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 nonbonding 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 condensed structural formula is a more compact way of drawing the structural formula of a molecule.
 A condensed structural formula is also referred to as a semistructural formula.
 In a condensed structural formula, or semistructural formula, covalent bonds are not always shown and atoms of the same type bonded to one another are grouped together.
When the formula is written in a line with covalent bonds shown, it is also referred to as a linear formula.
 If there is repetition of a group in a chain, then brackets [square brackets] are used to indicate the repetition of a group in a chain^{1}
A subscript number written to the right of the final bracket tells us how many times this group of atoms is found in this position in the molecule.
 If groups of atoms are attached to a chain, then parentheses (round brackets) are used to enclose groups attached to a chain
 For the sake of clarity, double and/or triple covalent bonds between carbon atoms are usually shown in the condensed structural formula, or semistructural formula.
 To write the condensed structural formula, or semistructural formula:
Step 1: Draw the 2dimensional structural formula for the molecule
Step 2: Identify the carbon backbone (or skeleton) and any branches or sidechains in the structural formula as well as any functional groups such as halogen (group 17) atoms.
Step 3: Identify groups of atoms along the carbon backbone in the 2dimensional structural formula
2D Structure 
Condensed Structure 

CH_{3} 

CH_{2} 

CH 

C 
Step 4: Redraw the formula by replacing the identified groups of atoms in the 2dimensional structural formula with their condensed structure, or semistructural formula.
Step 5: Remove single carboncarbon covalent bonds from the backbone or skeleton (this step is not absolutely necessary).
Step 6: If the molecule contains branches or sidechains:
(a) condense the structure as for the carbon backbone
(b) use parentheses to enclose groups of atoms attached to the chain
(c) use a subscript number to indicate the number of times this group occurs at that position
(d) write this group to the right of the carbon atom of the backbone at the position it is located.
Condensed Structural Formula: Short Straight Chain Hydrocarbons
Write the condensed structural formula, or semistructural formula, for the 2dimensional structural formula shown below:
 H    H    H   
H  C    C    C  H 
  H    H    H  
Step 1: Draw the 2dimensional structural formula for the molecule
 H    H    H   
H  C    C    C  H 
  H    H    H  
Step 2: Identify the carbon backbone (or skeleton) and any branches or sidechains in the structural formula
 H    H    H   
H  C    C    C  H 
  H    H    H  
This molecule does not have any branches or sidechains.
Step 3: Identify groups of atoms along the carbon backbone in the 2dimensional structural formula
 H    H    H   
H  C    C    C  H 
  H    H    H  
This molecule contains three groups of atoms, CH_{3}, CH_{2} and CH_{3}:
Step 4: Redraw the formula by replacing the identified groups of atoms in the 2dimensional structural formula with their condensed structure.
2dimensional structural formula: 
 H    H    H   
H  C    C    C  H 
  H    H    H  

