This structure has two open positions at the middle carbon (standard valence of carbon is four):

By default, a substructure search on the above structure retrieves the following:

Substituted structure matches with	Example
Bonds at open positions
An electron at an open position (radical)	Note: The single electron is not drawn in the structure.
A pair of electrons (-) or no electrons (+) at an open position

Tools and structure drawing techniques are available to control substitution:

• Variables menu
• R-group tool
• Lock Atoms tool
• Drawing hydrogen attachments

The Variables menu and R-group tool control substitution by specifying substituents at open positions.

• The Variables menu   contains a list of predefined variable symbols, e.g., X = all halogen atoms.
• The R-group tool  is used to create customized variable groups containing combinations of atoms, shortcuts, and variables.
  
  

The Lock Atoms tool  locks an open position from substitution. It prevents retrieval of structures having a bond to a non-hydrogen atom at an open position.

Using the Lock Atoms tool on the middle carbon	Retrieves matches
	Without substitution at the open positions on the middle carbon. Matches with a hydrogen attachment, one electron (radicals), two electrons (anions), and no electrons (cations) are allowed.

Drawing a hydrogen attachment to an open position controls substitution at that position.

Drawing a H attachment on the middle carbon	Retrieves matches
	With substitution at the single, open position on the middle carbon

Terminal shortcuts - such as Me - are locked from substitution by default.

To allow substitution at a terminal position, draw the open position explicitly.