Question

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

Short answer

To identify the structural features of various hydrocarbons, consider the following: (a) Alkanes have only single bonds between carbon atoms and follow the general formula CnH2n+2, e.g., methane (CH4). (b) Cycloalkanes are ring-structured alkanes with the general formula CnH2n, e.g., cyclohexane (C6H12). (c) Alkenes contain a carbon-carbon double bond and follow the general formula CnH2n, e.g., ethene (C2H4). (d) Alkynes contain a carbon-carbon triple bond and follow the general formula CnH2n-2, e.g., ethyne (C2H2). (e) Saturated hydrocarbons have only single bonds between carbon atoms, including alkanes and cycloalkanes, e.g., methane (CH4) and cyclohexane (C6H12). (f) Aromatic hydrocarbons possess a planar ring of carbon atoms with delocalized electrons, e.g., benzene (C6H6), which has alternating single and double bonds.

Step-by-step solution

(a) Identifying an alkane

Alkanes are hydrocarbons that only contain single bonds between carbon atoms. They can be in the form of chains or rings. These molecules can be represented by the general formula CnH2n+2, where n is the number of carbon atoms in the molecule. An example of an alkane is methane (CH4).

(b) Identifying a cycloalkane

Cycloalkanes are alkanes that have their carbon atoms connected in rings instead of linear chains. They have the general formula of CnH2n, where n is the number of carbon atoms in the ring. An example of a cycloalkane is cyclohexane (C6H12).

(c) Identifying an alkene

Alkenes are hydrocarbons containing a carbon-carbon double bond. Their general formula is CnH2n, where n is the number of carbon atoms in the molecule. An example of an alkene is ethene (C2H4), also known as ethylene.

(d) Identifying an alkyne

Alkynes are hydrocarbons containing a carbon-carbon triple bond. Their general formula is CnH2n-2, where n is the number of carbon atoms in the molecule. An example of an alkyne is ethyne (C2H2), also known as acetylene.

(e) Identifying a saturated hydrocarbon

Saturated hydrocarbons are hydrocarbons that only contain single bonds between carbon atoms. These include both alkanes and cycloalkanes. Examples of saturated hydrocarbons are methane (CH4) and cyclohexane (C6H12).

(f) Identifying an aromatic hydrocarbon

Aromatic hydrocarbons are hydrocarbons that contain a planar ring of carbon atoms, with a delocalized system of electrons. The most common example is benzene (C6H6), where the six carbon atoms form a ring, and the delocalized electrons are shared evenly among the carbon atoms, creating alternating single and double bonds.

Key concepts

Alkanes

Alkanes are often the simplest form of hydrocarbons, as they consist only of carbon and hydrogen atoms with single bonds linking each carbon atom. This specific structure allows alkanes to be known as "saturated hydrocarbons." The reason they're called saturated is that each carbon atom is bonded to as many hydrogen atoms as possible. These molecules can take the form of straight chains or even branched chains.

For example, methane (\(CH_4\)) and ethane (\(C_2H_6\)) are both simple alkanes. The general formula for alkanes is \(C_nH_{2n+2}\), where \(n\) represents the number of carbon atoms.

• Single carbon-carbon bonds
• Saturated structure
• General formula \(C_nH_{2n+2}\)

Keep in mind, alkanes are relatively unreactive due to the stability of their single bonds.

Alkenes

Alkenes distinguish themselves from alkanes by possessing at least one carbon-carbon double bond. This double bond introduces unsaturation into the molecule, making alkenes more reactive than their alkane counterparts.

While alkenes also consist of carbon and hydrogen atoms, their general formula is \(C_nH_{2n}\), meaning they have fewer hydrogen atoms compared to alkanes for the same number of carbon atoms.

An example is ethene (\(C_2H_4\)), one of the simplest alkenes. Alkenes are often the starting material for the production of addition polymers like polyethylene.

• Presence of carbon-carbon double bond
• Reactive due to unsaturation
• General formula \(C_nH_{2n}\)

The double bond provides alkenes with unique chemical properties, enabling reactions like hydrogenation and polymerization.

Alkynes

Alkynes are yet another form of hydrocarbons characterized by their carbon-carbon triple bonds. This triple bond introduces even more unsaturation than the double bond in alkenes, making alkynes generally more reactive.

In terms of chemical formula, alkynes are represented by \(C_nH_{2n-2}\), which clearly shows the reduction in hydrogen atoms as compared to both alkanes and alkenes.

• Presence of carbon-carbon triple bond
• Even more reactive due to high unsaturation
• General formula \(C_nH_{2n-2}\)

An example of an alkyne is ethyne (\(C_2H_2\)), commonly known as acetylene, which is often used in welding operations.

Saturated Hydrocarbons

Saturated hydrocarbons, as their name suggests, contain only single bonds between carbon atoms. They are essentially the same as alkanes and include cyclic variations known as cycloalkanes.

Since these hydrocarbons have no multiple bonds, they are relatively stable and less chemically reactive. Examples include methane (\(CH_4\)) and cyclohexane (\(C_6H_{12}\)).

• Exclusive presence of single bonds
• High stability, low reactivity
• Include both alkane and cycloalkane forms

Saturated hydrocarbons are broadly used as fuels due to their stability, burning to produce energy and carbon dioxide.

Aromatic Hydrocarbons

Aromatic hydrocarbons are distinguished by their ring structure and a unique electron delocalization within the ring system. This means that the electrons are not confined to single bonds but are free to circulate around the ring, creating a stable configuration.

The most well-known aromatic hydrocarbon is benzene (\(C_6H_6\)), which has a hexagonal ring with hydrogen atoms anchored to each carbon. This structure presents alternating single and double bonds, adding to its stability and reducing its reactivity.

• Closed ring structure
• Electron delocalization
• Examples include benzene

Aromatic hydrocarbons are crucial in the manufacturing of dyes, plastics, and pharmaceuticals due to their stable and versatile chemical properties.