# 2-dimensional (Full Display) Structural Formula Tutorial

## Key Concepts

• A molecular formula tells us how many atoms of each element make up the molecule.

• A Lewis (electron dot) structure shows us how the bonding and non-bonding valence electrons are arranged around the atoms.

• A structural formula shows us how the covalent bonds are arranged around the atoms in a molecule.

• A full display structural formula shows every atom and every covalent bond between atoms in a molecule.

• In a structural formula, each bonding (shared) pair of electrons in the Lewis (electron dot) structure is replaced with a single line to represent the covalent bond between the two atoms.

• A 2-dimensional structural formula represents all the covalent bonds in a molecule as if the molecule were flat (that is, 2-dimensional).

• A 2-dimensional structural formula may also be referred to as a 2-dimensional full display structural formula.

• For any given molecular formula, it may be possible to draw more than one valid 2-dimensional (full display) structural formula.

## Drawing a 2-Dimensional (Full Display) Structural Formula for Hydrocarbon Molecules

2-Dimensional (full display) structural formula are commonly used in organic (carbon) chemistry.

The elements you are most likely to encounter in organic (carbon) molecules are carbon, hydrogen, oxygen, nitrogen, bromine and chlorine.

The Lewis (electron dot) structures for atoms of these elements are shown below:

Elementscarbonhydrogenoxygennitrogenbrominechlorine
Lewis Structure
 . .C. .
H.
 : .O. :
 . ..N. .
 : ..Br.. .
 : ..Cl.. .

If you have been given the molecular formula of molecule of a hydrocarbon, you can use this to draw possible 2-dimensional full display structural formulae:

Step 1: Draw a chain of the required number of carbon atoms (given in the molecular formula)

Step 2: Place the 4 electrons around each carbon atom as in the Lewis Structures given above.

Step 3: Replace any shared pair of electrons with a line representing the covalent bond.

Step 4: Place a hydrogen atom wherever there is an unpaired electron.

Step 5: Check that you have the right number of hydrogen atoms, if you have too many H atoms, remove hydrogen atoms in pairs from adjacent carbon atoms in the structure and then pair up the remaining unshared electrons between adjacent carbon atoms.

Step 6: Replace any shared pair of electrons between atoms with a line representing the covalent bond between them.

### Example

Draw the 2-dimensional (full display) structural formula for a molecule with molecular formula C2H6

Step 1: Draw a chain of the required number of carbon atoms

C2H6 contains 2 carbon atoms:

 C C

Step 2: Place the 4 electrons around each carbon atom as in the Lewis Structures given above.

 . .C. .. .C. .

Step 3: Replace any shared pair of electrons with a line representing the covalent bond.

 . .C. - .C. .

Step 4: Place a hydrogen atom and its valence electron wherever there is an unpaired electron on a carbon atom.

 H H .. .. H .. C - C .. H .. .. H H

Step 5: Check that you have the right number of carbon and hydrogen atoms:
Number of carbon atoms = 2 (and there are 2 carbon atoms in the molecular formula C2H6)
Number of hydrogen atoms = 6 (and there are 6 hydrogen atoms in the molecular formula C2H6)

Step 6: Replace any shared pair of electrons between atoms with a line representing the covalent bond between them to complete the 2-dimensional full display structural formula.

 H H | | H - C - C - H | | H H

## Drawing 2-Dimensional (full display) Structural Formula for Halogenated Hydrocarbon Molecules

If you have been given the molecular formula for a molecule of a halogenated hydrocarbon (a molecule containing carbon, a halogen such as bromine or chlorine, and maybe some hydrogen), you can use this to draw possible 2-dimensional full display structural formulae:

Step 1: Draw a chain of the required number of carbon atoms (given in the molecular formula)

Step 2: Place the 4 electrons around each carbon atom as in the Lewis Structures given above.

Step 3: Replace any shared pair of electrons with a line representing the covalent bond.

Step 4: Place a hydrogen atom and its valence electron wherever there is an unpaired electron on a carbon atom (just pretend that there are no halogen atoms present in the molecule at this stage).

Step 5: Replace one hydrogen atom and its valence electron in the structure with one halogen atom and its valence electrons.
Repeat this for the required number of halogen atoms.

Step 6: Check that you have the right number of hydrogen atoms, if you have too many H atoms, remove hydrogen atoms in pairs from adjacent carbon atoms in the structure and then pair up the remaining unshared electrons between adjacent carbon atoms.

Step 7: Remove any non-bonding pairs of electrons (those around halogen atoms that are not used to bond with carbon atoms)

Step 8: Replace any shared pair of electrons between atoms with a line representing the covalent bond between them.

### Example :

Draw the 2-dimensional (full display) structural formula for a molecule with molecular formula C2H4Br2

Step 1: Draw a chain of the required number of carbon atoms

C2H4Br2 contains 2 carbon atoms:

 C C

Step 2: Place the 4 electrons around each carbon atom as in the Lewis Structures given above.

 . .C. .. .C. .

Step 3: Replace any shared pair of electrons with a line representing the covalent bond.

 . .C. - .C. .

Step 4: Place a hydrogen atom and its valence electron wherever there is an unpaired electron on a carbon atom.

 H H .. .. H .. C - C .. H .. .. H H

Step 5: Replace one hydrogen atom in the structure with one halogen atom.
Repeat this for the required number of halogen atoms.

