Organic Chemistry Quick Review

Overview

Introductory organic chemistry review covering bonding patterns, Lewis structures, bond polarity, ionic vs covalent bonding, hydrocarbon types, bond length/strength, sigma and pi bonds, hybridization, formal charge, and key functional groups with examples.

Element Bonding Preferences and Octet Rule

• Group 1 (e.g., H) forms 1 bond; Be 2; B 3; C 4; N 3; O 2; halogens 1.
• Second-row elements (C, N, O, F) prefer 8 electrons (octet) for stability.
• Knowing preferred bonds helps draw correct Lewis structures.

Covalent vs Ionic Bonds

• Covalent bonds share electrons; sharing can be equal or unequal.
• Nonpolar covalent: equal sharing (e.g., H2, C–H generally ~ΔEN 0.4).
• Polar covalent: unequal sharing; ΔEN ≥ 0.5 (e.g., C–F).
• Ionic bonds: electrons transferred; metals form cations, nonmetals form anions.
• Electrostatic attraction between opposite charges holds ionic crystals together.

Bond Polarity and Electronegativity

• Electronegativity examples: C 2.5; H 2.1; F 4.0.
• Polarized bond: partial charges due to electronegativity difference (δ+ and δ−).
• Hydrogen bonding: when H bonded directly to N, O, or F; explains water’s high boiling point.
• Hydrocarbons (C–H, C–C only) tend to be nonpolar molecules (e.g., CH4).

Drawing Lewis Structures: Core Steps

• Place central atom with typical valence (e.g., C makes 4 bonds).
• Add terminal atoms per bond preferences (H 1, halogens 1).
• Add multiple bonds to satisfy octets when needed (double, triple).
• Add lone pairs to complete octets on electronegative atoms.

Hydrocarbons: Alkanes, Alkenes, Alkynes

• Alkanes: saturated; formula CnH2n+2 (e.g., C1 to C10 names).
• Alkenes: contain at least one C=C double bond (unsaturated).
• Alkynes: contain C≡C triple bond (unsaturated).
• Ethane (C2H6), Ethene (C2H4), Ethyne (C2H2); common names: ethene = ethylene, ethyne = acetylene.

Bond Length, Strength, and Bond Order

• Single > double > triple in length; triple is shortest, single longest.
• Approximate C–C lengths: single 154 pm (1.54 Å); double 133 pm; triple 120 pm.
• Strength: triple strongest, single weakest (more bonds harder to break).
• Bond order: single 1; double 2; triple 3.

Sigma (σ) and Pi (π) Bonds

• Single bond: 1 σ.
• Double bond: 1 σ + 1 π.
• Triple bond: 1 σ + 2 π.
• σ bonds are stronger than π bonds (bond-to-bond comparison).

Hybridization: Atoms and Bonds

• Count groups (atoms + lone pairs) around atom to assign hybridization.
• 4 groups → sp3; 3 groups → sp2; 2 groups → sp.
• Bond hybridization labeling: combine the two atoms’ hybridizations (e.g., sp3–s for C–H).

Formal Charge and Electron Counting

• Formal charge = valence electrons − (bonds + dots).
• Lone pair = 2 nonbonding electrons; a bond = 2 bonding electrons.
• C with 3 bonds, 0 dots: +1 (carbocation).
• C with 3 bonds, 1 dot: 0 (radical, often neutral).
• C with 3 bonds, 2 dots: −1 (carbanion).
• Example ions: sulfur with 1 bond and 3 lone pairs → −1; ammonium N with 4 bonds → +1.
• Bonding electrons count by 2 per bond; nonbonding by 2 per lone pair.

Functional Groups and Representative Structures

• Alcohol: –OH on sp3 carbon (e.g., CH3CH2OH, ethanol).
• Aldehyde: carbonyl (C=O) at chain end with H (–CHO) (e.g., ethanal; acetaldehyde).
• Ketone: carbonyl within chain (no H on carbonyl carbon) (e.g., CH3–CO–CH3, propanone).
• Ether: R–O–R′ (e.g., CH3–O–CH3, dimethyl ether).
• Ester: carbonyl adjacent to –OR (two oxygens, one in C=O) (e.g., CH3–C(=O)–O–CH3, methyl ethanoate).
• Carboxylic acid: carbonyl + hydroxyl (–COOH) (e.g., pentanoic acid from five carbons).

Structural Expansion Tips

• CH3 (methyl) groups usually at ends; CH2 groups in chain middles.
• CH groups branch points; often carry substituents (e.g., Br, OH).
• Ensure each C has 4 bonds, O has 2 bonds, H has 1 bond; add lone pairs on electronegative atoms.

Common Alkane Names and Formulae

Bond Properties Summary

Hybridization Quick Guide

Key Terms & Definitions

• Polar covalent bond: electrons shared unequally; partial charges form.
• Nonpolar covalent bond: electrons shared equally; no net dipole in bond.
• Hydrogen bond: special interaction when H is bonded to N, O, or F.
• Cation: positively charged ion; anion: negatively charged ion.
• Carbocation: positively charged carbon species.
• Carbanion: negatively charged carbon species.
• Radical: species with an unpaired electron; often neutral.
• Carbonyl: C=O functional moiety; in aldehydes, ketones, acids, esters.

Action Items / Next Steps

• Practice drawing Lewis structures ensuring correct valences and octets.
• Identify bond polarity using electronegativity differences; mark partial charges.
• Memorize alkane names up to 10 carbons; apply CnH2n+2.
• Drill recognizing functional groups: alcohol, aldehyde, ketone, ether, ester, carboxylic acid.
• Compute formal charges using valence − (bonds + dots); verify neutrality/charge.
• Count σ and π bonds in given molecules; relate to bond order and reactivity.
• Assign hybridization by counting groups; label bond hybridization where asked.