Question

Hydrocarbons are often represented by line drawings. How is it possible to look at a line drawing and deduce the molecular formula of the compound?

Short answer

To deduce the molecular formula from a hydrocarbon's line drawing or skeletal structure, follow these steps: 1. Count the number of carbon atoms in the drawing, represented by vertices or line intersections. 2. Determine the number of hydrogen atoms for each carbon by recognizing the missing bonds it has; each carbon atom forms a total of four bonds. 3. Combine the number of carbon and hydrogen atoms to create the molecular formula. Hydrocarbons are organic compounds with carbon (C) and hydrogen (H) atoms, and their molecular formula indicates the number of each atom present in the molecule. In line drawings, carbon atoms are represented by vertices or intersections of lines, while hydrogen atoms are implied and covalent bonds are represented by lines connecting the atoms.

Step-by-step solution

Understanding hydrocarbons and their molecular formulas

Hydrocarbons are organic compounds consisting of carbon (C) and hydrogen (H) atoms. The molecular formula indicates the number of each type of atom present in the molecule. The most basic hydrocarbons are called alkanes, with the molecular formula CnH2n+2, where 'n' represents the number of carbon atoms. For example, methane (CH4) has one carbon atom and four hydrogen atoms, while ethane (C2H6) has two carbon atoms and six hydrogen atoms.

Understanding line drawings and skeletal structures

In line drawings or skeletal structures, carbon atoms are represented by the vertices of the lines and the intersections of two lines, while hydrogen atoms are not shown explicitly but implied. Each line represents a covalent bond formed between two carbon atoms or between a carbon and a hydrogen atom. For example, here's the line drawing of propane (C3H8): ``` H H H | | | H--C-C-C--H | | | H H H ```

Counting carbon atoms in the line drawing

In order to deduce the molecular formula from the hydrocarbon's skeletal structure, start by counting the number of carbon atoms in the drawing. Each vertex or intersection of two lines represents one carbon atom. Using the propane example, there are three carbon atoms at the vertices of the three connected lines: ``` H H H | | | H--C-C-C--H | | | H H H ```

Deduce the number of hydrogen atoms

Each carbon atom forms four bonds. In a line drawing, these bonds are formed with either neighboring carbon atoms or implied hydrogen atoms. Deduce the number of hydrogen atoms for each carbon by recognizing the number of missing bonds it has. For example, in propane, we can see that: 1. The first carbon (C1) has one bond with a neighboring carbon (C2); thus, it forms three bonds with three hydrogen atoms. 2. The second carbon (C2) has two bonds with neighboring carbons (C1 and C3); thus, it forms two bonds with two hydrogen atoms. 3. The third carbon (C3) has one bond with a neighboring carbon (C2); thus, it forms three bonds with three hydrogen atoms. In total, there are eight hydrogen atoms present in the propane line drawing: ``` H H H | | | H--C-C-C--H | | | H H H ```

Determine the molecular formula

Combine the number of carbon and hydrogen atoms counted in the above steps to create the molecular formula. In the case of propane, the molecular formula would be C3H8. By following these steps, you can determine the molecular formula for a hydrocarbon compound from its line drawing or skeletal structure.

Key concepts

Understanding Hydrocarbons

Hydrocarbons are fundamental organic compounds consisting solely of carbon (C) and hydrogen (H) atoms. These molecules form the basis of organic chemistry and are primarily classified by the types of bonds between carbon atoms. They range from simple structures like methane, with a single carbon atom, to complex polymers and long-chain molecules found in fuels, plastics, and natural materials like plant oils and waxes.

One of the main family of hydrocarbons are the alkanes, known for their single (sigma) bonds between carbon atoms, making them saturated hydrocarbons. Alkanes have a straightforward and systematic naming convention based on the number of carbon atoms present, and they adhere to a general molecular formula.

Line Drawing Chemical Structures

Line drawings, also known as structural formulas, are a simplified representation of molecules used extensively in organic chemistry. They provide a clear and concise way to visualize the structure of a compound. Each line represents a bond between atoms, and each intersection or terminus of lines corresponds to a carbon atom.

• To determine the hydrogen atoms, one must consider that carbon forms four bonds
• Implied hydrogen atoms are deduced from the number of existing bonds to carbon
• Double and triple bonds are represented by double and triple lines, respectively

Learning to interpret these drawings is a fundamental skill for anyone studying chemistry, as it allows the visualization of molecules without drawing all the atoms explicitly.

Skeletal Structures of Organic Compounds

Skeletal structures are a more stylised version of line drawings, where the focus is on the framework of carbon atoms within the molecule. This form of representation omits the hydrogen atoms bonded to carbon, as well as the carbon atoms themselves, assuming that each vertex or end of a line is a carbon atom. Other elements are generally drawn, though. This method greatly simplifies complex organic structures and is particularly useful for quickly sketching molecules with a pen and paper.

• The skeleton shows the arrangement of carbon atoms
• Hydrogen atoms are assumed to fill in the remaining bonds to carbon
• Functional groups and heteroatoms (atoms other than carbon and hydrogen) are explicitly drawn

The skeletal structure technique is very descriptive despite its simplicity and requires practice to master effectively.

Alkanes Molecular Formula

Alkanes are the simplest form of hydrocarbons and follow a distinct molecular formula, which is designated as \( C_nH_{2n+2} \). Here, 'n' represents the number of carbon atoms, and the formula reflects that each carbon atom forms four bonds. For alkanes in a straight chain, this means two of the bonds are to adjacent carbon atoms (except for the terminal carbons), and the remaining bonds are to hydrogen atoms. For example, the smallest alkane, methane, has one carbon atom and four hydrogen atoms, resulting in the molecular formula CH4. Ethane follows, with a molecular formula of C2H6, and the pattern continues predictably through larger alkanes such as propane (C3H8) and butane (C4H10).

• In line drawings, the end carbon atoms will have three hydrogen bonds
• The middle carbon atoms will have two hydrogen bonds
• Ring structures or branches within a molecule alter the hydrogen count accordingly

Understanding this formula and its implications allows chemists to quickly determine the number of hydrogen atoms in an alkane and to visualize or sketch its structure.