thioethers (sulfides). The example that comes up most frequently is dimethyl sulfide.

Nitro- Strong electron withdrawal occurs in nitro groups. The most straightforward nitroalkane is the solvent nitromethane.

Imines- Aldehydes and ketones' nitrogen-containing counterparts are imines. 

Azides pop up from time to time.  The “A” in the anti-HIV drug AZT stands for azido.