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

The structural features that help identify hydrocarbons are: (a) Alkane: Presence of only single C-C bonds, general formula \(C_nH_{2n+2}\). (b) Cycloalkane: Closed ring structure with only single C-C bonds, general formula \(C_nH_{2n}\). (c) Alkene: At least one C=C bond, general formula \(C_nH_{2n}\). (d) Alkyne: At least one C≡C bond, general formula \(C_nH_{2n-2}\). (e) Saturated hydrocarbon: Only single C-C bonds and absence of double/triple bonds (e.g., alkanes, cycloalkanes). (f) Aromatic hydrocarbon: Flat ring structure with alternating single and double bonds following the Hückel rule (4n+2 π electrons).

Step-by-step solution

1. Identifying Alkane

An alkane is a hydrocarbon in which all carbon-carbon (C-C) bonds are single bonds. It has the general formula \(C_nH_{2n+2}\), where n is the number of carbons. To identify an alkane, look for the presence of only single C-C bonds and a corresponding number of hydrogen atoms.

2. Identifying Cycloalkane

A cycloalkane is a hydrocarbon in which the carbon atoms are arranged in a closed ring structure, and all C-C bonds in the ring are single bonds. Its general formula is \(C_nH_{2n}\), where n is the number of carbon atoms in the ring. To identify a cycloalkane, look for a closed ring structure containing only single C-C bonds.

3. Identifying Alkene

An alkene is a hydrocarbon that has at least one carbon-carbon double bond (C=C). Its general formula is \(C_nH_{2n}\), where n is the number of carbon atoms. To identify an alkene, look for the presence of a C=C bond in the molecule.

4. Identifying Alkyne

An alkyne is a hydrocarbon that has at least one carbon-carbon triple bond (C≡C). Its general formula is \(C_nH_{2n-2}\), where n is the number of carbon atoms. To identify an alkyne, look for the presence of a C≡C bond in the molecule.

5. Identifying Saturated Hydrocarbon

A saturated hydrocarbon is a hydrocarbon in which all carbon-carbon bonds are single bonds, and there are no double or triple bonds. Alkanes and cycloalkanes are examples of saturated hydrocarbons. To identify a saturated hydrocarbon, look for the presence of only single C-C bonds and an absence of double or triple C-C bonds.

6. Identifying Aromatic Hydrocarbon

An aromatic hydrocarbon is a hydrocarbon with a planar (flat) ring structure containing alternating single and double bonds, which follow the Hückel rule of having 4n+2 π electrons in the ring, where n is a whole number. The most common example of an aromatic hydrocarbon is benzene, with the formula \(C_6H_6\). To identify an aromatic hydrocarbon, look for a flat ring structure with alternating single and double bonds that follow the Hückel rule.