Chapter 22: Organic Chemistry – Structure, Nomenclature, and Reactions

Organic Chemistry: Introduction and Scope

Definition and Importance

Organic chemistry is the branch of chemistry that studies compounds containing carbon, except for a few simple compounds such as CO, CO2, carbonates, and carbides. Organic compounds are central to life and technology, forming the basis of biomolecules, fuels, plastics, and pharmaceuticals.

• Odorants are often organic molecules responsible for fragrances and odors.

• Organic chemistry explores the properties, structures, and reactions of carbon-containing compounds.

Unique Properties of Carbon

Bonding and Structure

• Carbon forms up to four strong covalent bonds with other atoms, including itself.

• It can create single, double, or triple bonds, and can catenate (form chains or rings).

• This versatility leads to millions of different organic compounds.

Hydrocarbons

Classification and Properties

• Hydrocarbons contain only carbon and hydrogen.

• They are divided into aliphatic (chains/rings, saturated or unsaturated) and aromatic (contain benzene rings) hydrocarbons.

• Hydrocarbons are generally insoluble in water due to nonpolar bonds.

Types of Hydrocarbons

Type	Bonding	General Formula	Example
Alkanes	Single bonds (saturated)	$\mathrm{C}_n\mathrm{H}_{2n+2}$	Methane ($\mathrm{CH}_4$)
Alkenes	At least one double bond (unsaturated)	$\mathrm{C}_n\mathrm{H}_{2n}$	Ethene ($\mathrm{C}_2\mathrm{H}_4$)
Alkynes	At least one triple bond (unsaturated)	$\mathrm{C}_n\mathrm{H}_{2n-2}$	Ethyne ($\mathrm{C}_2\mathrm{H}_2$)
Aromatic	Benzene ring(s)	Varies	Benzene ($\mathrm{C}_6\mathrm{H}_6$)

Additional info: The formulas above apply to noncyclic structures with no more than one multiple bond.

Formulas and Models

• Molecular formula: Shows the number and type of atoms (e.g., $\mathrm{C}_2\mathrm{H}_6$).

• Structural formula: Shows how atoms are bonded (e.g., $\mathrm{CH}_3\mathrm{CH}_3$).

• Condensed formula: Groups hydrogens with their attached carbons (e.g., $\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_3$).

• Line (skeletal) formula: Shows carbon skeleton as lines; hydrogens are implied.

Isomerism

Structural Isomers

• Compounds with the same molecular formula but different connectivity.

• Example: $\mathrm{C}_4\mathrm{H}_{10}$ can be butane or isobutane.

Stereoisomers

• Same connectivity, different spatial arrangement.

• Geometric (cis–trans) isomers: Restricted rotation around double bonds leads to different spatial arrangements.

• Optical isomers (enantiomers): Non-superimposable mirror images, often due to a chiral center (carbon with four different substituents).

Optical Activity

• Enantiomers rotate plane-polarized light in opposite directions: dextrorotatory (right), levorotatory (left).

• A racemic mixture contains equal amounts of both enantiomers and does not rotate light.

Physical Properties of Hydrocarbons

• Boiling and melting points increase with molar mass.

• Hydrocarbons are nonpolar and less dense than water.

• Dispersion forces are the main intermolecular attractions.

Naming Hydrocarbons (IUPAC System)

Alkanes

• Find the longest continuous carbon chain (base name).

• Name substituents (alkyl groups) and number the chain from the end nearest a branch.

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

• Write the name as: (substituent number)-(substituent name)(base name).

Alkenes and Alkynes

• Base chain must include the double or triple bond.

• Number the chain from the end closest to the multiple bond.

• Use the suffix –ene for alkenes and –yne for alkynes.

• Indicate the position of the multiple bond with a number.

Aromatic Hydrocarbons

• Base name is benzene; substituents are named as prefixes.

• For disubstituted benzenes, use ortho- (1,2-), meta- (1,3-), para- (1,4-) or numbers.

• When benzene is a substituent, it is called a phenyl group.

Hydrocarbon Reactions

Combustion

• Highly exothermic reaction with oxygen, producing CO2 and H2O.

• General equation: $\mathrm{C}_x\mathrm{H}_y + O_2 \rightarrow CO_2 + H_2O$

Halogenation (Alkanes)

• Substitution of hydrogen with a halogen (e.g., Cl, Br), initiated by heat or UV light.

• Example: $\mathrm{CH}_4 + Cl_2 \rightarrow \mathrm{CH}_3Cl + HCl$

Addition Reactions (Alkenes and Alkynes)

• Hydrogenation: Addition of H2 to convert unsaturated to saturated hydrocarbons (requires catalyst).

• Halogenation: Addition of X2 (X = Cl, Br, etc.).

• Hydrohalogenation: Addition of HX (X = halogen).

Aromatic Substitution

• Benzene undergoes substitution rather than addition due to resonance stabilization.

