First-year courses · Sciences

Introductory organic chemistry

Functional groups, IUPAC naming basics, isomers, chirality and R/S, resonance, and SN1, SN2, E1 and E2 reactions explained in introductory organic flashcards.

45 cards5 topicsAbout 20 minutes a roundTypical introductory organic chemistry unit or first organic chemistry course (structure, stereochemistry, substitution and elimination)

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About this deck

Organic chemistry rewards pattern recognition, and this deck is built to train the patterns you meet first. It covers the main functional groups, the basics of IUPAC naming, degrees of unsaturation, constitutional isomers and stereoisomers, chirality centres, R and S assignment, enantiomers, diastereomers and meso compounds, conformations, resonance and curved arrows, acidity, nucleophiles and electrophiles, carbocation stability, and the four core reaction types: SN1, SN2, E1 and E2.

You should already know Lewis structures, formal charge, hybridization and electronegativity from general chemistry. Being able to read skeletal (line-angle) structures, where each corner or line end is a carbon and hydrogens on carbon are not drawn, will help a great deal.

Organic is visual, so keep paper beside you. For a functional group card, draw an example molecule before flipping; for a stereochemistry card, draw the molecule and assign the configuration yourself. For the Substitution and elimination tag, practise by naming the substrate type (methyl, primary, secondary, tertiary), the nucleophile or base strength and the solvent, then predicting the mechanism; only then flip. Mixing those four mechanism cards repeatedly is the best way to stop confusing them.

Frequent mistakes: assigning R or S correctly but forgetting to reverse the answer when the lowest-priority group points toward you; moving atoms instead of electrons when drawing resonance structures; and predicting SN2 on a tertiary carbon, where crowding blocks the backside attack. Students also often confuse ketones with aldehydes; an aldehyde carbonyl carbon carries at least one hydrogen.

Before you start: First-year general chemistry, especially Lewis structures, VSEPR, hybridization and acid-base ideas.

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Cover the definitions with a sheet of paper and test yourself, or use the study modes above.

Functional groups

Hydrocarbon: A compound made only of carbon and hydrogen; families include alkanes, alkenes, alkynes and aromatics.Example: Hexane (C6H14) and benzene (C6H6).
Alkane: A saturated hydrocarbon with only C-C single bonds; acyclic members have the formula CnH2n+2.Example: Propane, C3H8.
Alkene: A hydrocarbon containing at least one carbon-carbon double bond (sp2 carbons, trigonal planar, about 120 degrees).Example: Ethene, CH2=CH2.
Alkyne: A hydrocarbon containing a carbon-carbon triple bond (sp carbons, linear, 180 degrees).Example: Ethyne (acetylene), HC triple-bonded to CH.
Aromatic ring: A flat, cyclic, fully conjugated ring with 4n + 2 pi electrons (Huckel's rule), giving unusual stability; benzene is the standard case.Example: Benzene has 6 pi electrons (n = 1).
Alcohol: A compound with an -OH group on an sp3 (saturated) carbon; classified as primary, secondary or tertiary by how many carbons attach to that carbon.Example: Ethanol, CH3CH2OH, is primary.
Ether: A compound with an oxygen bonded to two carbon groups, R-O-R'.Example: Diethyl ether, CH3CH2-O-CH2CH3.
Aldehyde: A carbonyl group (C=O) whose carbon is bonded to at least one hydrogen, written R-CHO.Example: Ethanal (acetaldehyde), CH3CHO.
Ketone: A carbonyl group bonded to two carbon groups, R-CO-R'.Example: Propanone (acetone), CH3COCH3.
Carboxylic acid: A carbonyl carbon that also carries an -OH group, -COOH; weakly acidic (typical pKa about 4 to 5).Example: Ethanoic (acetic) acid, CH3COOH, pKa 4.76.
Ester: A carbonyl carbon bonded to an -OR group, R-COO-R'; often fruity smelling and made from an acid and an alcohol.Example: Ethyl ethanoate, CH3COOCH2CH3.
Amine: An organic derivative of ammonia with one, two or three carbon groups on nitrogen; weakly basic.Example: Methylamine, CH3NH2.
Amide: A carbonyl carbon bonded to a nitrogen, R-CO-NR2; the link in proteins (peptide bonds).Example: Acetamide, CH3CONH2.
Alkyl halide: A compound with a halogen (F, Cl, Br, I) bonded to an sp3 carbon; the usual substrate for substitution and elimination.Example: 2-bromopropane is a secondary example.

Naming

IUPAC parent chain: The longest continuous carbon chain (or the one containing the main functional group) gives the base name; other groups become substituents.Example: CH3CH(CH3)CH2CH2CH3 has a five-carbon chain: a pentane.
Lowest locants rule: Number the main chain from the end that gives the main functional group, then substituents, the lowest possible numbers.Example: 2-methylpentane, not 4-methylpentane.
Chain-length prefixes: meth (1), eth (2), prop (3), but (4), pent (5), hex (6), hept (7), oct (8), non (9), dec (10).Example: Heptane has 7 carbons.
Functional group suffixes: -ol for alcohols, -al for aldehydes, -one for ketones, -oic acid for carboxylic acids, -amine for amines.Example: Propan-2-ol (isopropyl alcohol); butanoic acid.
Degree of unsaturation: The number of rings plus pi bonds, from (2C + 2 + N - H - X)/2 for a formula with C, H, N and halogens X.Example: C6H6: (12 + 2 - 6)/2 = 4 (one ring, three pi bonds).

