Unit 7: Drawing & Representing Compounds
Understanding the different conventions for representing the structure of organic molecules.
7.3 Displayed Formulae
The displayed formula (or graphical formula) is the most detailed representation of a molecule. It shows all the atoms and all the covalent bonds connecting them. Each bond is represented by a line.
This type of formula is very clear and unambiguous, showing exactly how the atoms are connected. However, it can be time-consuming to draw for larger molecules.
Example: Displayed formula for propane ($C_3H_8$)
Solved Examples:
- What is the main feature of a displayed formula?
Solution: It shows every single atom and every single covalent bond. - Draw the displayed formula for ethane ($C_2H_6$).
Solution: A two-carbon chain with each carbon bonded to three hydrogens and to the other carbon. - How many bonds are shown around each carbon atom in a displayed formula of an alkane?
Solution: Four single bonds. - Draw the displayed formula for ethene ($C_2H_4$).
Solution: A two-carbon chain connected by a double bond, with each carbon also bonded to two hydrogens. - What is a disadvantage of using displayed formulae?
Solution: They can be large and cumbersome to draw for complex molecules. - Draw the displayed formula for methanol ($CH_3OH$).
Solution: A carbon atom bonded to three hydrogen atoms and one oxygen atom, which is in turn bonded to one hydrogen atom. - How is a double bond represented in a displayed formula?
Solution: With two parallel lines between the atoms. - Draw the displayed formula for 1-chloropropane ($CH_3CH_2CH_2Cl$).
Solution: A three-carbon chain, with a chlorine atom on one of the end carbons, and all other bonds filled with hydrogens. - Does a displayed formula show the 3D shape of a molecule?
Solution: No, it is a 2D representation and does not typically show the correct bond angles or molecular geometry. - How many hydrogen atoms would be in the displayed formula of butane ($C_4H_{10}$)?
Solution: Ten.
7.4 Structural Formulae
The structural formula is a condensed way of representing a molecule. It shows the atoms in the order they are bonded but omits most or all of the bond lines. It provides enough information to show the structure without being as cluttered as a displayed formula.
- Unbranched chains: Written as a sequence, e.g., butane is $CH_3CH_2CH_2CH_3$. This can be further condensed to $CH_3(CH_2)_2CH_3$.
- Branched chains: Branches are shown in parentheses after the carbon atom they are attached to. For example, methylpropane is $CH_3CH(CH_3)CH_3$.
- Functional groups: Double bonds are shown explicitly. Other groups like -OH or -Cl are written next to the carbon they are attached to.
Solved Examples:
- Write the structural formula for pentane.
Solution: $CH_3CH_2CH_2CH_2CH_3$ or $CH_3(CH_2)_3CH_3$. - What does the notation $(CH_3)$ mean in $CH_3CH(CH_3)CH_3$?
Solution: It indicates a methyl ($CH_3$) group that is a branch attached to the preceding carbon atom. - Write the structural formula for propan-2-ol.
Solution: $CH_3CH(OH)CH_3$. - Convert the displayed formula of ethene into a structural formula.
Solution: $CH_2=CH_2$. - What is the structural formula for 2,2-dimethylpropane?
Solution: $C(CH_3)_4$. - Write the structural formula for but-2-ene.
Solution: $CH_3CH=CHCH_3$. - What is the main advantage of a structural formula over a displayed formula?
Solution: It is more compact and quicker to write while still showing the connectivity of the atoms. - Write the structural formula for 1,2-dichloroethane.
Solution: $CH_2ClCH_2Cl$. - How is a triple bond shown in a structural formula?
Solution: With a triple bond symbol (≡), for example, ethyne is $CH≡CH$. - From the structural formula $CH_3CH_2CH(CH_3)CH_3$, what is the longest carbon chain?
Solution: The longest chain has four carbons (butane), with a methyl group on the second carbon.
7.5 Skeletal Formulae
The skeletal formula (or stick formula) is a highly simplified representation used extensively in organic chemistry.
Rules for Drawing Skeletal Formulae:
- Carbon atoms are not shown. They are represented by the vertices (corners) and ends of lines.
- Hydrogen atoms attached to carbon atoms are not shown. It is assumed that each carbon atom is bonded to enough hydrogens to give it a total of four bonds.
