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Organic Chemistry Unit 1: Structure, bonding and functional groups

Organic molecules are built from carbon frameworks, and their shape, charge distribution and functional groups control their reactions.

Covers: Hybridization and molecular shape, drawing structures, formal charge and resonance, polarity and IMFs, functional groups, acids and bases in organic chemistry

Study guide

Organic chemistry is the chemistry of carbon compounds. Carbon forms four bonds, nitrogen three, oxygen two and hydrogen and halogens one. Hybridization explains shape: sp³ atoms are tetrahedral (109.5°), sp² atoms are trigonal planar (120°) with one π bond, and sp atoms are linear (180°) with two π bonds. A single bond is one σ bond, a double bond is one σ and one π, and a triple bond is one σ and two π. Chemists draw molecules with Lewis structures, condensed formulas and bond-line (skeletal) structures, where each vertex is a carbon and hydrogens on carbon are implied.

Formal charge = valence electrons − (nonbonding electrons + bonds). Resonance structures show electrons delocalized over several atoms; the real molecule is a hybrid, and curved arrows show electron movement from an electron source (lone pair or π bond) to a destination. Major resonance contributors have more full octets, fewer formal charges and negative charges on more electronegative atoms. Polarity arises from electronegativity differences and molecular shape, and it affects boiling points and solubility through intermolecular forces.

Functional groups are atoms that give molecules characteristic reactivity: alkanes, alkenes (C=C), alkynes (C≡C), aromatic rings, alkyl halides, alcohols (–OH), ethers (R–O–R), amines (–NH₂), aldehydes (–CHO), ketones (R–CO–R), carboxylic acids (–COOH), esters (–COOR) and amides (–CONR₂). Acid–base reactions are central: the stronger acid has the lower pKa, and equilibrium favors the weaker acid and weaker base. Factors that stabilize a conjugate base (and make an acid stronger) are summarized by ARIO: atom (larger and more electronegative atoms hold negative charge better), resonance, induction (nearby electronegative atoms) and orbital (more s-character holds charge better, so sp > sp² > sp³).

The heat of combustion or heat of hydrogenation can compare the stability of isomers. The more stable isomer releases less heat. More branched alkanes have lower heats of combustion than their straight-chain isomers. More substituted, trans or conjugated alkenes have lower heats of hydrogenation. Amines are bases because nitrogen has a lone pair. Alkylamines (conjugate acid pKa about 10–11) are more basic than ammonia, while aniline (about 4.6) is much weaker because its lone pair is delocalized into the ring. Amides and pyrrole are essentially non-basic, because their nitrogen lone pair is tied up in resonance (in pyrrole, in the aromatic ring).

A Brønsted acid donates a proton, and a Lewis acid accepts an electron pair. Carbocations, BF₃, AlCl₃ and FeBr₃ are common Lewis acids. In curved-arrow terms, every nucleophile is acting as a Lewis base and every electrophile as a Lewis acid.

Key ideas

• Hybridization (sp³, sp², sp) determines geometry and bond angles.
• Bond-line structures imply carbons and hydrogens.
• Resonance spreads electrons over several atoms; curved arrows show electron flow.
• Functional groups determine reactivity.
• ARIO explains acid strength through conjugate base stability.

Formulas and rules

Common mistakes

• Drawing curved arrows from atoms instead of from electrons (lone pairs or bonds).
• Thinking resonance structures interconvert; the real structure is a single hybrid.

Words to know

Hybridization

Mixing atomic orbitals to form new hybrid orbitals.

sp³ hybridized

Tetrahedral atom with 109.5° bond angles.

sp² hybridized

Trigonal planar atom with 120° bond angles.

Sigma bond

A bond formed by head-on orbital overlap.

Pi bond

A bond formed by side-by-side p orbital overlap.

Bond-line structure

A drawing where vertices are carbons and C–H bonds are implied.

Formal charge

The charge assigned to an atom in a Lewis structure.

Resonance

Delocalization of electrons shown by multiple structures.

Curved arrow

An arrow showing the movement of an electron pair.

Functional group

A group of atoms that gives characteristic reactivity.

Alcohol

A compound with an –OH group on an sp³ carbon.

Carbonyl group

A C=O group.

Aldehyde

A carbonyl compound with at least one H on the carbonyl carbon.

Ketone

A carbonyl compound with two carbon groups on the carbonyl carbon.

Carboxylic acid

A compound with a –COOH group.

Ester

A compound with a –COOR group.

Amide

A carbonyl compound bonded to nitrogen.

Amine

A compound containing a basic nitrogen with carbon groups.

pKa

A measure of acid strength; lower means stronger.

Inductive effect

Electron withdrawal through sigma bonds.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.