Introduction to Organic Chemistry: Structure, Nomenclature, and Properties

Organic Chemistry: Introduction and Historical Context

What Do I Smell? The Role of Organic Molecules

Organic chemistry is the study of carbon-containing compounds, many of which are responsible for the scents and flavors we experience daily. These molecules interact with our sensory receptors, producing characteristic smells.

• Benzyl acetate is an organic compound responsible for the sweet smell of jasmine.

• Organic molecules such as triethylamine are responsible for the smell of dead fish.

• Other examples include vanillin (vanilla) and cinnamaldehyde (cinnamon).

Example: When you smell jasmine, benzyl acetate molecules bind to receptors in your nose, triggering a nerve signal to your brain.

Vitalism: The Difference between Organic and Inorganic Compounds

Historical Classification and Properties

By the late 18th century, chemists classified compounds as either organic (from living things) or inorganic (from non-living sources).

• Organic compounds (e.g., sugar) were thought to be unique to living organisms and easily decomposed.

• Inorganic compounds (e.g., salt) were obtained from the earth and were more difficult to decompose.

• Organic compounds were believed to require a vital force for their synthesis, a concept known as vitalism.

Disproving Vitalism: Laboratory Synthesis of Urea

In 1828, Friedrich Wöhler synthesized urea (an organic compound) from ammonium cyanate (an inorganic compound), proving that organic compounds could be created in the laboratory without a vital force.

• Reaction:

$\mathrm{NH_4OCN} \xrightarrow{\text{Heat}} \mathrm{H_2NCONH_2}$

• This experiment opened the field of organic chemistry to scientific investigation.

Chemistry and the Origin of Life

Stanley Miller and Harold Urey's 1953 experiment simulated early Earth conditions and demonstrated that organic molecules, including amino acids (the building blocks of proteins), could form under prebiotic conditions.

• Organic compounds can be synthesized from simple molecules under the right conditions.

• This supports the idea that life's building blocks could arise through chemical evolution.

Carbon: A Versatile Atom

Bonding and Structural Diversity

Carbon forms more compounds than any other element due to its four valence electrons, allowing it to form four covalent bonds. Carbon atoms can bond to each other, creating chains, branches, and rings.

• Examples: Propane (chain), isobutane (branched), cyclohexane (ring).

Molecular Geometry

• Tetrahedral geometry: Four single bonds around carbon (e.g., methane, $\mathrm{CH_4}$).

• Trigonal planar geometry: Double bond and two single bonds (e.g., ethene, $\mathrm{C_2H_4}$).

• Linear geometry: Triple bond and a single bond (e.g., ethyne, $\mathrm{C_2H_2}$).

Hydrocarbons: Compounds Containing Only Carbon and Hydrogen

Classification and General Formulas

Hydrocarbons are classified into four main groups:

• Alkanes: Saturated hydrocarbons (single bonds), general formula $\mathrm{C_nH_{2n+2}}$

• Alkenes: Unsaturated hydrocarbons (at least one double bond), general formula $\mathrm{C_nH_{2n}}$

• Alkynes: Unsaturated hydrocarbons (at least one triple bond), general formula $\mathrm{C_nH_{2n-2}}$

• Aromatic hydrocarbons: Contain benzene rings

Physical Properties and Isomerism

• As the number of carbon atoms increases in n-alkanes, boiling points increase.

• Isomers are compounds with the same molecular formula but different structures (e.g., butane and isobutane).

Naming Hydrocarbons

Alkanes

• The base name is determined by the number of carbons in the longest chain (prefix + -ane).

• Branches (alkyl groups) are named as substituents.

• Number the chain to give substituents the lowest possible numbers.

• Use prefixes di-, tri-, tetra- for multiple identical substituents.

Example: 2,4-dimethylpentane

Alkenes and Alkynes

• Base name ends in -ene (alkenes) or -yne (alkynes).

• Number the chain to give the double or triple bond the lowest possible number.

• Indicate the position of the multiple bond before the base name.

Example: 3,4-dimethyl-3-hexene

Hydrocarbon Reactions

Combustion

All hydrocarbons undergo combustion in oxygen to produce carbon dioxide and water, releasing energy.

