1. What organic chemistry studies

Organic chemistry is the study of carbon-containing compounds, especially those built around covalent carbon-carbon and carbon-heteroatom bonds. It matters because carbon can form chains, rings, branched frameworks, and multiple bonds, giving rise to an enormous number of molecules with distinct properties.

At the study level, the subject usually reduces to four questions:

1. What is the structure?

2. How reactive is it?

3. What product forms from a given set of conditions?

4. Why does that product dominate?

The answer is usually determined by a small set of ideas:

• Bonding and hybridization

• Functional groups

• Resonance and inductive effects

• Stereochemistry

• Mechanistic steps

• Thermodynamic and kinetic control

Organic chemistry is less about memorizing isolated reactions and more about recognizing patterns.

2. Atomic structure and bonding

Carbon bonding

Carbon is tetravalent and most commonly forms four bonds by using hybrid orbitals.

Single bonds are sigma bonds. Double bonds contain one sigma and one pi bond. Triple bonds contain one sigma and two pi bonds.

Sigma and pi bonds

Sigma bonds are formed by end-to-end orbital overlap and allow free rotation unless restricted by the molecular framework. Pi bonds are formed by side-by-side overlap and restrict rotation.

This is why alkenes can have cis/trans or E/Z isomerism, while ordinary alkanes usually cannot.

Resonance

Resonance describes delocalization of electrons over multiple atoms. A resonance hybrid is more stable than any one contributing structure when electron density can be spread out.

Useful indicators of resonance stabilization:

• Lone pair adjacent to a pi bond

• Positive charge adjacent to a pi bond

• Negative charge adjacent to a pi bond

• Aromatic ring conjugation

Inductive effects

Electron-withdrawing groups pull electron density through sigma bonds. Electron-donating groups push electron density through sigma bonds or resonance. These effects influence acidity, basicity, and reaction rate.

Examples of strong electron-withdrawing groups:

• Carbonyls

• Halogens

• Nitro groups

• Trifluoromethyl groups

3. Functional groups and naming

Functional groups determine the identity and reactivity of a molecule. When solving problems, identify the highest-priority functional group first, then build the rest of the analysis around it.

Naming strategy

Use this sequence:

1. Find the longest carbon chain or parent ring.

2. Identify the highest-priority functional group.

3. Number the parent to give the principal group the lowest possible locant.

4. Name substituents alphabetically.

5. Use prefixes for multiplicity such as di-, tri-, tetra-.

Common suffix priorities in introductory organic chemistry generally follow the pattern:

carboxylic acid > ester > amide > aldehyde > ketone > alcohol > amine > alkene > alkyne > alkane

This priority determines the suffix, not whether the molecule contains lower-priority groups.

Ring and aromatic naming

Benzene derivatives are common. If a benzene ring contains a principal functional group, that group defines the parent name. Otherwise, the ring can be named as a phenyl substituent or as a substituted benzene.

4. Isomerism and stereochemistry

Constitutional isomers

Constitutional isomers have the same molecular formula but different connectivity.

Example:

• Butane and isobutane both have formula $C_4H_{10}$, but the atoms are connected differently.

Stereoisomers

Stereoisomers have the same connectivity but differ in 3D arrangement.

Enantiomers

Enantiomers are non-superimposable mirror images. They usually have identical physical properties in achiral environments except for optical rotation and behavior in chiral systems.

Diastereomers

Diastereomers are stereoisomers that are not mirror images. They usually have different physical and chemical properties.

Chirality

A molecule is chiral if it is not superimposable on its mirror image.

Common causes of chirality:

• A tetrahedral carbon bonded to four different groups

• Axial chirality in some biaryl systems

• Helical chirality in some extended structures

R/S assignment

Use the Cahn-Ingold-Prelog rules:

1. Rank substituents by atomic number.

2. Orient the lowest-priority group away from you.

3. Trace 1 -> 2 -> 3.

4. Clockwise is $R$, counterclockwise is $S$.

E/Z alkenes

Use E/Z when cis/trans is insufficient.

• If the highest-priority groups on each alkene carbon are on the same side, the alkene is $Z$.

• If they are on opposite sides, the alkene is $E$.

Conformations

Conformations differ by rotation about single bonds.

Important examples:

• Staggered conformations are lower in energy than eclipsed conformations.

• In cyclohexane, the chair conformation is more stable than the boat conformation.

• Axial substituents can experience 1,3-diaxial strain.

5. Acidity, basicity, and electron flow

Organic chemistry problems often begin with a proton transfer or end with one. Acidity and basicity control what species are present and therefore what can react.

Definitions

• An acid donates a proton.

• A base accepts a proton.

• A nucleophile donates an electron pair to form a bond.