Isomers and stereochemistry

Constitutional isomers: Compounds with the same molecular formula but atoms connected in a different order.Example: Butane and 2-methylpropane are both C4H10.
Stereoisomers: Compounds with the same formula and the same connectivity that differ only in the 3D arrangement of atoms.Example: cis- and trans-2-butene.
Cis-trans isomers: Stereoisomers arising from restricted rotation (a double bond or ring): matching groups on the same side (cis) or opposite sides (trans).Example: cis-2-butene has both methyl groups on the same side of C=C.
E and Z labels: Rank the two groups on each alkene carbon by CIP priority: higher-priority groups on the same side is Z, opposite sides is E.Example: In 1-bromo-1-chloropropene the label depends on where Br sits relative to the CH3.
Chirality centre: A tetrahedral atom, usually carbon, bonded to four different groups; it makes the molecule non-superimposable on its mirror image (unless the molecule is meso).Example: C2 of butan-2-ol bears H, OH, CH3 and CH2CH3.
Enantiomers: Stereoisomers that are non-superimposable mirror images; they share most physical properties but rotate plane-polarized light in opposite directions.Example: (R)- and (S)-butan-2-ol.
Diastereomers: Stereoisomers that are not mirror images of each other; they have different physical properties.Example: (2R,3R) and (2R,3S) forms of 2,3-dibromopentane.
Meso compound: A molecule with chirality centres that is achiral overall because an internal mirror plane makes it identical to its mirror image.Example: (2R,3S)-tartaric acid is meso and optically inactive.
CIP priority rules: Rank attached atoms by atomic number; if tied, compare the next set of atoms outward; double bonds count as duplicated atoms.Example: -OH > -CH2CH3 > -CH3 > -H.
R and S configuration: With priority 4 pointing away, trace 1 to 2 to 3: clockwise is R, counterclockwise is S. If priority 4 points toward you, reverse the result.Example: Tracing clockwise with H toward the viewer means the centre is actually S.
Racemic mixture: A 50:50 mixture of two enantiomers; it shows no net optical rotation.Example: SN1 reactions at a chirality centre often give nearly racemic products.
Maximum number of stereoisomers: A molecule with n chirality centres has at most 2^n stereoisomers; meso forms reduce the count.Example: Two centres: up to 4; tartaric acid has only 3 because one is meso.
Conformations: Shapes a molecule takes by rotating around single bonds; staggered forms are lower in energy than eclipsed ones, and chair cyclohexane prefers bulky groups equatorial.Example: Methylcyclohexane is mostly in the chair with CH3 equatorial.

Resonance and acidity

Resonance hybrid: The real electron distribution of a species that cannot be shown by one Lewis structure; it is a weighted average of the contributing structures, not a flip between them.Example: In the acetate ion, both C-O bonds are identical, each about 1.5 bonds.
Curved arrow notation: Arrows showing electron movement: tail at the electrons (lone pair or bond), head where they end up. Full arrows mean pairs; half arrows mean single electrons.Example: A lone pair on O- pushing into a C=O while the pi bond moves onto the other O.
Rules for drawing resonance contributors: Only electrons (pi bonds and lone pairs) move; atoms and sigma bonds stay put, total charge is unchanged, and second-row atoms never exceed an octet.Example: Moving an H to draw a new form makes a tautomer, a different compound.
Factors that stabilize a conjugate base: Acid strength rises when the negative charge sits on a more electronegative or larger atom, is spread by resonance, or is pulled by nearby electron-withdrawing groups.Example: Acetic acid (pKa 4.8) is far stronger than ethanol (pKa about 16) because acetate is resonance stabilized.

Substitution and elimination

Nucleophile: An electron-rich species with a lone pair or pi bond that donates electrons to form a new bond; a Lewis base.Example: OH-, CN-, NH3 and I-.
Electrophile: An electron-poor species or atom that accepts an electron pair; a Lewis acid.Example: The carbon bonded to Br in CH3Br; a carbocation.
Leaving group: The atom or group that departs with the bonding electron pair; good ones are weak bases, stable after leaving.Example: I- > Br- > Cl- >> F-; OH- is poor unless protonated to water.
Carbocation stability: Tertiary > secondary > primary > methyl, because alkyl groups donate electron density (hyperconjugation and induction); resonance also stabilizes.Example: (CH3)3C+ forms far more easily than CH3+.
SN2 mechanism: One-step substitution: the nucleophile attacks from the back as the leaving group leaves, inverting configuration. Rate = k[substrate][Nu]; best for methyl and primary, polar aprotic solvents.Example: CH3CH2Br + OH- gives CH3CH2OH in one concerted step.
SN1 mechanism: Two-step substitution: the leaving group departs to form a carbocation, then a nucleophile adds. Rate = k[substrate]; favoured by tertiary substrates and polar protic solvents; gives racemization.Example: (CH3)3CBr in water gives (CH3)3COH.
E2 mechanism: One-step elimination: a strong base removes a beta hydrogen anti-periplanar to the leaving group as the double bond forms. Rate = k[substrate][base].Example: 2-bromobutane with ethoxide gives mostly 2-butene.
E1 mechanism: Two-step elimination through a carbocation, then loss of a beta proton; favoured by tertiary substrates, weak bases and heat; often competes with SN1.Example: Heating tert-butyl alcohol in acid gives 2-methylpropene.
Zaitsev's rule: In most eliminations the major product is the more substituted, more stable alkene; very bulky bases can instead favour the less substituted (Hofmann) product.Example: 2-bromobutane gives more 2-butene than 1-butene with ethoxide.