linear formula: (partial condensed formula) 
CH_{3}CH_{2}CH_{3} 
Step 5: Remove single carboncarbon covalent bonds from the backbone or skeleton (this step is not absolutely necessary).
linear formula: (partial condensed formula) 
CH_{3}CH_{2}CH_{3} 
Condensed Structural Formula, or semistructural formula: 
CH_{3}CH_{2}CH_{3} 
We could condense this formula even further by recognising that the carbon atom of the CH_{2} group is the centre of the molecule and that there are 2 CH_{3} groups attached to it.
Since these two CH_{3} groups are identical, we can enclose them in parentheses, (CH_{3}).
There are two of these CH_{3} groups attached to the same carbon atom, so we use a subscript 2 after the closing parenthesis: (CH_{3})_{2}
The new condensed (or semi) structural formula becomes (CH_{3})_{2}CH_{2}
Condensed Structural Formula: Long Straight Chain Hydrocarbons
The process for writing the condensed structural formula, or semistructural formula, for a long straight chain hydrocarbon is very similar to that used above for short straight chain hydrocarbons.
The only difference is that the condensed structural formula, or semistructural formula, can be made even more compact by using brackets and subscript numbers to indicate the length of carbon chain between the terminal (end) carbon atoms.
The 2dimensional (full display) structural formula for a molecule containing a long hydrocarbon chain is shown below:
 H    H    H    H    H    H    H    H    H    H    
H  C    C    C    C    C    C    C    C    C    C    H 
  H    H    H    H    H    H    H    H    H    H   
Step 1: Draw the 2dimensional structural formula for the molecule
 H    H    H    H    H    H    H    H    H    H    
H  C    C    C    C    C    C    C    C    C    C    H 
  H    H    H    H    H    H    H    H    H    H   
Step 2: Identify the carbon backbone (or skeleton) and any branches or sidechains in the structural formula
 H    H    H    H    H    H    H    H    H    H    
H  C    C    C    C    C    C    C    C    C    C    H 
  H    H    H    H    H    H    H    H    H    H   
There are no branches or sidechains in this molecule.
Step 3: Identify groups of atoms along the carbon backbone in the 2dimensional structural formula
 H    H    H    H    H    H    H    H    H    H    
H  C    C    C    C    C    C    C    C    C    C    H 
  H    H    H    H    H    H    H    H    H    H   
This molecule contains two CH_{3} groups and eight CH_{2} groups
Step 4: Redraw the formula by replacing the identified groups of atoms in the 2dimensional structural formula with their condensed structure.
2dimensional structural formula: 
 H    H    H    H    H    H    H    H    H    H    
H  C    C    C    C    C    C    C    C    C    C    H 
  H    H    H    H    H    H    H    H    H    H   

linear formula: (partial condensed formula) 
CH_{3}CH_{2}CH_{2}CH_{2}CH_{2}CH_{2}CH_{2}CH_{2}CH_{2}CH_{3} 
Step 5: Remove single carboncarbon covalent bonds from the backbone or skeleton (this step is not absolutely necessary).
CH_{3}CH_{2}CH_{2}CH_{2}CH_{2}CH_{2}CH_{2}CH_{2}CH_{2}CH_{3}
The condensed structural formula, or semistructural formula, above can be further condensed by recognising that the eight CH_{2} groups occur in a long chain (no branches or side chains) so they could be represented as [CH_{2}]_{8}
The condensed structural formula, or semistructural formula, then becomes CH_{3}[CH_{2}]_{8}CH_{3}
Condensed Structural Formula: BranchedChain Hydrocarbons
Write the condensed structural formula, or semistructural formula, for the 2dimensional structural formula shown below:
   H     
 H    HCH    H   
H  C    C    C  H 
  H    HCH    H  
    H    
Step 1: Draw the 2dimensional structural formula for the molecule
   H     
 H    HCH    H   
H  C    C    C  H 
  H    HCH    H  
    H    
Step 2: Identify the carbon backbone (or skeleton) and any branches or sidechains in the structural formula
   H     
 H    HCH    H   
H  C    C    C  H 
  H    HCH    H  
    H    
Step 3: Identify groups of atoms along the carbon backbone in the 2dimensional structural formula
   H     
 H    HCH    H   
H  C    C    C  H 
  H    HCH    H  
    H    
This molecule contains three groups along its carbon backbone: CH_{3}, C and CH_{3}
Step 4: Redraw the formula by replacing the identified groups of atoms in the 2dimensional structural formula with their condensed structure.
2dimensional structural formula: 
   H     
 H    HCH    H   
H  C    C    C  H 
  H    HCH    H  
    H    

Partial condensed structure: 
   H     
   HCH     
 CH_{3}    C    CH_{3}  
    HCH    
    H    

Step 5: Remove single carboncarbon covalent bonds from the backbone or skeleton (this step is not absolutely necessary).
Partial condensed structure: (for backbone) 
   H     
   HCH     
 CH_{3}    C    CH_{3}  
    HCH    
    H    

