Organic Chemistry I – Structures: Hydrocarbons, Functional Groups, and Isomerism

Introduction to Organic Chemistry

Organic chemistry is the study of compounds containing carbon. Organic compounds are essential to life and industry, including substances such as proteins, sugars, DNA, vitamins, alcohols, pharmaceuticals, rubbers, plastics, soaps, gasoline, explosives, and artificial flavourings.

The Chemistry of Carbon

• Carbon can form a diverse array of molecules because:

  • 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 composed primarily of C, H, O, and N.

• Functional groups add character and different properties to hydrocarbons.

Hydrocarbons

Greek Prefixes for Carbon Chain Length

Greek prefixes are used to indicate the number of carbon atoms in an organic 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

Representing Organic Molecules

• Structural formula: Shows all atoms and bonds explicitly.

• Condensed structural formula: Groups atoms to simplify the structure.

• Carbon skeleton (bond-line) formula: Shows only the carbon backbone and functional groups; hydrogens on carbons are implied.

For large molecules, Lewis structures become cumbersome. Bond-line structures are more efficient and commonly used in organic chemistry.

Bond-Line and 3D Notation

• Bond-line structures omit hydrogen atoms bonded to carbon and show the carbon framework as lines.

• Dash-wedge notation is used to represent three-dimensionality:

  • Line: Bond in the plane of the page.

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

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

Example: Drawing Line Structures

• Hexane: $\mathrm{CH_3CH_2CH_2CH_2CH_2CH_3}$

• Diethyl ether: $\mathrm{CH_3CH_2OCH_2CH_3}$

• Cyclopentanone: Five-membered ring with a ketone group.

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

Categories of Organic Molecules

• Aromatic: Contain benzene ring-like structures (e.g., benzene, phenyl group).

• Aliphatic: All other organic molecules (alkanes, alkenes, alkynes, etc.).

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

Alkanes

• Alkanes are saturated hydrocarbons (only single bonds).

• All carbons are sp3-hybridized.

• Can be straight-chain, branched, or cyclic.

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

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

Alkenes and Alkynes

• Alkenes are unsaturated hydrocarbons with at least one double C=C bond.

• Alkynes are unsaturated hydrocarbons with at least one triple C≡C bond.

• Alkenes and alkynes are more reactive than alkanes due to the presence of π bonds.

Aromatic Molecules

• Aromatic compounds contain benzene-like rings, which are highly stable due to resonance.

• Examples: benzene, toluene, phenol, aniline, benzoic acid, benzaldehyde.

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules.

• Halides: Alkanes with a halogen (F, Cl, Br, I) attached. General formula: R–X.

• Alcohols: Contain an –OH group. Classified as primary (1°), secondary (2°), or tertiary (3°) based on the number of carbon substituents on the carbon bearing the –OH.

• Thiols: Contain an –SH group. Also classified as 1°, 2°, or 3°.

• Ethers: Contain an R–O–R' structure. Bent geometry around oxygen.

• Thioethers: Contain an R–S–R' structure. Bent geometry around sulfur.

• Aldehydes: Contain a terminal carbonyl group (–CHO).

• Ketones: Contain an internal carbonyl group (C=O bonded to two carbons).

• Carboxylic acids: Contain a –COOH group. Acidic in nature.

• Esters: Contain a –COOR group. Common in artificial flavourings.

• Amines: Nitrogen analogues of alcohols. Classified as 1°, 2°, or 3°.

• Amides: Nitrogen analogues of carboxylic acids. Also classified as 1°, 2°, or 3°.

Examples of Functional Groups

• Primary amine: –NH2 attached to one carbon.

• Secondary amine: –NH attached to two carbons.

• Tertiary amine: N attached to three carbons.

• Functional group identification: For example, in aromatic molecules, you may find phenol (–OH on benzene), benzaldehyde (–CHO on benzene), benzoic acid (–COOH on benzene), amine (–NH2), or amide (–CONH2).

Isomerism

Isomers are compounds with the same chemical formula but different arrangements of atoms. There are three major types:

• Constitutional isomers: Atoms are connected in a different order.

• Geometric (diastereomers): Same connectivity, different spatial arrangement (e.g., cis/trans isomers).

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

Constitutional Isomers

• Can be identified from Lewis structures by different connectivity.

• Example: Butane and isobutane (C4H10).

Configurational (Stereoisomers)

• Same connectivity, different arrangement in space.

• Includes geometric and optical isomers.

Geometric Isomers

• Different spatial arrangement around a double bond or ring (cis/trans or E/Z).

Optical Isomers (Enantiomers)

• Non-superimposable mirror images.

• Chirality arises when a carbon has four different groups attached (chiral center).

• Enantiomers have identical physical properties except for the direction in which they rotate plane-polarized light and their reactions with other chiral substances.

• Many drugs are chiral; often, only one enantiomer is biologically active.

Summary Table: Types of Isomerism

Type	Description	Example
Constitutional	Different connectivity	Butane vs. isobutane
Geometric	Same connectivity, different spatial arrangement (cis/trans)	cis-2-butene vs. trans-2-butene
Optical	Non-superimposable mirror images (enantiomers)	L-alanine vs. D-alanine

Key Formulas and Concepts

• General formula for alkanes: $C_nH_{2n+2}$

• General formula for cycloalkanes: $C_nH_{2n}$

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

Applications and Examples

• Functional group identification is essential for predicting chemical reactivity and properties.

• Isomerism explains the diversity of organic compounds with the same molecular formula.

• Bond-line structures are the standard for representing complex organic molecules efficiently.

Additional info: The notes above are based on standard introductory organic chemistry content, with some inferred context for completeness and clarity.