Question

Based on the molecular formula, determine whether each compound is an alkane, alkene, or alkyne. (Assume that the hydrocarbons are non- cyclic and there is no more than one multiple bond.) \begin{equation}\begin{array}{llllll}{\text { a. }} & {\mathrm{C}_{8} \mathrm{H}_{16}} & {\text { b. } \mathrm{C}_{4} \mathrm{H}_{6}} & {\text { c. }} & {\mathrm{C}_{7} \mathrm{H}_{16}} & {\text { d. }} & {\mathrm{C}_{2} \mathrm{H}_{2}}\end{array}\end{equation}

Short answer

a. Alkene, b. Alkyne, c. Alkane, d. Alkyne

Step-by-step solution

- Use the General Formula for Alkanes

The general formula for alkanes is \(C_nH_{2n+2}\). To test if each compound is an alkane, substitute the number of carbon atoms (n) into this formula and see if it matches the provided hydrocarbon.

- Check for Alkenes Using their Formula

The general molecular formula for alkenes is \(C_nH_{2n}\). This formula accounts for one double bond in the molecule. If the molecular formula of the compound matches this pattern, it is an alkene.

- Identify Alkynes with their Characteristic Formula

Alkynes have a molecular formula of \(C_nH_{2n-2}\), representing one triple bond within the molecule. Compare each compound's molecular formula with this pattern to determine if it is an alkyne.

- Determine the Compound Classes

Apply the steps above to classify each provided compound:\ (a) \(C_8H_{16}\) fits the formula for alkenes (\(C_nH_{2n}\)).\ (b) \(C_4H_6\) fits the formula for alkynes (\(C_nH_{2n-2}\)).\ (c) \(C_7H_{16}\) follows the alkane formula (\(C_nH_{2n+2}\)).\ (d) \(C_2H_2\) matches the alkyne formula (\(C_nH_{2n-2}\)).

Key concepts

Alkane General Formula

Understanding the backbone of organic chemistry starts with alkanes, which are the simplest hydrocarbons consisting solely of carbon (C) and hydrogen (H) atoms connected by single bonds. When we move to write down the general formula for alkanes, we arrive at a simple yet potent equation: \( C_nH_{2n+2} \). This formula gives us the means to quickly determine if a molecule falls into the alkane category.

For example, consider a hydrocarbon compound with seven carbon atoms. Plugging in '7' for 'n' in the alkane formula, we get \( C_7H_{16} \), a perfect match for an alkane's molecular structure. It's crucial to note that alkanes are saturated hydrocarbons, meaning they have the maximum number of hydrogen atoms possible on their carbon skeleton. This maximization of hydrogen presence makes alkanes relatively less reactive compared to their unsaturated counterparts - alkenes and alkynes, which have double and triple bonds, respectively.

Alkene Molecular Formula

Moving along the hydrocarbon spectrum, we encounter alkenes, which intrigue chemists with their characteristic double bonds. Their general molecular formula is written as \( C_nH_{2n} \), and it signifies the presence of exactly one double bond within the molecule. This subtle structural change has profound implications, leading to fewer hydrogen atoms than in an alkane.

To illustrate, let's look at a molecule with eight carbon atoms. Applying the alkene formula, we get \( C_8H_{16} \), thereby classifying it under the realm of alkenes. This is also where the notion of 'unsaturation' seeps in; alkenes have two fewer hydrogen atoms than the corresponding alkanes, setting the stage for a varied range of chemical reactions, thanks to that double bond's presence.

Alkyne Identification

Alkynes mark their territory in the hydrocarbon family with the distinct presence of a carbon-carbon triple bond. This defining feature is mirrored in their general molecular formula, \( C_nH_{2n-2} \), hinting at even fewer hydrogen atoms than alkenes.

Take a molecule with two carbon atoms; plug those numbers into the alkyne formula and you end up with \( C_2H_2 \), a textbook example of an alkyne. Identifying an alkyne is therefore straightforward - the number of hydrogen atoms is precisely two less than double the number of carbon atoms. This shortage of hydrogen introduces a high degree of unsaturation, rendering alkynes exceptionally reactive and versatile for synthetic applications.