Functional Groups and Formal Charge

Learning Goals

• Recognize the most common functional groups in Organic Chemistry I.
• Connect functional groups to expected reactivity.
• Define formal charge.
• Use the formal charge formula correctly.
• Count valence electrons, lone pairs, and bonds.
• Recognize common charge patterns for C, N, O, and halogens.
• Avoid common incorrect structures.
• Identify missing charges in drawings.

Section 1

What Are Functional Groups?

A functional group is a specific atom or group of atoms that gives an organic molecule predictable chemical behavior. Carbon skeletons can be large or small, but the functional group is usually the part of the molecule that controls acidity, basicity, polarity, boiling point, and reaction type.

This is the part I really want to land, because it is what keeps organic chemistry from being pure memorization. You are not learning molecules one at a time. Once you recognize the functional group, you can predict what kind of chemistry that molecule is likely to undergo.

Section 2

Functional Groups to Know

These are the functional groups I want you recognizing early. You do not need to know every reaction yet, but you should be able to spot the pattern in a structure.

Alkane

Pattern: C-C and C-H single bonds only

Recognize it: No heteroatoms, no π bonds, no special reactive group.

Example: CH₃CH₃

Alkene

Pattern: C=C

Recognize it: A carbon-carbon double bond.

Example: CH₂=CH₂

Alkyne

Pattern: C≡C

Recognize it: A carbon-carbon triple bond.

Example: HC≡CH

Alkyl halide

Pattern: C-X

Recognize it: A carbon bonded to F, Cl, Br, or I.

Example: CH₃Br

Alcohol

Pattern: R-OH

Recognize it: An OH group attached to carbon.

Example: CH₃OH

Ether

Pattern: R-O-R

Recognize it: An oxygen single-bonded between two carbons.

Example: CH₃OCH₃

Amine

Pattern: R-NH₂, R₂NH, or R₃N

Recognize it: A nitrogen bonded to carbon and/or hydrogen.

Example: CH₃NH₂

Aldehyde

Pattern: R-CHO

Recognize it: A carbonyl with at least one hydrogen attached to the carbonyl carbon.

Example: CH₃CHO

Ketone

Pattern: R-CO-R

Recognize it: A carbonyl carbon bonded to two carbons.

Example: CH₃COCH₃

Carboxylic acid

Pattern: R-COOH

Recognize it: A carbonyl and OH on the same carbon.

Example: CH₃COOH

Ester

Pattern: R-COOR

Recognize it: A carbonyl attached to an OR group.

Example: CH₃COOCH₃

Amide

Pattern: R-CONH₂, R-CONHR, or R-CONR₂

Recognize it: A carbonyl attached directly to nitrogen.

Example: CH₃CONH₂

Section 3

Why Formal Charge Matters

A formal charge is the charge assigned to an atom when the electrons in bonds are split equally between bonded atoms. It is not always the same as the real physical charge distribution, but it is the system organic chemists use to keep track of electrons in drawings.

Section 4

The Formal Charge Formula

The formula works because each bond contributes one electron to the atom when you calculate formal charge. Lone-pair electrons get counted fully, because those belong entirely to that atom.

Section 5

Valence Electrons to Know

For most Organic Chemistry I problems, these are the main valence electron counts you need.

Section 6

Oxygen Patterns

Oxygen normally has 6 valence electrons. The three patterns I want you to know cold are neutral, negative, and positive.

Neutral oxygen

2 bonds + 2 lone pairs

FC = 6 − 4 − 2 = 0

Negative oxygen

1 bond + 3 lone pairs

FC = 6 − 6 − 1 = −1

Positive oxygen

3 bonds + 1 lone pair

FC = 6 − 2 − 3 = +1

Section 7

Nitrogen Patterns

Nitrogen normally has 5 valence electrons. Its charge comes down to how many bonds and lone pairs it carries.

Neutral nitrogen

3 bonds + 1 lone pair

FC = 5 − 2 − 3 = 0

Positive nitrogen

4 bonds + 0 lone pairs

FC = 5 − 0 − 4 = +1

Negative nitrogen

2 bonds + 2 lone pairs

FC = 5 − 4 − 2 = −1

Section 8

Carbon Patterns

Carbon normally has 4 valence electrons. Neutral carbon has 4 bonds. For Organic Chemistry I, charged carbons can ONLY have 3 bonds. The thing that flips it positive or negative is whether there is a lone pair or not. In both examples below, the middle carbon only has 3 bonds (2 visible C-C bonds, and one invisible C-H bond).

Carbocation

3 bonds + no lone pair

FC = 4 − 0 − 3 = +1

Carbanion

3 bonds + 1 lone pair

FC = 4 − 2 − 3 = −1

Section 9

Fast Recognition Table

You should know how to calculate formal charge, but my goal for you is to reach the point where you recognize these common patterns on sight, no formula needed.

Section 10

Common Mistakes