• An electrophile accepts an electron pair.

pKa as a stability measure

Lower pKa means stronger acid. Stronger acids have more stable conjugate bases.

General stability trends for conjugate bases:

1. More electronegative atom bearing the charge

2. More resonance delocalization

3. More s-character at the charged atom

4. Greater inductive withdrawal nearby

5. Larger atom size for a given charge type

Examples:

• Terminal alkynes are more acidic than alkenes and alkanes because $sp$ carbon has more s-character.

• Carboxylic acids are much more acidic than alcohols because the carboxylate conjugate base is resonance-stabilized.

Curved arrows

Curved arrows track electron movement, not atom movement. A valid arrow starts at an electron pair or bond and ends at an atom or bond where electrons will go.

Common mistakes:

• Starting an arrow at an atom instead of electrons

• Breaking octets without justification

• Creating impossible valence states for second-row atoms

Hard and soft intuition

In many cases, harder and smaller nucleophiles favor more polar, less polarizable centers, while softer and more polarizable nucleophiles are better matched to softer electrophiles. This idea becomes especially useful in advanced synthesis and carbonyl chemistry.

6. Reaction mechanisms

Mechanisms explain how a reaction happens step by step. A mechanism should conserve atoms, charge, and valence at every stage.

The main mechanistic families

Rate and selectivity

Mechanistic outcomes depend on:

• Substrate structure

• Leaving group quality

• Nucleophile/base strength

• Solvent

• Temperature

• Steric hindrance

• Carbocation or radical stability

Energy profile thinking

Fast reactions usually have a lower activation energy. The major product is often the one formed by the lowest-energy pathway unless reversibility or equilibrium changes the picture.

Useful distinction:

• Kinetic control: product forms fastest

• Thermodynamic control: product is most stable

7. Core reaction families

Substitution reactions

SN2

SN2 is a one-step backside attack with inversion of configuration.

Conditions that favor SN2:

• Methyl or primary substrates

• Strong nucleophile

• Polar aprotic solvent

• Minimal steric hindrance

SN1

SN1 proceeds through a carbocation intermediate.

Conditions that favor SN1:

• Tertiary substrates

• Good leaving group

• Polar protic solvent

• Weak to moderate nucleophile is often sufficient

The carbocation can rearrange by hydride or alkyl shift if a more stable cation is available.

Elimination reactions

E2

E2 is a one-step elimination that requires an anti-periplanar arrangement between the leaving group and the beta hydrogen.

It is favored by:

• Strong base

• Heat

• Secondary or tertiary substrates

• Good leaving group

E1

E1 proceeds through a carbocation and often competes with SN1. Heat often shifts the balance toward elimination.

Addition to alkenes

Common alkene additions include:

• Hydrogenation

• Hydrohalogenation

• Halogenation

• Halohydrin formation

• Hydroboration-oxidation

• Oxymercuration-demercuration

Regiochemistry matters:

• Markovnikov addition places the proton on the carbon that already has more hydrogens, leaving the positive or substituted center on the more substituted carbon in the intermediate or product logic.

• Anti-Markovnikov additions occur in some radical or hydroboration pathways.

Radical chemistry

Radical reactions involve single-electron steps.

Key points:

• Initiation creates radicals.

• Propagation keeps the chain going.

• Termination combines radicals.

Radical stability often follows:

tertiary > secondary > primary > methyl, with allylic and benzylic radicals additionally stabilized by resonance.

8. Carbonyl chemistry

Carbonyl compounds are central because the C=O bond is polarized and the carbon is electrophilic.

Why carbonyls react

The carbonyl carbon is electron-poor because oxygen withdraws electron density and stabilizes the resulting oxyanion after nucleophilic attack.

Nucleophilic addition

Aldehydes and ketones usually undergo nucleophilic addition because they have no good leaving group on the carbonyl carbon.

Common transformations:

• Reduction to alcohols

• Cyanohydrin formation

• Grignard or organolithium addition

• Hemiacetal and acetal formation

• Imine and enamine formation

Carboxylic acid derivatives

Acyl derivatives usually undergo nucleophilic acyl substitution.

Relative reactivity often decreases in this order:

acyl chloride > anhydride > ester > amide > carboxylate

The trend follows leaving group ability and resonance stabilization.

Alpha chemistry

Hydrogens alpha to carbonyls can be acidic because the resulting enolate is resonance-stabilized.

Enols and enolates are important in:

• Aldol reactions

• Claisen condensations

• Alkylation at alpha carbon

• Racemization at stereogenic alpha centers

Aldol logic

An aldol reaction forms a new carbon-carbon bond by coupling an enolate with a carbonyl compound. The product often contains both an alcohol and a carbonyl group.

The most important practical step is deciding whether the carbonyl partner can form an enolate and whether the reaction conditions favor self-condensation or crossed condensation.