$\mathrm{C_xH_y} + O_2 \rightarrow CO_2 + H_2O$

Substitution (Alkanes)

In substitution reactions, hydrogen atoms are replaced by other atoms (commonly halogens).

$\mathrm{CH_4} + Cl_2 \rightarrow CH_3Cl + HCl$

Addition (Alkenes and Alkynes)

Atoms add across double or triple bonds, converting unsaturated hydrocarbons to saturated ones.

$\mathrm{CH_2=CH_2} + Cl_2 \rightarrow CH_2ClCH_2Cl$

Hydrogenation

Hydrogen is added across double or triple bonds, converting unsaturated fats (liquids) to saturated fats (solids).

Aromatic Hydrocarbons

Benzene and Its Structure

Benzene ($\mathrm{C_6H_6}$) is a stable aromatic hydrocarbon with a ring structure and resonance stabilization. All carbon-carbon bonds are equivalent, intermediate between single and double bonds.

Naming Aromatic Compounds

• Monosubstituted benzenes: (substituent)benzene (e.g., bromobenzene).

• Disubstituted benzenes: Numbered to give the lowest possible numbers; prefixes ortho- (1,2-), meta- (1,3-), para- (1,4-) are also used.

• Benzene as a substituent is called a phenyl group.

Functional Groups and Families of Organic Compounds

Overview of Functional Groups

Functional groups are characteristic atoms or groups of atoms that define the chemical behavior of organic compounds. The letter R represents a hydrocarbon group.

Functional Group	General Formula	Example
Alcohol	R–OH	Ethanol
Ether	R–O–R'	Diethyl ether
Aldehyde	R–CHO	Formaldehyde
Ketone	R–CO–R'	Acetone
Carboxylic Acid	R–COOH	Acetic acid
Ester	R–COOR'	Ethyl acetate
Amine	R–NH2	Methylamine

Alcohols

• Contain the –OH group; named with the suffix -ol.

• Number the chain to give the –OH group the lowest possible number.

• Example: 2-pentanol

Ethers

• General formula R–O–R'; common names use the format (alkyl)(alkyl) ether.

• Diethyl ether is a common laboratory solvent and former anesthetic.

Aldehydes and Ketones

• Both contain the carbonyl group (C=O).

• Aldehydes: R–CHO; named with the suffix -al.

• Ketones: R–CO–R'; named with the suffix -one.

• Many have distinctive odors (e.g., formaldehyde, carvone).

Carboxylic Acids and Esters

• Carboxylic acids: R–COOH; named with the suffix -oic acid.

• Esters: R–COOR'; named as alkyl alkanoates; responsible for fruity smells.

• Esters are formed from the reaction of a carboxylic acid and an alcohol.

$\mathrm{RCOOH} + \mathrm{R'OH} \rightarrow \mathrm{RCOOR'} + H_2O$

Amines

• Organic compounds containing nitrogen; derivatives of ammonia (NH3).

• Named according to the hydrocarbon groups attached to nitrogen, with the suffix -amine.

• Often have strong, unpleasant odors (e.g., trimethylamine, cadaverine).

Polymers

Structure and Types

Polymers are long, chainlike molecules made of repeating units called monomers.

• Addition polymers: Monomers link without eliminating atoms (e.g., polyethylene).

• Condensation polymers: Monomers link with the elimination of a small molecule, often water (e.g., nylon, Kevlar).

Polymer	Type	Uses
Polyethylene	Addition	Packaging, bottles
Polypropylene	Addition	Rope, kitchenware
Polystyrene	Addition	Packaging, foam cups
Nylon	Condensation	Fibers, fabrics
Kevlar	Condensation	Bulletproof vests

Kevlar: A Case Study

• Kevlar is a condensation polymer with aromatic rings and amide linkages.

• Its parallel, cross-linked chains provide high strength, chemical resistance, and flame resistance.

• Used in bulletproof vests and other protective gear.

Summary of Key Concepts

• Organic chemistry focuses on carbon-containing compounds and their reactions.

• Vitalism was disproved by the laboratory synthesis of organic compounds.

• Hydrocarbons are classified by their bonding and structure (alkanes, alkenes, alkynes, aromatics).

• Functional groups define the chemical properties of organic molecules.

• Polymers are important materials made from organic monomers.