Functional Groups and Families of Organic Compounds

Definition

• A functional group is a specific group of atoms that imparts characteristic chemical properties to an organic molecule.

• General symbol: R (for hydrocarbon chain).

Family	Functional Group	General Formula	Example
Alcohols	–OH (hydroxyl)	R–OH	Ethanol
Aldehydes	–CHO (carbonyl at end)	R–CHO	Formaldehyde
Ketones	–CO– (carbonyl within chain)	R–CO–R'	Acetone
Carboxylic Acids	–COOH (carboxyl)	R–COOH	Acetic acid
Esters	–COOR	R–COOR'	Ethyl acetate
Ethers	–O–	R–O–R'	Diethyl ether
Amines	–NH2, –NHR, –NR2	R–NH2	Methylamine
Amides	–CONH2	R–CONH2	Acetamide

Alcohols

• Contain the hydroxyl (–OH) group.

• Named by replacing the –e ending of the parent alkane with –ol.

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

• Common examples: Ethanol (drinking alcohol), isopropyl alcohol (rubbing alcohol), methanol (wood alcohol).

Reactions of Alcohols

• Substitution: Replacement of –OH with another group.

• Elimination (dehydration): Removal of water to form alkenes (acid-catalyzed).

• Oxidation: Primary alcohols → aldehydes → carboxylic acids; secondary alcohols → ketones.

Aldehydes and Ketones

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

• Aldehydes: Carbonyl at the end of the chain (–CHO).

• Ketones: Carbonyl within the chain (–CO–).

• Named by replacing the –e ending with –al (aldehydes) or –one (ketones).

• Many have distinctive odors and flavors (e.g., benzaldehyde in almonds, carvone in spearmint).

Reactions

• Oxidation of alcohols forms aldehydes and ketones.

• Reduction converts them back to alcohols.

• Addition reactions occur at the polar carbonyl group.

Carboxylic Acids and Esters

Carboxylic Acids

• Contain the carboxyl group (–COOH).

• Named by replacing the –e ending with –oic acid.

• Always at the end of the chain; highest naming precedence.

• Examples: Acetic acid (vinegar), formic acid (insect stings), citric acid (citrus fruits).

Esters

• Formed by condensation of a carboxylic acid and an alcohol (acid-catalyzed).

• Named as alkyl (from alcohol) + carboxylate (from acid, ending –oate).

• Often have sweet odors (e.g., in fruits and perfumes).

Condensation Reactions

• Organic reactions where a small molecule (often water) is eliminated as two molecules combine.

• Example: Synthesis of esters and amides.

Ethers

• General formula: R–O–R'

• Named as (alkyl group 1) (alkyl group 2) ether (common names).

• Diethyl ether is a common laboratory solvent with a low boiling point.

Amines

• Organic derivatives of ammonia (NH3), with one or more hydrogens replaced by alkyl groups.

• Named by listing alkyl groups attached to nitrogen, followed by –amine.

• Often have strong, unpleasant odors (e.g., from decomposing proteins).

• Act as weak bases; react with acids to form ammonium salts and with carboxylic acids to form amides (condensation reaction).

Summary Table: Key Functional Groups

Class	Functional Group	Suffix/Prefix	Example Name
Alcohol	–OH	-ol	Ethanol
Aldehyde	–CHO	-al	Ethanal
Ketone	–CO–	-one	Propanone
Carboxylic Acid	–COOH	-oic acid	Ethanoic acid
Ester	–COOR	-oate	Ethyl ethanoate
Ether	–O–	ether	Diethyl ether
Amine	–NH2	-amine	Methylamine
Amide	–CONH2	-amide	Acetamide

Key Equations and Concepts

• General formulas:

  • Alkanes: $\mathrm{C}_n\mathrm{H}_{2n+2}$

  • Alkenes: $\mathrm{C}_n\mathrm{H}_{2n}$

  • Alkynes: $\mathrm{C}_n\mathrm{H}_{2n-2}$

• Combustion reaction:

  • $\mathrm{C}_x\mathrm{H}_y + O_2 \rightarrow CO_2 + H_2O$

• Halogenation (alkane):

  • $\mathrm{RH} + X_2 \rightarrow \mathrm{RX} + HX$

• Hydrogenation (alkene/alkyne):

  • $\mathrm{RCH=CHR'} + H_2 \rightarrow \mathrm{RCH_2CH_2R'}$

• Esterification (condensation):

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

Applications and Examples

• Fragrances and flavors: Many are due to specific organic molecules (e.g., carvone in spearmint, benzaldehyde in almonds).

• Fuels: Hydrocarbons are the main component of gasoline, natural gas, etc.

• Polymers: Ethylene is used to make polyethylene plastic.

• Medicines: Aspirin is an ester (acetylsalicylic acid).

Additional info: This summary covers the main concepts, nomenclature, and reactions of organic chemistry as presented in a general chemistry course, with emphasis on structure, isomerism, and functional groups.