- All other atoms (heteroatoms like O, N, Cl) and any hydrogens attached to them are shown.
- Bonds are drawn as lines, with correct representations for double and triple bonds. The lines are drawn in a zig-zag pattern to represent the tetrahedral bond angles of carbon.
Example: Skeletal formula for pentan-2-ol, $CH_3CH(OH)CH_2CH_2CH_3$
Solved Examples:
- What does a vertex (corner) represent in a skeletal formula?
Solution: A carbon atom. - Draw the skeletal formula for hexane.
Solution: A zig-zag line with six carbon positions (five vertices and one end). - How are hydrogen atoms represented in a skeletal formula?
Solution: Hydrogen atoms bonded to carbons are not shown at all. Hydrogens bonded to other elements (like oxygen) are shown. - Draw the skeletal formula for but-2-ene.
Solution: A four-carbon zig-zag with a double bond between the second and third carbons. - What is the main advantage of skeletal formulae?
Solution: They are very quick and easy to draw, especially for large, complex molecules and ring structures. - Identify the molecule represented by a simple square in skeletal notation.
Solution: Cyclobutane. - How many hydrogen atoms are on the second carbon of the skeletal structure for pentane?
Solution: Two. The carbon is bonded to two other carbons, so it must have two hydrogens to make four bonds. - Draw the skeletal formula for 2-methylbutane.
Solution: A four-carbon zig-zag chain with a single line (a methyl group) branching off from the second carbon. - What atoms must always be explicitly shown in a skeletal formula?
Solution: Any atom that is not carbon, and any hydrogen atom attached to a non-carbon atom. - Draw the skeletal formula for cyclohexene.
Solution: A hexagon with one double bond.
7.6 Molecular and Empirical Formulae
Molecular Formula
The molecular formula shows the actual number of atoms of each element in one molecule of a compound. It provides no information about the structure or bonding.
Example: The molecular formula for both butane and methylpropane is $C_4H_{10}$.
Empirical Formula
The empirical formula shows the simplest whole-number ratio of atoms of each element in a compound.
Example: The molecular formula of ethene is $C_2H_4$. The simplest ratio is 1:2, so its empirical formula is $CH_2$. The molecular formula of propene is $C_3H_6$. Its empirical formula is also $CH_2$.
Different compounds can have the same empirical formula. The molecular formula is always a whole-number multiple of the empirical formula.
Solved Examples:
- What is the molecular formula of hexane?
Solution: $C_6H_{14}$. - What is the empirical formula of hexane ($C_6H_{14}$)?
Solution: The ratio is 6:14, which simplifies to 3:7. The empirical formula is $C_3H_7$. - A molecule has an empirical formula of $CH_2$ and a molar mass of 42 g/mol. What is its molecular formula? (Ar C=12, H=1)
Solution: The mass of the empirical formula unit ($CH_2$) is 12 + 2(1) = 14 g/mol. The multiple is 42 / 14 = 3. The molecular formula is $(CH_2)_3$, which is $C_3H_6$. - What is the main limitation of a molecular formula?
Solution: It does not show the structure of the molecule. Different molecules (isomers) can have the same molecular formula. - What is the empirical formula of glucose, $C_6H_{12}O_6$?
Solution: The ratio is 6:12:6, which simplifies to 1:2:1. The empirical formula is $CH_2O$. - Can a molecule have the same molecular and empirical formula?
Solution: Yes. For example, the molecular formula of propane is $C_3H_8$. The ratio 3:8 cannot be simplified, so its empirical formula is also $C_3H_8$. - What is the molecular formula for the molecule represented by the structural formula $CH_3CH(OH)CH_3$?
Solution: Count the atoms: 3 carbons, 8 hydrogens, 1 oxygen. The molecular formula is $C_3H_8O$. - Give the empirical formula for ethyne, $C_2H_2$.
Solution: $CH$. - Which type of formula is determined directly from percentage composition data?
Solution: The empirical formula. - Two compounds have the molecular formula $C_2H_6O$. What is the relationship between them?
Solution: They are structural isomers (ethanol, $CH_3CH_2OH$, and methoxymethane, $CH_3OCH_3$).