# Condensed Structural Formula (Semi-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 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 semi-structural formula.

• In a condensed structural formula, or semi-structural 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 chain1
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 semi-structural formula.

• To write the condensed structural formula, or semi-structural formula:

Step 1: Draw the 2-dimensional structural formula for the molecule

Step 2: Identify the carbon backbone (or skeleton) and any branches or side-chains 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 2-dimensional structural formula

2-D Structure Condensed Structure
 H| H- C - |H
CH3
 H| - C - |H
CH2
 | - C - |H
CH
 | - C - |
C

Step 4: Re-draw the formula by replacing the identified groups of atoms in the 2-dimensional structural formula with their condensed structure, or semi-structural formula.

Step 5: Remove single carbon-carbon covalent bonds from the backbone or skeleton (this step is not absolutely necessary).

Step 6: If the molecule contains branches or side-chains:

(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 semi-structural formula, for the 2-dimensional structural formula shown below:

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

Step 1: Draw the 2-dimensional 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 side-chains in the structural formula

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

This molecule does not have any branches or side-chains.

Step 3: Identify groups of atoms along the carbon backbone in the 2-dimensional structural formula

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

This molecule contains three groups of atoms, CH3, CH2 and CH3:

Step 4: Re-draw the formula by replacing the identified groups of atoms in the 2-dimensional structural formula with their condensed structure.

2-dimensional structural formula:
 H| H| H| H- C - C - C -H |H |H |H
linear formula:
(partial condensed formula)
CH3-CH2-CH3

Step 5: Remove single carbon-carbon covalent bonds from the backbone or skeleton (this step is not absolutely necessary).

linear formula:(partial condensed formula) CH3-CH2-CH3 CH3CH2CH3

We could condense this formula even further by recognising that the carbon atom of the CH2 group is the centre of the molecule and that there are 2 CH3 groups attached to it.
Since these two CH3 groups are identical, we can enclose them in parentheses, (CH3).
There are two of these CH3 groups attached to the same carbon atom, so we use a subscript 2 after the closing parenthesis: (CH3)2
The new condensed (or semi) structural formula becomes (CH3)2CH2

## Condensed Structural Formula: Long Straight Chain Hydrocarbons

The process for writing the condensed structural formula, or semi-structural 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 semi-structural 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 2-dimensional (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 2-dimensional 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 side-chains 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 side-chains in this molecule.

Step 3: Identify groups of atoms along the carbon backbone in the 2-dimensional 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 CH3 groups and eight CH2 groups

Step 4: Re-draw the formula by replacing the identified groups of atoms in the 2-dimensional structural formula with their condensed structure.

2-dimensional 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)
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3

Step 5: Remove single carbon-carbon covalent bonds from the backbone or skeleton (this step is not absolutely necessary).

CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3

The condensed structural formula, or semi-structural formula, above can be further condensed by recognising that the eight CH2 groups occur in a long chain (no branches or side chains) so they could be represented as [CH2]8

The condensed structural formula, or semi-structural formula, then becomes CH3[CH2]8CH3

## Condensed Structural Formula: Branched-Chain Hydrocarbons

Write the condensed structural formula, or semi-structural formula, for the 2-dimensional structural formula shown below:

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

Step 1: Draw the 2-dimensional structural formula for the molecule

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

Step 2: Identify the carbon backbone (or skeleton) and any branches or side-chains in the structural formula

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

Step 3: Identify groups of atoms along the carbon backbone in the 2-dimensional structural formula

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

This molecule contains three groups along its carbon backbone: CH3, C and CH3

Step 4: Re-draw the formula by replacing the identified groups of atoms in the 2-dimensional structural formula with their condensed structure.

2-dimensional structural formula:
 H| H| H-C-H| H| H- C - C - C -H |H |H-C-H |H |H
Partial condensed structure:
 H| H-C-H| CH3 - C - CH3 |H-C-H |H

Step 5: Remove single carbon-carbon covalent bonds from the backbone or skeleton (this step is not absolutely necessary).

Partial condensed structure:
(for backbone)
 H| H-C-H| CH3 - C - CH3 |H-C-H |H
condensed structure for backbone
 H| H-C-H| CH3CCH3 |H-C-H |H

Step 6:This molecule contains branches or side-chains:

(a) Condense the structure for each branch or side-chain as for the carbon backbone:

 CH3| CH3CCH3 |CH3

(b) We can see that there are actually four CH3 groups bonded to the same carbon atom, so we use parantheses to enclose this group of atoms: (CH3)

(c) This CH3 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: (CH3)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 semi-structural formula: C(CH3)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 semi-structural formula, for the 2-dimensional structural formula shown below:

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

Step 1: Draw the 2-dimensional 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 side-chains 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 2-dimensional structural formula

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

This molecule contains three groups of atoms along the carbon backbone, CH3, CH and CH3:

Step 4: Re-draw the formula by replacing the identified groups of atoms in the 2-dimensional structural formula with their condensed structure.

 Cl| CH3 -CH- CH3

Step 5: Remove single carbon-carbon covalent bonds from the backbone or skeleton (this step is not absolutely necessary).

 Cl| CH3 CH CH3

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 semi-structural formula, is CH3CHClCH3

It is possible to condense this structural formula even further by recognising that there is a central carbon atom to which both CH3 groups are attached.
We then enclose the CH3 groups in paretheses, (CH3) and use the subscript number 2 to indicate that there are two of these groups attached to the same carbon atom, (CH3)2
The new condensed structural formula, or semi-structural formula, is therefore (CH3)2CHCl

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

Step 1: Identify any branches or side-chains in the condensed structural formula, or semi-structural formula.

Re-write the formula showing these branches (side-chains) as 2-dimensional structures

Step 2: Identify any functional groups indicated by parentheses or any halogen atoms in the condensed structural formula, or semi-structural formula.

Re-write 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 2-dimensional structural formula by showing the covalent bonds between all atoms.

### Example

Draw the 2-dimensional (full display) structural formula for the molecule with the condensed structural formula, or semi-structural formula, (CH3)2CH(OH)

Step 1: Identify any branches or side-chains in the condensed structural formula, or semi-structural formula.

Treat the CH3 groups in parentheses as branches or side-chains

Re-write the formula showing these branches (side-chains) as 2-dimensional structures

 H| H| H-C-H| H- C - CH (OH) |H

Step 2: Identify any functional groups indicated by parentheses, or any halogen atoms in the condensed structural formula, or semi-structural formula.

This molecule contains an OH functional group as indicated by (OH) bonded to the 2nd C of the backbone:

 H| H| H-3C-H| H- 1C - 2CH (OH) |H

Re-write the partial structural formula showing how these atoms are bonded to the carbon backbone (skeleton).

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

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

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

What would you like to do now?

1The use of parentheses (), brackets [] and braces {} in linear formula is described in the IUPAC "Preferred names in the nomenclature of organic compounds" (Draft 7 October 2004)

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