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Formal Charge

Calculating Formal Charge in Chemistry

This note covers the concept of formal charge calculation in the context of organic chemistry, illustrated through various examples.

What is Formal Charge?

Formal charge is a way to track electron distribution in a molecule to understand the stability and reactivity of atoms within the molecule.

Formula for Calculating Formal Charge

Formal Charge = (Number of Valence Electrons in Free Atom) - (Number of Bonds + Number of Dots (Lone Pair Electrons))

Examples

Example 1: Oxygen in Methanol

• Structure: CH3OH with oxygen having 3 lone pairs.
• Valence Electrons: Oxygen (Group 6A) has 6 valence electrons.
• Bonds and Dots: Oxygen has 1 bond and 6 dots (3 lone pairs).
• Calculation: 6 - (1 + 6) = -1
• Formal Charge: Oxygen has a formal charge of -1.

Example 2: Carbocation

• Structure: Carbon with 3 bonds.
• Valence Electrons: Carbon (Group 4A) has 4 valence electrons.
• Bonds and Dots: Carbon has 3 bonds and 0 dots.
• Calculation: 4 - (3 + 0) = +1
• Formal Charge: Carbon has a formal charge of +1 and is called a carbocation (carbon cation).

Example 3: Bromine with a Double Bond

• Structure: Bromine double bonded to carbon with 2 lone pairs on bromine.
• Valence Electrons: Bromine (Group 7A) has 7 valence electrons.
• Bonds and Dots: Bromine has 2 bonds and 4 dots (2 lone pairs).
• Calculation: 7 - (2 + 4) = +1
• Formal Charge: Bromine has a formal charge of +1.

Example 4: Nitrogen with Four Methyl Groups

• Structure: Nitrogen attached to four CH3 groups.
• Lewis Structure: Nitrogen in the center with four single bonds to CH3 groups.
• Valence Electrons: Nitrogen (Group 5A) has 5 valence electrons.
• Bonds and Dots: Nitrogen has 4 bonds and 0 dots.
• Calculation: 5 - (4 + 0) = +1
• Formal Charge: Nitrogen has a formal charge of +1.

Example 5: Carbon in a Double Bond with a Lone Pair

• Structure: CH2=CH with a lone pair on the second carbon.
• Lewis Structure: First carbon has 2 single bonds to hydrogen and a double bond to the second carbon. The second carbon has a single bond to hydrogen and a lone pair.
• Valence Electrons: Carbon (Group 4A) has 4 valence electrons.
• Bonds and Dots: The second carbon has 3 bonds (1 single + 1 double) and 2 dots (1 lone pair).
• Calculation: 4 - (3 + 2) = -1
• Formal Charge: The second carbon has a formal charge of -1.