Question

Indicate how alkanes, alkenes, and alkynes differ by giving the structural formula of a compound in each class that contains three carbon atoms.

Short answer

Alkanes: propane (CH₃-CH₂-CH₃); alkenes: propene (CH₂=CH-CH₃); alkynes: propyne (CH≡C-CH₃).

Step-by-step solution

Define Alkanes

Alkanes are hydrocarbons containing only single covalent bonds between carbon atoms. The general formula for alkanes is \(C_nH_{2n+2}\). For three carbon atoms, the alkane is propane with the formula \(C_3H_8\). Its structural formula is CH₃-CH₂-CH₃.

Define Alkenes

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. The general formula for alkenes is \(C_nH_{2n}\). For three carbon atoms, the alkene is propene with the formula \(C_3H_6\). Its structural formula is CH₂=CH-CH₃.

Define Alkynes

Alkynes are hydrocarbons with at least one carbon-carbon triple bond. The general formula for alkynes is \(C_nH_{2n-2}\). For three carbon atoms, the alkyne is propyne with the formula \(C_3H_4\). Its structural formula is CH≡C-CH₃.

Key concepts

Alkanes

Alkanes are a type of hydrocarbon, which means they are organic compounds consisting solely of carbon and hydrogen atoms. These compounds are characterized by having single covalent bonds between the carbon atoms in their structures.

• The general formula for alkanes is \(C_nH_{2n+2}\), where \(n\) represents the number of carbon atoms.
• This formula works because each carbon atom forms four single covalent bonds, typically with hydrogen, to fill its valency.
• An example of an alkane with three carbon atoms is propane with the chemical formula \(C_3H_8\).
• The structural formula for propane is CH₃-CH₂-CH₃.

Alkanes are also known as saturated hydrocarbons because all carbon bonds are single, meaning they hold as many hydrogen atoms as possible. This saturation makes them generally less reactive than other types of hydrocarbons, like alkenes or alkynes.

Alkenes

Alkenes are also hydrocarbons, but unlike alkanes, they contain at least one double bond between carbon atoms. This double bond significantly affects the chemical properties of the compound.

• The general formula for alkenes is \(C_nH_{2n}\), indicating fewer hydrogen atoms compared to alkanes of the same carbon length due to the presence of a double bond.
• For example, propene is an alkene with three carbon atoms, and its formula is \(C_3H_6\).
• The structural formula is CH₂=CH-CH₃, showing the presence of a double bond.

Because of the double bond, alkenes are known as unsaturated hydrocarbons. This unsaturation allows for reactions such as addition, where atoms can add across the double bond, breaking it to form single bonds. This makes alkenes more chemically reactive than their alkane counterparts.

Alkynes

Alkynes are a class of hydrocarbons that contain at least one triple bond between carbon atoms. This triple bond gives alkynes unique chemical properties.

• The general formula for alkynes is \(C_nH_{2n-2}\), reflecting even fewer hydrogen atoms due to the presence of a triple bond.
• An example of an alkyne with three carbon atoms is propyne, which has the formula \(C_3H_4\).
• The structural formula for propyne is CH≡C-CH₃.

Alkynes are also considered unsaturated hydrocarbons, similar to alkenes, but with even greater reactivity due to the triple bond. This bond can undergo a variety of reactions, including additions, which are key in many chemical processes and industrial applications.