Different Types of Formulae

Introduction to Organic Chemistry

What are the two main ways of referring to organic compounds?

By their formulae or by their names.

What are the five main types of formula used for organic compounds?

Displayed formula, structural formula, skeletal formula, molecular formula and empirical formula.

Define a displayed formula.

A formula that shows every atom and every bond in a molecule.

What information does a displayed formula show?

Every atom and every bond separately.

Why are displayed formulae useful?

They show exactly how all the atoms are bonded together.

What is an important exam rule when asked for a displayed formula?

Show every atom and every bond; a skeletal formula will not gain full marks.

Define a structural formula.

A formula that shows unambiguously how the atoms in a molecule are joined together.

How does a structural formula simplify a displayed formula?

Atoms bonded to each carbon are grouped together instead of showing every individual bond.

What is the structural formula of butane?

CH₃CH₂CH₂CH₃.

Can bonds between carbon atoms be shown in a structural formula?

Yes. For example, butane can be written as CH₃–CH₂–CH₂–CH₃.

What does the word unambiguously mean when describing a structural formula?

The formula makes it clear exactly how the atoms are connected.

Define a skeletal formula.

A formula that shows the bonds between carbon atoms as lines, with carbon atoms and hydrogens attached to carbon not normally shown.

How are carbon atoms represented in a skeletal formula?

Each line ending and each junction or change in direction represents a carbon atom.

Are carbon symbols normally shown in skeletal formulae?

No.

Are hydrogen atoms attached to carbon normally shown in skeletal formulae?

No.

How do you determine the number of hydrogens attached to a carbon in a skeletal formula?

Add enough hydrogen atoms so that the carbon forms four bonds in total.

Are atoms other than carbon and hydrogen shown in skeletal formulae?

Yes.

What does each line in a skeletal formula represent?

A covalent bond.

How can you count carbon atoms in a skeletal formula?

Count every line end and every junction or vertex that represents a carbon.

What do the two ends of the skeletal formula of butane represent?

CH₃ groups.

What do the two internal junctions in the skeletal formula of butane represent?

CH₂ groups.

Why is a skeletal formula useful?

It provides a simpler way of representing the structure of an organic molecule.

Define a molecular formula.

A formula that shows the actual number of atoms of each element in a molecule.

Does a molecular formula show how the atoms are joined together?

No, it only shows the number of atoms of each element.

What is the molecular formula of butane?

C₄H₁₀.

Why can one molecular formula sometimes represent more than one compound?

Different compounds can contain the same numbers of each type of atom but have the atoms arranged differently.

What is the molecular formula of chloroethane?

C₂H₅Cl.

Define an empirical formula.

A formula that shows the numbers of each type of atom in the simplest whole-number ratio.

How is an empirical formula obtained from a molecular formula?

Divide all the subscripts by their highest common factor.

What is the empirical formula of C₄H₁₀?

C₂H₅.

Why is the empirical formula of butane C₂H₅ rather than C₄H₁₀?

The ratio 4:10 can be simplified to 2:5.

What is the empirical formula of C₂H₆?

CH₃.

What is the empirical formula of C₆H₁₂O₆?

CH₂O.

What is the empirical formula of C₂H₅Cl?

C₂H₅Cl.

Why are the molecular and empirical formulae of chloroethane the same?

The subscripts 2:5:1 cannot be simplified to a smaller whole-number ratio.

When are the molecular formula and empirical formula identical?

When the numbers of atoms in the molecular formula have no common factor greater than 1.

What does the word empirical mean in chemistry?

As found from practical evidence or experimental results.

What is the difference between a displayed formula and a structural formula?

A displayed formula shows every atom and every bond, whereas a structural formula groups atoms together while still showing unambiguously how they are connected.

What is the difference between a molecular formula and an empirical formula?

A molecular formula gives the actual numbers of each type of atom, whereas an empirical formula gives their simplest whole-number ratio.

What is the difference between a molecular formula and a structural formula?

A molecular formula only gives the numbers of each type of atom, whereas a structural formula shows how the atoms are connected.

What is the difference between a displayed formula and a skeletal formula?

A displayed formula shows every atom and bond, whereas a skeletal formula omits carbon symbols and hydrogens bonded to carbon and represents the carbon framework using lines.

Which type of formula shows every atom and every bond?

Displayed formula.

Which type of formula shows unambiguously how atoms are joined together?

Structural formula.

Which type of formula uses a zig-zag line to represent the carbon skeleton?

Skeletal formula.

Which type of formula gives the actual numbers of atoms in a molecule?

Molecular formula.

Which type of formula gives the simplest whole-number ratio of atoms?

Empirical formula.

What is the structural formula of chloroethane?

CH₃CH₂Cl.

What is the molecular formula of chloroethane?

C₂H₅Cl.

What is the empirical formula of chloroethane?

C₂H₅Cl.

Why must chlorine be shown explicitly in the skeletal formula of chloroethane?

Only carbon atoms and hydrogens attached to carbon are omitted; other elements must be shown.

How many carbon atoms are represented by a skeletal formula containing five line ends/junctions corresponding to carbons?

Five carbon atoms.

What valency does carbon normally have when interpreting organic formulae?

Four.

How can you check the number of hidden hydrogens in a skeletal formula?

Ensure that each carbon has four bonds in total.