Question

What structural features help us identify a compound as (a) an alkane, (b) a cycloalkane, (c) an alkene, (d) an alkyne, (e) a saturated hydrocarbon, (f) an aromatic hydrocarbon?

Short answer

(a) Alkanes have only single covalent bonds between adjacent carbon atoms with the general formula \(C_nH_{2n+2}\). (b) Cycloalkanes are closed carbon rings with single covalent bonds and follow the general formula \(C_nH_{2n}\). (c) Alkenes contain at least one carbon-carbon double bond with the general formula \(C_nH_{2n}\). (d) Alkynes have at least one carbon-carbon triple bond and follow the general formula \(C_nH_{2n-2}\). (e) Saturated hydrocarbons have only single covalent bonds between adjacent carbon atoms and can be either alkanes or cycloalkanes. (f) Aromatic hydrocarbons consist of closed carbon rings with conjugated double bonds, the most common of which is benzene (C6H6).

Step-by-step solution

Alkane's Structural Features

An alkane is a saturated hydrocarbon containing only single covalent bonds between adjacent carbon atoms. The general formula for an alkane is \(C_nH_{2n+2}\), where n represents the number of carbon atoms in the compound. A key structural feature of alkanes is that they have no double or triple bonds, making them less reactive compared to other types of hydrocarbons.

Cycloalkane's Structural Features

Cycloalkanes are cyclic hydrocarbons, forming rings by connecting the carbon atoms with single covalent bonds. The general formula for cycloalkanes is \(C_nH_{2n}\), where n represents the number of carbon atoms in the ring. The primary structural feature of cycloalkanes is the presence of a closed carbon ring without any double or triple bonds between the carbon atoms.

Alkene's Structural Features

Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond. Their general formula is \(C_nH_{2n}\), where n represents the number of carbon atoms in the compound. The main structural feature of alkenes is the presence of at least one carbon-carbon double bond, making them more reactive than alkanes or cycloalkanes.

Alkyne's Structural Features

Alkynes are another type of unsaturated hydrocarbons, and they contain at least one carbon-carbon triple bond. The general formula for alkynes is \(C_nH_{2n-2}\), where n represents the number of carbon atoms in the compound. The distinct structural feature of an alkyne is the presence of at least one carbon-carbon triple bond, making them even more reactive than alkenes or alkanes.

Saturated Hydrocarbon's Structural Features

Saturated hydrocarbons are compounds that have only single covalent bonds between adjacent carbon atoms, and they can be either straight-chain or cyclic. Both alkanes (\(C_nH_{2n+2}\)) and cycloalkanes (\(C_nH_{2n}\)) are examples of saturated hydrocarbons. The defining structural feature of saturated hydrocarbons is the absence of any double or triple bonds between carbon atoms.

Aromatic Hydrocarbon's Structural Features

Aromatic hydrocarbons are a class of cyclic hydrocarbons with unique electronic structures, and they are known for their stability and distinctive aroma. The most common aromatic hydrocarbon is benzene (C6H6), which consists of a hexagonal carbon ring with alternating single and double bonds. The key structural feature of aromatic hydrocarbons is the presence of a closed carbon ring with conjugated double bonds, which form a delocalized electron cloud that stabilizes the molecule.

Key concepts

Alkanes

Alkanes are the simplest form of hydrocarbons and are composed entirely of carbon and hydrogen atoms. They are known for their single bonds; every carbon atom is bonded to another with a single covalent bond.

This structure gives alkanes their general formula of \(C_nH_{2n+2}\). For example, methane (\(CH_4\)), ethane (\(C_2H_6\)), and propane (\(C_3H_8\)) are alkanes with one, two, and three carbon atoms, respectively. A distinctive characteristic of alkanes is their lack of reactivity, which is due to the strength and non-polarity of the C-H bonds.

Alkanes are also referred to as saturated hydrocarbons because they have the maximum number of hydrogen atoms per carbon, indicating no more can be added without breaking the existing structure. Alkanes are typically found in natural gas and petroleum and are used as fuels and lubricants.

Cycloalkanes

Cycloalkanes represent a type of hydrocarbons that have their carbon atoms arranged in a ring-like structure, connected by single bonds. Their general formula is \(C_nH_{2n}\), which reflects the fact that there are two fewer hydrogen atoms than in an open-chain alkane with the same number of carbon atoms.

One common example of a cycloalkane is cyclohexane (\(C_6H_{12}\)), which consists of a six-membered carbon ring. Cycloalkanes can exhibit different conformations or shapes, such as 'chair' or 'boat' for cyclohexane, which can affect their chemical properties. Although they are saturated hydrocarbons like the alkanes, the ring structure introduces strain and therefore influences reactivity compared to their straight-chain counterparts.

Alkenes

Alkenes are characterized by the presence of at least one carbon-carbon double bond, which classifies them as unsaturated hydrocarbons. This double bond alters the general formula to \(C_nH_{2n}\), showing that they have fewer hydrogen atoms than alkanes.

The double bond in alkenes is a region of high electron density, making these compounds more reactive than alkanes. For instance, ethene (or ethylene, \(C_2H_4\)) can readily participate in addition reactions, where other atoms or groups add across the double bond. Alkenes play a vital role in the polymer industry; ethylene is used to make polyethylene, one of the most common plastics.

Alkynes

Alkynes are hydrocarbons with one or more carbon-carbon triple bonds, distinguishing them with a general formula of \(C_nH_{2n-2}\). The triple bond is composed of one sigma and two pi bonds, creating a linear geometry around the carbons.

The triple bond is even more reactive than the double bond found in alkenes, making alkynes capable of engaging in a variety of chemical reactions. A notable example of an alkyne is ethyne (also known as acetylene, \(C_2H_2\)), which is widely used in welding torches for its high combustion temperature. Alkynes are less common in nature than alkanes and alkenes but are important intermediates in synthetic organic chemistry.

Saturated Hydrocarbons

Saturated hydrocarbons, as the name suggests, are fully 'saturated' with hydrogen atoms, indicating that they contain only single bonds between carbon atoms. This group includes both alkanes and cycloalkanes, which share the key feature of having a maximum hydrogen to carbon ratio.

This saturation means these hydrocarbons are relatively nonreactive, stable, and are often used as solvents and in fuels. Saturated hydrocarbons are prevalent in various natural sources, such as crude oil, and are a key part of the energy sector where they are used as a source of heat and power.

Aromatic Hydrocarbons

Aromatic hydrocarbons comprise a unique class that contains a ring with delocalized electrons, resulting from alternating single and double bonds, known as conjugation. This structure imparts exceptional stability to the molecule due to the electron delocalization.

The most classic example of an aromatic hydrocarbon is benzene \((C_6H_6)\), with its six-carbon ring and alternating double bonds. This conjugation allows for the electrons to be shared across the entire ring, which is often represented by a circle inside the hexagonal structure of benzene. Aromatic hydrocarbons are widely used in chemical synthesis and are found in substances ranging from pharmaceuticals to dyes and plastics.