C2H4Br2 contains 2 halogen atoms, that is, 2 bromine atoms.
Replace 2 hydrogen atoms in the structure with two bromine atoms:

 ..:Br: ..:Br: .. .. H .. C - C .. H .. .. H H
OR
 ..:Br: H .. .. ..:Br.. .. C - C .. H .. .. H H

Step 6: Check that you have the right number of carbon, hydrogen and halogen atoms:
Number of carbon atoms = 2 (and there are 2 carbon atoms in the molecular formula C2H4Br2)
Number of hydrogen atoms = 4 (and there are 4 hydrogen atoms in the molecular formula C2H4Br2)
Number of halogen (bromine) atoms = 2 (and there are 2 bromine atoms in the molecular formula C2H4Br2)

Step 7: Remove any non-bonding pair of electrons (those around the halogen atoms that are not used to bond with carbon atoms)

 Br Br .. .. H .. C - C .. H .. .. H H
OR
 Br H .. .. Br .. C - C .. H .. .. H H

Step 8: Replace any shared pair of electrons between atoms with a line representing the covalent bond between them to complete the 2-dimensional full display structural formula.

 Br Br | | H - C - C - H | | H H
OR
 Br H | | Br - C - C - H | | H H

## Drawing 2-Dimensional (full display) Structural Formula for Organic Compounds Containing Oxygen

An oxygen atom has 6 valence electrons. In order to complete its octet of electrons it will share two of its electrons, that is, oxygen will form 2 covalent bonds in organic (carbon) molecules.
This results in a number of different ways in which oxygen atoms can be included in the structural formula of organic molecules:
Lewis Structure 2-D Structural Formula
O shares 1 electron with 2 different C atoms:
 . .C. .. ..O.. .. .C. .
 C - O - C
O shares 1 electron with a C atom and 1 electron with a H atom
 . .C. .. ..O.. .. H
 C - O - H
O shares 2 electrons with the same C atom
 .C. .... ..O..
 C = O

It is therefore useful to place oxygen atoms in the structure before hydrogen atoms.

If you have been given the molecular formula for an organic (carbon) molecule containing oxygen:

Step 1: Draw chains of the required number of carbon atoms (given in the molecular formula)

Step 2: Position oxygen atoms with their valence electrons:

• Oxygen atoms between carbon atoms in the chain

• Oxygen atoms above or below a single carbon atom in the chain

Step 3: Complete the partial structure by placing hydogen atoms wherever there is an unpaired electron.

Step 4: Check the number of carbon, oxygen and hydrogen atoms.

Step 5: If you have too many hydrogen atoms try:

• removing 2 hydrogen atoms, one from each adjacent carbon atom, to create a double bond between carbon atoms (C=C)

• removing 1 hydrogen atom from the carbon atom in the chain that has the oxygen atom, then create a double bond between the oxygen atom and the carbon atom (C=O)

Keep repeating this process until you have the required number of carbon, hydrogen and oxygen atoms (as given in the molecular formula).

Step 6: Replace all shared pairs of electrons with lines (-) representing covalent bonds, and remove any non-bonding pairs of electrons, to complete the 2-dimensional (full display) structural formula

### Example:

Draw the 2-dimensional (full display) structural formula for a molecule with the molecular formula C2H6O

Step 1: Draw chains of the required number of carbon atoms (given in the molecular formula)
C2H6O contains 2 carbon atoms:

 . .C. .. .C. .

Step 2: Position oxygen atoms with their valence electrons:

(a) O between C atoms in the chain (b) O above or below a single C atom in the chain
 . .C. : ..O.. : .C. .
 : .O : . ..C. : .C. .

Step 3: Complete the partial structure by placing hydogen atoms wherever there is an unpaired electron.

(a) O between C atoms in the chain (b) O above or below a single C atom in the chain
 H: H..C..H : ..O.. : H..C..H :H
 H.. :O: H H: ..C.. : ..C.. :H H H

Step 4/5: Check the number of carbon, oxygen and hydrogen atoms.

(a) O between C atoms in the chain (b) O above or below a single C atom in the chain

 H: H..C..H : ..O.. : H..C..H :H
 H.. :O: H H: ..C.. : ..C.. :H H H
no. C atoms22
no. O atoms11
no. H atoms66

Both (a) and (b) have the molecular formula C2H6O so they are both possible structures.

Step 6: Replace all shared pairs of electrons with lines (-) representing covalent bonds, and remove any non-bonding pairs of electrons, to complete the 2-dimensional (full display) structural formula

(a) O between C atoms in the chain (b) O above or below a single C atom in the chain

 H- H|C|H - O - H|C|H -H
 H| O H H- |C| - |C| -H H H

## Converting 2-Dimensional (full display) Structural Formula to Molecular Formula

If you have been given the 2-dimensional (full display) structural formula of an organic molecule you can convert this to a molecular formula:

Step 1: List the symbols for the elements present in the molecule in the order:
C, H, O
or
C, H, halogen

Step 2: Count the number of atoms of each element present in the structural formula.

Step 3: Write the number of atoms of each element as a subscript number to the right of its symbol

Step 4: If any of the numbers are 1, remove the number 1 then write the molecular formula.

### Example of Converting a 2-Dimensional (full display) Structural Formula to a Molecular Formula

Give the molecular formula for the molecule shown below:

 H| H| H-C-H| H| H- C - C - C - O -H |H |H |H

Step 1: List the symbols for the elements present in the molecule in the order: C, H, O

Elements present are: C H O

Step 2: Count the number of atoms of each element present in the structural formula.

 4 C atoms H| H| H-C1-H| H| H- C2 - C3 - C4 - O -H |H |H |H

 10 H atoms H8| H10| H9-C-H7| H6| H1- C - C - C - O -H5 |H2 |H3 |H4

 1 O atom H| H| H-C-H| H| H- C - C - C - O1 -H |H |H |H

Step 3: Write the number of atoms of each element as a subscript number to the right of its symbol

C4 H10 O1

Step 4: If any of the numbers are 1, remove the number 1 then write the molecular formula.

C4H10O is the molecular formula for the 2-dimensional (full display) structural formula given.

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