CHEM 3451 Chapter 2: Molecular Representations

Vocabulary flashcards covering organic chemistry molecular representations, bond-line conventions, functional groups, formal charges, resonance theory, and electron delocalization.

Bond-line structure

A molecular representation where a zigzag line represents a carbon backbone, each corner or endpoint represents a carbon atom, hydrogen atoms on carbons are omitted, and heteroatoms with their attached hydrogens are explicitly drawn.

Constitutional isomers

Compounds with the exact same molecular formula (such as $C_3H_8O$) that differ in connectivity, rendering a simple molecular formula inadequate to distinguish between them.

Carbocation

A positively charged carbon atom that can have a maximum of 3 bonds and does not satisfy the octet rule.

Carbanion

A negatively charged carbon atom that possesses 3 bonds and one lone pair of electrons, thereby satisfying the octet rule.

Wedge (stereochemical bond)

A solid triangular bond notation used in three-dimensional structures to signify a group projecting out of the plane toward the viewer.

Dash (stereochemical bond)

A hatched bond notation used in three-dimensional structures to signify a group projecting into the plane away from the viewer.

Chiral carbon

A carbon atom bonded to four distinctly different groups.

Resonance

The representation of a molecule using two or more bond-line structures to describe $\pi$ molecular orbitals and electron density delocalized across more than two nuclei.

Resonance hybrid

A single composite structure representing the blend of all resonance forms that most closely resembles the major contributing structures.

Curved arrows (in resonance)

Arrows indicating the redistribution of electron pair density between resonance contributors, where the tail starts where electrons originate and the head points to where they relocate.

Rules of curved arrows for resonance

Two strict prohibitions: $\sigma$ bonds must never be broken, and the octet rule must never be exceeded for second-row elements.

Allylic lone pair pattern

A resonance pattern where a lone pair resides on an atom adjacent to a $\pi$ bond, delocalizing into the $\pi$ system using two curved arrows.

Allylic carbocation pattern

A resonance pattern characterized by a positive charge adjacent to a $\pi$ bond, allowing electron redistribution across three carbon atoms using one curved arrow.

Lone pair adjacent to $C^+$ pattern

A resonance pattern where a lone pair next to a carbocation moves to form a double bond using one curved arrow, converting a higher-energy localized state into a lower-energy delocalized state.

$\pi$ bond between atoms of differing electronegativity pattern

A resonance pattern where $\pi$ electrons move entirely toward the more electronegative atom using a single curved arrow.

Conjugated $\pi$ bonds in a ring pattern

A resonance pattern (such as in benzene) where alternating double bonds enclosed in a ring shift simultaneously using three curved arrows.

Rule 1 for assessing resonance significance

The resonance contributor with the greatest number of octet atoms is the most significant contributor.

Rule 2 for assessing resonance significance

The resonance structure possessing fewer formal charges is more significant than structures with more formal charges.

Rule 3 for assessing resonance significance

The resonance structure that places a negative formal charge on the more electronegative atom (or positive charge on the less electronegative atom) is more significant.

Delocalized lone pair

A lone pair located in an unhybridized p orbital adjacent to a parallel p orbital that participates in resonance and confers stability.

Localized lone pair

A lone pair that cannot overlap with an adjacent p orbital (such as the $sp^2$ hybrid orbital lone pair in pyridine) and therefore cannot participate in resonance.

Formal charge on oxygen: 1 bond and 3 lone pairs

Corresponds to a formal charge of $-1$ ($\ominus$).

Formal charge on oxygen: 2 bonds and 2 lone pairs

Corresponds to a neutral formal charge of zero (no charge).

Formal charge on oxygen: 3 bonds and 1 lone pair

Corresponds to a formal charge of $+1$ ($\oplus$).

Alkyl halide

A functional group consisting of an alkyl group attached to a halogen ($R-X$, where $X = \text{Cl}$, $\text{Br}$, or $\text{I}$).

Alkene

A hydrocarbon containing a carbon-carbon double bond ($C=C$).

Alkyne

A hydrocarbon containing a carbon-carbon triple bond ($C\equiv C$).

Alcohol

A functional group characterized by a hydroxyl group bound to a carbon chain ($R-OH$).

Ether

A functional group where an oxygen atom is singly bonded to two alkyl groups ($R-O-R$).

Thiol

A functional group composed of a sulfhydryl group bound to an alkyl group ($R-SH$).

Sulfide

A functional group where a sulfur atom is singly bonded to two alkyl groups ($R-S-R$).

Aromatic (arene)

A cyclic, fully conjugated, planar ring system, such as benzene or methylbenzene.

Ketone

A functional group with a carbonyl carbon bonded to two carbon-containing groups ($R-CO-R$).

Aldehyde

A functional group with a carbonyl carbon bonded to at least one hydrogen atom and one alkyl group ($R-CHO$).

Carboxylic acid

A functional group featuring a carbonyl group attached directly to a hydroxyl group ($R-COOH$).

Acyl halide

A functional group containing a carbonyl carbon bonded directly to a halogen atom ($R-COX$).

Anhydride

A functional group containing two carbonyl groups bridged by a single oxygen atom ($R-COOOC-R$).

Ester

A functional group containing a carbonyl carbon bonded to an alkoxy group ($R-COOR$).

Amide

A functional group featuring a carbonyl carbon bonded to a nitrogen atom ($R-CONR_2$).

Amine

A compound containing a basic nitrogen atom with a lone pair bonded to alkyl groups or hydrogens ($R-NR_2$).