A Level Chemistry OCR Practice Exam

What is an unsaturated hydrocarbon?

1. A hydrocarbon with only single bonds

2. A hydrocarbon containing carbon-to-carbon multiple bonds

3. A hydrocarbon with no functional groups

4. A hydrocarbon that is gaseous at room temperature

The correct answer is: A hydrocarbon containing carbon-to-carbon multiple bonds

An unsaturated hydrocarbon is defined as a hydrocarbon that contains carbon-to-carbon multiple bonds, which can either be double or triple bonds. These types of bonds result in fewer hydrogen atoms attached to the carbon framework compared to saturated hydrocarbons, which only contain single bonds and are fully saturated with hydrogen atoms. The presence of these multiple bonds also influences the chemical reactivity of unsaturated hydrocarbons, making them more reactive than their saturated counterparts. This is important in organic chemistry, as unsaturation allows for various reactions, such as addition reactions, where new atoms can be added to the molecule. In contrast, hydrocarbons characterized by only single bonds, with no multiple bonds, are termed saturated. Thus, options relating to single bonds or descriptions such as entirely gaseous at room temperature do not define unsaturation accurately, nor do they consider the reactivity associated with multiple bonds. Additionally, the description of having no functional groups does not pertain to the unsaturation concept, as hydrocarbons can be classified as unsaturated regardless of the presence of functional groups.