10.2.1 - Overview of Common Functional Groups

Alkanes are the simplest type of organic compounds with only single bonds between carbon atoms. Their general formula is CnH2n+2, meaning that for every n carbon atoms, there are 2n+2 hydrogen atoms. Naming alkanes involves finding the longest chain of carbon atoms and identifying any branches or substituents. For example, the molecule with the formula CH3–CH2–CH2–CH3 is named butane because it has a four-carbon chain.

Reactivity of alkanes is quite low; they do not easily react with many chemical substances. However, they can undergo a process called free-radical halogenation, where they react with halogens (like chlorine or bromine) in the presence of light, and they can combust in oxygen to produce carbon dioxide and water.

Alkenes are organic compounds that have at least one double bond between two carbon atoms, represented as C=C. These double bonds create a planar structure around them, leading to specific shapes and angles. When naming alkenes, the goal is to include the double bond in the longest chain while ensuring that the double bond gets the lowest number. For instance, CH2=CH–CH2–CH3 is called 1-butene because the double bond is between the first and second carbons. Alkenes are more reactive than alkanes because they can undergo various types of reactions, particularly electrophilic addition, where different atoms or groups can add across the double bond.

Alkynes are characterized by a triple bond between two carbon atoms (C≡C), resulting in a linear structure around the triple bond. Naming alkynes follows a similar system to alkenes, but they end with the suffix ‘-yne’. For example, CH≡C–CH2–CH3 is called 1-butyne. Alkynes are more reactive than alkenes and can participate in addition reactions where the triple bond breaks to form new bonds. They can also undergo oxidations that can lead to the formation of carboxylic acids or ketones depending on the reactants used in the reaction.

Aromatic compounds, like benzene, feature a stable structure formed by six carbon atoms arranged in a ring with alternating double bonds. This special bonding arrangement results in the delocalization of pi electrons across the ring, contributing to the overall stability and unique chemical properties. Naming these compounds typically starts with 'benzene' for the base structure, with other groups attached (substituents) denoting positions using specific locants. For example, toluene has a methyl group attached to benzene.

When reacting, aromatic compounds generally undergo a process known as electrophilic aromatic substitution rather than addition reactions. This means instead of double bonds being added like alkenes, a hydrogen atom on the benzene ring is replaced with another atom or group, allowing the compound to retain its aromatic structure throughout the reaction.

Alcohols are organic compounds characterized by the presence of one or more hydroxyl (–OH) groups attached to a carbon atom. Their general formula is represented as R–OH, where R can be an alkyl group (a segment of a hydrocarbon) or an aryl group (derived from aromatic compounds). Depending on how many carbon atoms are attached to the carbon bearing the –OH group, alcohols can be classified as primary, secondary, or tertiary. For instance, ethanol is a primary alcohol because the -OH group is attached to a carbon that is only connected to one other carbon, while isopropanol is a secondary alcohol because it’s attached to a carbon connected to two other carbon atoms.

The presence of the hydroxyl group leads to significant physical properties, including relatively high boiling points compared to hydrocarbons because of the hydrogen bonding between alcohol molecules. Reactivity-wise, alcohols can undergo various transformations, including oxidation to aldehydes and carboxylic acids, and even dehydration to form alkenes under the right conditions.

Ethers are characterized by the presence of an oxygen atom bonded to two alkyl or aryl groups, which gives them the general formula R–O–R′. When naming ethers, the names of the two groups attached to the oxygen are combined, and the term 'ether' is added at the end. For example, a compound with the formula CH3–O–CH2CH3 can be systematically named ethoxyethane. One key feature of ethers is their relatively low boiling points compared to alcohols because they cannot form hydrogen bonds with themselves, leading to lesser intermolecular forces. Consequently, ethers are excellent organic solvents that dissolve a wide range of substances.

In terms of reactivity, ethers are generally stable to many reactions but can be cleaved into alcohols and alkyl halides in the presence of strong acids (like HI or HBr). They can also undergo autoxidation under radical conditions, which may produce hazardous peroxides.

Aldehydes and ketones contain a carbonyl group (C=O), but their structures differ slightly. In aldehydes, the carbonyl group is always at the end of the carbon chain and is bonded to at least one hydrogen atom, while ketones have the carbonyl group within the carbon chain attached to two other carbon atoms. Naming these compounds is straightforward; for aldehydes, '–al' is added to the root name, while for ketones, '–one' replaces the '–e' in the alkane name. For example, CH3CHO becomes ethanal.

Aldehydes and ketones have moderate boiling points due to their polar nature, and they serve well as solvents for both polar and nonpolar substances. Their reactivity primarily involves nucleophilic addition at the carbonyl carbon, leading to various reactions, such as forming alcohols upon reduction and undergoing oxidation to form carboxylic acids.

Carboxylic acids feature a carboxyl group that combines a carbonyl (C=O) and a hydroxyl (–OH) group on the same carbon, which gives them unique properties. Carbon in the carboxyl group is sp2-hybridized, resulting in strong hydrogen bonding between the molecules. When it comes to naming, the '–e' suffix of the parent alkane is replaced with '–oic acid', such as CH3–COOH being known as ethanoic acid (commonly known as acetic acid).

