Organic Chemistry and Bonding: Key Concepts and Formulas

Length affects strength, with longer being weaker and shorter equaling stronger bonds.

How does bond length affect strength?

Formal Charge formula

FC = #of valence electrons - [lone pair e- + 1/2bonding e-]

Sigma Bond

a bond formed when two atomic orbitals combine to form a molecular orbital that is symmetrical around the axis connecting the two atomic nuclei

Pi Bond

a bond that is formed when parallel orbitals overlap to share electrons.

Polar Bond

a covalent bond in which electrons are shared unequally

Degree of unsaturation formula

(2C+2+N-H-X)/2

structual isomers

differ in covalent arrangements of atoms and possible location of single and double bonds

Resonance hybrid

average of the resonance forms shown by the individual Lewis structures

delocalization of electrons

Refers to bonding electrons distributed among more than two atoms that are bonded together; occurs in species that exhibit resonance.

Lewis Acid

electron pair acceptor

Lewis Base

electron pair donor

Bronstead Acid

proton donor

Bronstead Base

proton acceptor

Acid strength in terms of equilibrium

If equilibrium lies far to the right, the acid is strong (dissociates almost completely).

If equilibrium lies to the left, the acid is weak (only partially dissociates).

Acid strength versus base strength

Strong acid ⇔ weak conjugate base

Weak acid ⇔ strong conjugate base

pKaH

pKa of the conjugate acid

Factors that influence acid strength

Electronegativity (Across a Period),Size of Atom (Down a Group),Resonance Stabilization,Inductive (Polar) Effect,Charge Effects,Hybridization of the Atom Bearing H

Electronegativity (Across a Period)

As electronegativity ↑ → conjugate base (A⁻) is more stable → acid strength ↑.

Size of Atom (Down a Group)

Larger atoms can spread out negative charge better → stronger acids.

Resonance Stabilization

Conjugate base stabilized by resonance → stronger acid.

Inductive (Polar) Effect

Atoms or groups that are electron-withdrawing (like -Cl, -NO₂, -CF₃, halogens in general) pull electron density through σ bonds toward themselves.

This "pull" stabilizes the negative charge on the conjugate base (A⁻).

A more stable conjugate base = a stronger acid.

Hybridization of the Atom Bearing H

More s-character = more electronegative = stronger acid.

Charge Effects

Positively charged species are more acidic (they want to lose H⁺).

Alkane

a hydrocarbon containing only single covalent bonds

General Formula For alkane

CnH2n+2

isopropyl

1-methylethyl

isobutyl

2-methylpropyl

secbutyl

1-methylpropyl

tertbutyl

1,1-dimethylethyl

Boiling point of alkanes

typically increases as the length of the carbon chain increases

melting point of alkanes

melting point increases as molecular weight increases

Newman projection

A method of visualizing a compound in which the line of sight is down a carbon-carbon bond axis.

dihedral angle

the angle between two specified groups in a Newman projection

eclipsed newman projection

-unfavorable

-steric hindrance

-less stable

-higher energy

guache staggered

two largest groups are 60 degrees apart

anti staggered conformation

the two largest groups are 180° apart, and strain is minimized

torsional strain

Caused by eclipsed bonds (bonding electron clouds repelling each other).

Steric Strain (a.k.a. Van der Waals repulsion)

Caused when bulky groups get too close, even if staggered.(CH₃ groups 180° apart) is lowest energy, gauche is higher, eclipsed highest.

Angle Strain

Occurs when bond angles deviate from the ideal tetrahedral 109.5°.