9. Spectroscopy and structure determination

Structure determination is usually a puzzle with several data sources.

IR spectroscopy

IR detects bond vibrations and is especially useful for identifying functional groups.

NMR spectroscopy

NMR is the most important structure tool in organic chemistry.

1H NMR

Useful outputs:

• Chemical shift gives electronic environment

• Integration gives relative number of hydrogens

• Splitting gives neighboring nonequivalent hydrogens

The $n+1$ rule is a first approximation: a proton with $n$ equivalent neighboring protons appears as an $(n+1)$-plet.

13C NMR

13C NMR shows the number and type of carbon environments. Carbonyl carbons appear far downfield compared with alkyl carbons.

Mass spectrometry

Mass spectrometry gives molecular mass and can reveal fragments that support a proposed structure.

Clue examples:

• A strong M+2 peak can suggest chlorine or bromine.

• Even/odd mass patterns can suggest nitrogen content in some cases.

Degree of unsaturation

The degree of unsaturation, also called index of hydrogen deficiency, helps count rings and pi bonds.

For a molecule with formula $C_cH_hN_nX_x$, where $X$ is a halogen:

Each ring or pi bond counts as one degree. A triple bond counts as two.

10. Organic chemistry in biology

Organic chemistry is the language of biology at the molecular level.

Amino acids and proteins

Amino acids contain both an amine and a carboxylic acid. In water they often exist as zwitterions. Peptide bonds are amide linkages, which are relatively stable because of resonance.

Important ideas:

• Side chains control polarity and charge

• Protein folding depends on hydrogen bonding, hydrophobic effects, ionic interactions, and disulfide bonds

• Enzyme active sites use acid-base, covalent, and metal-ion catalysis

Carbohydrates

Carbohydrates are polyhydroxy aldehydes, ketones, or derivatives. They commonly exist as cyclic hemiacetals or hemiketals.

Key ideas:

• Anomers differ at the anomeric carbon

• Mutarotation reflects ring opening and re-closing

• Glycosidic bonds link monosaccharides

Lipids

Lipids are often built from long hydrocarbon chains and ester or amide linkages.

Examples:

• Fatty acids

• Triacylglycerols

• Phospholipids

• Steroids

The balance between polar head groups and nonpolar tails drives membrane structure and transport behavior.

Nucleic acids

DNA and RNA are polymers of nucleotides connected by phosphodiester bonds. Base pairing depends on hydrogen bonding and pi stacking. Mutations often change molecular recognition rather than simply adding or removing atoms.

11. Problem-solving workflow

When a problem looks unfamiliar, use the same sequence every time.

Step 1: Identify the functional group

Name every functional group first. This narrows the reaction class dramatically.

Step 2: Count electrons and charges

Check formal charge, lone pairs, and possible resonance forms.

Step 3: Look for the reactive site

Ask whether the key atom is:

• Nucleophilic

• Electrophilic

• Acidic

• Basic

• Radical-prone

Step 4: Check stereochemistry

Decide whether the problem cares about:

• Configuration ($R/S$)

• Alkene geometry ($E/Z$)

• Conformation

• Stereoselectivity

Step 5: Apply conditions

Reagents matter as much as the substrate. The same starting material can give different products under acidic, basic, oxidative, reductive, or radical conditions.

Step 6: Verify the product

Confirm that:

• Atom count is conserved

• Charge is conserved

• Octets are reasonable

• Stereochemistry matches the mechanism

• Regiochemistry matches the pathway

Common traps

• Confusing SN1 with SN2

• Forgetting carbocation rearrangements

• Ignoring anti-periplanar geometry in E2

• Misreading resonance as atom movement

• Missing the role of acid or base in carbonyl chemistry

• Treating a functional group as if it were isolated from the rest of the molecule

12. Formula summary

Core relations

Quick reactivity summary

Aromaticity checklist

A ring is aromatic when it is:

• Cyclic

• Planar or nearly planar

• Fully conjugated

• Contains $4n+2$ pi electrons

If it is cyclic and conjugated but has $4n$ pi electrons, it is antiaromatic if planar and destabilized if forced toward planarity.

Final study rule

When learning a new reaction, always ask:

1. What is the nucleophile?

2. What is the electrophile?

3. What is the leaving group?

4. What intermediate forms, if any?

5. What controls regioselectivity and stereoselectivity?

If you can answer those five questions, most organic chemistry problems become manageable.

Sources

• OpenStax University Physics

• Physics LibreTexts

• Halliday, Resnick, and Walker, Fundamentals of Physics

• Serway and Jewett, Physics for Scientists and Engineers

• Griffiths, Introduction to Electrodynamics

• Griffiths, Introduction to Quantum Mechanics

• Taylor, Classical Mechanics

• Parell GitHub repository