condensed structure for backbone 
H  
HCH  
CH_{3}CCH_{3
}   HCH 
 H 

Step 6:This molecule contains branches or sidechains:
(a) Condense the structure for each branch or sidechain as for the carbon backbone:
CH_{3}  
CH_{3}CCH_{3
}   CH_{3} 
(b) We can see that there are actually four CH_{3} groups bonded to the same carbon atom, so we use parantheses to enclose this group of atoms: (CH_{3})
(c) This CH_{3} group occurs four times on the same carbon atom, so we use a subscript number to indicate the number of times this group occurs at that position: (CH_{3})_{4}
(d) Write this group to the right of the carbon atom of the backbone at the position it is located.
Condensed Structural Formula, or semistructural formula: C(CH_{3})_{4}
Halogenated Hydrocarbons
A halogenated hydrocarbon is a molecule that contains carbon, hydrogen and a halogen (Group 17) atom such as fluorine, chlorine, bromine or iodine.
Write the condensed structural formula, or semistructural formula, for the 2dimensional structural formula shown below:
 H    Cl    H   
H  C    C    C  H 
  H    H    H  
Step 1: Draw the 2dimensional structural formula for the molecule
 H    Cl    H   
H  C    C    C  H 
  H    H    H  
Step 2: Identify the carbon backbone (or skeleton) and any branches or sidechains as well as functional groups such as halogen atoms in the structural formula
 H    Cl    H   
H  C    C    C  H 
  H    H    H  
Step 3: Identify groups of atoms along the carbon backbone in the 2dimensional structural formula
 H    Cl    H   
H  C    C    C  H 
  H    H    H  
This molecule contains three groups of atoms along the carbon backbone, CH_{3}, CH and CH_{3}:
Step 4: Redraw the formula by replacing the identified groups of atoms in the 2dimensional structural formula with their condensed structure.
Step 5: Remove single carboncarbon covalent bonds from the backbone or skeleton (this step is not absolutely necessary).
Step 6:This molecule contains one halogen atom (chlorine, Cl):
Write the symbol for this chlorine atom to the right of the carbon atom of the backbone to which it is attached :
Condensed Structural Formula, or semistructural formula, is CH_{3}CHClCH_{3}
It is possible to condense this structural formula even further by recognising that there is a central carbon atom to which both CH_{3} groups are attached.
We then enclose the CH_{3} groups in paretheses, (CH_{3}) and use the subscript number 2 to indicate that there are two of these groups attached to the same carbon atom, (CH_{3})_{2}
The new condensed structural formula, or semistructural formula, is therefore (CH_{3})_{2}CHCl
Converting a Condensed Structural Formula to a 2Dimensional (full display) Structural Formula
Step 1: Identify any branches or sidechains in the condensed structural formula, or semistructural formula.
Rewrite the formula showing these branches (sidechains) as 2dimensional structures
Step 2: Identify any functional groups indicated by parentheses or any halogen atoms in the condensed structural formula, or semistructural formula.
Rewrite the partial structural formula showing how these atoms are bonded to the carbon backbone (skeleton).
Step 3: The carbon backbone (skeleton) should now be obvious so you can insert covalent bonds between the carbon atoms making up the backbone.
Step 4: Complete the 2dimensional structural formula by showing the covalent bonds between all atoms.
Example
Draw the 2dimensional (full display) structural formula for the molecule with the condensed structural formula, or semistructural formula, (CH_{3})_{2}CH(OH)
Step 1: Identify any branches or sidechains in the condensed structural formula, or semistructural formula.
Treat the CH_{3} groups in parentheses as branches or sidechains
Rewrite the formula showing these branches (sidechains) as 2dimensional structures
   H   
 H    HCH   
H  C    CH  (OH) 
  H    
Step 2: Identify any functional groups indicated by parentheses, or any halogen atoms in the condensed structural formula, or semistructural formula.
This molecule contains an OH functional group as indicated by (OH) bonded to the 2^{nd} C of the backbone:
   H   
 H    H^{3}CH   
H  ^{1}C    ^{2}CH  (OH) 
  H    
Rewrite the partial structural formula showing how these atoms are bonded to the carbon backbone (skeleton).
   H   
 H    HCH   
H  C    CH  OH 
  H    
Step 3: The carbon backbone (skeleton) should now be obvious so you can insert covalent bonds between the carbon atoms making up the backbone.
The structure we have drawn so far is probably more familiar to you if you draw it like the one below on the right 
   H   
 H    HCH   
H  C    CH  OH 
  H    


   H     
 H    O    H   
H  C    CH    C  H 
  H      H  

Step 4: Complete the 2dimensional structural formula by showing the covalent bonds between all the atoms.
   H     
 H    O    H   
H  C    C    C  H 
  H    H    H  
