Organic Chemistry I – Structures: Hydrocarbons and Functional Groups

Introduction to Organic Chemistry

Definition and Scope

Organic chemistry is the study of molecules that contain carbon. Organic compounds are fundamental to life and technology, and include a wide variety of substances:

• Proteins, Sugars, DNA, Vitamins, Alcohols, Pharmaceuticals

• Rubbers, Plastics, Soaps, Gasoline, Explosives, Artificial flavourings

The Chemistry of Carbon

Carbon is unique in its ability to form a diverse array of molecules due to several key properties:

• It can form long chains (catenation).

• It can form single, double, and triple bonds.

• C–C and C–H bonds are very stable.

• Organic molecules are primarily composed of C, H, O, and N.

• Functional groups add character and different properties to hydrocarbons.

Hydrocarbons

Classification and Nomenclature

Hydrocarbons are organic molecules composed entirely of carbon and hydrogen. Greek prefixes are used to indicate the number of carbon atoms in a molecule:

# of C	Greek prefix	# of C	Greek prefix
1	meth	6	hex
2	eth	7	hept
3	prop	8	oct
4	but	9	non
5	pent	10	dec

Representations of Organic Molecules

Organic molecules can be represented in several ways, each providing different levels of detail:

• Structural formula: Shows all atoms and bonds explicitly.

• Condensed structural formula: Groups atoms together, omitting some bonds for simplicity.

• Carbon skeleton (bond-line) formula: Shows only the carbon backbone and functional groups, omitting hydrogen atoms bonded to carbon.

For large molecules, Lewis structures are often too cumbersome, so bond-line structures are preferred for clarity and simplicity.

Bond-Line Structures and 3D Notation

Bond-line structures omit hydrogen atoms bonded to carbon and focus on the connectivity of the carbon skeleton. To represent three-dimensionality:

• Line: Bond lies in the plane of the page.

• Dash: Bond goes into the page (away from the viewer).

• Wedge: Bond comes out of the page (towards the viewer).

Examples of Hydrocarbon Structures

• Hexane: $\mathrm{CH_3CH_2CH_2CH_2CH_2CH_3}$

• Diethylether: $\mathrm{CH_3CH_2OCH_2CH_3}$

• Cyclopentanone: Cyclic structure with a ketone functional group.

• 4-ethyl-2-methylhexane: Branched alkane with ethyl and methyl substituents.

Categories of Organic Molecules

• Aromatic: Molecules containing benzene ring-like structures.

• Aliphatic: All other hydrocarbons (straight-chain, branched, or cyclic).

Both types can be modified with functional groups, which are generally the site of chemical reactivity.

Types of Hydrocarbons

Alkanes

Alkanes are saturated hydrocarbons containing only single bonds. All carbons are sp3-hybridized.

• General formula for non-cyclic alkanes: $\mathrm{C_nH_{2n+2}}$

• General formula for cyclic alkanes: $\mathrm{C_nH_{2n}}$

• Can be straight chain, branched, or cyclic.

Alkenes and Alkynes

Alkenes and alkynes are unsaturated hydrocarbons:

• Alkenes: Contain at least one double carbon-carbon bond.

• Alkynes: Contain at least one triple carbon-carbon bond.

These unsaturated hydrocarbons are more reactive than alkanes due to the presence of multiple bonds.

Aromatic Molecules

Aromatic molecules have benzene-like structures, characterized by a ring of six carbon atoms with alternating double bonds (delocalized electrons).

• Common aromatic compounds: benzene, toluene, phenol, aniline, benzoic acid, benzaldehyde, benzamide.

• Substituted aromatics: 2-chlorophenol, 3-phenylpentanone.

Tabular Comparison of Molecular Representations

The following table compares different ways to represent molecules with the same molecular formula:

Molecular Formula	C3H6O acetone	C2H3N acetonitrile	C3H7NO2 L-alanine
Lewis Structure	Shows all atoms and bonds explicitly	Shows all atoms and bonds explicitly	Shows all atoms and bonds explicitly
Condensed Structural Formula	CH3COCH3	CH3CN	CH3CH(NH2)COOH
Bond-Line Structure	Shows basic bond angles, omits H atoms, uses wedges/hash to show spatial relationships	Shows basic bond angles, omits H atoms, uses wedges/hash to show spatial relationships	Shows basic bond angles, omits H atoms, uses wedges/hash to show spatial relationships

Additional info: The table above is inferred from the context and typical organic chemistry conventions.

Summary Table: Hydrocarbon Types

Type	Bond Type	General Formula	Example
Alkane	Single bonds	$\mathrm{C_nH_{2n+2}}$	Hexane
Alkene	Double bond(s)	$\mathrm{C_nH_{2n}}$	Ethene
Alkyne	Triple bond(s)	$\mathrm{C_nH_{2n-2}}$	Ethyne
Aromatic	Alternating double bonds (delocalized)	Variable	Benzene

Key Points for Study

• Organic chemistry centers on carbon-containing molecules.

• Hydrocarbons are classified as alkanes, alkenes, alkynes, and aromatics.

• Structural representations vary in detail and utility; bond-line structures are most efficient for large molecules.

• Functional groups determine chemical reactivity and properties.

• Understanding nomenclature and structure is foundational for further study in organic chemistry.