Organic Chemistry: Functional Groups, Resonance, and Stability

Functional Groups in Organic Chemistry

Overview of Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Recognizing and naming functional groups is essential for understanding organic chemistry.

• Alkyl Groups: Derived from alkanes by removal of a hydrogen atom. Common examples include methyl (–CH3), ethyl (–CH2CH3), propyl (–CH2CH2CH3), isopropyl, tert-butyl, and butyl.

• Non-hydrogen group: Refers to groups attached to a carbon skeleton that are not hydrogen.

Alkane Prefixes Table

The following table lists the prefixes used for naming carbon chains in organic molecules:

Prefix	Carbons	Prefix	Carbons
meth-	1	undec-	11
eth-	2	dodec-	12
prop-	3	tridec-	13
but-	4	tetradec-	14
pent-	5	pentadec-	15
hex-	6	hexadec-	16
hept-	7	heptadec-	17
oct-	8	octadec-	18
non-	9	nonadec-	19
dec-	10	eicos-	20

Additional info: These prefixes are used in IUPAC nomenclature to indicate the number of carbon atoms in the main chain.

Identifying Functional Groups in Molecules

Functional Group Analysis Example

To determine which functional groups are present in a molecule, examine the structure for characteristic atoms and bonding patterns.

• Aldehyde: Contains a carbonyl group (C=O) bonded to at least one hydrogen. Look for the structure $\ce{R-CHO}$.

• Ketone: Contains a carbonyl group (C=O) bonded to two carbons. Structure: $\ce{R-CO-R'}$.

• Phenyl: A benzene ring attached as a substituent ($\ce{C6H5-}$).

• Sulfide: Contains a sulfur atom bonded to two carbons ($\ce{R-S-R'}$).

• Ether: Contains an oxygen atom bonded to two carbons ($\ce{R-O-R'}$).

• Carbon Center (degree designation): Refers to the number of carbon atoms attached to a central carbon (primary, secondary, tertiary, quaternary).

Example: In the provided molecule, the following groups are identified:

• Ketone (C=O between two carbons)

• Phenyl (benzene ring)

• Sulfide (S atom between two carbons)

• Quaternary carbon center (carbon attached to four other carbons)

Additional info: Aldehyde and ether are not present in the example molecule.

Resonance Structures and Stability

Resonance Contributors and Hybrids

Resonance describes the delocalization of electrons in molecules that cannot be represented by a single Lewis structure. Resonance contributors are individual Lewis structures, while the resonance hybrid is the true representation of the molecule.

• Resonance Contributor: A possible Lewis structure showing electron arrangement.

• Resonance Hybrid: The actual structure, which is a weighted average of all contributors.

• Arrow Pushing: Curved arrows are used to show the movement of electrons between resonance forms.

Example: The resonance hybrid of a molecule may be depicted with dashed lines to indicate delocalized bonds and partial charges ($\delta^+$ or $\delta^-$) where applicable.

Resonance Preferences

Not all resonance structures contribute equally to the resonance hybrid. The following rules help determine the major contributor:

• Rule 1: Complete Valence Shells – Structures in which all atoms have filled valence shells are favored.

• Rule 2: Negative Charge on Electronegative Atoms – Structures with negative charge on more electronegative atoms (e.g., O, N) are preferred.

• Rule 3: Least Separation of Unlike Charges – Structures with minimal charge separation are more stable.

• Rule 4: Maximize Covalent Bonds – Structures with more covalent bonds are favored.

Example: For the nitrosyl cation and formaldehyde, the major contributor is the one with complete valence shells and negative charge on the more electronegative atom.

Resonance Stability Table

Preference	Major Contributor	Minor Contributor
Complete valence shells	All atoms filled	Unfilled valence shells
Negative charge on electronegative atom	Oxygen	Carbon
Least charge separation	No separation	Separated charges
Maximize covalent bonds	More bonds	Fewer bonds

Additional info: Resonance structures of neutral molecules generally have a maximum of two charges, except for nitro groups.

Arrow Pushing in Resonance

Curly Arrow Conventions

Curly arrows are used to show the movement of electrons in resonance structures. The arrows start at the electron source (lone pair or pi bond) and point to the electron sink (atom or bond).

• Allylic Lone Pair: Use two curly arrows to show electron movement.

• Allylic Cation: Use one curly arrow from the pi bond to the cation.

• Pi-Bond Between Atoms of Different Electronegativity: Arrows show electron movement toward the more electronegative atom.

• Lone Pair Adjacent to Cation: Arrow from lone pair to cation.

Example: Connecting resonance structures A–D via arrow pushing demonstrates electron delocalization.

Carbocation Stability and Hyperconjugation

Hyperconjugation

Hyperconjugation is the stabilizing interaction that results from the overlap of sigma bonds (usually C–H or C–C) with an adjacent empty p-orbital in a carbocation. This delocalization increases carbocation stability.

• Primary (1°) Carbocation: Least stable, minimal hyperconjugation.

• Secondary (2°) Carbocation: Moderately stable, more hyperconjugation.

• Tertiary (3°) Carbocation: Most stable, maximum hyperconjugation.

Example: The stability order is: $\text{3°} > \text{2°} > \text{1°}$.

Additional info: Charge-separated resonance structures may be stabilized by hyperconjugation and other effects.

Summary Table: Functional Groups and Their Identification

Functional Group	Key Structure	How to Identify
Aldehyde	$\ce{R-CHO}$	C=O with H attached
Ketone	$\ce{R-CO-R'}$	C=O between two carbons
Phenyl	$\ce{C6H5-}$	Benzene ring as substituent
Sulfide	$\ce{R-S-R'}$	S atom between two carbons
Ether	$\ce{R-O-R'}$	O atom between two carbons