They have higher boiling points compared to similarly sized compounds due to the extensive hydrogen bonding. Carboxylic acids can donate a proton (H+) making them weak acids, and they can react in various ways, including neutralization with strong bases, leading to the formation of carboxylate salts, or esterification by reacting with alcohols to yield esters.

Esters are organic compounds characterized by a carbonyl group next to an ether linkage (R–COO–R′). They are typically formed through the reaction of carboxylic acids and alcohols. Naming esters involves replacing the ‘–oic acid’ suffix with ‘–oate’ and indicating the substituent from the alcohol first; for example, CH3COOCH2CH3 is ethyl ethanoate (also known as ethyl acetate).

Esters are known for their pleasant, fruity smells and are commonly found in many fruits and artificial flavors. Lower molecular weight esters typically exist as liquids at room temperature and can act as good solvents. Regarding their reactivity, esters undergo hydrolysis reactions that can result in the formation of carboxylic acids and alcohols. They can also participate in transesterification processes or be reduced to alcohols.

Amines are organic compounds characterized by the presence of an amino group (–NH2), which consists of a nitrogen atom bonded to one or more carbon-containing groups. Depending on whether the nitrogen is bonded to one (primary), two (secondary), or three (tertiary) carbon groups, amines can be classified accordingly. The naming of amines follows a straightforward rule of adding ‘–amine’ to the longest carbon chain containing the nitrogen. For example, CH3–NH2 is called methanamine.

Physically, primary and secondary amines can form hydrogen bonds, leading to higher boiling points compared to tertiary amines, which do not have hydrogen bonding capabilities. Amines are also basic in nature, able to accept protons due to the lone pair of electrons on the nitrogen atom. Reactivity-wise, amines can participate in various reactions, including acylation to form amides and nucleophilic substitutions with alkyl halides.

Amides feature a carbonyl group (C=O) adjacent to a nitrogen atom (–N–), and they are formed from the reaction of carboxylic acids and amines. The carbon atom is sp2-hybridized, while the nitrogen can adopt either sp2 or sp3 hybridization depending on resonance structures. Amides are named by replacing ‘–oic acid’ with ‘–amide’ when derived from carboxylic acids. For example, CH3–CONH2 is called ethanamide.

Due to strong hydrogen bonding capabilities, amides display higher boiling and melting points than their corresponding amines and carboxylic acids. Reactively, they can undergo hydrolysis to yield carboxylic acids and amines, and they can also be reduced to form amines. Furthermore, amides are significant in biological systems as they play essential roles in forming peptide bonds between amino acids in proteins.

Nitriles are compounds that contain a cyano group (–C≡N), characterized by a triple bond between carbon and nitrogen, with the carbon being sp-hybridized. They are named by replacing the '–e' ending of the corresponding alkane with '–nitrile'. For example, CH3–C≡N is called ethanenitrile (or acetonitrile). Nitriles are moderately polar and generally have higher boiling points compared to their hydrocarbon counterparts due to the strong bond formed between carbon and nitrogen.

Chemically, nitriles can be hydrolyzed under acidic or basic conditions to yield carboxylic acids, and they can be reduced to form primary amines. They can also participate in nucleophilic addition reactions where compounds like Grignard reagents can add to the cyano carbon, leading to the formation of ketones upon hydrolysis.

Halides are compounds formed when a carbon atom is bonded to a halogen atom (X), which can be chlorine, bromine, iodine, or fluorine. The structure can either be alkyl (with sp3 hybridized carbon) or aryl (with sp2 hybridized carbon). Naming halides typically involves prefixing ‘chloro-’, ‘bromo-’, ‘iodo-’, or ‘fluoro-’ to the name of the parent hydrocarbon, and indicating positions of the halides with locants (numerical identifiers).

The reactivity of halides is significant; they can engage in nucleophilic substitution reactions (SN1 or SN2) and elimination reactions (E1 or E2) to form alkenes. Additionally, they can also form Grignard reagents, highly reactive species for further synthetic processes. However, aryl halides have distinct behavior and can react through nucleophilic aromatic substitution under specific conditions.

Phenols are characterized by having a hydroxyl (–OH) group directly attached to an aromatic ring, which enables resonance stabilization. The naming typically starts from the base name ‘phenol’, followed by the identification of any substituents. For example, 2-methylphenol is also known as o-cresol. Due to the delocalization of electrons, phenols exhibit stronger acidity compared to regular alcohols, allowing for versatile chemical behavior.

Regarding reactivity, phenols can easily undergo electrophilic aromatic substitution reactions, which can alter the aromatic ring while maintaining its structure. Additionally, phenols can be oxidized to form quinones or participate in esterification reactions to form phenolic esters.

Aromatic heterocycles are compounds where one or more heteroatoms (like nitrogen, oxygen, or sulfur) are incorporated into aromatic rings, altering their properties compared to regular benzenes. Pyridine is a six-membered ring with nitrogen that demonstrates basic characteristics but directs electrophilic substitutions in a meta position due to the electron-withdrawing effect of the nitrogen atom. In comparison, pyrrole features a nitrogen in a five-membered ring, where its lone pair contributes to aromaticity, making it less basic than pyridine. Furan and thiophene also represent five-membered aromatic heterocycles with oxygen and sulfur, respectively, participating in similar reactions but exhibiting increased reactivity due to their electron-rich nature.