Problem 11 - Chemistry The Central Science

Question

For each of the following hydrocarbons, state how many carbon atoms are in each molecule: \begin{equation}\begin{array}{l}{\text { (a) methane }} \\ {\text { (b) decane }} \\ {\text { (c) } 2 \text { -methylhexane }} \\ {\text { (d) neopentane }} \\ {\text { (e) acetylene }}\end{array}\end{equation}

Step-by-Step Solution

Answer

There are \(1\) carbon atom in methane (a), \(10\) carbon atoms in decane (b), \(7\) carbon atoms in 2-methylhexane (c), \(5\) carbon atoms in neopentane (d), and \(2\) carbon atoms in acetylene (e).

Step 1: (a) Methane

Methane is the simplest hydrocarbon, and its name comes from the prefix "meth-" indicating that there is only one carbon atom, and the suffix "-ane" indicating that it is an alkane. Therefore, there is \(1\) carbon atom in methane.

Step 2: (b) Decane

For decane, the prefix "dec-" indicates that the hydrocarbon contains 10 carbon atoms, and the suffix "-ane" indicates that it is an alkane. Therefore, there are \(10\) carbon atoms in decane.

Step 3: (c) 2-Methylhexane

In this case, the name of the hydrocarbon is descriptive of its structure. The prefix "hex-" indicates that the main chain contains 6 carbon atoms. The "2-methyl" part of the name tells us that at position 2 on the main chain, there is a methyl group (which is a carbon atom with three hydrogen atoms) attached. Therefore, the total number of carbon atoms in 2-methylhexane is \(6 + 1 = 7\).

Step 4: (d) Neopentane

Neopentane is also known as 2,2-dimethylpropane. The prefix "prop-" indicates that the main chain contains 3 carbon atoms. "2,2-dimethyl" tells us that there are two methyl groups (each representing a single carbon atom) attached to the main chain at position 2. Therefore, the total number of carbon atoms in neopentane is \(3 + 2 = 5\).

Step 5: (e) Acetylene

Acetylene, also known as ethyne, is an alkyne (a hydrocarbon containing a carbon-carbon triple bond). The prefix "eth-" in "ethyne" indicates that there are two carbon atoms in the molecule, and the suffix "-yne" indicates that it is an alkyne. Therefore, there are \(2\) carbon atoms in acetylene.

Key Concepts

Carbon Atom Counting

Counting carbon atoms in hydrocarbons is a fundamental skill in organic chemistry. Each syllable and prefix in a hydrocarbon's name carries specific information about its structure. For instance, prefixes in the IUPAC naming system are crucial for identifying the number of carbon atoms in the main chain of a hydrocarbon.

When a hydrocarbon name includes the prefix "meth-", like in methane, it signifies a single carbon atom. Similarly, the prefix "eth-" denotes two carbon atoms, as seen in ethyne (acetylene).

• "Meth-" implies 1 carbon
• "Eth-" implies 2 carbons
• "Prop-" signifies 3 carbons
• "But-" is used for 4 carbons
• "Pent-" indicates 5 carbons
• Other prefixes represent higher numbers, such as "dec-", which signifies 10 carbons in decane.

In more complex molecules, like 2-methylhexane, extra names such as "methyl" may suggest additional side chains. The main chain length and these side groups help ascertain the total carbon count in the molecule.

Alkanes

Alkanes are the simplest class of hydrocarbons. They consist entirely of single bonds and are often referred to as saturated hydrocarbons. The suffix "-ane" in a hydrocarbon's name indicates that it is an alkane.

An alkane's formula follows the general expression \( C_nH_{2n+2} \). Here, \( n \) is the number of carbon atoms in the molecule. For example, if \( n = 2 \), the alkane would be ethane, with the molecular formula \( C_2H_6 \).

• Methane (\( CH_4 \)) is the simplest alkane with one carbon atom.
• Decane has 10 carbons, forming a larger alkane molecule.

Understanding and identifying alkanes helps in recognizing their uncomplicated structure, which consists of linear or branched chains, such as in neopentane, better known as 2,2-dimethylpropane.

Alkynes

Alkynes are a class of hydrocarbons characterized by the presence of at least one carbon-carbon triple bond. This feature makes them unsaturated, as they contain fewer hydrogen atoms compared to alkanes or alkenes. The suffix "-yne" is indicative of an alkyne.

In the context of molecular formulas, the general pattern for alkynes is \( C_nH_{2n-2} \). For instance, if \( n = 2 \), ethyne (commonly called acetylene) has the chemical formula \( C_2H_2 \). This reflects the triple bond's presence, which creates a strong bond between the two carbon atoms.

• Alkynes, having triple bonds, differ distinctly from alkanes which only have single bonds.
• The presence of a "-yne" suffix always signifies a triple bond in the molecule.

Acetylene is a simple alkyne and serves as a foundational model for understanding more complex alkyne structures.